RNAs from pathogens inhibit plant immunity

Provided are pathogen-resistant plants comprising a heterologous expression cassette, the expression cassette comprising a promoter operably linked to a polynucleotide that is complementary to, or mediates destruction, of a plant immunity suppressing sRNA of a pathogen, wherein the plant is less susceptible to the pathogen compared to a control plant lacking the expression cassette. Methods of making and cultivating pathogen-resistant plants are also provided.

The Sequence Listing named “081906-1099170-214920US_SEQ.txt”, created on Sep. 24, 2018, 224,616 bytes, machine format IBM-PC, MS-Windows operating system, is hereby incorporated by reference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

Botrytis cinereais a fungal pathogen that infects almost all vegetable and fruit crops and annually causes $10-100 billion losses worldwide. With its broad host range,B. cinereais a useful model for studying the pathogenicity of aggressive fungal pathogens. Many pathogens of plants and animals deliver effectors into host cells to suppress host immunity (H. Ashida et al.,Curr. Opin. Microbiol.14, 16 (2011); M. Rafiqi et al.,Curr. Opin. Plant Biol.15, 477 (2012); T. O. Bozkurt et al.,Curr. Opin. Plant Biol.15, 483 (2012); H. Hilbi, et al.,Traffic13, 1187 (2012)).

sRNAs induce gene silencing by binding to Argonaute (AGO) proteins and directing the RNA-induced silencing complex (RISC) to genes with complementary sequences. sRNAs from both plant and animal hosts have been recognized as regulators in host-microbial interaction (5-8). Although sRNAs are also present in various fungi and oomycetes, including many pathogens (9-14), it has not been clear whether they regulate host-pathogen interaction.

BRIEF SUMMARY OF THE INVENTION

The present application provides for plants (or a plant cell, seed, flower, leaf, fruit, or other plant part from such plants or processed food or food ingredient from such plants) comprising a heterologous expression cassette, the expression cassette comprising a promoter operably linked to a polynucleotide that is complementary to, or mediates destruction, of a plant immunity suppressing sRNA of a pathogen, wherein the plant is less susceptible to the pathogen compared to a control plant lacking the expression cassette.

In some embodiments, the polynucleotide encodes a short tandem target mimic (STTM) of the sRNA. In some embodiments, the STTM is engineered from primers (a forward primer and a reverse primer) listed in Table 2. In some embodiments, the polynucleotide encodes an antisense nucleic acid that is complementary to the sRNA.

The present application also provides for plants (or a plant cell, seed, flower, leaf, fruit, or other plant part from such plants or processed food or food ingredient from such plants) comprising a heterologous expression cassette, the expression cassette comprising a promoter operably linked to a polynucleotide that is an sRNA-resistant target that encodes a protein that functions in plant immunity, wherein the promoter is heterologous to the polynucleotide. In some embodiments, a plant into which the expression cassette has been introduced has enhanced pathogen resistance compared to a control plant lacking the expression cassette.

In some embodiments, the polynucleotide is substantially (e.g., at least 60, 70, 75, 80, 85, 90, or 95%) identical to any of SEQ ID NOS:4-13. In some embodiments, the polynucleotide is an sRNA-resistant target encoding mitogen activated protein kinase 1 (MPK1), mitogen activated protein kinase 2 (MPK2), peroxiredoxin (PRXIIF), cell-wall associated kinase (WAK), or tomato mitogen activated protein kinase kinase kinase 4 (MAPKKK4). In some embodiments, the polynucleotide is an sRNA-resistant target of a gene listed inFIG. 1, Table 1, or Table 3. In some embodiments, the polynucleotide is resistant to gene silencing by an sRNA listed in Table 1. In some embodiments, the polynucleotide is resistant to gene silencing by Bc-siR3.1, Bc-siR3.2, or Bc-siR5.

In some embodiments, the sRNA comprises a sequence listed in Table 1. In some embodiments, the sRNA comprises the sequence of Bc-siR3.1, Bc-siR3.2, or Bc-siR5.

In some embodiments, the pathogen isBotrytis. In some embodiments, the pathogen isBotrytis cinera.

In some embodiments, the promoter is an inducible promoter. In some embodiments, the promoter is pathogen inducible. In some embodiments, the promoter is induced upon infection byBotrytis. In some embodiments, the promoter is substantially (e.g., at least 60, 70, 75, 80, 85, 90, or 95%) identical toArabidopsisBIK1 (SEQ ID NO:1),ArabidopsisPDF1.2 (SEQ ID NO:2), or tomato TPK1b (SEQ ID NO:3). In some embodiments, the promoter is stress-inducible. In some embodiments, the promoter is tissue-specific. In some embodiments, the promoter is specifically expressed in the epidermis. In some embodiments, the promoter is substantially (e.g., at least 60, 70, 75, 80, 85, 90, or 95%) identical toArabidopsisML1 (SEQ ID NO:14) or tomato ML1 (SEQ ID NO:15).

