Patent Publication Number: US-2021180076-A1

Title: Compositions and methods for genome editing in plants

Description:
GOVERNMENT SUPPORT 
     This invention was made with government support under IOS1758745 awarded by National Science Foundation and 20183352228789 awarded by USDA National Institute of Food and Agriculture. The government has certain rights in the invention. 
     CROSS REFERENCE T0 RELATED APPLICATIONS 
     This application claims priority to provisional application U.S. Ser. No. 62/949,230, filed Dec. 17, 2019, which is incorporated herein by reference in its entirety. 
    
    
     SEQUENCE LISTING 
     The instant application contains a Sequence Listing which has been submitted in ASCII format via EFS-Web and is hereby incorporated by reference in its entirety. Said ASCII copy, created on Dec. 15, 2020, is named 2020-12-15QI_P13394US01_SEQLISTING_ST25.txt and is 436,494 bytes in size. 
     TECHNICAL FIELD 
     The present invention relates to compositions and methods for editing genomic sequences and for modulating gene expression in plants. 
     BACKGROUND 
     Methodologies for specific gene targeting or precise genome editing are of great importance to functional characterization of plant genes and genetic improvement of agricultural crops. Plants with stably modified genomic DNA can have new traits such as herbicide tolerance, insect resistance, or accumulation of valuable proteins including pharmaceutical proteins and industrial enzymes imparted to them. The expression of native plant genes may be up- or down-regulated or otherwise altered, their expression may be abolished entirely, DNA sequences may be altered (e.g., through point mutations, insertions, or deletions), or new non-native genes may be inserted into a plant genome to impart new traits to the plant. 
     The most common methods for modifying plant genomic DNA tend to modify the DNA at random sites within the genome. In many cases, however, it is desirable to modify the genomic DNA at a pre-determined target site in the plant genome of interest, e.g., to avoid disruption of native plant genes or to insert a transgene cassette at a genomic locus that is known to provide robust gene expression. Only recently have technologies for targeted modification of plant genomic DNA become available. CRISPR-Cas9 and Cas12a, as RNA-guided endonuclease systems, have become leading sequence-specific nucleases (SSNs) in plant genome engineering. Cas12b (formerly C2c1), a class 2 type V-B CRISPR system, was recently demonstrated as a new SSN for mammalian genome editing. Similar to Cas12a (formerly Cpfl, a class 2 type V-A system), Cas12b prefers T-rich PAMs (protospacer adjacent motifs) and generates staggered ends of DNA double-strand breaks (DSBs). Similar to Cas9 (a class 2 type II system), Cas12b requires a CRISPR RNA (crRNA) and a trans-activating crRNA (tracrRNA), which can be combined as a single guide RNA (sgRNA), for DNA targeting. By contrast, Cas12a only requires a crRNA. Hence, Cas12b is more amendable than Cas12a with versatile guide RNA engineering. In addition, Cas12b is significantly smaller than Cas9 and Cas12a in protein size. In human and mouse cells, AaCas12b can barely tolerate single base pair mismatches in the protospacer, suggesting it has high targeting specificity. 
     It is an objective of the present disclosure to provide Cas12b systems for plant genome engineering. Additional objectives, features, and advantages will become apparent based on the disclosure contained herein. 
     SUMMARY 
     The presently disclosed subject matter relates generally to genome engineering. In certain embodiments, the disclosed subject matter relates to compositions and methods for editing genome sequences in a cell. The cell may be a prokaryotic cell or a eukaryotic cell. The cell may be a non-mammalian cell. Preferably, the cell is a plant cell. 
     In certain embodiments, the compositions relate to CRISPR Cas12b nucleases, for example,  Alicyclobacillus acidoterrestris  Cas12b (AacCas12b),  Alicyclobacillus acidiphilus  Cas12b (AaCas12b),  Bacillus thermoamylovorans  Cas12b (BthCas12b), and  Bacillus hisashii  Cas12b (BhCas12b). Applicants have surprisingly found that Cas12b from  Alicyclobacillus acidiphilus  provides superior editing efficiency in plant cells. The methods produce double-stranded breaks (DSBs) at a target site in a genomic DNA sequence, resulting in mutation, insertion, and/or deletion of DNA sequences at the target site in a genome. In certain embodiments, the methods may include multiplexed genome editing. 
     Compositions comprise DNA constructs comprising nucleotide sequences that encode a Cas12b protein operably linked to a promoter that is operable in the cells of interest. Particular Cas12b protein sequences are set forth in SEQ ID NOs: 5-8; particular Cas12b protein-encoding polynucleotide sequences are set forth in SEQ ID NOs: 1-4. The DNA constructs comprising polynucleotide sequences that encode the Cas12b proteins of the invention, or the Cas12b proteins of the invention themselves, can be used to direct the modification of genomic DNA at genomic loci. Methods to use these DNA constructs to modify genomic DNA sequences are described herein. In certain embodiments, the DNA constructs are vectors for delivery of Cas12b to plant cells. Modified plants and plant cells, including  Oryza sativa  and  Oryza sativa  cells, are also encompassed. 
     Compositions and methods for modulating the expression of genes are also provided. The methods target protein(s) to sites in a genome to effect an up- or down-regulation of a gene or genes whose expression is regulated by the targeted site in the genome. Compositions comprise DNA constructs comprising nucleotide sequences that encode a modified Cas12b protein with diminished or abolished nuclease activity, optionally fused to a transcriptional activation or repression domain. Methods to use these DNA constructs to modify gene expression are described herein. 
     While multiple embodiments are disclosed, still other embodiments of the inventions will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. Accordingly, the figures and detailed description are to be regarded as illustrative in nature and not restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The following drawings form part of the specification and are included to further demonstrate certain embodiments or various aspects of the invention. In some instances, embodiments of the invention can be best understood by referring to the accompanying figures in combination with the detailed description presented herein. The description and accompanying figures may highlight a certain specific example, or a certain aspect of the invention. However, one skilled in the art will understand that portions of the example or aspect may be used in combination with other examples or aspects of the invention. 
         FIG. 1  is an illustration of the dual Pol II promoter system for expression of Cas12b and sgRNA. Note the sgRNA is flanked by HH and HDV ribozymes for precise processing. 
         FIG. 2  shows a comparison of mutation frequencies by AacCas12b, AaCas12b and BthCas12b at two target sites. Data were generated from high-throughput sequencing. Error bars represent standard deviations of two or three biological replicates. 
         FIG. 3A  and  FIG. 3B  show a comparison of deletion sizes by AacCas12b and AaCas12b at the OsEPFL9-sgRNA02 site and the OsGS3-sgRNA02 site. Data were generated from high-throughput sequencing. Error bars represent standard deviations of two or three biological replicates.  FIG. 3C  shows deletion sizes at OsEPFL9-sgRNA02 by BthCas12b. Error bars represent standard deviations of two biological replicates. 
         FIG. 4A  and  FIG. 4B  show a comparison of deletion position by AaCas12b at the OsEPFL9-sgRNA02 site and OsGS3-sgRNA02 site (SEQ ID NOs: 56-57). PAM and protospacer sequences are circled and underlined, respectively. Data were generated from high-throughput sequencing. Error bars represent standard deviations of two or three biological replicates.  FIG. 4C  and  FIG. 4D  show deletion position at OsEPFL9-sgRNA02 and OsGS3-sgRNA02 sites by AacCas12b (SEQ ID NOs: 56-57). Error bars represent standard deviations of two biological replicates.  FIG. 4E  shows deletion position at OsEPFL9-sgRNA02 by BthCas12b (SEQ ID NO: 56). Error bars represent standard deviations of two biological replicates. 
         FIG. 5A  and  FIG. 5B  show mutation frequency and deletion sizes at OsPDS-sgRNA01 by AacCas12b in rice protoplasts. Error bars represent standard deviations of two biological replicates. 
         FIG. 6  shows a comparison of mutation frequencies by AacCas12b and AaCas12b at 10 sites with ATTV, CTTA and GTTG PAMs. Data were generated from high-throughput sequencing. Error bars represent standard deviations of two or three biological replicates. 
         FIG. 7A  shows targeted mutagenesis by AaCas12b at additional three PAM sites. Data were generated from high-throughput sequencing. Error bars represent standard deviations of two or three biological replicates.  FIG. 7B  shows deletion sizes and position at OsR005-sgRNA02, OsDEP1-sgRNA01 and OsmiR528-sgRNA02 sites by AaCas12b in rice protoplasts (SEQ ID NOs: 90-92). Error bars represent standard deviations of two biological replicates. 
         FIG. 8A  and  FIG. 8B  show off-targeting analysis with mismatch (MM) sgRNAs at the OsEPFL9-sgRNA02 site and the 0512g24050-sgRNA01 site by AaCas12b (SEQ ID NOs: 58-71). Data were generated from RFLP analysis. Error bars represent standard deviations of two or three biological replicates. 
         FIG. 9  shows a comparison of protospacer length for targeted mutagenesis at the OsEPFL9-sgRNA02 site by AaCas12b (SEQ ID NOs: 64 and 72-76). Data were generated from RFLP analysis. Error bars represent standard deviations of two or three biological replicates. 
         FIG. 10A  and  FIG. 10B  show a summary of the genotyping results on stable transgenic T0 lines at the OsEPFL9-crRNA02 site by AacCas12b and AaCas12b. Genotypes of five example mutants are shown for each Cas12b (SEQ ID NOs: 77-89). The PAM sequence (GTTG) and the target sequence are in grey. The NcoI enzyme site used in RFLP analysis is underlined. 
         FIG. 11  is an illustration of the dual Pol II promoter based multiplexed Cas12b systems for AacCas12b and AaCas12b. 
         FIG. 12A  and  FIG. 12B  are a summary of the genotyping results on multiplexed stable transgenic T0 lines by AacCas12b and AaCas12b at three sites: OsR005-sgRNA02 (L4), OsEPFL9-sgRNA02 (L10) and OsGS3-sgRNA02 (L12). RFLP analysis of independent T0 lines (shown below) and Sanger sequencing were both used for genotyping. The plus sign ‘+’ indicates heterozygous or homozygous mutants confirmed by both methods. 
         FIG. 13  is an illustration of protein domains of AacCas12b. The three amino acid mutations used to inactivate Cas12b nuclease activity are indicated. 
         FIG. 14A  and  FIG. 14B  shows RFLP analysis of nuclease activity for protein variants of AacCas12b and AaCas12b in rice protoplasts. 
         FIG. 15  is an illustration of nine sgRNAs that direct targeted transcriptional repression at 0504g39780. Relative targeting positions and PAM sites are indicated. These sgRNAs target either the non-template strand (NT) or the template strand (T) of the DNA. 
         FIG. 16A ,  FIG. 16B , and  FIG. 16C  show qRT-PCR data showing targeted repression of 0s04g39780 in rice protoplasts. dAacCas12b, dAaCas12b and dBthCas12b were compared at three different PAMs at different target positions. Student&#39;s t-test: *P&lt;0.05, **P&lt;0.01, ***P&lt;0.001, compared to the control. 
         FIG. 17A  shows illustrations of three synthetic transcriptional repressors based on AacCas12b, AaCas12b and BthCas12b.  FIG. 17B  is qRT-PCR data showing targeted repression by dCas12b-SRDX repressors in rice protoplasts. OsTubulin was used as the endogenous control gene. The gene expression level of the wild type was normalized as 1. Error bars represent standard deviations of two biological replicates. 
         FIG. 18  shows a comparison of AaCas12b (with three sgRNA scaffolds) and BhCas12b systems at ATTA-01, ATTC-01, ATTG-02 and GTTG-01 sites in rice protoplasts. 
         FIG. 19  shows a comparison of four MS2-containing Aac sgRNA scaffolds at ATTG-02 and GTTG-01 sites in rice protoplasts. 
         FIG. 20  shows schematics of four activation systems tested, including dFnCas12a-TV (I), dAaCas12b-TV with three different sgRNA scaffolds (II), dAaCas12b-TV with four different sgRNA scaffolds containing an MS2 aptamer to recruit MS2-TV (III), and dAaCas12b-TV with four different sgRNA scaffolds containing an MS2 aptamer to recruit MS2-VPR (IV). 
         FIG. 21A  and  FIG. 21B  show qRT-PCR data showing targeted activation of OsER1 and OsGW7 in rice protoplasts. A total of 12 activations systems were tested. A sgRNA was used to direct each Cas12 activation system to the promoter of interest. OsTubulin was used as the endogenous control gene. The gene expression level of the wild type was normalized as 1. Student&#39;s t test: *P&lt;0.1, **P&lt;0.02, compared to the control. Error bars represent standard deviations of three biological replicates. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to Cas12b-mediated genome editing in plants. Methods and compositions are provided herein for the control of gene expression involving sequence targeting, such as genome perturbation or gene-editing, that relate to the CRISPR-Cas12b system and components thereof. The CRISPR enzymes of the invention are selected from a Cas12b enzyme. The methods and compositions include nucleic acids to bind target DNA sequences. Also provided are nucleic acids encoding the Cas12b polypeptides, as well as methods of using Cas12b polypeptides to modify chromosomal (i.e., genomic) or organellar DNA sequences of host cells including plant cells. The Cas12b polypeptides interact with specific guide RNAs (gRNAs), which direct the Cas12b endonuclease to a specific target site, at which site the Cas12b endonuclease introduces a double-stranded break that can be repaired by a DNA repair process such that the DNA sequence is modified. The methods disclosed herein can be used to target and modify specific chromosomal sequences and/or introduce exogenous sequences at targeted locations in the genome of plant cells. The methods can further be used to introduce sequences or modify regions within organelles (e.g., chloroplasts and/or mitochondria). Furthermore, the targeting is specific with limited off target effects. 
     So that the present invention may be more readily understood, certain terms are first defined. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which embodiments of the invention pertain. Many methods and materials similar, modified, or equivalent to those described herein can be used in the practice of the embodiments of the present invention without undue experimentation, the preferred materials and methods are described herein. In describing and claiming the embodiments of the present invention, the following terminology will be used in accordance with the definitions set out below. 
     It is to be understood that all terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting in any manner or scope. For example, as used in this specification and the appended claims, the singular forms “a,” “an” and “the” can include plural referents unless the content clearly indicates otherwise. Similarly, the word “or” is intended to include “and” unless the context clearly indicate otherwise. The word “or” means any one member of a particular list and also includes any combination of members of that list. Further, all units, prefixes, and symbols may be denoted in its SI accepted form. 
     Numeric ranges recited within the specification are inclusive of the numbers defining the range and include each integer within the defined range. Throughout this disclosure, various aspects of this invention are presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges, fractions, and individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6, and decimals and fractions, for example, 1.2, 3.8, 1½, and 4¾ This applies regardless of the breadth of the range. 
     The term “about,” as used herein, refers to variation in the numerical quantity that can occur, for example, through typical measuring techniques and equipment, with respect to any quantifiable variable, including, but not limited to, mass, volume, time, and temperature. Further, given solid and liquid handling procedures used in the real world, there is certain inadvertent error and variation that is likely through differences in the manufacture, source, or purity of the ingredients used to make the compositions or carry out the methods and the like. The term “about” also encompasses these variations. Whether or not modified by the term “about,” the claims include equivalents to the quantities. 
     The methods and compositions of the present invention may comprise, consist essentially of, or consist of the components and ingredients of the present invention as well as other ingredients described herein. As used herein, “consisting essentially of” means that the methods, systems, apparatuses and compositions may include additional steps, components or ingredients, but only if the additional steps, components or ingredients do not materially alter the basic and novel characteristics of the claimed methods, systems, apparatuses, and compositions. 
     Cas12b Endonucleases 
     The terms “CRISPR-Cas protein”, “CRISPR protein”, “Cas protein”, “Cas effector protein”, “CRISPR enzyme”, and “Cas enzyme” may be used interchangeably herein. Provided herein are Cas12b proteins, and fragments and variants thereof, for use in modifying genomes including plant genomes. The present disclosure encompasses the use of a Cas12b effector protein, derived from a Cas12b locus denoted as subtype V-B. Such effector proteins are also referred to as C2c1. Cas12b is a large protein (about 1100-1300 amino acids) that contains a RuvC-like nuclease domain homologous to the corresponding domain of Cas9 along with a counterpart to the characteristic arginine-rich cluster of Cas9. However, Cas12b lacks the HNH nuclease domain that is present in all Cas9 proteins, and the RuvC-like domain is contiguous in the Cas12b sequence, in contrast to Cas9 where it contains long inserts including the HNH domain. Accordingly, in particular embodiments, the CRISPR-Cas enzyme comprises only a RuvC-like nuclease domain. 
     Cas12b creates a staggered cut at the target locus, with a 5′ overhang, or a “sticky end” at the PAM distal side of the target sequence. In some embodiments, the 5′ overhang is 7 nt. See Lewis and Ke, Mol Cell. 2017 Feb. 2; 65(3):377-379. Cas12b creates double strand breaks at the distal end of PAM, in contrast to cleavage at the proximal end of PAM created by Cas9. Cas12a and Cas12b are both Type V CRISPR-Cas proteins that share structure similarity. Unlike Cas9, which generates blunt cuts at the proximal end of PAM, Cas12a and Cas12b generate staggered cuts at the distal end of PAM. 
     Cas12b polypeptides can be wild type Cas12b polypeptides, modified Cas12b polypeptides, or a fragment of a wild type or modified Cas12b polypeptide. The Cas12b polypeptide can be modified to increase nucleic acid binding affinity and/or specificity, alter an enzymatic activity, and/or change another property of the protein. For example, nuclease (i.e., DNase, RNase) domains of the Cas12b polypeptide can be modified, deleted, or inactivated. Alternatively, the Cas12b polypeptide can be truncated to remove domains that are not essential for the function of the protein. 
     In some embodiments, the Cas12b polypeptide can be derived from a wild type Cas12b polypeptide or fragment thereof. In other embodiments, the Cas12b polypeptide can be derived from a modified Cas12b polypeptide. For example, the amino acid sequence of the Cas12b polypeptide can be modified to alter one or more properties (e.g., nuclease activity, affinity, stability, etc.) of the protein. Alternatively, domains of the Cas12b polypeptide not involved in RNA-guided cleavage can be eliminated from the protein such that the modified Cas12b polypeptide is smaller than the wild type Cas12b polypeptide. 
     In some embodiments, the Cas12b polypeptide can be modified to inactivate the nuclease domain so that it is no longer functional. In some embodiments in which one of the nuclease domains is inactive, the Cas12b polypeptide does not cleave double-stranded DNA. In specific embodiments, the mutated Cas12b polypeptide comprises one or more mutations in a position corresponding to amino acid positions D570, E848, or D977 in  Alicyclobacillus acidiphilus  Cas12b when aligned for maximum identity that reduces or eliminates the nuclease activity. 