In another aspect, the present invention provides for expression cassettes comprising: a promoter operably linked to a polynucleotide that is complementary to, or mediates destruction, of a plant immunity suppressing sRNA of a pathogen, wherein the plant is less susceptible to the pathogen compared to a control plant lacking the expression cassette; or comprising a promoter operably linked a polynucleotide that is an sRNA-resistant target that encodes a protein that functions in plant immunity, wherein the promoter is heterologous to the polynucleotide. Isolated nucleic acids comprising said expression cassettes are also provided.

In still another aspect, the present invention provides for expression vectors comprising an expression cassette as described herein.

In another aspect, methods of making a pathogen-resistant plant are provided. In some embodiments, the method comprises:introducing the nucleic acid comprising an expression cassette as described herein into a plurality of plants; andselecting a plant comprising the expression cassette

In yet another aspect, methods of cultivating a plurality of pathogen-resistant plants are provided.

DEFINITIONS

The term “pathogen-resistant” or “pathogen resistance” refers to an increase in the ability of a plant to prevent or resist pathogen infection or pathogen-induced symptoms. Pathogen resistance can be increased resistance relative to a particular pathogen species or genus (e.g.,Botrytis), increased resistance to multiple pathogens, or increased resistance to all pathogens (e.g., systemic acquired resistance).

The term “plant immunity suppressing sRNA” refers to an sRNA that induces gene silencing in a plant of one or more genes that function or are predicted to function in plant immunity. For example, in some embodiments a plant immunity suppressing sRNA is an sRNA that induces gene silencing of a mitogen-activated protein kinase (e.g., MPK1, MPK2, or MAPKKK4), an oxidative stress-related gene (e.g., periredoxin (PRXIIF), or a cell wall-associated kinase (WAK). Exemplary plant immunity suppressing sRNAs are listed, for example, inFIG. 16and Table 1.

The term “sRNA” refers to “small RNA,” a short non-coding RNA sequence. In some embodiments, an sRNA sequence comprises less than about 250 nucleotides (e.g., less than 250 nucleotides, less than 200 nucleotides, less than 150 nucleotides, less than 100 nucleotides, or less than 50 nucleotides). In some embodiments, an sRNA sequence comprises about 50-250 nucleotides, about 15-250 nucleotides, about 20-200 nucleotides, about 50-200 nucleotides, about 20-100 nucleotides, about 20-50 nucleotides, or about 20-30 nucleotides. In some embodiments, a sRNA sequence induces gene silencing, e.g., in a host plant. For example, in some embodiments a sRNA sequence induces gene silencing by directing a host's (e.g., host plant's) RNA-induced silencing complex (RISC) to genes with complementary sequences (“target genes”).

The term “sRNA-resistant target,” as used with reference to a polynucleotide sequence, refers to a polynucleotide sequence having a synonymous mutation relative to a sRNA target gene, wherein the polynucleotide sequence of the sRNA-resistant target comprises one or more nucleotide mutations relative to the polynucleotide sequence of the sRNA target gene that decreases the ability of the sRNA (e.g., a pathogen sRNA) to induce gene silencing of the sRNA-resistant target gene and wherein the amino acid sequence (e.g., protein sequence) that is encoded by the polynucleotide sequence of the sRNA-resistant target is identical to the amino acid sequence that is encoded by the polynucleotide sequence of the sRNA target gene. In some embodiments, the polynucleotide sequence of the sRNA-resistant target comprises 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more nucleotide mutations relative to the polynucleotide sequence of the sRNA target gene.

The term “nucleic acid” or “polynucleotide” refers to a single or double-stranded polymer of deoxyribonucleotide or ribonucleotide bases read from the 5′ to the 3′ end. Nucleic acids may also include modified nucleotides that permit correct read through by a polymerase and do not significantly alter expression of a polypeptide encoded by that nucleic acid.

The phrase “nucleic acid encoding” or “polynucleotide encoding” refers to a nucleic acid which directs the expression of a specific protein or peptide. The nucleic acid sequences include both the DNA strand sequence that is transcribed into RNA and the RNA sequence that is translated into protein. The nucleic acid sequences include both the full length nucleic acid sequences as well as non-full length sequences derived from the full length sequences. It should be further understood that the sequence includes the degenerate codons of the native sequence or sequences which may be introduced to provide codon preference in a specific host cell.

Two nucleic acid sequences or polypeptides are said to be “identical” if the sequence of nucleotides or amino acid residues, respectively, in the two sequences is the same when aligned for maximum correspondence as described below. “Percentage of sequence identity” is determined by comparing two optimally aligned sequences over a comparison window, wherein the portion of the polynucleotide or polypeptide sequence in the comparison window may comprise additions or deletions (i.e., gaps) as compared to the reference sequence (which does not comprise additions or deletions) for optimal alignment of the two sequences. The percentage is calculated by determining the number of positions at which the identical nucleic acid base or amino acid residue occurs in both sequences to yield the number of matched positions, dividing the number of matched positions by the total number of positions in the window of comparison and multiplying the result by 100 to yield the percentage of sequence identity. When percentage of sequence identity is used in reference to proteins or peptides, it is recognized that residue positions that are not identical often differ by conservative amino acid substitutions, where amino acid residues are substituted for other amino acid residues with similar chemical properties (e.g., charge or hydrophobicity) and therefore do not change the functional properties of the molecule. Where sequences differ in conservative substitutions, the percent sequence identity may be adjusted upwards to correct for the conservative nature of the substitution. Means for making this adjustment are well known to those of skill in the art. Typically this involves scoring a conservative substitution as a partial rather than a full mismatch, thereby increasing the percentage sequence identity. Thus, for example, where an identical amino acid is given a score of 1 and a non-conservative substitution is given a score of zero, a conservative substitution is given a score between zero and 1. The scoring of conservative substitutions is calculated according to, e.g., the algorithm of Meyers & Miller,Computer Applic. Biol. Sci.4:11-17 (1988) e.g., as implemented in the program PC/GENE (Intelligenetics, Mountain View, Calif., USA).

The term “substantial identity” or “substantially identical,” as used in the context of polynucleotide or polypeptide sequences, refers to a sequence that has at least 60% sequence identity to a reference sequence. Alternatively, percent identity can be any integer from 60% to 100%. Exemplary embodiments include at least: 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99%, as compared to a reference sequence using the programs described herein; preferably BLAST using standard parameters, as described below. One of skill will recognize that these values can be appropriately adjusted to determine corresponding identity of proteins encoded by two nucleotide sequences by taking into account codon degeneracy, amino acid similarity, reading frame positioning and the like.

A “comparison window,” as used herein, includes reference to a segment of any one of the number of contiguous positions selected from the group consisting of from 20 to 600, usually about 50 to about 200, more usually about 100 to about 150 in which a sequence may be compared to a reference sequence of the same number of contiguous positions after the two sequences are optimally aligned. Methods of alignment of sequences for comparison are well-known in the art. Optimal alignment of sequences for comparison may be conducted by the local homology algorithm of Smith and WatermanAdd. APL. Math.2:482 (1981), by the homology alignment algorithm of Needleman and WunschJ. Mol. Biol.48:443 (1970), by the search for similarity method of Pearson and LipmanProc. Natl. Acad. Sci. (U.S.A.) 85: 2444 (1988), by computerized implementations of these algorithms (GAP, BESTFIT, BLAST, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group (GCG), 575 Science Dr., Madison, Wis.), or by manual alignment and visual inspection.

The BLAST algorithm also performs a statistical analysis of the similarity between two sequences (see, e.g., Karlin & Altschul,Proc. Nat'l. Acad. Sci. USA90:5873-5787 (1993)). One measure of similarity provided by the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability by which a match between two nucleotide or amino acid sequences would occur by chance. For example, a nucleic acid is considered similar to a reference sequence if the smallest sum probability in a comparison of the test nucleic acid to the reference nucleic acid is less than about 0.01, more preferably less than about 10−5, and most preferably less than about 10−20.

The term “complementary to” is used herein to mean that a polynucleotide sequence is complementary to all or a portion of a reference polynucleotide sequence. In some embodiments, a polynucleotide sequence is complementary to at least 15, at least 20, at least 25, at least 30, at least 40, at least 50, at least 75, at least 100, at least 125, at least 150, at least 175, at least 200, or more contiguous nucleotides of a reference polynucleotide sequence. In some embodiments, a polynucleotide sequence is “substantially complementary” to a reference polynucleotide sequence if at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% of the polynucleotide sequence is complementary to the reference polynucleotide sequence.

A polynucleotide sequence is “heterologous” to an organism or a second polynucleotide sequence if it originates from a foreign species, or, if from the same species, is modified from its original form. For example, when a promoter is said to be operably linked to a heterologous coding sequence, it means that the coding sequence is derived from one species whereas the promoter sequence is derived another, different species; or, if both are derived from the same species, the coding sequence is not naturally associated with the promoter (e.g., is a genetically engineered coding sequence, e.g., from a different gene in the same species, or an allele from a different ecotype or variety).

An “expression cassette” refers to a nucleic acid construct, which when introduced into a host cell, results in transcription and/or translation of a RNA or polypeptide, respectively. Antisense constructs or sense constructs that are not or cannot be translated are expressly included by this definition. One of skill will recognize that the inserted polynucleotide sequence need not be identical, but may be only substantially similar to a sequence of the gene from which it was derived.

The term “promoter,” as used herein, refers to a polynucleotide sequence capable of driving transcription of a coding sequence in a cell. Thus, promoters used in the polynucleotide constructs of the invention include cis-acting transcriptional control elements and regulatory sequences that are involved in regulating or modulating the timing and/or rate of transcription of a gene. For example, a promoter can be a cis-acting transcriptional control element, including an enhancer, a promoter, a transcription terminator, an origin of replication, a chromosomal integration sequence, 5′ and 3′ untranslated regions, or an intronic sequence, which are involved in transcriptional regulation. These cis-acting sequences typically interact with proteins or other biomolecules to carry out (turn on/off, regulate, modulate, etc.) gene transcription. A “plant promoter” is a promoter capable of initiating transcription in plant cells. A “constitutive promoter” is one that is capable of initiating transcription in nearly all tissue types, whereas a “tissue-specific promoter” initiates transcription only in one or a few particular tissue types. An “inducible promoter” is one that initiates transcription only under particular environmental conditions or developmental conditions.