     The nuclease domain can be modified using well-known methods, such as site-directed mutagenesis, PCR-mediated mutagenesis, and total gene synthesis, as well as other methods known in the art. Cas12b proteins with inactivated nuclease domains (dCas12b proteins) can be used to modulate gene expression without modifying DNA sequences. In certain embodiments, a dCas12b protein may be targeted to particular regions of a genome such as promoters for a gene or genes of interest through the use of appropriate gRNAs. The dCas12b protein can bind to the desired region of DNA and may interfere with RNA polymerase binding to this region of DNA and/or with the binding of transcription factors to this region of DNA. This technique may be used to up- or down-regulate the expression of one or more genes of interest. In certain other embodiments, the dCas12b protein may be fused to a repressor domain to further downregulate the expression of a gene or genes whose expression is regulated by interactions of RNA polymerase, transcription factors, or other transcriptional regulators with the region of chromosomal DNA targeted by the gRNA. In certain other embodiments, the dCas12b protein may be fused to an activation domain to effect an upregulation of a gene or genes whose expression is regulated by interactions of RNA polymerase, transcription factors, or other transcriptional regulators with the region of chromosomal DNA targeted by the gRNA. 
     The Cas12b polypeptides disclosed herein can further comprise at least one nuclear localization signal (NLS). In general, an NLS comprises a stretch of basic amino acids. Nuclear localization signals are known in the art (see, e.g., Lange et al.,  J. Biol. Chem . (2007) 282:5101-5105). The NLS can be located at the N-terminus, the C-terminus, or in an internal location of the Cas12b polypeptide. 
     The Cas12b polypeptide disclosed herein can further comprise at least one plastid targeting signal peptide, at least one mitochondrial targeting signal peptide, or a signal peptide targeting the Cas12b polypeptide to both plastids and mitochondria. Plastid, mitochondrial, and dual-targeting signal peptide localization signals are known in the art (see, e.g., Nassoury and Morse (2005)  Biochim Biophys Acta  1743:5-19; Kunze and Berger (2015)  Front Physiol  6:259; Herrmann and Neupert (2003)  IUBMB Life  55:219-225; Soll (2002)  Curr Opin Plant Biol  5:529-535; Carrie and Small (2013)  Biochim Biophys Acta  1833:253-259; Carrie et al. (2009)  FEBS J  276:1187-1195; Silva-Filho (2003)  Curr Opin Plant Biol  6:589-595; Peeters and Small (2001)  Biochim Biophys Acta  1541:54-63; Murcha et al. (2014)  J Exp Bot  65:6301-6335; Mackenzie (2005)  Trends Cell Biol  15:548-554; Glaser et al. (1998)  Plant Mol Biol  38:311-338). The plastid, mitochondrial, or dual-targeting signal peptide can be located at the N-terminus, the C-terminus, or in an internal location of the Cas12b polypeptide. 
     In still other embodiments, the Cas12b polypeptide can also comprise at least one marker domain. Non-limiting examples of marker domains include fluorescent proteins, purification tags, and epitope tags. In certain embodiments, the marker domain can be a fluorescent protein. Non limiting examples of suitable fluorescent proteins include green fluorescent proteins (e.g., GFP, GFP-2, tagGFP, turboGFP, EGFP, Emerald, Azami Green, Monomeric Azami Green, CopGFP, AceGFP, ZsGreen1), yellow fluorescent proteins (e.g. YFP, EYFP, Citrine, Venus, YPet, PhiYFP, ZsYellow1), blue fluorescent proteins (e.g. EBFP, EBFP2, Azurite, mKalamal, GFPuv, Sapphire, T-sapphire), cyan fluorescent proteins (e.g. ECFP, Cerulean, CyPet, AmCyanl, Midoriishi-Cyan), red fluorescent proteins (mKate, mKate2, mPlum, DsRed monomer, mCherry, mRFP1, DsRed-Express, DsRed2, DsRed-Monomer, HcRed-Tandem, HcRedl, AsRed2, eqFP611, mRasberry, mStrawberry, Jred), and orange fluorescent proteins (mOrange, mKO, Kusabira-Orange, Monomeric Kusabira-Orange, mTangerine, tdTomato) or any other suitable fluorescent protein. In other embodiments, the marker domain can be a purification tag and/or an epitope tag. Exemplary tags include, but are not limited to, glutathione-S-transferase (GST), chitin binding protein (CBP), maltose binding protein, thioredoxin (TRX), poly(NANP), tandem affinity purification (TAP) tag, myc, AcV5, AU1, AUS, E, ECS, E2, FLAG, HA, nus, Softag 1, Softag 3, Strep, SBP, Glu-Glu, HSV, KT3, S, 51, T7, V5, VSV-G, 6×His, biotin carboxyl carrier protein (BCCP), and calmodulin. 
     In certain embodiments, the Cas12b polypeptide may be part of a protein-RNA complex comprising a guide RNA. The guide RNA interacts with the Cas12b polypeptide to direct the Cas12b polypeptide to a specific target site, wherein the 5′ end of the guide RNA can base pair with a specific protospacer sequence of the nucleotide sequence of interest in the plant genome, whether part of the nuclear, plastid, and/or mitochondrial genome. As used herein, the term “DNA-targeting RNA” refers to a guide RNA that interacts with the Cas12b polypeptide and the target site of the nucleotide sequence of interest in the genome of a cell. A DNA-targeting RNA, or a DNA polynucleotide encoding a DNA-targeting RNA, can comprise: a first segment comprising a nucleotide sequence that is complementary to a sequence in the target DNA, and a second segment that interacts with a Cas12b polypeptide. 
     The polynucleotides encoding Cas12b polypeptides disclosed herein can be used to isolate corresponding sequences from other prokaryotic or eukaryotic organisms, or from metagenomically-derived sequences whose native host organism is unclear or unknown. In this manner, methods such as PCR, hybridization, and the like can be used to identify such sequences based on their sequence homology or identity to the sequences set forth herein. Sequences isolated based on their sequence identity to the entire Cas12b sequences set forth herein or to variants and fragments thereof are encompassed by the present invention. Such sequences include sequences that are orthologs of the disclosed Cas12b sequences. “Orthologs” is intended to mean genes derived from a common ancestral gene and which are found in different species as a result of speciation. Genes found in different species are considered orthologs when their nucleotide sequences and/or their encoded protein sequences share at least about 75%, about 80%, about 85%, about 90%, about 91%, about 92%, about 93%, about 94%, about 95%, about 96%, about 97%, about 98%, about 99%, or greater sequence identity. Functions of orthologs are often highly conserved among species. Thus, isolated polynucleotides that encode polypeptides having Cas12b endonuclease activity and which share at least about 75% or more sequence identity to the sequences disclosed herein, are encompassed by the present invention. 
     In particular embodiments, the Cas12b protein from an organism from a genus comprising  Alicyclobacillus, Desulfovibrio, Desulfonatronum, Opitutaceae, Tuberibacillus, Bacillus, Brevibacillus, Candidatus, Desulfatirhabdium, Citrobacter, Elusimicrobia, Methylobacterium, Omnitrophica, Phycisphaerae, Planctomycetes, Spirochaetes , and  Verrucomicrobiaceae.    
     In further particular embodiments, the Cas12b protein is from a species selected from  Alicyclobacillus acidoterrestris  (e.g., ATCC 49025),  Alicyclobacillus contaminans  (e.g., DSM 17975),  Alicyclobacillus macrosporangiidus  (e.g. DSM 17980),  Bacillus hisashii  strain C4 , Candidatus Lindowbacteria  bacterium RIFCSPLOW02 , Desulfovibrio inopinatus  (e.g., DSM 10711),  Desulfonatronum thiodismutans  (e.g., strain MLF-1),  Elusimicrobia bacterium  RIFOXYA12, Omnitrophica WOR 2 bacterium RIFCSPHIGH02 , Opitutaceae  bacterium TAVS,  Phycisphaerae bacterium  ST-NAGAB-D1 , Planctomycetes bacterium  RBG 13 46 10 , Spirochaetes bacterium  GWB1 27 13 , Verrucomicrobiaceae bacterium  UBA2429 , Tuberibacillus calidus  (e.g., DSM 17572),  Bacillus thermoamylovorans  (e.g., strain B4166),  Brevibacillus  sp. CF112 , Bacillus  sp. NSP2.1,  Desulfatirhabdium butyrativorans  (e.g., DSM 18734),  Alicyclobacillus herbarius  (e.g., DSM 13609),  Citrobacter freundii  (e.g., ATCC 8090),  Brevibacillus agri  (e.g., BAB-2500),  Methylobacterium nodulans (e.g., ORS 2060). 
     In some embodiments, the Cas12b from  Alicyclobacillus acidoterrestris, Alicyclobacillus acidiphilus, Bacillus thermoamylovorans , or  Bacillus hisashii . In a preferred embodiment, the Cas12b from  Alicyclobacillus acidiphilus . Applicants have surprisingly found that Cas12b from  Alicyclobacillus acidiphilus  provides superior editing efficiency in plant cells. 
     As used herein, Cas12b endonuclease activity refers to CRISPR endonuclease activity wherein, a guide RNA (gRNA) associated with a Cas12b polypeptide causes the Cas12b-gRNA complex to bind to a pre-determined nucleotide sequence that is complementary to the gRNA; and wherein Cas12b activity can introduce a double-stranded break at or near the site targeted by the gRNA. In certain embodiments, this double-stranded break may be a staggered DNA double-stranded break. As used herein a “staggered DNA double-stranded break” can result in a double strand break with about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9, or about 10 nucleotides of overhang on either the 3′ or 5′ ends following cleavage. In specific embodiments, the Cas12b polypeptide introduces a staggered DNA double-stranded break with a 5′ overhang. The double strand break can occur at or near the sequence to which the DNA-targeting RNA (e.g., guide RNA) sequence is targeted. 
     Fragments and variants of the Cas12b polynucleotides and Cas12b amino acid sequences encoded thereby that retain Cas12b nuclease activity are encompassed herein. By “Cas12b nuclease activity” is intended the binding of a pre-determined DNA sequence as mediated by a guide RNA. In embodiments wherein the Cas12b nuclease retains a functional RuvC domain, Cas12b nuclease activity can further comprise double-strand break induction. By “fragment” is intended a portion of the polynucleotide or a portion of the amino acid sequence. “Variants” is intended to mean substantially similar sequences. For polynucleotides, a variant comprises a polynucleotide having deletions (i.e., truncations) at the 5′ and/or 3′ end; deletion and/or addition of one or more nucleotides at one or more internal sites in the native polynucleotide; and/or substitution of one or more nucleotides at one or more sites in the native polynucleotide. As used herein, a “native” polynucleotide or polypeptide comprises a naturally occurring nucleotide sequence or amino acid sequence, respectively. Generally, variants of a particular polynucleotide of the invention will have at least about 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to that particular polynucleotide as determined by sequence alignment programs and parameters as described elsewhere herein. 
     “Variant” amino acid or protein is intended to mean an amino acid or protein derived from the native amino acid or protein by deletion (so-called truncation) of one or more amino acids at the N-terminal and/or C-terminal end of the native protein; deletion and/or addition of one or more amino acids at one or more internal sites in the native protein; or substitution of one or more amino acids at one or more sites in the native protein. Variant proteins encompassed by the present invention are biologically active, that is they continue to possess the desired biological activity of the native protein. Biologically active variants of a native polypeptide will have at least about 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to the amino acid sequence for the native sequence as determined by sequence alignment programs and parameters described herein. A biologically active variant of a protein of the invention may differ from that protein by as few as 1-15 amino acid residues, as few as 1-10, such as 6-10, as few as 5, as few as 4, 3, 2, or even 1 amino acid residue. 
     Variant sequences may also be identified by analysis of existing databases of sequenced genomes. In this manner, corresponding sequences can be identified and used in the methods of the invention. 
     Methods of alignment of sequences for comparison are well known in the art. Thus, the determination of percent sequence identity between any two sequences can be accomplished using a mathematical algorithm. Non-limiting examples of such mathematical algorithms are the algorithm of Myers and Miller (1988)  CABIOS  4:11-17; the local alignment algorithm of Smith et al. (1981)  Adv. Appl. Math.  2:482; the global alignment algorithm of Needleman and Wunsch (1970)  J Mol. Biol.  48:443-453; the search-for-local alignment method of Pearson and Lipman (1988)  Proc. Natl. Acad. Sci.  85:2444-2448; the algorithm of Karlin and Altschul (1990)  Proc. Natl. Acad. Sci. USA  87:2264-2268, modified as in Karlin and Altschul (1993)  Proc. Natl. Acad. Sci. USA  90:5873-5877. 
     Computer implementations of these mathematical algorithms can be utilized for comparison of sequences to determine sequence identity. Such implementations include, but are not limited to: CLUSTAL in the PC/Gene program (available from Intelligenetics, Mountain View, Calif.); the ALIGN program (Version 2.0) and GAP, BESTFIT, BLAST, FASTA, and TFASTA in the GCG Wisconsin Genetics Software Package, Version 10 (available from Accelrys Inc., 9685 Scranton Road, San Diego, Calif., USA). Alignments using these programs can be performed using the default parameters. The CLUSTAL program is well described by Higgins et al. (1988)  Gene  73:237-244; Higgins et al. (1989)  CABIOS  5:151-153; Corpet et al. (1988)  Nucleic Acids Res.  16:10881-90; Huang et al. (1992) C ABIOS  8:155-65; and Pearson et al. (1994)  Meth. Mol. Biol.  24:307-331. The ALIGN program is based on the algorithm of Myers and Miller (1988) supra. A PAM120 weight residue table, a gap length penalty of 12, and a gap penalty of 4 can be used with the ALIGN program when comparing amino acid sequences. The MUSCLE algorithm for multiple sequence alignment may be used for comparisons of multiple nucleic acid or protein sequences (Edgar (2004)  Nucleic Acids Research  32:1792-1797). The BLAST programs of Altschul et al (1990)  J Mol. Biol.  215:403 are based on the algorithm of Karlin and Altschul (1990) supra. BLAST nucleotide searches can be performed with the BLASTN program, score=100, wordlength=12, to obtain nucleotide sequences homologous to a nucleotide sequence encoding a protein of the invention. BLAST protein searches can be performed with the BLASTX program, score=50, wordlength=3, to obtain amino acid sequences homologous to a protein or polypeptide of the invention. To obtain gapped alignments for comparison purposes, Gapped BLAST (in BLAST 2.0) can be utilized as described in Altschul et al. (1997)  Nucleic Acids Res.  25:3389. Alternatively, PSI-BLAST (in BLAST 2.0) can be used to perform an iterated search that detects distant relationships between molecules. See Altschul et al. (1997) supra. When utilizing BLAST, Gapped BLAST, PSI-BLAST, the default parameters of the respective programs (e.g., BLASTN for nucleotide sequences, BLASTX for proteins) can be used. See the website at www.ncbi.nlm.nih.gov. Alignment may also be performed manually by inspection. 
     The nucleic acid molecules encoding Cas12b polypeptides, or fragments or variants thereof, can be codon optimized for expression in a plant of interest or other cell or organism of interest. A “codon-optimized gene” is a gene having its frequency of codon usage designed to mimic the frequency of preferred codon usage of the host cell. Nucleic acid molecules can be codon optimized, either wholly or in part. Because any one amino acid (except for methionine and tryptophan) is encoded by a number of codons, the sequence of the nucleic acid molecule may be changed without changing the encoded amino acid. Codon optimization is when one or more codons are altered at the nucleic acid level such that the amino acids are not changed but expression in a particular host organism is increased. Those having ordinary skill in the art will recognize that codon tables and other references providing preference information for a wide range of organisms are available in the art (see, e.g., Zhang et al. (1991)  Gene  105:61-72; Murray et al. (1989)  Nucl. Acids Res.  17:477-508). Methodology for optimizing a nucleotide sequence for expression in a plant is provided, for example, in U.S. Pat. No. 6,015,891, and the references cited therein. 
     Fusion Proteins 
     Fusion proteins are provided herein comprising a Cas12b polypeptide, or a fragment or variant thereof, and an effector domain. The Cas12b polypeptide can be directed to a target site by a guide RNA, at which site the effector domain can modify or effect the targeted nucleic acid sequence. The effector domain can be a cleavage domain, an epigenetic modification domain, a transcriptional activation domain, a transcriptional repressor domain, a deaminase domain, or a reverse transcriptase. The fusion protein can further comprise at least one additional domain chosen from a nuclear localization signal, plastid signal peptide, mitochondrial signal peptide, signal peptide capable of protein trafficking to multiple subcellular locations, a cell-penetrating domain, or a marker domain, any of which can be located at the N-terminus, C-terminus, or an internal location of the fusion protein. The Cas12b polypeptide can be located at the N-terminus, the C-terminus, or in an internal location of the fusion protein. The Cas12b polypeptide can be directly fused to the effector domain, or can be fused with a linker. In specific embodiments, the linker sequence fusing the Cas12b polypeptide with the effector domain can be at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 40, or 50 amino acids in length. For example, the linker can range from 1-5, 1-10, 1-20, 1-50, 2-3, 3-10, 3-20, 5-20, or 10-50 amino acids in length. 
     In some embodiments, the Cas12b polypeptide of the fusion protein can be derived from a wild type Cas12b protein. The Cas12b-derived protein can be a modified variant or a fragment. In some embodiments, the Cas12b polypeptide can be modified to contain a nuclease domain (e.g. a RuvC or RuvC-like domain) with reduced or eliminated nuclease activity. For example, the Cas12b-derived polypeptide can be modified such that the nuclease domain is deleted or mutated such that it is no longer functional (i.e., the nuclease activity is absent). Particularly, a Cas12b polypeptide can have polypeptide comprises one or more mutations in a position corresponding to amino acid positions D570, E848, or D977 in  Alicyclobacillus acidiphilus  Cas12b (SEQ ID NO: 7) when aligned for maximum identity that reduces or eliminates the nuclease activity. 
     The nuclease domain can be inactivated by one or more deletion mutations, insertion mutations, and/or substitution mutations using known methods, such as site-directed mutagenesis, PCR-mediated mutagenesis, and total gene synthesis, as well as other methods known in the art. In an exemplary embodiment, the Cas12b polypeptide of the fusion protein is modified by mutating the RuvC-like domain such that the Cas12b polypeptide has no nuclease activity. 
     The fusion protein also comprises an effector domain located at the N-terminus, the C-terminus, or in an internal location of the fusion protein. In some embodiments, the effector domain is a cleavage domain. As used herein, a “cleavage domain” refers to a domain that cleaves DNA. The cleavage domain can be obtained from any endonuclease or exonuclease. Non-limiting examples of endonucleases from which a cleavage domain can be derived include, but are not limited to, restriction endonucleases and homing endonucleases. See, for example, New England Biolabs Catalog or Belfort et al. (1997)  Nucleic Acids Res.  25:3379-3388. Additional enzymes that cleave DNA are known (e.g., 51 Nuclease; mung bean nuclease; pancreatic DNase I; micrococcal nuclease; yeast HO endonuclease). See also Linn et al. (eds.) Nucleases, Cold Spring Harbor Laboratory Press, 1993. One or more of these enzymes (or functional fragments thereof) can be used as a source of cleavage domains. 
     In some embodiments, the cleavage domain can be derived from a type II-S endonuclease. Type II-S endonucleases cleave DNA at sites that are typically several base pairs away from the recognition site and, as such, have separable recognition and cleavage domains. These enzymes generally are monomers that transiently associate to form dimers to cleave each strand of DNA at staggered locations. Non-limiting examples of suitable type II-S endonucleases include BfiI, BpmI, BsaI, BsgI, BsmBI, BsmI, BspMI, FokI, MbolI, and SapI. 