The term “plant” includes whole plants, shoot vegetative organs and/or structures (e.g., leaves, stems and tubers), roots, flowers and floral organs (e.g., bracts, sepals, petals, stamens, carpels, anthers), ovules (including egg and central cells), seed (including zygote, embryo, endosperm, and seed coat), fruit (e.g., the mature ovary), seedlings, plant tissue (e.g., vascular tissue, ground tissue, and the like), cells (e.g., guard cells, egg cells, trichomes and the like), and progeny of same. The class of plants that can be used in the method of the invention is generally as broad as the class of higher and lower plants amenable to transformation techniques, including angiosperms (monocotyledonous and dicotyledonous plants), gymnosperms, ferns, and multicellular algae. It includes plants of a variety of ploidy levels, including aneuploid, polyploid, diploid, haploid, and hemizygous.

DETAILED DESCRIPTION OF THE INVENTION

As described in the Examples section below, it has been surprisingly discovered that small RNAs (sRNAs) from a plant pathogen can suppress genes involved in plant immunity. Without being bound to a particular theory, it is believed that the pathogen sRNAs suppress immunity in a host plant by using the host plant's own gene silencing mechanisms to suppress genes that function in plant immunity.

Thus, one aspect of the present invention relates to enhancing a plant's pathogen resistance by blocking, attenuating, or targeting for destruction the pathogen sRNAs. In some embodiments, a pathogen sRNA is blocked, attenuated, or targeted for destruction using a complementary polynucleotide sequence (e.g., an antisense nucleic acid sequence that is complementary or substantially complementary to the sRNA) or using a short tandem target mimic (STTM) targeting the sRNA. In some embodiments, the complementary polynucleotide sequence or STTM that targets the pathogen sRNA is expressed in a plant (e.g., in an expression cassette operably linked to a promoter), wherein the plant is less susceptible to the pathogen as compared to a control plant in which complementary polynucleotide sequence or STTM is not expressed.

In another aspect, the present invention relates to enhancing a plant's pathogen resistance by expressing sRNA-resistant target genes involved in plant immunity in plants to overcome the effect of the pathogen sRNAs. In some embodiments, the sRNA-resistant target genes are expressed under the control of a promoter (e.g., a pathogen-inducible promoter, a stress-inducible promoter, or a tissue-specific promoter).

II. Pathogen sRNAs and Attenuation of Pathogen sRNAs

In one aspect, methods of blocking or attenuating plant immunity-suppressing sRNAs of pathogens are provided. In some embodiments, the method comprises expressing in a plant a polynucleotide that is complementary or substantially complementary to the pathogen sRNA or that mediates destruction of the pathogen sRNA. In some embodiments, the polynucleotide encodes a short tandem target mimic (STTM) targeting the sRNA. In some embodiments, the polynucleotide encodes an antisense nucleic acid that is complementary or substantially complementary to the sRNA. In some embodiments, the method comprises expressing in the plant the polynucleotide that is complementary or substantially complementary to the pathogen sRNA or that mediates destruction of the pathogen sRNA under the control of a promoter, e.g., a constitutively active promoter, an inducible promoter, or tissue-specific promoter (e.g., a stress inducible promoter, a pathogen inducible promoter, or an epidermis-specific promoter).

In another aspect, plants having blocked or attenuated function of pathogen sRNAs are provided. In some embodiments, the plant comprises a heterologous expression cassette, the expression cassette comprising a promoter operably linked to a polynucleotide that is complementary or substantially complementary to the pathogen sRNA or that mediates destruction of the pathogen sRNA, wherein the plant is less susceptible to the pathogen relative to a control plant lacking the expression cassette. In some embodiments, the expression cassette comprises a polynucleotide that encodes a short tandem target mimic (STTM) targeting the sRNA. In some embodiments, the expression cassette comprises a polynucleotide that encodes an antisense nucleic acid that is complementary or substantially complementary to the sRNA. In some embodiments, the expression cassette comprises a promoter that is an inducible promoter (e.g., stress inducible or pathogen inducible). In some embodiments, the expression cassette comprises a promoter that is a constitutively active promoter. In some embodiments, the promoter is tissue-specific (e.g., epidermis-specific).

In yet another aspect, expression cassettes comprising a promoter operably linked to a polynucleotide that is complementary to, or mediates destruction, of a plant immunity suppressing sRNA of a pathogen, wherein the promoter is heterologous to the polynucleotide, or isolated nucleic acids comprising said expression cassettes, are provided. In some embodiments, the expression cassette comprises a polynucleotide that encodes a short tandem target mimic (STTM) targeting the sRNA. In some embodiments, the expression cassette comprises a polynucleotide that encodes an antisense nucleic acid that is complementary or substantially complementary to the sRNA. In some embodiments, the expression cassette comprises a promoter that is an inducible promoter (e.g., stress inducible or pathogen inducible). In some embodiments, the expression cassette comprises a promoter that is a constitutively active promoter. In some embodiments, the promoter is tissue-specific (e.g., epidermis-specific). In some embodiments, a plant in which the expression cassette is introduced is less susceptible to the pathogen compared to a control plant lacking the expression cassette.