     In certain embodiments, the type II-S cleavage can be modified to facilitate dimerization of two different cleavage domains (each of which is attached to a Cas12b polypeptide or fragment thereof). In embodiments wherein the effector domain is a cleavage domain the Cas12b polypeptide can be modified as discussed herein such that its endonuclease activity is eliminated. For example, the Cas12b polypeptide can be modified by mutating the RuvC-like domain such that the polypeptide no longer exhibits endonuclease activity. 
     In other embodiments, the effector domain of the fusion protein can be an epigenetic modification domain. In general, epigenetic modification domains alter histone structure and/or chromosomal structure without altering the DNA sequence. Changes in histone and/or chromatin structure can lead to changes in gene expression. Examples of epigenetic modification include, without limit, acetylation or methylation of lysine residues in histone proteins, and methylation of cytosine residues in DNA. Non-limiting examples of suitable epigenetic modification domains include histone acetyltansferase domains, histone deacetylase domains, histone methyltransferase domains, histone demethylase domains, DNA methyltransferase domains, and DNA demethylase domains. 
     In embodiments in which the effector domain is a histone acetyltansferase (HAT) domain, the HAT domain can be derived from EP300 (i.e., E1A binding protein p300), CREBBP (i.e., CREB-binding protein), CDY1, CDY2, CDYL1, CLOCK, ELP3, ESA1, GCNS (KAT2A), HAT1, KAT2B, KATS, MYST1, MYST2, MYST3, MYST4, NCOA1, NCOA2, NCOA3, NCOAT, P/CAF, Tip60, TAFII250, or TF3C4. In embodiments wherein the effector domain is an epigenetic modification domain, the Cas12b polypeptide can be modified as discussed herein such that its endonuclease activity is eliminated. For example, the Cas12b polypeptide can be modified by mutating the RuvC-like domain such that the polypeptide no longer possesses nuclease activity. 
     In some embodiments, the effector domain of the fusion protein can be a transcriptional activation domain. In general, a transcriptional activation domain interacts with transcriptional control elements and/or transcriptional regulatory proteins (i.e., transcription factors, RNA polymerases, etc.) to increase and/or activate transcription of one or more genes. In some embodiments, the transcriptional activation domain can be, without limit, a herpes simplex virus VP16 activation domain, VP64 (which is a tetrameric derivative of VP16), a NFκKB p65 activation domain, p53 activation domains 1 and 2, a CREB (cAMP response element binding protein) activation domain, an E2A activation domain, and an NFAT (nuclear factor of activated T-cells) activation domain. In other embodiments, the transcriptional activation domain can be Ga14, Gcn4, MLL, Rtg3, Gln3, Oaf1, Pip2, Pdr1, Pdr3, Pho4, and Leu3. The transcriptional activation domain may be wild type, or it may be a modified version of the original transcriptional activation domain. 
     In some embodiments, the effector domain of the fusion protein is a VP16 or VP64 transcriptional activation domain. In an exemplary embodiment, the transcriptional activation domain is TV or VPR. In embodiments wherein the effector domain is a transcriptional activation domain, the Cas12b polypeptide can be modified as discussed herein such that its endonuclease activity is eliminated. For example, the Cas12b polypeptide can be modified by mutating the RuvC-like domain such that the polypeptide no longer possesses nuclease activity. 
     In still other embodiments, the effector domain of the fusion protein can be a transcriptional repressor domain. In general, a transcriptional repressor domain interacts with transcriptional control elements and/or transcriptional regulatory proteins (i.e., transcription factors, RNA polymerases, etc.) to decrease and/or terminate transcription of one or more genes. Non-limiting examples of suitable transcriptional repressor domains include inducible cAMP early repressor (ICER) domains, Kruppel-associated box A (KRAB-A) repressor domains, YY1 glycine rich repressor domains, Spl-like repressors, E(spl) repressors, IκB repressor, and MeCP2. In an exemplary embodiment, the transcriptional repressor domain is an SRDX repressor domain. 
     In embodiments wherein the effector domain is a transcriptional repressor domain, the Cas12b polypeptide can be modified as discussed herein such that its endonuclease activity is eliminated. For example, the Cas12b polypeptide can be modified by mutating the RuvC-like domain such that the polypeptide no longer possesses nuclease activity. 
     In some embodiments, the effector domain of the fusion protein can be a nucleotide deaminase or a catalytic domain thereof. The nucleotide deaminase may be an adenosine deaminase or a cytidine deaminase. In general, a Cas12b fused with a deaminase domain can target a sequence in the genome of a plant through the direction of a guide RNA to perform base editing, including the introduction of C to T or A to G substitutions. In some embodiments, the adenosine deaminase can be, without limit, a member of the enzyme family known as adenosine deaminases that act on RNA (ADARs), a member of the enzyme family known as adenosine deaminases that act on tRNA (ADATs), or an adenosine deaminase domain-containing (ADAD) family member. In some embodiments, the cytidine deaminase can be, without limit, a member of the enzyme family known as apolipoprotein B mRNA-editing complex (APOBEC) family deaminase, an activation-induced deaminase (AID), or a cytidine deaminase 1 (CDA1). 
     In embodiments wherein the effector domain is a deaminase domain, the Cas12b polypeptide can be modified as discussed herein such that its endonuclease activity is eliminated. For example, the Cas12b polypeptide can be modified by mutating the RuvC-like domain such that the polypeptide no longer possesses nuclease activity. In some embodiments, the Cas12b polypeptide has nickase activity. 
     In some embodiments, the effector domain of the fusion protein can be a reverse transcriptase for prime editing. Prime editing of a target sequence enables the incorporation of a nucleotide change including a single-nucleotide change (e.g., any transition or any transversion), an insertion of one or more nucleotides, or a deletion of one or more nucleotides. A Cas12b fused with a reverse transcriptase is guided to a specific DNA sequence by a modified guide RNA, named a pegRNA. The pegRNA is altered (relative to a standard guide RNA) to comprise an extended portion that provides a DNA synthesis template sequence which encodes a single strand DNA flap, which is homologous to a strand of the targeted endogenous DNA sequence to be edited, but which contains the desired one or more nucleotide changes and which, following synthesis by the reverse transcriptase, becomes incorporated into the target DNA molecule. Prime editing is disclosed in, for example, PCT Publication WO/2020/191248, the entire contents of which is hereby incorporated by reference. 
     In embodiments wherein the effector domain is a reverse transcriptase, the Cas12b polypeptide can be modified as discussed herein such that its endonuclease activity is eliminated. For example, the Cas12b polypeptide can be modified by mutating the RuvC-like domain such that the polypeptide no longer possesses nuclease activity. In some embodiments, the Cas12b polypeptide has nickase activity. 
     In some embodiments, the fusion protein further comprises at least one additional domain. Non-limiting examples of suitable additional domains include nuclear localization signals, cell-penetrating or translocation domains, and marker domains. 
     When the effector domain of the fusion protein is a cleavage domain, a dimer comprising at least one fusion protein can form. The dimer can be a homodimer or a heterodimer. In some embodiments, the heterodimer comprises two different fusion proteins. In other embodiments, the heterodimer comprises one fusion protein and an additional protein. 
     The dimer can be a homodimer in which the two fusion protein monomers are identical with respect to the primary amino acid sequence. In one embodiment where the dimer is a homodimer, the Cas12b polypeptide can be modified such that the endonuclease activity is eliminated. In certain embodiments wherein the Cas12b polypeptide is modified such that endonuclease activity is eliminated, each fusion protein monomer can comprise an identical Cas12b polypeptide and an identical cleavage domain. The cleavage domain can be any cleavage domain, such as any of the exemplary cleavage domains provided herein. In such embodiments, specific guide RNAs would direct the fusion protein monomers to different but closely adjacent sites such that, upon dimer formation, the nuclease domains of the two monomers would create a double stranded break in the target DNA. 
     The dimer can also be a heterodimer of two different fusion proteins. For example, the Cas12b polypeptide of each fusion protein can be derived from a different Cas12b polypeptide or from an orthologous Cas12b polypeptide. For example, each fusion protein can comprise a Cas12b polypeptide derived from a different source. In these embodiments, each fusion protein would recognize a different target site (i.e., specified by the protospacer and/or PAM sequence). For example, the guide RNAs could position the heterodimer to different but closely adjacent sites such that their nuclease domains produce an effective double stranded break in the target DNA. 
     Alternatively, two fusion proteins of a heterodimer can have different effector domains. In embodiments in which the effector domain is a cleavage domain, each fusion protein can contain a different modified cleavage domain. In these embodiments, the Cas12b polypeptide(s) can be modified such that their endonuclease activities are eliminated. The two fusion proteins forming a heterodimer can differ in both the Cas12b polypeptide domain and the effector domain. 
     In any of the above-described embodiments, the homodimer or heterodimer can comprise at least one additional domain chosen from nuclear localization signals (NLSs), plastid signal peptides, mitochondrial signal peptides, signal peptides capable of trafficking proteins to multiple subcellular locations, cell-penetrating, translocation domains and marker domains, as detailed above. In any of the above-described embodiments, one or both of the Cas12b polypeptides can be modified such that endonuclease activity of the polypeptide is eliminated or modified. 
     The heterodimer can also comprise one fusion protein and an additional protein. For example, the additional protein can be a nuclease. In one embodiment, the nuclease is a zinc finger nuclease. A zinc finger nuclease comprises a zinc finger DNA binding domain and a cleavage domain. A zinc finger recognizes and binds three (3) nucleotides. A zinc finger DNA binding domain can comprise from about three zinc fingers to about seven zinc fingers. The zinc finger DNA binding domain can be derived from a naturally occurring protein or it can be engineered. See, for example, Beerli et al. (2002)  Nat. Biotechnol.  20:135-141; Pabo et al. (2001)  Ann. Rev. Biochem.  70:313-340; Isalan et al. (2001)  Nat. Biotechnol.  19:656-660; Segal et al. (2001)  Curr. Opin. Biotechnol.  12:632-637; Choo et al. (2000)  Curr. Opin. Struct. Biol.  10:411-416; Zhang et al. (2000)  J. Biol. Chem.  275(43):33850-33860; Doyon et al. (2008)  Nat. Biotechnol.  26:702-708; and Santiago et al. (2008)  Proc. Natl. Acad. Sci. USA  105:5809-5814. The cleavage domain of the zinc finger nuclease can be any cleavage domain detailed herein. In some embodiments, the zinc finger nuclease can comprise at least one additional domain chosen from nuclear localization signals, plastid signal peptides, mitochondrial signal peptides, signal peptides capable of trafficking proteins to multiple subcellular locations, cell-penetrating or translocation domains, which are detailed herein. 
     In certain embodiments, any of the fusion proteins detailed above or a dimer comprising at least one fusion protein may be part of a protein-RNA complex comprising at least one guide RNA. A guide RNA interacts with the Cas12b polypeptide of the fusion protein to direct the fusion protein to a specific target site, wherein the 5′ end of the guide RNA base pairs with a specific protospacer sequence. 
     Nucleic Acids Encoding Cas12b Polypeptides or Fusion Proteins 
     Nucleic acids encoding any of the Cas12b polypeptides or fusion proteins described herein are provided. The nucleic acid can be RNA or DNA. Examples of polynucleotides that encode Cas12b polypeptides are set forth in SEQ ID NOs: 1-4. In one embodiment, the nucleic acid encoding the Cas12b polypeptide or fusion protein is mRNA. The mRNA can be 5′ capped and/or 3′ polyadenylated. In another embodiment, the nucleic acid encoding the Cas12b polypeptide or fusion protein is DNA. The DNA can be present in a vector. 
     Nucleic acids encoding the Cas12b polypeptide or fusion proteins can be codon optimized for efficient translation into protein in the plant cell of interest. Programs for codon optimization are available in the art (e.g., OPTIMIZER at genomes.urv.es/OPTIMIZER; OptimumGene™ from GenScript at genscript.com/codon_opt.html). 
     In certain embodiments, DNA encoding the Cas12b polypeptide or fusion protein can be operably linked to at least one promoter sequence. The DNA coding sequence can be operably linked to a promoter control sequence for expression in a host cell of interest. In some embodiments, the host cell is a plant cell. “Operably linked” is intended to mean a functional linkage between two or more elements. For example, an operable linkage between a promoter and a coding region of interest (e.g., region coding for a Cas12b polypeptide or guide RNA) is a functional link that allows for expression of the coding region of interest. Operably linked elements may be contiguous or non-contiguous. When used to refer to the joining of two protein coding regions, by operably linked is intended that the coding regions are in the same reading frame. 
     The promoter sequence can be constitutive, regulated, growth stage-specific, or tissue-specific. It is recognized that different applications can be enhanced by the use of different promoters in the nucleic acid molecules to modulate the timing, location and/or level of expression of the Cas12b polypeptide and/or guide RNA. Such nucleic acid molecules may also contain, if desired, a promoter regulatory region (e.g., one conferring inducible, constitutive, environmentally- or developmentally-regulated, or cell- or tissue-specific/selective expression), a transcription initiation start site, a ribosome binding site, an RNA processing signal, a transcription termination site, and/or a polyadenylation signal. 
     In some embodiments, the nucleic acid molecules provided herein can be combined with constitutive, tissue-preferred, developmentally-preferred or other promoters for expression in plants. Examples of constitutive promoters functional in plant cells include the cauliflower mosaic virus (CaMV) 35S transcription initiation region, the 1′- or 2′-promoter derived from T-DNA of  Agrobacterium tumefaciens , the ubiquitin 1 promoter, the Smas promoter, the cinnamyl alcohol dehydrogenase promoter (U.S. Pat. No. 5,683,439), the Nos promoter, the pEmu promoter, the rubisco promoter, the GRP1-8 promoter and other transcription initiation regions from various plant genes known to those of skill. If low level expression is desired, weak promoter(s) may be used. Weak constitutive promoters include, for example, the core promoter of the Rsyn7 promoter (WO 99/43838 and U.S. Pat. No. 6,072,050), the core 35S CaMV promoter, and the like. Other constitutive promoters include, for example, U.S. Pat. Nos. 5,608,149; 5,608,144; 5,604,121; 5,569,597; 5,466,785; 5,399,680; 5,268,463; and 5,608,142. See also, U.S. Pat. No. 6,177,611, herein incorporated by reference. 
     Examples of inducible promoters are the Adhl promoter which is inducible by hypoxia or cold stress, the Hsp70 promoter which is inducible by heat stress, the PPDK promoter and the pepcarboxylase promoter which are both inducible by light. Also useful are promoters which are chemically inducible, such as the In2-2 promoter which is safener induced (U.S. Pat. No. 5,364,780), the ERE promoter which is estrogen induced, and the Axigl promoter which is auxin induced and tapetum specific but also active in callus (PCT US01/22169). 
     Examples of promoters under developmental control in plants include promoters that initiate transcription preferentially in certain tissues, such as leaves, roots, fruit, seeds, or flowers. A “tissue specific” promoter is a promoter that initiates transcription only in certain tissues. Unlike constitutive expression of genes, tissue-specific expression is the result of several interacting levels of gene regulation. As such, promoters from homologous or closely related plant species can be preferable to use to achieve efficient and reliable expression of transgenes in particular tissues. In some embodiments, the expression comprises a tissue-preferred promoter. A “tissue preferred” promoter is a promoter that initiates transcription preferentially, but not necessarily entirely or solely in certain tissues. 
     In some embodiments, the nucleic acid molecules encoding a Cas12b polypeptide and/or guide RNA comprise a cell type specific promoter. A “cell type specific” promoter is a promoter that primarily drives expression in certain cell types in one or more organs. Some examples of plant cells in which cell type specific promoters functional in plants may be primarily active include, for example, BETL cells, vascular cells in roots, leaves, stalk cells, and stem cells. The nucleic acid molecules can also include cell type preferred promoters. A “cell type preferred” promoter is a promoter that primarily drives expression mostly, but not necessarily entirely or solely in certain cell types in one or more organs. Some examples of plant cells in which cell type preferred promoters functional in plants may be preferentially active include, for example, BETL cells, vascular cells in roots, leaves, stalk cells, and stem cells. The nucleic acid molecules described herein can also comprise seed-preferred promoters. In some embodiments, the seed-preferred promoters have expression in embryo sac, early embryo, early endosperm, aleurone, and/or basal endosperm transfer cell layer (BETL). 
     Examples of seed-preferred promoters include, but are not limited to, 27 kD gamma zein promoter and waxy promoter, Boronat, A. et al. (1986)  Plant Sci.  47:95-102; Reina, M. et al.  Nucl. Acids Res.  18(21):6426; and Kloesgen, R. B. et al. (1986)  Mol. Gen. Genet.  203:237-244. Promoters that express in the embryo, pericarp, and endosperm are disclosed in U.S. Pat. No. 6,225,529 and PCT publication WO 00/12733. The disclosures for each of these are incorporated herein by reference in their entirety. 
     Promoters that can drive gene expression in a plant seed-preferred manner with expression in the embryo sac, early embryo, early endosperm, aleurone and/or basal endosperm transfer cell layer (BETL) can be used in the compositions and methods disclosed herein. Such promoters include, but are not limited to, promoters that are naturally linked to  Zea mays  early endosperm 5 gene,  Zea mays  early endosperm 1 gene,  Zea mays  early endosperm 2 gene, GRMZM2G124663, GRMZM2G006585, GRMZM2G120008, GRMZM2G157806, GRMZM2G176390, GRMZM2G472234, GRMZM2G138727,  Zea mays  CLAVATA1 , Zea mays MRP1 , Oryza sativa  PR602 , Oryza sativa  PR9a,  Zea mays  BET1 , Zea mays  BETL-2 , Zea mays  BETL-3 , Zea mays  BETL-4 , Zea mays  BETL-9 , Zea mays  BETL-10 , Zea mays  MEG1 , Zea mays  TCCR1 , Zea mays  ASP1 , Oryza sativa  ASP1 , Triticum durum  PR60 , Triticum durum  PR91 , Triticum durum  GL7, AT3G10590, AT4G18870, AT4G21080, AT5G23650, AT3G05860, AT5G42910, AT2G26320, AT3G03260, AT5G26630, AtIPT4, AtIPT8, AtLEC2, LFAH12. Additional such promoters are described in U.S. Pat. Nos. 7,803,990, 8,049,000, 7,745,697, 7,119,251, 7,964,770, 7,847,160, 7,700,836, U.S. Patent Application Publication Nos. 20100313301, 20090049571, 20090089897, 20100281569, 20100281570, 20120066795, 20040003427; PCT Publication Nos. WO/1999/050427, WO/2010/129999, WO/2009/094704, WO/2010/019996 and WO/2010/147825, each of which is herein incorporated by reference in its entirety for all purposes. Functional variants or functional fragments of the promoters described herein can also be operably linked to the nucleic acids disclosed herein. 