In some embodiments, the plant immunity suppressing sRNA is from a viral, bacterial, fungal, nematode, or insect pathogen. In some embodiments, the sRNA is from a fungal pathogen. Examples of plant fungal pathogens include, but are not limited to,Botyritis, Magnaporthe, Sclerotinia, Puccinia, Fusarium, Mycosphaerella, Blumeria, Colletotrichum, Ustilago, andMelampsora. See, e.g., Dean et al.,Mol Plant Pathol13:804 (2012). In some embodiments, the pathogen isBotyritis. In some embodiments, the pathogen isBotyritis cinera.

In some embodiments, the pathogen sRNA comprises a sequence of about 15-250 nucleotides that specifically targets (e.g., induces gene silencing of) a gene encoding a protein that functions or is predicted to function in plant immunity. In some embodiments, the pathogen sRNA comprises a sequence of about 15-250 nucleotides that specifically targets a gene that encodes mitogen activated protein kinase 1 (MPK1), mitogen activated protein kinase 2 (MPK2), peroxiredoxin (PRXIIF), cell-wall associated kinase (WAK), or mitogen activated protein kinase kinase kinase 4 (MAPKKK4). In some embodiments, the pathogen sRNA comprises a sequence of about 15-250 nucleotides that specifically targets any of SEQ ID NOs:4-13 or a portion thereof.

In some embodiments, the function of a pathogen sRNA as described herein in a plant is blocked, attenuated, or reduced by expressing in the plant a polynucleotide that is complementary or substantially complementary to the sRNA or that mediates the destruction of the sRNA. As used herein, the term “mediates destruction of an sRNA” refers to inducing or promoting the degradation of a small RNA (e.g., by a small RNA degrading nuclease). In some embodiments, the polynucleotide encodes a short tandem target mimic (STTM) that targets the sRNA. In some embodiments, the polynucleotide encodes an antisense nucleic acid that is complementary or substantially complementary to the sRNA.

Short Tandem Target Mimics

In some embodiments, a short tandem target mimic (STTM) construct is used to block or attenuate function or activity of the pathogen sRNA. STTMs are composed of two short polynucleotide sequences mimicking small RNA target sites (e.g., one or more pathogen sRNA sites as described herein), separated by a linker of an empirically determined optimal size. STTMs trigger efficient degradation of targeted sRNAs by small RNA degrading nucleases. See Yan et al.,Plant Cell24:415-427 (2012).

Typically, the STTM is designed to have two noncleavable sRNA binding sites separated by a spacer. The two noncleavable sRNA binding sites can be either identical (to target one specific sRNA) or slightly different to target two slightly different sRNAs. The optimal length of the spacer is typically from about 48 to 88 nucleotides, although shorter or longer spacer sequences can be used. The sequences of the spacer should be relatively AT rich and able to form a stable stem. Methods of designing and testing STTM constructs are described, e.g., in Yan et al.,Plant Cell24:415-427 (2012), and in Tang et al.,Methods58:118-125 (2012), incorporated by reference herein.

Antisense Technology

In some embodiments, antisense technology is used to block or attenuate function or activity of the pathogen sRNA. The antisense nucleic acid sequence that is transformed into plants is substantially identical to the pathogen sRNA sequence to be blocked. In some embodiments, the antisense polynucleotide sequence is complementary to the pathogen sRNA sequence to be blocked. However, the sequence does not have to be perfectly identical to inhibit expression. Thus, in some embodiments, an antisense polynucleotide sequence that is substantially complementary (e.g., at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% complementary) to the pathogen sRNA sequence to be blocked can be used (e.g., in an expression cassette under the control of a heterologous promoter, which is then transformed into plants such that the antisense nucleic acid is produced). In some embodiments, the antisense polynucleotide is expressed under the control of a promoter as described in Section IV below, e.g., a constitutively active promoter, an inducible promoter, or a tissue-specific promoter.

Other methods of using oligonucleotide or polynucleotide constructs for blocking the function of small RNAs as described herein can also be used, such as target mimicry (see, e.g., Franco-Zorrilla et al.,Nat Genet.39:1033-1037 (2007)) and “sponges” (see, e.g., Ebert et al.,Nat. Methods4:721-726 (2007)).

III. Expression of sRNA-Resistant Targets

In another aspect, methods of making plants that are resistant to one or more pathogen sRNAs are provided. In some embodiments, the method comprises:introducing into a plant a heterologous expression cassette comprising a promoter operably linked to a polynucleotide that is an sRNA-resistant target that encodes a protein that functions in plant immunity, wherein the promoter is heterologous to the polynucleotide; andselecting a plant comprising the expression cassette.

In another aspect, expression cassettes comprising a promoter operably linked to a polynucleotide encoding a sRNA-resistant target, isolated nucleic acids comprising said expression cassettes, or plants comprising said expression cassettes, are provided. In some embodiments, a plant into which the expression cassette has been introduced has enhanced pathogen resistance relative to a control plant lacking the expression cassette. In some embodiments, a plant into which the expression cassette has been introduced has enhanced resistance to a fungal pathogen (e.g.,Botrytis, e.g.,B. cinera) relative to a control plant lacking the expression cassette.