     Chemical-regulated promoters can be used to modulate the expression of a gene through the application of an exogenous chemical regulator. Depending upon the objective, the promoter may be a chemical-inducible promoter, where application of the chemical induces gene expression, or a chemical-repressible promoter, where application of the chemical represses gene expression. Chemical-inducible promoters are known in the art and include, but are not limited to, the maize In2-2 promoter, which is activated by benzenesulfonamide herbicide safeners, the maize GST promoter, which is activated by hydrophobic electrophilic compounds that are used as pre-emergent herbicides, and the tobacco PR-la promoter, which is activated by salicylic acid. Other chemical-regulated promoters of interest include steroid-responsive promoters (see, for example, the glucocorticoid-inducible promoter in Schena et al. (1991)  Proc. Natl. Acad. Sci. USA  88:10421-10425 and McNellis et al. (1998)  Plant  14(2):247-257) and tetracycline-inducible and tetracycline-repressible promoters (see, for example, Gatz et al. (1991)  Mol. Gen. Genet.  227:229-237, and U.S. Pat. Nos. 5,814,618 and 5,789,156), herein incorporated by reference. 
     Tissue-preferred promoters can be utilized to target enhanced expression of an expression construct within a particular tissue. In certain embodiments, the tissue-preferred promoters may be active in plant tissue. Tissue-preferred promoters are known in the art. See, for example, Yamamoto et al. (1997)  Plant  12(2):255-265; Kawamata et al. (1997)  Plant Cell Physiol.  38(7):792-803; Hansen et al. (1997)  Mol. Gen Genet.  254(3):337-343; Russell et al. (1997)  Transgenic Res.  6(2): 157-168; Rinehart et al. 
     (1996)  Plant Physiol.  112(3): 1331-1341; Van Camp et al. (1996)  Plant Physiol.  112(2):525-535; Canevascini et al. (1996)  Plant Physiol.  112(2):513-524; Yamamoto et al. (1994)  Plant Cell Physiol.  35(5):773-778; Lam (1994)  Results Probl. Cell Differ.  20:181-196; Orozco et al. (1993)  Plant Mol Biol.  23(6):1129-1138; Matsuoka et al. (1993)  Proc Natl. Acad. Sci. USA  90(20):9586-9590; and Guevara-Garcia et al. (1993)  Plant J.  4(3):495-505. Such promoters can be modified, if necessary, for weak expression. 
     Leaf-preferred promoters are known in the art. See, for example, Yamamoto et al. (1997)  Plant J.  12(2):255-265; Kwon et al. (1994)  Plant Physiol.  105:357-67; Yamamoto et al. (1994)  Plant Cell Physiol.  35(5):773-778; Gotor et al. (1993)  Plant J.  3:509-18; Orozco et al. (1993)  Plant Mol. Biol.  23(6):1129-1138; and Matsuoka et al. (1993)  Proc. Natl. Acad. Sci. USA  90(20):9586-9590. In addition, the promoters of cab and rubisco can also be used. See, for example, Simpson et al. (1958)  EMBO J  4:2723-2729 and Timko et al. (1988)  Nature  318:57-58. 
     Root-preferred promoters are known and can be selected from the many available from the literature or isolated de novo from various compatible species. See, for example, Hire et al. (1992)  Plant Mol. Biol.  20(2):207-218 (soybean root-specific glutamine synthetase gene); Keller and Baumgartner (1991)  Plant Cell  3(10):1051-1061 (root-specific control element in the GRP 1.8 gene of French bean); Sanger et al. (1990)  Plant Mol. Biol.  14(3):433-443 (root-specific promoter of the mannopine synthase (MAS) gene of  Agrobacterium tumefaciens ); and Miao et al. (1991)  Plant Cell  3(1):11-22 (full-length cDNA clone encoding cytosolic glutamine synthetase (GS), which is expressed in roots and root nodules of soybean). See also Bogusz et al. (1990)  Plant Cell  2(7):633-641, where two root-specific promoters isolated from hemoglobin genes from the nitrogen-fixing nonlegume  Parasponia andersonii  and the related non-nitrogen-fixing nonlegume Trema tomentosa are described. The promoters of these genes were linked to a β-glucuronidase reporter gene and introduced into both the nonlegume  Nicotiana tabacum  and the legume  Lotus corniculatus , and in both instances root-specific promoter activity was preserved. Leach and Aoyagi (1991) describe their analysis of the promoters of the highly expressed roIC and roID root-inducing genes of  Agrobacterium rhizogenes  (see Plant Science (Limerick) 79(1):69-76). They concluded that enhancer and tissue-preferred DNA determinants are dissociated in those promoters. Teen et al. (1989) used gene fusion to lacZ to show that the  Agrobacterium  T-DNA gene encoding octopine synthase is especially active in the epidermis of the root tip and that the TR2′ gene is root specific in the intact plant and stimulated by wounding in leaf tissue, an especially desirable combination of characteristics for use with an insecticidal or larvicidal gene (see  EMBO J.  8(2):343-350). The TRY gene, fused to nptII (neomycin phosphotransferase II) showed similar characteristics. Additional root-preferred promoters include the VfENOD-GRP3 gene promoter (Kuster et al. (1995)  Plant Mol. Biol.  29(4):759-772); and roM promoter (Capana et al. (1994)  Plant Mol. Biol.  25(4):681-691. See also U.S. Pat. Nos. 5,837,876; 5,750,386; 5,633,363; 5,459,252; 5,401,836; 5,110,732; and 5,023,179. The phaseolin gene (Murai et al. (1983)  Science  23:476-482 and Sengopta-Gopalen et al. (1988)  PNAS  82:3320-3324. The promoter sequence can be wild type or it can be modified for more efficient or efficacious expression. 
     The nucleic acid sequences encoding the Cas12b polypeptide or fusion protein can be operably linked to a promoter sequence that is recognized by a phage RNA polymerase for in vitro mRNA synthesis. In such embodiments, the in vitro-transcribed RNA can be purified for use in the methods of genome modification described herein. For example, the promoter sequence can be a T7, T3, or SP6 promoter sequence or a variation of a T7, T3, or SP6 promoter sequence. In some embodiments, the sequence encoding the Cas12b polypeptide or fusion protein can be operably linked to a promoter sequence for in vitro expression of the Cas12b polypeptide or fusion protein in plant cells. In such embodiments, the expressed protein can be purified for use in the methods of genome modification described herein. 
     In certain embodiments, the DNA encoding the Cas12b polypeptide or fusion protein also can be linked to a polyadenylation signal (e.g., SV40 polyA signal and other signals functional in the cells of interest) and/or at least one transcriptional termination sequence. Additionally, the sequence encoding the Cas12b polypeptide or fusion protein also can be linked to a sequence encoding at least one nuclear localization signal, at least one plastid signal peptide, at least one mitochondrial signal peptide, at least one signal peptide capable of trafficking proteins to multiple subcellular locations, at least one cell-penetrating domain, and/or at least one marker domain, described elsewhere herein. The DNA encoding the Cas12b polypeptide or fusion protein can be present in a vector. Suitable vectors include plasmid vectors, phagemids, cosmids, artificial/mini-chromosomes, transposons, and viral vectors (e.g., lentiviral vectors, adeno-associated viral vectors, etc.). In one embodiment, the DNA encoding the Cas12b polypeptide or fusion protein is present in a plasmid vector. Non-limiting examples of suitable plasmid vectors include pUC, pBR322, pET, pBluescript, pCAMBIA, and variants thereof. The vector can comprise additional expression control sequences (e.g., enhancer sequences, Kozak sequences, polyadenylation sequences, transcriptional termination sequences, etc.), selectable marker sequences (e.g., antibiotic resistance genes), origins of replication, and the like. Additional information can be found in “Current Protocols in Molecular Biology” Ausubel et al., John Wiley &amp; Sons, New York, 2003 or “Molecular Cloning: A Laboratory Manual” Sambrook &amp; Russell, Cold Spring Harbor Press, Cold Spring Harbor, N.Y., 3rd edition, 2001. In some embodiments, the vector comprising the sequence encoding the Cas12b polypeptide or fusion protein is selected from SEQ ID NOs: 27-46. 
     In some embodiments, the expression vector comprising the sequence encoding the Cas12b polypeptide or fusion protein can further comprise a sequence encoding a guide RNA. The sequence encoding the guide RNA can be operably linked to at least one transcriptional control sequence for expression of the guide RNA in the plant or plant cell of interest. For example, DNA encoding the guide RNA can be operably linked to a promoter sequence that is recognized by RNA polymerase III (Pol III). Examples of suitable Pol III promoters include, but are not limited to, mammalian U6, U3, H1, and 7SL RNA promoters and rice U6 and U3 promoters. 
     Methods for Modifying a Nucleotide Sequence in a Genome 
     Methods are provided herein for modifying a nucleotide sequence of a genome. Non-limiting examples of genomes include cellular, nuclear, organellar, and plasmid genomes. The methods comprise introducing into a genome host (e.g., a cell or organelle) one or more DNA-targeting polynucleotides such as a DNA-targeting RNA (“guide RNA,” “gRNA,” “CRISPR RNA,” or “crRNA”) or a DNA polynucleotide encoding a DNA-targeting RNA, wherein the DNA-targeting polynucleotide comprises: (a) a first segment comprising a nucleotide sequence that is complementary to a sequence in the target DNA; and (b) a second segment that interacts with a Cas12b polypeptide and also introducing to the genome host a Cas12b polypeptide, or a polynucleotide encoding a Cas12b polypeptide, wherein the a Cas12b polypeptide comprises: (a) a polynucleotide-binding portion that interacts with the gRNA or other DNA-targeting polynucleotide; and (b) an activity portion that exhibits site-directed enzymatic activity. The genome host can then be cultured under conditions in which the Cas12b polypeptide is expressed and cleaves the nucleotide sequence that is targeted by the gRNA. Finally, a genome host comprising the modified nucleotide sequence can be selected. 
     The methods disclosed herein comprise introducing into a genome host at least one Cas12b polypeptide or a nucleic acid encoding at least one Cas12b polypeptide, as described herein. In some embodiments, the Cas12b polypeptide can be introduced into the genome host as an isolated protein. In such embodiments, the Cas12b polypeptide can further comprise at least one cell-penetrating domain, which facilitates cellular uptake of the protein. In some embodiments, the Cas12b polypeptide can be introduced into the genome host as a nucleoprotein in complex with a guide polynucleotide (for instance, as a ribonucleoprotein in complex with a guide RNA). In other embodiments, the Cas12b polypeptide can be introduced into the genome host as an mRNA molecule that encodes the Cas12b polypeptide. In still other embodiments, the Cas12b polypeptide can be introduced into the genome host as a DNA molecule comprising an open reading frame that encodes the Cas12b polypeptide. In general, DNA sequences encoding the Cas12b polypeptide or fusion protein described herein are operably linked to a promoter sequence that will function in the genome host. The DNA sequence can be linear, or the DNA sequence can be part of a vector. In still other embodiments, the Cas12b polypeptide or fusion protein can be introduced into the genome host as an RNA-protein complex comprising the guide RNA or a fusion protein and the guide RNA. 
     In certain embodiments, mRNA encoding the Cas12b polypeptide may be targeted to an organelle (e.g., plastid or mitochondria). In certain embodiments, mRNA encoding one or more guide RNAs may be targeted to an organelle (e.g., plastid or mitochondria). In certain embodiments, mRNA encoding the Cas12b polypeptide and one or more guide RNAs may be targeted to an organelle (e.g., plastid or mitochondria). Methods for targeting mRNA to organelles are known in the art (see, e.g., U.S. Patent Application 2011/0296551; U.S. Patent Application No. 2011/0321187; Gomez and Pallas (2010) PLoS One 5:e12269), and are incorporated herein by reference. 
     In certain embodiments, DNA encoding the Cas12b polypeptide can further comprise a sequence encoding a guide RNA. In general, each of the sequences encoding the Cas12b polypeptide and the guide RNA is operably linked to one or more appropriate promoter control sequences that allow expression of the Cas12b polypeptide and the guide RNA, respectively, in the genome host. The DNA sequence encoding the Cas12b polypeptide and the guide RNA can further comprise additional expression control, regulatory, and/or processing sequence(s). The DNA sequence encoding the Cas12b polypeptide and the guide RNA can be linear or can be part of a vector. 
     Methods described herein further can also comprise introducing into a genome host at least one guide RNA or DNA encoding at least one polynucleotide such as a guide RNA. A guide RNA interacts with the Cas12b polypeptide to direct the Cas12b polypeptide to a specific target site, at which site the guide RNA base pairs with a specific DNA sequence in the targeted site. Guide RNAs can comprise three regions: a first region that is complementary to the target site in the targeted DNA sequence, a second region that forms a stem loop structure, and a third region that remains essentially single-stranded. The first region of each guide RNA is different such that each guide RNA guides a Cas12b polypeptide to a specific target site. The second and third regions of each guide RNA can be the same in all guide RNAs. 
     One region of the guide RNA is complementary to a sequence (i.e., protospacer sequence) at the target site in the targeted DNA such that the first region of the guide RNA can base pair with the target site. In various embodiments, the first region of the guide RNA can comprise from about 8 nucleotides to more than about 30 nucleotides. For example, the region of base pairing between the first region of the guide RNA and the target site in the nucleotide sequence can be about 8, about 9, about 10, about 11, about 12, about 13, about 14, about 15, about 16, about 17, about 18, about 19, about 20, about 22, about 23, about 24, about 25, about 27, about 30 or more than 30 nucleotides in length. In an exemplary embodiment, the first region of the guide RNA is about 23, 24, or 25 nucleotides in length. The guide RNA also can comprise a second region that forms a secondary structure. In some embodiments, the secondary structure comprises a stem or hairpin. The length of the stem can vary. For example, the stem can range from about 5, to about 6, to about 10, to about 15, to about 20, to about 25 base pairs in length. The stem can comprise one or more bulges of 1 to about 10 nucleotides. The overall length of the second region can range from about 14 to about 25 nucleotides in length. In certain embodiments, the loop is about 3, 4, or 5 nucleotides in length and the stem comprises about 5, 6, 7, 8, 9, or 10 base pairs. 
     The guide RNA can also comprise a third region that remains essentially single-stranded. Thus, the third region has no complementarity to any nucleotide sequence in the cell of interest and has no complementarity to the rest of the guide RNA. The length of the third region can vary. In general, the third region is more than about 4 nucleotides in length. For example, the length of the third region can range from about 5 to about 60 nucleotides in length. The combined length of the second and third regions (also called the universal or scaffold region) of the guide RNA can range from about 30 to about 120 nucleotides in length. In one aspect, the combined length of the second and third regions of the guide RNA range from about 40 to about 45 nucleotides in length. 
     In some embodiments, the guide RNA comprises a single molecule comprising all three regions. In other embodiments, the guide RNA can comprise two separate molecules. The first RNA molecule can comprise the first region of the guide RNA and one half of the “stem” of the second region of the guide RNA. The second RNA molecule can comprise the other half of the “stem” of the second region of the guide RNA and the third region of the guide RNA. Thus, in this embodiment, the first and second RNA molecules each contain a sequence of nucleotides that are complementary to one another. For example, in one embodiment, the first and second RNA molecules each comprise a sequence (of about 6 to about 25 nucleotides) that base pairs to the other sequence to form a functional guide RNA. 
     In certain embodiments, the guide RNA can be introduced into the genome host as an RNA molecule. The RNA molecule can be transcribed in vitro. Alternatively, the RNA molecule can be chemically synthesized. In other embodiments, the guide RNA can be introduced into the genome host as a DNA molecule. In such cases, the DNA encoding the guide RNA can be operably linked to one or more promoter sequences for expression of the guide RNA in the genome host. For example, the RNA coding sequence can be operably linked to a promoter sequence that is recognized by RNA polymerase III (Pol III). 
     The DNA molecule encoding the guide RNA can be linear or circular. In some embodiments, the DNA sequence encoding the guide RNA can be part of a vector. Suitable vectors include plasmid vectors, phagemids, cosmids, artificial/mini-chromosomes, transposons, and viral vectors. In an exemplary embodiment, the DNA encoding the guide RNA is present in a plasmid vector. Non-limiting examples of suitable plasmid vectors include pUC, pBR322, pET, pBluescript, pCAMBIA, and variants thereof. The vector can comprise additional expression control sequences (e.g., enhancer sequences, Kozak sequences, polyadenylation sequences, transcriptional termination sequences, etc.), selectable marker sequences (e.g., antibiotic resistance genes), origins of replication, and the like. In some embodiments, the vector comprising sequence encoding the guide RNA is selected from SEQ ID NOs: 47-55. 
     In embodiments in which both the Cas12b polypeptide and the guide RNA are introduced into the genome host as DNA molecules, each can be part of a separate molecule (e.g., one vector containing Cas12b polypeptide or fusion protein coding sequence and a second vector containing guide RNA coding sequence) or both can be part of the same molecule (e.g., one vector containing coding (and regulatory) sequence for both the Cas12b polypeptide or fusion protein and the guide RNA). 
     Cas12b proteins are RNA guided nucleases. Its cleavage relies on a tracr RNA to recruit a guide RNA comprising a guide sequence and a direct repeat, where the guide sequence hybridizes with the target nucleotide sequence to form a DNA/RNA heteroduplex. A Cas12b polypeptide in conjunction with a guide RNA is directed to a target site in a genome host, wherein the Cas12b polypeptide introduces a double-stranded break in the targeted DNA. The target site has no sequence limitation except that the sequence is immediately preceded (upstream) by a consensus sequence. This consensus sequence is also known as a protospacer adjacent motif (PAM). It is well-known in the art that a suitable PAM sequence must be located at the correct location relative to the targeted DNA sequence to allow the Cas12b nuclease to produce the desired double-stranded break. Cas12b PAM sequences are T-rich sequences. In some embodiments, the PAM sequence is 5′ VTTV 3′, wherein V is A, C, or G. In a particular embodiment, the PAM sequence is 5′ ATTV 3′ or 5′ GTTG 3′. As detailed herein, the first region of the guide RNA is complementary to the protospacer of the target sequence. Typically, the first region of the guide RNA is about 19 to 21 nucleotides in length. 
     The target site can be in the coding region of a gene, in an intron of a gene, in a control region of a gene, in a non-coding region between genes, etc. The gene can be a protein coding gene or an RNA coding gene. The gene can be any gene of interest as described herein. 
     In some embodiments, the methods disclosed herein further comprise introducing at least one donor polynucleotide into a genome host. A donor polynucleotide comprises at least one donor sequence. In some aspects, a donor sequence of the donor polynucleotide corresponds to an endogenous or native sequence found in the targeted DNA. For example, the donor sequence can be essentially identical to a portion of the DNA sequence at or near the targeted site, but which comprises at least one nucleotide change. Thus, the donor sequence can comprise a modified version of the wild type sequence at the targeted site such that, upon integration or exchange with the native sequence, the sequence at the targeted location comprises at least one nucleotide change. For example, the change can be an insertion of one or more nucleotides, a deletion of one or more nucleotides, a substitution of one or more nucleotides, or combinations thereof. As a consequence of the integration of the modified sequence, the genome host can produce a modified gene product from the targeted chromosomal sequence. 
     The donor sequence of the donor polynucleotide can alternatively correspond to an exogenous sequence. As used herein, an “exogenous” sequence refers to a sequence that is not native to the genome host, or a sequence whose native location in the genome host is in a different location. For example, the exogenous sequence can comprise a protein coding sequence, which can be operably linked to an exogenous promoter control sequence such that, upon integration into the genome, the genome host is able to express the protein coded by the integrated sequence. For example, the donor sequence can be any gene of interest, such as those encoding agronomically important traits as described elsewhere herein. Alternatively, the exogenous sequence can be integrated into the targeted DNA sequence such that its expression is regulated by an endogenous promoter control sequence. In other iterations, the exogenous sequence can be a transcriptional control sequence, another expression control sequence, or an RNA coding sequence. Integration of an exogenous sequence into a targeted DNA sequence is termed a “knock in.” The donor sequence can vary in length from several nucleotides to hundreds of nucleotides to hundreds of thousands of nucleotides. 
     In some embodiments, the donor sequence in the donor polynucleotide is flanked by an upstream sequence and a downstream sequence, which have substantial sequence identity to sequences located upstream and downstream, respectively, of the targeted site. Because of these sequence similarities, the upstream and downstream sequences of the donor polynucleotide permit homologous recombination between the donor polynucleotide and the targeted sequence such that the donor sequence can be integrated into (or exchanged with) the targeted DNA sequence. 
     The upstream sequence, as used herein, refers to a nucleic acid sequence that shares substantial sequence identity with a DNA sequence upstream of the targeted site. Similarly, the downstream sequence refers to a nucleic acid sequence that shares substantial sequence identity with a DNA sequence downstream of the targeted site. As used herein, the phrase “substantial sequence identity” refers to sequences having at least about 75% sequence identity. Thus, the upstream and downstream sequences in the donor polynucleotide can have about 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% sequence identity with sequence upstream or downstream to the targeted site. In an exemplary embodiment, the upstream and downstream sequences in the donor polynucleotide can have about 95% or 100% sequence identity with nucleotide sequences upstream or downstream to the targeted site. In one embodiment, the upstream sequence shares substantial sequence identity with a nucleotide sequence located immediately upstream of the targeted site (i.e., adjacent to the targeted site). In other embodiments, the upstream sequence shares substantial sequence identity with a nucleotide sequence that is located within about one hundred (100) nucleotides upstream from the targeted site. Thus, for example, the upstream sequence can share substantial sequence identity with a nucleotide sequence that is located about 1 to about 20, about 21 to about 40, about 41 to about 60, about 61 to about 80, or about 81 to about 100 nucleotides upstream from the targeted site. In one embodiment, the downstream sequence shares substantial sequence identity with a nucleotide sequence located immediately downstream of the targeted site (i.e., adjacent to the targeted site). In other embodiments, the downstream sequence shares substantial sequence identity with a nucleotide sequence that is located within about one hundred (100) nucleotides downstream from the targeted site. Thus, for example, the downstream sequence can share substantial sequence identity with a nucleotide sequence that is located about 1 to about 20, about 21 to about 40, about 41 to about 60, about 61 to about 80, or about 81 to about 100 nucleotides downstream from the targeted site. 
     Each upstream or downstream sequence can range in length from about 20 nucleotides to about 5000 nucleotides. In some embodiments, upstream and downstream sequences can comprise about 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2800, 3000, 3200, 3400, 3600, 3800, 4000, 4200, 4400, 4600, 4800, or 5000 nucleotides. In exemplary embodiments, upstream and downstream sequences can range in length from about 50 to about 1500 nucleotides. 
     Donor polynucleotides comprising the upstream and downstream sequences with sequence similarity to the targeted nucleotide sequence can be linear or circular. In embodiments in which the donor polynucleotide is circular, it can be part of a vector. For example, the vector can be a plasmid vector. 
     In certain embodiments, the donor polynucleotide can additionally comprise at least one targeted cleavage site that is recognized by the Cas12b polypeptide. The targeted cleavage site added to the donor polynucleotide can be placed upstream or downstream or both upstream and downstream of the donor sequence. For example, the donor sequence can be flanked by targeted cleavage sites such that, upon cleavage by the Cas12b polypeptide, the donor sequence is flanked by overhangs that are compatible with those in the nucleotide sequence generated upon cleavage by the Cas12b polypeptide. Accordingly, the donor sequence can be ligated with the cleaved nucleotide sequence during repair of the double stranded break by a non-homologous repair process. Generally, donor polynucleotides comprising the targeted cleavage site(s) will be circular (e.g., can be part of a plasmid vector). 
     The donor polynucleotide can be a linear molecule comprising a short donor sequence with optional short overhangs that are compatible with the overhangs generated by the Cas12b polypeptide. In such embodiments, the donor sequence can be ligated directly with the cleaved chromosomal sequence during repair of the double-stranded break. In some instances, the donor sequence can be less than about 1,000, less than about 500, less than about 250, or less than about 100 nucleotides. In certain cases, the donor polynucleotide can be a linear molecule comprising a short donor sequence with blunt ends. In other iterations, the donor polynucleotide can be a linear molecule comprising a short donor sequence with 5′ and/or 3′ overhangs. The overhangs can comprise 1, 2, 3, 4, or 5 nucleotides. 
     In some embodiments, the donor polynucleotide will be DNA. The DNA may be single-stranded or double-stranded and/or linear or circular. The donor polynucleotide may be a DNA plasmid, a bacterial artificial chromosome (BAC), a yeast artificial chromosome (YAC), a viral vector, a linear piece of DNA, a PCR fragment, a naked nucleic acid, or a nucleic acid complexed with a delivery vehicle such as a liposome or poloxamer. In certain embodiments, the donor polynucleotide comprising the donor sequence can be part of a plasmid vector. In any of these situations, the donor polynucleotide comprising the donor sequence can further comprise at least one additional sequence. 
     In some embodiments, the method can comprise introducing one Cas12b polypeptide (or encoding nucleic acid) and one guide RNA (or encoding DNA) into a genome host, wherein the Cas12b polypeptide introduces one double-stranded break in the targeted DNA. In embodiments in which an optional donor polynucleotide is not present, the double-stranded break in the nucleotide sequence can be repaired by a non-homologous end-joining (NHEJ) repair process. Because NHEJ is error-prone, deletions of at least one nucleotide, insertions of at least one nucleotide, substitutions of at least one nucleotide, or combinations thereof can occur during the repair of the break. Accordingly, the targeted nucleotide sequence can be modified or inactivated. For example, a single nucleotide change (SNP) can give rise to an altered protein product, or a shift in the reading frame of a coding sequence can inactivate or “knock out” the sequence such that no protein product is made. In embodiments in which the optional donor polynucleotide is present, the donor sequence in the donor polynucleotide can be exchanged with or integrated into the nucleotide sequence at the targeted site during repair of the double-stranded break. For example, in embodiments in which the donor sequence is flanked by upstream and downstream sequences having substantial sequence identity with upstream and downstream sequences, respectively, of the targeted site in the nucleotide sequence, the donor sequence can be exchanged with or integrated into the nucleotide sequence at the targeted site during repair mediated by homology-directed repair process. Alternatively, in embodiments in which the donor sequence is flanked by compatible overhangs (or the compatible overhangs are generated in situ by the Cas12b polypeptide) the donor sequence can be ligated directly with the cleaved nucleotide sequence by a non-homologous repair process during repair of the double-stranded break. Exchange or integration of the donor sequence into the nucleotide sequence modifies the targeted nucleotide sequence or introduces an exogenous sequence into the targeted nucleotide sequence. 
     The methods disclosed herein can also comprise introducing one or more Cas12b polypeptides (or encoding nucleic acids) and two guide polynucleotides (or encoding DNAs) into a genome host, wherein the Cas12b polypeptides introduce two double-stranded breaks in the targeted nucleotide sequence. The two breaks can be within several base pairs, within tens of base pairs, or can be separated by many thousands of base pairs. In embodiments in which an optional donor polynucleotide is not present, the resultant double-stranded breaks can be repaired by a non-homologous repair process such that the sequence between the two cleavage sites is lost and/or deletions of at least one nucleotide, insertions of at least one nucleotide, substitutions of at least one nucleotide, or combinations thereof can occur during the repair of the break(s). In embodiments in which an optional donor polynucleotide is present, the donor sequence in the donor polynucleotide can be exchanged with or integrated into the targeted nucleotide sequence during repair of the double-stranded breaks by either a homology-based repair process (e.g., in embodiments in which the donor sequence is flanked by upstream and downstream sequences having substantial sequence identity with upstream and downstream sequences, respectively, of the targeted sites in the nucleotide sequence) or a non-homologous repair process (e.g., in embodiments in which the donor sequence is flanked by compatible overhangs). 
     Methods for Modifying a Nucleotide Sequence in a Plant Genome 
     Plant cells possess nuclear, plastid, and mitochondrial genomes. The compositions and methods of the present invention may be used to modify the sequence of the nuclear, plastid, and/or mitochondrial genome, or may be used to modulate the expression of a gene or genes encoded by the nuclear, plastid, and/or mitochondrial genome. Accordingly, by “chromosome” or “chromosomal” is intended the nuclear, plastid, or mitochondrial genomic DNA. “Genome” as it applies to plant cells encompasses not only chromosomal DNA found within the nucleus, but organelle DNA found within subcellular components (e.g., mitochondria or plastids) of the cell. Any nucleotide sequence of interest in a plant cell, organelle, or embryo can be modified using the methods described herein. In specific embodiments, the methods disclosed herein are used to modify a nucleotide sequence encoding an agronomically important trait, such as a plant hormone, plant defense protein, a nutrient transport protein, a biotic association protein, a desirable input trait, a desirable output trait, a stress resistance gene, a disease/pathogen resistance gene, a male sterility, a developmental gene, a regulatory gene, a gene involved in photosynthesis, a DNA repair gene, a transcriptional regulatory gene or any other polynucleotide and/or polypeptide of interest. Agronomically important traits such as oil, starch, and protein content can also be modified. Modifications include increasing content of oleic acid, saturated and unsaturated oils, increasing levels of lysine and sulfur, providing essential amino acids, and also modification of starch. Hordothionin protein modifications are described in U.S. Pat. Nos. 5,703,049, 5,885,801, 5,885,802, and 5,990,389, herein incorporated by reference. Another example is lysine and/or sulfur rich seed protein encoded by the soybean 2S albumin described in U.S. Pat. No. 5,850,016, and the chymotrypsin inhibitor from barley, described in Williamson et al. (1987)  Eur. J. Biochem.  165:99-106, the disclosures of which are herein incorporated by reference. 
     The Cas12b polypeptide (or encoding nucleic acid), the guide RNA(s) (or encoding DNA), and the optional donor polynucleotide(s) can be introduced into a plant cell, organelle, or plant embryo by a variety of means, including transformation. Transformation protocols as well as protocols for introducing polypeptides or polynucleotide sequences into plants may vary depending on the type of plant or plant cell, i.e., monocot or dicot, targeted for transformation. Suitable methods of introducing polypeptides and polynucleotides into plant cells include microinjection (Crossway et al. (1986)  Biotechniques  4:320-334), electroporation (Riggs et al. (1986)  Proc. Natl. Acad. Sci. USA  83:5602-5606 , Agrobacterium -mediated transformation (U.S. Pat. No. 5,563,055 and U.S. Pat. No. 5,981,840), direct gene transfer (Paszkowski et al. (1984)  EMBO  13:2717-2722), and ballistic particle acceleration (see, for example, U.S. Pat. Nos. 4,945,050; 5,879,918; 5,886,244; and, 5,932,782; Tomes et al. (1995) in Plant Cell, Tissue, and Organ Culture: Fundamental Methods, ed. Gamborg and Phillips (Springer-Verlag, Berlin); McCabe et al. (1988)  Biotechnology  6:923-926); and Led transformation (WO 00/28058). Also see Weissinger et al. (1988)  Ann. Rev. Genet.  22:421-477; Sanford et al. (1987)  Particulate Science and Technology  5:27-37 (onion); Christou et al. (1988)  Plant Physiol.  87:671-674 (soybean); McCabe et al. (1988)  Bio/Technology  6:923-926 (soybean); Finer and McMullen (1991)  In Vitro Cell Dev. Biol.  27P:175-182 (soybean); Singh et al. (1998)  Theor. Appl. Genet.  96:319-324 (soybean); Datta et al. (1990)  Biotechnology  8:736-740 (rice); Klein et al. (1988) Proc. Natl. Acad. Sci. USA 85:4305-4309 (maize); Klein et al. (1988)  Biotechnology  6:559-563 (maize); U.S. Pat. Nos. 5,240,855; 5,322,783; and, 5,324,646; Klein et al. (1988)  Plant Physiol.  91:440-444 (maize); Fromm et al. (1990)  Biotechnology  8:833-839 (maize); Hooykaas-Van Slogteren et al. (1984) Nature (London) 311:763-764; U.S. Pat. No. 5,736,369 (cereals); Bytebier et al. (1987)  Proc. Natl. Acad. Sci. USA  84:5345-5349 (Liliaceae); De Wet et al. (1985) in The Experimental Manipulation of Ovule Tissues, ed. Chapman et al. (Longman, New York), pp. 197-209 (pollen); Kaeppler et al. (1990)  Plant Cell Reports  9:415-418 and Kaeppler et al. (1992)  Theor. Appl. Genet.  84:560-566 (whisker-mediated transformation); D&#39;Halluin et al. (1992)  Plant Cell  4:1495-1505 (electroporation); Li et al. (1993)  Plant Cell Reports  12:250-255 and Christou and Ford (1995)  Annals of Botany  75:407-413 (rice); Osjoda et al. (1996)  Nature Biotechnology  14:745-750 (maize via  Agrobacterium tumefaciens ); all of which are herein incorporated by reference. Site-specific genome editing of plant cells by biolistic introduction of a ribonucleoprotein comprising a nuclease and suitable guide RNA has been demonstrated (Svitashev et al (2016)  Nat Commun  7:13274); these methods are herein incorporated by reference. “Stable transformation” is intended to mean that the nucleotide construct introduced into a plant integrates into the genome of the plant and is capable of being inherited by the progeny thereof The nucleotide construct may be integrated into the nuclear, plastid, or mitochondrial genome of the plant. Methods for plastid transformation are known in the art (see, e.g.,  Chloroplast Biotechnology: Methods and Protocols  (2014) Pal Maliga, ed. and U.S. Patent Application No. 2011/0321187), and methods for plant mitochondrial transformation have been described in the art (see, e.g., U.S. Patent Application No. 2011/0296551), herein incorporated by reference. 
     The cells that have been transformed may be grown into plants (i.e., cultured) in accordance with conventional ways. See, for example, McCormick et al. (1986)  Plant Cell Reports  5:81-84. In this manner, the present invention provides transformed seed having a nucleic acid modification stably incorporated into their genome. 
     “Introduced” in the context of inserting a nucleic acid fragment (e.g., a recombinant DNA construct) into a cell, means “transfection” or “transformation” or “transduction” and includes reference to the incorporation of a nucleic acid fragment into a plant cell where the nucleic acid fragment may be incorporated into the genome of the cell (e.g., nuclear chromosome, plasmid, plastid chromosome or mitochondrial chromosome), converted into an autonomous replicon, or transiently expressed (e.g., transfected mRNA). 
     The present invention may be used for transformation of any plant species, including, but not limited to, monocots and dicots (i.e., monocotyledonous and dicotyledonous, respectively). Examples of plant species of interest include, but are not limited to, corn ( Zea mays ),  Brassica  sp. (e.g.,  B. napus, B. rapa, B. juncea ), particularly those Brassica species useful as sources of seed oil, alfalfa ( Medicago sativa ), rice ( Oryza sativa ), rye ( Secale cereale ), sorghum ( Sorghum bicolor, Sorghum vulgare ), camelina ( Camelina sativa ), millet (e.g., pearl millet ( Pennisetum glaucum ), proso millet ( Panicum miliaceum ), foxtail millet ( Setaria italica ), finger millet ( Eleusine coracana )), sunflower ( Helianthus annuus ), quinoa ( Chenopodium quinoa ), chicory ( Cichorium intybus ), lettuce ( Lactuca sativa ), safflower ( Carthamus tinctorius ), wheat ( Triticum aestivum ), soybean ( Glycine max ), tobacco ( Nicotiana tabacum ), potato ( Solanum tuberosum ), peanuts ( Arachis hypogaea ), cotton ( Gossypium barbadense, Gossypium hirsutum ), sweet potato ( Ipomoea batatus ), cassava ( Manihot esculenta ), coffee ( Coffea  spp.), coconut ( Cocos nucifera ), pineapple ( Ananas comosus ), citrus trees ( Citrus  spp.), cocoa ( Theobroma cacao ), tea ( Camellia sinensis ), banana ( Musa  spp.), avocado ( Persea americana ), fig ( Ficus casica ), guava ( Psidium guajava ), mango ( Mangifera indica ), olive ( Olea europaea ), papaya ( Carica papaya ), cashew ( Anacardium occidentale ), macadamia ( Macadamia integrifolia ), almond ( Prunus amygdalus ), sugar beets ( Beta vulgaris ), sugarcane ( Saccharum  spp.), oil palm ( Elaeis guineensis ), poplar ( Populus  spp.), eucalyptus ( Eucalyptus  spp.), oats ( Avena sativa ), barley ( Hordeum vulgare ), vegetables, ornamentals, and conifers. In some embodiments, the plant is rice ( Oryza sativa ). 
     The Cas12b polypeptides (or encoding nucleic acid), the guide RNA(s) (or DNAs encoding the guide RNA), and the optional donor polynucleotide(s) can be introduced into the plant cell, organelle, or plant embryo simultaneously or sequentially. The ratio of the Cas12b polypeptides (or encoding nucleic acid) to the guide RNA(s) (or encoding DNA) generally will be about stoichiometric such that the two components can form an RNA-protein complex with the target DNA. In one embodiment, DNA encoding a Cas12b polypeptide and DNA encoding a guide RNA are delivered together within the plasmid vector. 
     The compositions and methods disclosed herein can be used to alter expression of genes of interest in a plant, such as genes involved in grain weight. Therefore, the expression of a gene encoding a protein involved in grain weight may be modulated as compared to a control plant. 
     A “subject plant or plant cell” is one in which genetic alteration, such as a mutation, has been effected as to a gene of interest, or is a plant or plant cell which is descended from a plant or cell so altered and which comprises the alteration. A “control” or “control plant” or “control plant cell” provides a reference point for measuring changes in phenotype of the subject plant or plant cell. Thus, the expression levels are higher or lower than those in the control plant depending on the methods of the invention. 