In some embodiments, the promoter is heterologous to the polynucleotide. In some embodiments, the polynucleotide encoding the sRNA-resistant target is operably linked to an inducible promoter. In some embodiments, the promoter is pathogen inducible (e.g., aBotrytisinducible promoter). In some embodiments, the promoter is stress inducible (e.g., an abiotic stress inducible promoter). In some embodiments, the promoter is tissue-specific (e.g., epidermis-specific).

In some embodiments, the polynucleotide is an sRNA-resistant target that encodes a protein that functions or is predicted to function in plant immunity. As used herein, an sRNA-resistant target is a polynucleotide sequence having a synonymous mutation of a sequence that is targeted by a pathogen sRNA. As used herein, the term “synonymous mutation” refers to a change, relative to a reference sequence, in a DNA sequence that encodes for a protein or peptide (e.g., at 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more nucleotides relative to the reference sequence), wherein the change does not alter the amino acid that is encoded. For example, in some embodiments, pathogen sRNAs target plant immunity genes such as mitogen-activated protein kinases (including but not limited to, mitogen-activated protein kinase 1 (MPK1) or mitogen-activated protein kinase 2 (MPK2)); accordingly, in some embodiments an sRNA-resistant target comprises a synonymous mutation of a plant gene that encodes a mitogen-activated protein kinase (e.g., a synonymous mutation of MPK1 or MPK2).

In some embodiments, a polynucleotide sequence is an sRNA-resistant target if the polynucleotide sequence if the amino acid encoded by the polynucleotide sequence is produced at a detectable level. In some embodiments, a polynucleotide sequence is an sRNA-resistant target if the polynucleotide sequence if the amount of amino acid produced by a plant expressing the polynucleotide sequence in the presence of a pathogen sRNA is decreased by no more than 50%, 40%, 30%, 20%, 10%, 5%, or less relative to the amount of amino acid produced by a control plant expressing the polynucleotide sequence in the absence of the pathogen sRNA. Whether a polynucleotide is an sRNA-resistant target can be tested, for example, using a coexpression assay inNicotiana benthamianain which the sRNA is coexpressed with a polynucleotide sequence (e.g., a target gene or a synonymous mutation of the target gene) and the level of gene silencing induced by sRNA is measured. See, e.g., Example 1.

In some embodiments, the polynucleotide encodes a protein that functions or is predicted to function in plant immunity. In some embodiments, the polynucleotide comprises an sRNA-resistant target gene or predicted target gene listed inFIG. 16, Table 1, or Table 3. In some embodiments, the polynucleotide comprises a synonymous mutation of an sRNA target gene that encodes mitogen activated protein kinase 1 (MPK1), mitogen activated protein kinase 2 (MPK2), peroxiredoxin (PRXIIF), cell-wall associated kinase (WAK), or mitogen activated protein kinase kinase kinase 4 (MAPKKK4). In some embodiments, the polynucleotide comprises a synonymous mutation of an sRNA target gene in tomato selected from Solyc08g081210.2.1, Solyc03g061650.1.1, Solyc01g108160.2.1, Solyc09g014790.2.1, Solyc03g112190.2.1, or Solyc07g066530.2.1. In some embodiments, the polynucleotide comprises a synonymous mutation of an sRNA target gene inVitisselected from VIT_10s0092g00240, VIT_12s0028g01140, VIT_06s0009g01890, VIT_10s0116g00190, VIT_05s0020g01790, VIT_01s0011g01000, VIT_05s0077g01510.

In some embodiments, the polynucleotide is substantially identical (e.g., at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical) to any of SEQ ID NOS:4-13, comprises 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more nucleotide mutations relative to SEQ ID NOS:4-13, and encodes an identical protein as SEQ ID NOS:4-13. Non-limiting examples of nucleotide mutations (synonymous mutations) that can be made in the sequences of SEQ ID NOS:4-13 are described below in Example 3, as shown in the alignments of sRNA sequences to wild-type target gene sequences and mutated target gene sequences.

In some embodiments, the sRNA-resistant target gene comprises a polynucleotide sequence that is resistant to gene silencing by an sRNA listed inFIG. 16or Table 1. In some embodiments, the sRNA-resistant target comprises a polynucleotide sequence that is resistant to gene silencing by Bc-siR3.1 (TTGTGGATCTTGTAGGTGGGC; SEQ ID NO:43), Bc-siR3.2 (TACATTGTGGATCTTGTAGGT; SEQ ID NO:44), or Bc-siR5 (TTTGACTCGGAATGTATACTT; SEQ ID NO:45).

IV. Polynucleotides and Recombinant Expression Vectors

The isolation of polynucleotides of the invention may be accomplished by a number of techniques. For instance, oligonucleotide probes based on the sequences disclosed here can be used to identify the desired polynucleotide in a cDNA or genomic DNA library from a desired plant species. To construct genomic libraries, large segments of genomic DNA are generated by random fragmentation, e.g. using restriction endonucleases, and are ligated with vector DNA to form concatemers that can be packaged into the appropriate vector. Alternatively, cDNA libraries from plants or plant parts (e.g., flowers) may be constructed.

The cDNA or genomic library can then be screened using a probe based upon a sequence disclosed here. Probes may be used to hybridize with genomic DNA or cDNA sequences to isolate homologous genes in the same or different plant species. Alternatively, antibodies raised against a polypeptide can be used to screen an mRNA expression library.

Alternatively, the nucleic acids of interest can be amplified from nucleic acid samples using amplification techniques. For instance, polymerase chain reaction (PCR) technology to amplify the sequences of the genes directly from mRNA, from cDNA, from genomic libraries or cDNA libraries. PCR and other in vitro amplification methods may also be useful, for example, to clone nucleic acid sequences that code for proteins to be expressed, to make nucleic acids to use as probes for detecting the presence of the desired mRNA in samples, for nucleic acid sequencing, or for other purposes. For a general overview of PCR see PCR Protocols: A Guide to Methods and Applications. (Innis, M, Gelfand, D., Sninsky, J. and White, T., eds.), Academic Press, San Diego (1990).

Polynucleotides can also be synthesized by well-known techniques as described in the technical literature. See, e.g., Carruthers et al., Cold Spring Harbor Symp.Quant. Biol.47:411-418 (1982), and Adams et al.,J. Am. Chem. Soc.105:661 (1983). Double stranded DNA fragments may then be obtained either by synthesizing the complementary strand and annealing the strands together under appropriate conditions, or by adding the complementary strand using DNA polymerase with an appropriate primer sequence.

Once a polynucleotide sequence that is complementary to the pathogen sRNA or that mediates destruction of the pathogen sRNA, or a polynucleotide that is a sRNA-resistant target, is obtained, it can be used to prepare an expression cassette for expression in a plant. In some embodiments, expression of the polynucleotide is directed by a heterologous promoter.

Any of a number of means well known in the art can be used to drive expression of the polynucleotide sequence of interest in plants. Any organ can be targeted, such as shoot vegetative organs/structures (e.g. leaves, stems and tubers), epidermis, roots, flowers and floral organs/structures (e.g. bracts, sepals, petals, stamens, carpels, anthers and ovules), seed (including embryo, endosperm, and seed coat) and fruit. Alternatively, expression can be conditioned to only occur under certain conditions (e.g., using an inducible promoter).

For example, a plant promoter fragment may be employed to direct expression of the polynucleotide sequence of interest in all tissues of a regenerated plant. Such promoters are referred to herein as “constitutive” promoters and are active under most environmental conditions and states of development or cell differentiation. Examples of constitutive promoters include the cauliflower mosaic virus (CaMV) 35S transcription initiation region, the 1′- or 2′-promoter derived from T-DNA ofAgrobacterium tumafaciens, and other transcription initiation regions from various plant genes known to those of skill.

Alternatively, the plant promoter may direct expression of the polynucleotide sequence of interest in a specific tissue (tissue-specific promoters) or may be otherwise under more precise environmental control (inducible promoters). Examples of tissue-specific promoters under developmental control include promoters that initiate transcription only in certain tissues, such as the epidermis, leaves, or guard cells (including but not limited to those described in WO/2005/085449; U.S. Pat. Nos. 6,653,535; 7,834,243; EP Patent No. 1 888 754; Li et al.,Sci China C Life Sci.2005 April; 48(2):181-6; Husebye, et al.,Plant Physiol, April 2002, Vol. 128, pp. 1180-1188; Plesch, et al.,Gene, Volume 249, Number 1, 16 May 2000, pp. 83-89(7), and Sessions et al.,Plant J, October 1999, Vol. 20, pp. 259-263, each of which is incorporated by reference). Examples of environmental conditions that may affect transcription by inducible promoters include the presence of a pathogen, anaerobic conditions, elevated temperature, or the presence of light.

In some embodiments, the promoter is an inducible promoter. In some embodiments, the promoter is stress inducible (e.g., inducible by abiotic stress). In some embodiments, the promoter is pathogen inducible. In some embodiments, the promoter is induced upon infection byBotrytis. Non-limiting examples of pathogen inducible promoters includeBotrytis-Induced Kinase 1 (BIK1) and the plant defensing gene PDF1.2. See, e.g., Penninckx et al.,Plant Cell10:2103-2113 (1998); see also Veronese et al.,Plant Cell18:257-273 (2006). In some embodiments, the promoter isA. thalianaBIK1 (SEQ ID NO:1) or is substantially identical toA. thalianaBIK1 (SEQ ID NO:1). In some embodiments, the promoter isA. thalianaPDF1.2 (SEQ ID NO:2) or is substantially identical toA. thalianaPDF1.2 (SEQ ID NO:2). In some embodiments, the promoter is TPK1b (SEQ ID NO:3) or is substantially identical to TPK1b (SEQ ID NO:3).

In some embodiments, the promoter is a tissue-specific promoter. In some embodiments, the promoter is specifically expressed in the epidermis. Non-limiting examples of epidermis-specific promoters include Meristem Layer 1 (ML1). See, e.g., Takada et al.,Development140:1919-1923 (2013). In some embodiments, the promoter is substantially (e.g., at least 60, 70, 75, 80, 85, 90, or 95%) identical toArabidopsisML1 (SEQ ID NO:14) or tomato ML1 (SEQ ID NO:15).