     A control plant or plant cell may comprise, for example: (a) a wild-type plant or cell, i.e., of the same genotype as the starting material for the genetic alteration which resulted in the subject plant or cell; (b) a plant or plant cell of the same genotype as the starting material but which has been transformed with a null construct (i.e. with a construct which has no known effect on the trait of interest, such as a construct comprising a marker gene); (c) a plant or plant cell which is a non-transformed segregant among progeny of a subject plant or plant cell; (d) a plant or plant cell genetically identical to the subject plant or plant cell but which is not exposed to conditions or stimuli that would induce expression of the gene of interest; or (e) the subject plant or plant cell itself, under conditions in which the gene of interest is not expressed. 
     While the invention is described in terms of transformed plants, it is recognized that transformed organisms of the invention also include plant cells, plant protoplasts, plant cell tissue cultures from which plants can be regenerated, plant calli, plant clumps, and plant cells that are intact in plants or parts of plants such as embryos, pollen, ovules, flowers, glumes, panicles, leaves, stems, roots, root tips, anthers, pistils and the like. Grain is intended to mean the mature seed produced by commercial growers for purposes other than growing or reproducing the species. Progeny, variants, and mutants of the regenerated plants are also included within the scope of the invention, provided that these parts comprise the introduced polynucleotides. 
     Derivatives of coding sequences can be made using the methods disclosed herein to increase the level of preselected amino acids in the encoded polypeptide. For example, the gene encoding the barley high lysine polypeptide (BHL) is derived from barley chymotrypsin inhibitor, U.S. application Ser. No. 08/740,682, filed Nov. 1, 1996, and WO 98/20133, the disclosures of which are herein incorporated by reference. Other proteins include methionine-rich plant proteins such as from sunflower seed (Lilley et al. (1989)  Proceedings of the World Congress on Vegetable Protein Utilization in Human Foods and Animal Feedstuffs , ed. Applewhite (American Oil Chemists Society, Champaign, Ill.), pp. 497-502; herein incorporated by reference); corn (Pedersen et al. (1986)  J. Biol. Chem.  261:6279; Kirihara et al. (1988)  Gene  71:359; both of which are herein incorporated by reference); and rice (Musumura et al. (1989)  Plant Mol. Biol.  12:123, herein incorporated by reference). Other agronomically important genes encode latex, Floury 2, growth factors, seed storage factors, and transcription factors. 
     The methods disclosed herein can be used to modify herbicide resistance traits including genes coding for resistance to herbicides that act to inhibit the action of acetolactate synthase (ALS), in particular the sulfonylurea-type herbicides (e.g., the acetolactate synthase (ALS) gene containing mutations leading to such resistance, in particular the S4 and/or Hra mutations), genes coding for resistance to herbicides that act to inhibit action of glutamine synthase, such as phosphinothricin or basta (e.g., the bar gene); glyphosate (e.g., the EPSPS gene and the GAT gene; see, for example, U.S. Publication No. 20040082770 and WO 03/092360); or other such genes known in the art. The bar gene encodes resistance to the herbicide basta, the nptll gene encodes resistance to the antibiotics kanamycin and geneticin, and the ALS-gene mutants encode resistance to the herbicide chlorsulfuron. Additional herbicide resistance traits are described for example in U.S. patent application Ser. No. 2016/0208243, herein incorporated by reference. 
     Sterility genes can also be modified and provide an alternative to physical detasseling. Examples of genes used in such ways include male tissue-preferred genes and genes with male sterility phenotypes such as QM, described in U.S. Pat. No. 5,583,210. Other genes include kinases and those encoding compounds toxic to either male or female gametophytic development. Additional sterility traits are described for example in U.S. Patent Application 2016/0208243, herein incorporated by reference. 
     The quality of grain can be altered by modifying genes encoding traits such as levels and types of oils, saturated and unsaturated, quality and quantity of essential amino acids, and levels of cellulose. In corn, modified hordothionin proteins are described in U.S. Pat. Nos. 5,703,049, 5,885,801, 5,885,802, and 5,990,389. 
     Commercial traits can also be altered by modifying a gene or that could increase for example, starch for ethanol production, or provide expression of proteins. Another important commercial use of modified plants is the production of polymers and bioplastics such as described in U.S. Pat. No. 5,602,321. Genes such as β-Ketothiolase, PHBase (polyhydroxyburyrate synthase), and acetoacetyl-CoA reductase (see Schubert et al. (1988)  J. Bacteriol.  170:5837-5847) facilitate expression of polyhyroxyalkanoates (PHAs). 
     Exogenous products include plant enzymes and products as well as those from other sources including prokaryotes and other eukaryotes. Such products include enzymes, cofactors, hormones, and the like. The level of proteins, particularly modified proteins having improved amino acid distribution to improve the nutrient value of the plant, can be increased. This is achieved by the expression of such proteins having enhanced amino acid content. 
     The methods disclosed herein can also be used for insertion of heterologous genes and/or modification of native plant gene expression to achieve desirable plant traits. Such traits include, for example, disease resistance, herbicide tolerance, drought tolerance, salt tolerance, insect resistance, resistance against parasitic weeds, improved plant nutritional value, improved forage digestibility, increased grain yield, cytoplasmic male sterility, altered fruit ripening, increased storage life of plants or plant parts, reduced allergen production, and increased or decreased lignin content. Genes capable of conferring these desirable traits are disclosed in U.S. Patent Application 2016/0208243, herein incorporated by reference. 
     Methods for Modulating Gene Expression 
     The methods disclosed herein further encompass modification of a nucleotide sequence or regulating expression of a nucleotide sequence in a genome host. The methods can comprise introducing into the genome host at least one fusion protein or nucleic acid encoding at least one fusion protein, wherein the fusion protein comprises a Cas12b polypeptide or a fragment or variant thereof and an effector domain, and (b) at least one guide RNA or DNA encoding the guide RNA, wherein the guide RNA guides the Cas12b polypeptide of the fusion protein to a target site in the targeted DNA and the effector domain of the fusion protein modifies the chromosomal sequence or regulates expression of one or more genes in near the targeted DNA sequence. 
     Fusion proteins comprising a Cas12b polypeptide or a fragment or variant thereof and an effector domain are described herein. In general, the fusion proteins disclosed herein can further comprise at least one nuclear localization signal, plastid signal peptide, mitochondrial signal peptide, or signal peptide capable of trafficking proteins to multiple subcellular locations. Nucleic acids encoding fusion proteins are described herein. In some embodiments, the fusion protein can be introduced into the genome host as an isolated protein (which can further comprise a cell-penetrating domain). Furthermore, the isolated fusion protein can be part of a protein-RNA complex comprising the guide RNA. In other embodiments, the fusion protein can be introduced into the genome host as a RNA molecule (which can be capped and/or polyadenylated). In still other embodiments, the fusion protein can be introduced into the genome host as a DNA molecule. For example, the fusion protein and the guide RNA can be introduced into the genome host as discrete DNA molecules or as part of the same DNA molecule. Such DNA molecules can be plasmid vectors. 
     In some embodiments, the method further comprises introducing into the genome host at least one donor polynucleotide as described elsewhere herein. Means for introducing molecules into genome hosts such as cells, as well as means for culturing cells (including cells comprising organelles) are described herein. 
     In certain embodiments in which the effector domain of the fusion protein is a cleavage domain, the method can comprise introducing into the genome host one fusion protein (or nucleic acid encoding one fusion protein) and two guide RNAs (or DNA encoding two guide RNAs). The two guide RNAs direct the fusion protein to two different target sites in the chromosomal sequence, wherein the fusion protein dimerizes (e.g., forms a homodimer) such that the two cleavage domains can introduce a double stranded break into the targeted DNA sequence. In embodiments in which the optional donor polynucleotide is not present, the double-stranded break in the targeted DNA sequence can be repaired by a non-homologous end-joining (NHEJ) repair process. Because NHEJ is error-prone, deletions of at least one nucleotide, insertions of at least one nucleotide, substitutions of at least one nucleotide, or combinations thereof can occur during the repair of the break. Accordingly, the targeted chromosomal sequence can be modified or inactivated. For example, a single nucleotide change (SNP) can give rise to an altered protein product, or a shift in the reading frame of a coding sequence can inactivate or “knock out” the sequence such that no protein product is made. In embodiments in which the optional donor polynucleotide is present, the donor sequence in the donor polynucleotide can be exchanged with or integrated into the targeted DNA sequence at the targeted site during repair of the double-stranded break. For example, in embodiments in which the donor sequence is flanked by upstream and downstream sequences having substantial sequence identity with upstream and downstream sequences, respectively, of the targeted site in the targeted DNA sequence, the donor sequence can be exchanged with or integrated into the targeted DNA sequence at the targeted site during repair mediated by homology-directed repair process. Alternatively, in embodiments in which the donor sequence is flanked by compatible overhangs (or the compatible overhangs are generated in situ by the Cas12b polypeptide) the donor sequence can be ligated directly with the cleaved targeted DNA sequence by a non-homologous repair process during repair of the double-stranded break. Exchange or integration of the donor sequence into the targeted DNA sequence modifies the targeted DNA sequence or introduces an exogenous sequence into the targeted DNA sequence. 
     In other embodiments in which the effector domain of the fusion protein is a cleavage domain, the method can comprise introducing into the genome host two different fusion proteins (or nucleic acid encoding two different fusion proteins) and two guide RNAs (or DNA encoding two guide RNAs). The fusion proteins can differ as detailed elsewhere herein. Each guide RNA directs a fusion protein to a specific target site in the targeted DNA sequence, wherein the fusion proteins can dimerize (e.g., form a heterodimer) such that the two cleavage domains can introduce a double stranded break into the targeted DNA sequence. In embodiments in which the optional donor polynucleotide is not present, the resultant double-stranded breaks can be repaired by a non-homologous repair process such that deletions of at least one nucleotide, insertions of at least one nucleotide, substitutions of at least one nucleotide, or combinations thereof can occur during the repair of the break. In embodiments in which the optional donor polynucleotide is present, the donor sequence in the donor polynucleotide can be exchanged with or integrated into the chromosomal sequence during repair of the double-stranded break by either a homology-based repair process (e.g., in embodiments in which the donor sequence is flanked by upstream and downstream sequences having substantial sequence identity with upstream and downstream sequences, respectively, of the targeted sites in the chromosomal sequence) or a non-homologous repair process (e.g., in embodiments in which the donor sequence is flanked by compatible overhangs). 
     In certain embodiments in which the effector domain of the fusion protein is a transcriptional activation domain or a transcriptional repressor domain, the method can comprise introducing into the genome host one fusion protein (or nucleic acid encoding one fusion protein) and one guide RNA (or DNA encoding one guide RNA). The guide RNA directs the fusion protein to a specific targeted DNA sequence, wherein the transcriptional activation domain or a transcriptional repressor domain activates or represses expression, respectively, of a gene or genes located near the targeted DNA sequence. That is, transcription may be affected for genes in close proximity to the targeted DNA sequence or may be affected for genes located at further distance from the targeted DNA sequence. It is well-known in the art that gene transcription can be regulated by distantly located sequences that may be located thousands of bases away from the transcription start site or even on a separate chromosome (Harmston and Lenhard (2013)  Nucleic Acids Res  41:7185-7199). 
     In alternate embodiments in which the effector domain of the fusion protein is an epigenetic modification domain, the method can comprise introducing into the genome host one fusion protein (or nucleic acid encoding one fusion protein) and one guide RNA (or DNA encoding one guide RNA). The guide RNA directs the fusion protein to a specific targeted DNA sequence, wherein the epigenetic modification domain modifies the structure of the targeted DNA sequence. Epigenetic modifications include acetylation, methylation of histone proteins and/or nucleotide methylation. In some instances, structural modification of the chromosomal sequence leads to changes in expression of the chromosomal sequence. 
     Plants Comprising a Genetic Modification 
     Provided herein are plants, plant cells, plant organelles, and plant embryos comprising at least one nucleotide sequence that has been modified using a Cas12b polypeptide-mediated or fusion protein-mediated process as described herein. Also provided are plants, plant cells, plant organelles, and plant embryos comprising at least one DNA or RNA molecule encoding Cas12b polypeptide or fusion protein targeted to a chromosomal sequence of interest or a fusion protein, at least one guide RNA, and optionally one or more donor polynucleotide(s). The genetically modified plants disclosed herein can be heterozygous for the modified nucleotide sequence or homozygous for the modified nucleotide sequence. Plant cells comprising one or more genetic modifications in organellar DNA may be heteroplasmic or homoplasmic. 
     The modified chromosomal sequence of the plants, plant cells, plant organelles, and plant embryos may be modified such that it is inactivated, has up-regulated or down-regulated expression, or produces an altered protein product, or comprises an integrated sequence. The modified chromosomal sequence may be inactivated such that the sequence is not transcribed and/or a functional protein product is not produced. Thus, a genetically modified plant comprising an inactivated chromosomal sequence may be termed a “knock out” or a “conditional knock out.” The inactivated chromosomal sequence can include a deletion mutation (i.e., deletion of one or more nucleotides), an insertion mutation (i.e., insertion of one or more nucleotides), or a nonsense mutation (i.e., substitution of a single nucleotide for another nucleotide such that a stop codon is introduced). As a consequence of the mutation, the targeted chromosomal sequence is inactivated and a functional protein is not produced. The inactivated chromosomal sequence comprises no exogenously introduced sequence. Also included herein are genetically modified plants in which two, three, four, five, six, seven, eight, nine, or ten or more chromosomal sequences are inactivated. 
     The modified chromosomal sequence can also be altered such that it codes for a variant protein product. For example, a genetically modified plant comprising a modified chromosomal sequence can comprise a targeted point mutation(s) or other modification such that an altered protein product is produced. In one embodiment, the chromosomal sequence can be modified such that at least one nucleotide is changed and the expressed protein comprises one changed amino acid residue (missense mutation). In another embodiment, the chromosomal sequence can be modified to comprise more than one missense mutation such that more than one amino acid is changed. Additionally, the chromosomal sequence can be modified to have a three nucleotide deletion or insertion such that the expressed protein comprises a single amino acid deletion or insertion. The altered or variant protein can have altered properties or activities compared to the wild type protein, such as altered substrate specificity, altered enzyme activity, altered kinetic rates, etc. 
     In some embodiments, the genetically modified plant can comprise at least one chromosomally integrated nucleotide sequence. A genetically modified eukaryote comprising an integrated sequence may be termed a “knock in” or a “conditional knock in.” The nucleotide sequence that is integrated sequence can, for example, encode an orthologous protein, an endogenous protein, or combinations of both. In one embodiment, a sequence encoding an orthologous protein or an endogenous protein can be integrated into a nuclear or organellar chromosomal sequence encoding a protein such that the chromosomal sequence is inactivated, but the exogenous sequence is expressed. In such a case, the sequence encoding the orthologous protein or endogenous protein may be operably linked to a promoter control sequence. Alternatively, a sequence encoding an orthologous protein or an endogenous protein may be integrated into a nuclear or organellar chromosomal sequence without affecting expression of a chromosomal sequence. The present disclosure also encompasses genetically modified plants in which two, three, four, five, six, seven, eight, nine, or ten or more sequences, including sequences encoding protein(s), are integrated into the genome. Any gene of interest as disclosed herein can be introduced integrated into the chromosomal sequence of the plant nucleus or organelle. In particular embodiments, genes that increase plant growth or yield are integrated into the chromosome. The chromosomally integrated sequence encoding a protein can encode the wild type form of a protein of interest or can encode a protein comprising at least one modification such that an altered version of the protein is produced. 
     In certain embodiments, the genetically modified plant can comprise at least one modified chromosomal sequence encoding a protein such that the expression pattern of the protein is altered. For example, regulatory regions controlling the expression of the protein, such as a promoter or a transcription factor binding site, can be altered such that the protein is over-expressed, or the tissue-specific or temporal expression of the protein is altered, or a combination thereof. Alternatively, the expression pattern of the protein can be altered using a conditional knockout system. A non-limiting example of a conditional knockout system includes a Cre-lox recombination system. A Cre-lox recombination system comprises a Cre recombinase enzyme, a site-specific DNA recombinase that can catalyze the recombination of a nucleic acid sequence between specific sites (lox sites) in a nucleic acid molecule. Methods of using this system to produce temporal and tissue specific expression are known in the art. 
     The following numbered paragraphs also form part of the present disclosure: 
     1. A method of modifying a nucleotide sequence at a target site in the genome of a plant cell, the method comprising: introducing into the plant cell (i) a DNA-targeting RNA, or a DNA polynucleotide encoding a DNA-targeting RNA; and (ii) a Cas12b polypeptide, or a polynucleotide encoding a Cas12b polypeptide, wherein the method modifies the nucleotide sequence at the target site. 
     2. The method of numbered paragraph 1, further comprising: culturing the plant cell to produce a plant under conditions in which the Cas12b polypeptide is expressed and cleaves the nucleotide sequence at the target site to produce a modified nucleotide sequence; and selecting a plant comprising said modified nucleotide sequence. 
     3. The method of numbered paragraph 2, wherein the cleaving of the nucleotide sequence at the target site comprises a staggered double strand break. 
     4. The method of any of numbered paragraphs 1-3, wherein said modified nucleotide sequence comprises insertion of heterologous DNA into the genome of the plant cell, deletion of a nucleotide sequence from the genome of the plant cell, or mutation of at least one nucleotide in the genome of the plant cell. 
     5. The method of any of numbered paragraphs 1-4, wherein the polynucleotide encoding the Cas12b polypeptide is selected from SEQ ID NOs: 1-4, or a sequence having at least 90% identity thereto. 
     6. The method of any of numbered paragraphs 1-5, wherein the Cas12b polypeptide is selected from SEQ ID NOs: 5-8, or a sequence having at least 90% identity thereto. 
     7. The method of any of numbered paragraphs 1-6, wherein the Cas12b polypeptide is from  Alicyclobacillus acidoterrestris, Bacillus thermoamylovorans, Alicyclobacillus acidiphilus , or  Bacillus hisashii.    
     8. The method of numbered paragraph 7, wherein the Cas12b polypeptide is from  Alicyclobacillus acidiphilus.    
     9. The method of any of numbered paragraphs 1-8, wherein said plant cell is from a monocotyledonous or a dicotyledonous species. 
     10. The method of any of numbered paragraphs 1-9, wherein said plant cell is an  Oryza sativa  cell. 
     11. The method of any of numbered paragraphs 1-10, wherein said DNA-targeting RNA is a guide RNA. 
     12. The method of any of numbered paragraphs 1-11, wherein the DNA-targeting RNA comprises: (a) a first segment comprising a nucleotide sequence that is complementary to a sequence in the target DNA; and (b) a second segment that interacts with a Cas12b polypeptide. 
     13. The method of any of numbered paragraphs 1-12, wherein the expression of the Cas12b polypeptide is under the control of an inducible promoter, a constitutive promoter, a cell type-specific promoter, or a developmentally-preferred promoter. 
     14. The method of any of numbered paragraphs 1-13, wherein the target site is located immediately 3′ of a PAM site in the genome of the plant cell. 
     15. The method of numbered paragraph 14, wherein the PAM site comprises VTTV. 
     16. The method of any of numbered paragraphs 1-15, wherein the polynucleotide sequence encoding the Cas12b polypeptide is codon-optimized for expression in a plant cell. 
     17. The method of any of numbered paragraphs 1-16, wherein polynucleotide encoding the Cas12b polypeptide is present in a vector. 
     18. The method of numbered paragraph 17, wherein the vector is a polynucleotide sequence selected from SEQ ID NOs: 27-30. 
     19. The method of any of numbered paragraphs 1-18, wherein the Cas12b polypeptide comprises: (a) an RNA-binding portion that interacts with the DNA-targeting RNA; and (b) an activity portion that exhibits site-directed enzymatic activity. 
     20. The method of any of numbered paragraphs 1-19, wherein the Cas12b polypeptide is fused to a deaminase domain. 
     21. The method of numbered paragraph 20, wherein the modified nucleotide sequence comprises a base edit at the target site. 
     22. The method of any of numbered paragraphs 1-21, wherein the Cas12b polypeptide is fused to a reverse transcriptase. 
     23. The method of any of numbered paragraphs 20-22, wherein the Cas12b polypeptide comprises one or more mutations that reduce or eliminate the nuclease activity of the Cas12b polypeptide. 
     24. The method of any of numbered paragraphs 20-23, wherein the Cas12b polypeptide has nickase activity. 
     25. The method of any of numbered paragraphs 20-24, wherein the polynucleotide encoding the Cas12b polypeptide is selected from SEQ ID NOs: 9-17, or a sequence having at least 90% identity thereto. 
     26. The method of any of numbered paragraphs 20-25, wherein the Cas12b polypeptide is selected from SEQ ID NOs: 18-26, or a sequence having at least 90% identity thereto. 
     27. The method of any of numbered paragraphs 20-26, wherein the Cas12b polypeptide comprises one or more mutations in a position corresponding to amino acid positions D570, E848, or D977 in  Alicyclobacillus acidiphilus  Cas12b that reduce or eliminate the nuclease activity. 
     28. A method of modulating the expression of a target gene in a plant cell, the method comprising: introducing into the plant cell (i) a DNA-targeting RNA, or a DNA polynucleotide encoding a DNA-targeting RNA; and (ii) a mutated Cas12b polypeptide, or a polynucleotide encoding a mutated Cas12b polypeptide, wherein the mutated Cas12b polypeptide comprises one or more mutations that reduce or eliminate the nuclease activity of the Cas12b polypeptide, wherein the method modulates the expression of the target gene. 
     29. The method of numbered paragraph 28, wherein the target gene is upregulated or downregulated. 
     30. The method of numbered paragraphs 28 or 29, wherein the polynucleotide encoding the mutated Cas12b polypeptide is selected from SEQ ID NOs: 9-17, or a sequence having at least 90% identity thereto. 
     31. The method of any of numbered paragraphs 28-30, wherein the mutated Cas polypeptide is selected from SEQ ID NOs: 18-26, or a sequence having at least 90% identity thereto. 
     32. The method of any of numbered paragraphs 28-31, wherein the mutated Cas polypeptide comprises one or more mutations in a position corresponding to amino acid positions D570, E848, or D977 in  Alicyclobacillus acidiphilus  Cas12b that reduce or eliminate the nuclease activity. 
     33. The method of any of numbered paragraphs 28-32, wherein the mutated Cas12b polypeptide is fused to a transcriptional activation domain or a transcriptional repression domain. 
     34. The method of any of numbered paragraphs 28-33, wherein said plant cell is from a monocotyledonous or a dicotyledonous species. 
     35. The method of any of numbered paragraphs 28-34, wherein said plant cell is an  Oryza sativa  cell. 
     36. The method of any of numbered paragraphs 28-35, wherein said DNA-targeting RNA is a guide RNA. 
     37. The method of any of numbered paragraphs 28-36, wherein the expression of the Cas12b polypeptide is under the control of an inducible promoter, a constitutive promoter, a cell type-specific promoter, or a developmentally-preferred promoter. 
     38. The method of any of numbered paragraphs 28-37, wherein the polynucleotide sequence encoding the Cas12b polypeptide is codon-optimized for expression in a plant cell. 
     39. The method of any of numbered paragraphs 28-38, wherein polynucleotide encoding the mutated Cas12b polypeptide is present in a vector. 
     40. The method of numbered paragraph 39, wherein the vector is a polynucleotide sequence selected from SEQ ID NOs: 31-46. 
     41. A plant cell produced by the method of any of numbered paragraphs 1-40. 
     42. A plant comprising the plant cell of numbered paragraph 41. 
     43. A nucleic acid molecule comprising a polynucleotide sequence encoding a Cas12b polypeptide, wherein the polynucleotide sequence has at least 90% identity with a sequence selected from SEQ ID NOs: 1-4 and 9-17. 
     44. A nucleic acid molecule comprising a polynucleotide sequence encoding an Cas12b polypeptide operably linked to a heterologous promoter that is operable in a plant cell, wherein the Cas12b polypeptide has at least 90% identity with SEQ ID NO: 7, and wherein the Cas12b polypeptide provides increased editing efficiency in a plant cell relative to AacCas12b, BthCas12b, and BhCas12b. 
     45. A nucleic acid molecule comprising a polynucleotide sequence encoding a 
     Cas12b polypeptide, wherein the Cas12b polypeptide has at least 90% identity with a sequence selected from SEQ ID NOs: 5-8 and 18-26, and wherein the polynucleotide comprises at least one base change so as not to be a naturally occurring sequence. 
     46. The nucleic acid molecule of any of numbered paragraphs 43-45, wherein the nucleic acid molecule is a vector. 
     47. The nucleic acid molecule of numbered paragraph 46, wherein the vector is a polynucleotide sequence selected from SEQ ID NOs: 27-46. 
     48. A plant or a plant cell comprising the nucleic acid molecule of any of numbered paragraphs 43-47. 
     All publications and patent applications mentioned in the specification are indicative of the level of skill of those skilled in the art to which this invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 
     Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be obvious that certain changes and modifications may be practiced within the scope of the appended claims. 
     The following examples are offered by way of illustration and not by way of limitation. 
     EXAMPLES 
     Example 1 
     Characterization of Cas12b 
     Structures for DNA targeting complexes of AacCas12b and BthCas12b have been recently resolved. We decided to test AacCas12b, AaCas12b and BthCas12b for their capability in plant genome editing. Since AaCas12b shares high sequence identity to AacCas12b, the AacCas12b sgRNA scaffold was used for both AacCas12b and AaCas12b. Similarly, a BthCas12b sgRNA scaffold was used for BthCas12b. These Cas12b DNA coding sequences were codon-optimized for rice, a major crop and test platform in this study. We adopted the dual Polymerase II (Pol II) promoter expression system and HH-HDV (hammerhead virus-hepatitis delta virus) dual ribozyme guide RNA processing system that we established for CRISPR-Cas12a ( FIG. 1 ). Previous in vitro assays established PAMs as TTN (N=A, T, G, C) for AacCas12b and ATTN for BthCas12b. We targeted two sites in OsEPFL9 and OsGS3 with GTTG and ATTC PAMs, respectively. To quantify the editing efficiencies of Cas12b nucleases, expression vectors were transfected into rice protoplasts. AaCas12b resulted in editing efficiency over 10% at both sites, higher than AacCas12b (˜5%) ( FIG. 2 ). BthCas12b displayed very low editing efficiency ( FIG. 2 ). AaCas12b, AacCas12b and BthCas12b mainly generated 4-14 bp deletions ( FIG. 3A-3C ), which are larger than those induced by Cas9 (1-3 bp). These deletions occurred about 12-24 nucleotides distal to the PAM sites ( FIG. 4A-4E ), consistent with the staggered DSBs generated in this region. Targeting an additional site in OsPDS with AacCas12b further confirmed this editing pattern ( FIG. 5A, 5B ). 
     To further investigate the PAM requirements for AacCas12b and AaCas12b in planta, we targeted a series of VTTV (V=A, C, G) PAM sites and assessed editing activity in rice protoplasts. While both AacCas12b and AaCas12b showed editing activity at five out of six ATTV sites, AaCas12b is generally more efficient and it resulted in over 50% mutation frequencies at ATTA-01 and ATTC-01 sites ( FIG. 6 ). Among two additional GTTG PAM sites, both AacCas12b and AaCas12b resulted in high editing efficiency (50%-60%) at one site (GTTG-01) but failed at the other site (GTTG-02) ( FIG. 6 ). Further testing suggested AaCas12b could edit CTTG and GTTC PAM sites ( FIG. 7A, 7B ). However, both Cas12b variants largely failed at an additional three CTTG and two GTTC PAM sites, as well as three CTTC and two GTTA PAM sites. Unlike Cas12a, AacCas12b and AaCas12b could barely edit six VTTTV PAM sites tested. Interestingly, AaCas12b could edit a TTTTV PAM site with ˜20% mutation frequency ( FIG. 7A, 7B ). Together, our data demonstrates AaCas12b and AacCas12b are potent SSNs for targeted mutagenesis in rice and they generally recognize VTTV PAMs, with more preference for ATTV and GTTG PAMs. Our observation is largely consistent with the observations of PAM requirements for Cas12b orthologs in human cells. 
     Initial comparison of three Cas12b orthologs suggested AaCas12b is superior to AacCas12b and BthCas12b for targeted mutagenesis in rice. We assessed targeting specificity of AaCas12b by using six crRNA protospacer sequences of OsEPFL9-sgRNA02 that carry double mismatch nucleotides (at positions 1-2, 5-6, 9-10, 13-14, 17-18 and 19-20). These six constructs were compared with the on-target control construct in rice protoplasts. The mutation frequency data suggest all these mismatch nucleotides had completely abolished editing activity at the target site ( FIG. 8A ). Similar results were obtained by targeting an independent site with Os12g24050-sgRNA01 ( FIG. 8B ), suggesting AaCas12b is a highly specific SSN in rice cells. Interestingly, a recent study in human and mouse cells suggests AaCas12b, unlike Cas9 and Cas12a, could barely tolerant single base mismatches at nearly every position of the protospacer, supporting its high specificity. We further shortened the length of the protospacer of OsEPFL9-sgRNA02 and found AaCas12b completely lost editing activity with protospacers of 18 nucleotides and shorter ( FIG. 9 ). While more study is warranted, this result is in sharp contrast to Cas9 and Cas12a, which generally still possess nuclease activity with 17-18-nucleotide protospacers. Together, our data suggest AaCas12b is a highly specific SSN for plant genome editing. 
     Example 2 
     CRISPR-Cas12b Plant Genome Editing 
     We next sought to generate rice mutants by Cas12b. Both AacCas12b and AaCas12b constructs targeting the OsEPFL9-sgRNA02 site were transformed into rice calli by  Agrobacterium.  Analysis of 22 individual T0 transgenic lines for AacCas12b revealed eight lines carried monoallelic mutations at the target site, representing a 36.4% mutation rate ( FIG. 10A ). Consistent with the protoplast data, AaCas12b had a higher mutation rate of 54.2% as 13 out of 24 T0 lines were mutants and six lines carried biallelic mutations ( FIG. 10B ). The mutations in these edited lines were predominantly large deletions ( FIG. 10A, 10B ). These results demonstrated that both AacCas12b and AaCas12b can effectively generate stable mutants in rice. 
     A major advantage of the CRISPR system is its flexibility of multiplexing. We constructed a multiplexed Cas12b system based on dual Pol II promoters and an HH-sgRNA-HDV array ( FIG. 11 ). We decided to simultaneously target three rice genes with three sgRNAs: OsR005-sgRNA02, OsEPFL9-sgRNA02 and OsGS3-sgRNA02. Two multiplexing constructs based on AacCas12b and AaCas12b were made for rice stable transformation. For each construct, we analyzed 24 independent T0 lines. For AacCas12b, one line (Line 17) carried a monoallelic mutation at the OsR005-sgRNA02 site; 12 (50%) lines had mutations at the OsEPFL9-sgRNA02 site and three lines had biallelic mutations; six (25%) lines had mutations at the OsGS3-sgRNA02 site and none carried biallelic mutations ( FIG. 12A ). Among them, four (Lines 1, 17, 20 and 22) are double mutants ( FIG. 12A ). These mutations were first identified by Restriction Fragment Length Polymorphism (RFLP) assays and later confirmed by Sanger sequencing. For AaCas12b, none of the 24 T0 plants assayed carried mutations at the OsR005-sgRNA02 site, consistent with the low editing activity for this sgRNA in protoplasts ( FIG. 7 ). However, AaCas12b resulted in very high mutation rates at both OsEPFL9 and OsGS3 genes: at the OsEPFL9-sgRNA02 site, 16 (66.7%) T0 lines were mutants and seven lines had biallelic mutations; At the OsGS3-sgRNA02 site, 17 (70.85%) T0 lines were mutants and 11 lines had biallelic mutations ( FIG. 12B ). Impressively, 16 lines were double mutants and seven were biallelic double mutants ( FIG. 12B ). These mutations, including a 118 bp large deletion, have been further validated by Sanger sequencing. To assess off-target effects in T0 lines, we randomly selected two double mutants each generated by AacCas12b and AaCas12b. In both cases, sequencing of 7-8 top putative off-target sites of OsEPFL9-sgRNA02 and OsGS3-sgRNA02 revealed no off-target mutations. Taken together, we have successfully demonstrated multiplexed genome editing by generating combinational mutants with highly specific AacCas12b and AaCas12b. 
     Example 3 
     Cas12b CRISPR Interference Systems 
     We previously established CRISPR interference (CRISPRi) systems in plants based on Cas9 and Cas12a, which recognize NGG (for SpCas9) and TTTV (for AsCas12a and LbCas12a) PAMs, respectively. As Cas12b orthologs have different PAM requirements, repurposing them for CRISPRi will greatly expand the targeting range for plant transcriptional repression. We introduced single amino acid mutations at RuvC-I (D570A), RuvC-II (E848A) and RuvC-III (D977A) in AacCas12b and the corresponding mutations in AaCas12b and BthCas12b ( FIG. 13 ). Assessment of these protein variants of AacCas12b and AaCas12b in rice protoplasts revealed that they indeed lost nuclease activity ( FIG. 14A, 14B ). We chose three of these deactivated Cas12b (dCas12b) proteins, AacCas12b-D570A, AaCas12b-D570A and BthCas12b-D573A, to test CRISPRi in rice cells. We targeted the rice gene 0504g39780 by focusing on three PAMs: ATTC, CTTG and GTTG. For each PAM, we designed three sgRNAs that target either the promoter or the coding sequence ( FIG. 15 ). The resulting 27 CRISPRi constructs were tested in rice protoplasts and the target gene expression was quantified by qRT-PCR. Three out of nine dBthCas12b constructs resulted in transcriptional repression ( FIG. 16A-16C ), indicating BthCas12b was able to bind well to some target sites. Both dAacCas12b and dAaCas12b induced transcriptional repression at nearly every target site with variable repression levels (25%-75%) ( FIG. 16A-16C ). Interestingly, targeted binding of dCas12b to the promoter region and the coding sequence can both robustly repress the target gene expression ( FIG. 16A-16C ). We further fused three copies of SRDX repressor domain to the C-termini of the dCas12b proteins and generated three synthetic transcriptional repressors ( FIG. 17A ). By targeting the CTTG PAM sites with the same sgRNAs, we found these dCas12b-SRDX repressors resulted in comparable levels of gene repression to dCas12b ( FIG. 17B ). The data suggest the CRISPRi effects are predominantly contributed by transcription interference through dCas12b binding, rather than through chromatin modifications by the SRDX repressor. 
     Example 4 
     Cas12b Transcriptional Activation Systems 
     Until now, there has been no successful report of Cas12a transcriptional activation systems in plants. We previously reported an improved Cas9 based transcriptional activation system that uses engineered sgRNA2.0 scaffold with MS2 aptamers for recruiting transcriptional activators. Such guide RNA engineering could be applied to Cas12b, but not Cas12a, because Cas12a uses very short crRNAs which are incompatible with MS2 aptamer insertions. To establish efficient Cas12b based transcriptional activation systems, we first sought to engineer the sgRNA scaffold to improve the overall editing efficiency. We tested AaCas12b genome editing with the artsgRNA13 scaffold and three engineered artsgRNA13 scaffolds with 1-2 MS2 insertions. No editing activity was detected with these new scaffolds in rice protoplasts. However, AaCas12b, when coupled with the scaffolds Aa1.2 and Aa3.8, showed comparable editing efficiencies with the Aac scaffold at four independent target sites ( FIG. 18 ). Recently, an engineered Cas12b from  Bacillus hisashii  (Bh), BhCas12b-v4, was reported for genome editing in human cells. We compared a rice codon-optimized BhCas12b-v4 with our AaCas12b systems and found AaCas12b showed equivalent or even better editing efficiency than BhCas12b-v4 ( FIG. 18 ). We continued our focus on AaCas12b and sought to use engineered sgRNAs to recruit more activators for developing Cas12b based transcriptional activation systems. Four sgRNA scaffolds (Aac.3, Aa1.2.3, Aa3.8.3 and Aa3.8.4) that contained one MS2 aptamer near the 3′ end were first tested for genome editing. While all four modified sgRNA scaffolds led to detectable editing activities at two target sites in rice protoplasts, Aa3.8.4 had the highest editing efficiency at ATTG-02 site ( FIG. 19 ). Next, we sought to develop Cas12b transcriptional activation systems based on these MS2-containing scaffolds. A potent transcriptional activator, TV, was fused to the C-terminus of dAaCas12b. A dFnCas12a-TV fusion was also generated for comparison between Cas12a and Cas12b. Two potent activators, TV and VPR, were tested for MS2 based recruitment respectively. A total of 12 transcriptional activation configurations based on four general systems were tested ( FIG. 20 ). Two genes, OsER1 and OsGW7, were separately targeted for transcriptional activation and in each case only one sgRNA was used. Two activation systems resulted in significant transcriptional activation, while the other 10 systems including dFnCas12a-TV failed to achieve so ( FIG. 21A and 21B ). The dAaCas12b-TV, when coupled with Aa3.8 sgRNA scaffold, resulted in 3 to 5-fold activation of both target genes. Stronger transcriptional activation (5 to 8-fold) was achieved with the transcriptional system that is based on dAaCas12b-TV and Aac.3 sgRNA scaffold-mediated recruitment of MS2-VPR. Hence, we demonstrated a potent AaCas12b transcriptional activation system with simultaneous recruitment of TV and VPR by the dAaCas12b protein and engineered Aac.3 sgRNA, respectively. 
     Example 5 
     Vector Sequences 
       
     
       
         
           
               
             
               
                   
               
             
            
               
                 Cas12b vectors 
               
            
           
           
               
               
            
               
                 pYPQ290 (AacCas12b) 
                 SEQ ID NO: 27 
               
               
                   
               
               
                 pYPQ291 (BthCas12b) 
                 SEQ ID NO: 28 
               
               
                   
               
               
                 pYPQ292 (AaCas12b) 
                 SEQ ID NO: 29 
               
               
                   
               
               
                 pYPQ293 (BhCas12b_v4) 
                 SEQ ID NO: 30 
               
               
                   
               
            
           
           
               
            
               
                 Catalytically dead Cas12b vectors 
               
            
           
           
               
               
            
               
                 pYPQ290-D570A 
                 SEQ ID NO: 31 
               
               
                   
               
               
                 pYPQ290-D977A 
                 SEQ ID NO: 32 
               
               
                   
               
               
                 pYPQ290-E848A 
                 SEQ ID NO: 33 
               
               
                   
               
               
                 pYPQ291-D573A 
                 SEQ ID NO: 34 
               
               
                   
               
               
                 pYPQ291-D951A 
                 SEQ ID NO: 35 
               
               
                   
               
               
                 pYPQ291-E827A 
                 SEQ ID NO: 36 
               
               
                   
               
               
                 pYPQ292-D570A 
                 SEQ ID NO: 37 
               
               
                   
               
               
                 pYPQ292-D977A 
                 SEQ ID NO: 38 
               
               
                   
               
               
                 pYPQ292-E848A 
                 SEQ ID NO: 39 
               
               
                   
               
            
           
           
               
            
               
                 Catalytically dead Cas12b fused with 
               
               
                 transcriptional repressor 
               
            
           
           
               
               
            
               
                 pYPQ290-D570A-SRDX 
                 SEQ ID NO: 40 
               
               
                   
               
               
                 pYPQ291-D573A-SRDX 
                 SEQ ID NO: 41 
               
               
                   
               
               
                 pYPQ292-D570A-SRDX 
                 SEQ ID NO: 42 
               
               
                   
               
            
           
           
               
            
               
                 Catalytically dead Cas12b fused with 
               
               
                 transcriptional activator 
               
            
           
           
               
               
            
               
                 pYPQ239A (dFnCas12a)-TV 
                 SEQ ID NO: 43 
               
               
                   
               
               
                 pYPQ292 (AaCas12b)-D570A-TV 
                 SEQ ID NO: 44 
               
               
                   
               
               
                 pYPQ292 (AaCas12b)-D570A-TV-MS2-TV 
                 SEQ ID NO: 45 
               
               
                   
               
               
                 pYPQ292 (AaCas12b)-D570A-TV-MS2-VPR 
                 SEQ ID NO: 46 
               
               
                   
               
            
           
           
               
            
               
                 sgRNA expression vectors 
               
            
           
           
               
               
            
               
                 pYPQ141-ZmUbi-RZ-Aac 
                 SEQ ID NO: 47 
               
               
                   
               
               
                 pYPQ141-ZmUbi-RZ-Bth 
                 SEQ ID NO: 48 
               
               
                   
               
               
                 pYPQ141-ZmUbi-RZ-Bh 
                 SEQ ID NO: 49 
               
               
                   
               
               
                 pYPQ141-ZmUbi-RZ-Aac.3 
                 SEQ ID NO: 50 
               
               
                   
               
               
                 pYPQ141-ZmUbi-RZ-Aa1.2 
                 SEQ ID NO: 51 
               
               
                   
               
               
                 pYPQ141-ZmUbi-RZ-Aa1.2.3 
                 SEQ ID NO: 52 
               
               
                   
               
               
                 pYPQ141-ZmUbi-RZ-Aa3.8 
                 SEQ ID NO: 53 
               
               
                   
               
               
                 pYPQ141-ZmUbi-RZ-Aa3.8.3 
                 SEQ ID NO: 54 
               
               
                   
               
               
                 pYPQ141-ZmUbi-RZ-Aa3.8.4 
                 SEQ ID NO: 55