In some embodiments, a polyadenylation region at the 3′-end of the coding region can be included. The polyadenylation region can be derived from a NH3 gene, from a variety of other plant genes, or from T-DNA.

The vector comprising the sequences will typically comprise a marker gene that confers a selectable phenotype on plant cells. For example, the marker may encode biocide resistance, particularly antibiotic resistance, such as resistance to kanamycin, G418, bleomycin, hygromycin, or herbicide resistance, such as resistance to chlorosluforon or Basta.

V. Production of Transgenic Plants

As detailed herein, embodiments of the present invention provide for transgenic plants comprising recombinant expression cassettes for expressing a polynucleotide sequence as described herein (e.g., a polynucleotide sequence that is complementary to the pathogen sRNA or that mediates destruction of the pathogen sRNA, or a polynucleotide encoding a sRNA-resistant target). In some embodiments, a transgenic plant is generated that contains a complete or partial sequence of a polynucleotide that is derived from a species other than the species of the transgenic plant. It should be recognized that transgenic plants encompass the plant or plant cell in which the expression cassette is introduced as well as progeny of such plants or plant cells that contain the expression cassette, including the progeny that have the expression cassette stably integrated in a chromosome.

In some embodiments, the transgenic plants comprising recombinant expression cassettes for expressing a polynucleotide sequence as described herein have increased or enhanced pathogen resistance compared to a plant lacking the recombinant expression cassette, wherein the transgenic plants comprising recombinant expression cassettes for expressing the polynucleotide sequence have about the same growth as a plant lacking the recombinant expression cassette. Methods for determining increased pathogen resistance are described, e.g., in Section VI below.

A recombinant expression vector as described herein may be introduced into the genome of the desired plant host by a variety of conventional techniques. For example, the DNA construct may be introduced directly into the genomic DNA of the plant cell using techniques such as electroporation and microinjection of plant cell protoplasts, or the DNA construct can be introduced directly to plant tissue using ballistic methods, such as DNA particle bombardment. Alternatively, the DNA construct may be combined with suitable T-DNA flanking regions and introduced into a conventionalAgrobacterium tumefacienshost vector. The virulence functions of theAgrobacterium tumefacienshost will direct the insertion of the construct and adjacent marker into the plant cell DNA when the cell is infected by the bacteria. While transient expression of the polynucleotide sequence of interest is encompassed by the invention, generally expression of construction of the invention will be from insertion of expression cassettes into the plant genome, e.g., such that at least some plant offspring also contain the integrated expression cassette.

Microinjection techniques are also useful for this purpose. These techniques are well known in the art and thoroughly described in the literature. The introduction of DNA constructs using polyethylene glycol precipitation is described in Paszkowski et al.EMBO J.3:2717-2722 (1984). Electroporation techniques are described in Fromm et al.Proc. Natl. Acad. Sci. USA82:5824 (1985). Ballistic transformation techniques are described in Klein et al.Nature327:70-73 (1987).

Agrobacterium tumefaciens-mediated transformation techniques, including disarming and use of binary vectors, are well described in the scientific literature. See, for example, Horsch et al.Science233:496-498 (1984), and Fraley et al.Proc. Natl. Acad. Sci. USA80:4803 (1983).

Transformed plant cells derived by any of the above transformation techniques can be cultured to regenerate a whole plant that possesses the transformed genotype and thus the desired phenotype such as enhanced pathogen resistance. Such regeneration techniques rely on manipulation of certain phytohormones in a tissue culture growth medium, typically relying on a biocide and/or herbicide marker which has been introduced together with the desired nucleotide sequences. Plant regeneration from cultured protoplasts is described in Evans et al., Protoplasts Isolation and Culture, Handbook of Plant Cell Culture, pp. 124-176, MacMillilan Publishing Company, New York, 1983; and Binding, Regeneration of Plants, Plant Protoplasts, pp. 21-73, CRC Press, Boca Raton, 1985. Regeneration can also be obtained from plant callus, explants, organs, or parts thereof. Such regeneration techniques are described generally in Klee et al.Ann. Rev. of Plant Phys.38:467-486 (1987).

One of skill will recognize that after the expression cassette is stably incorporated in transgenic plants and confirmed to be operable, it can be introduced into other plants by sexual crossing. Any of a number of standard breeding techniques can be used, depending upon the species to be crossed.

The expression cassettes of the invention can be used to confer enhanced pathogen resistance on essentially any plant. Thus, the invention has use over a broad range of plants, including species from the genera Asparagus,Atropa, Avena, Brassica, Citrus, Citrullus, Capsicum, Cucumis, Cucurbita, Daucus, Fragaria, Glycine, Gossypium, Helianthus, Heterocallis, Hordeum, Hyoscyamus, Lactuca, Linum, Lolium, Lycopersicon, Malus, Manihot, Majorana, Medicago, Nicotiana, Oryza, Panieum, Pannesetum, Persea, Pisum, Pyrus, Prunus, Raphanus, Secale, Senecio, Sinapis, Solanum, Sorghum, Trigonella, Triticum, Vitis, Vigna, andZea. In some embodiments, the plant is a tomato plant. In some embodiments, the plant is a vining plant, e.g., a species from the genusVitis. In some embodiments, the plant is an ornamental plant. In some embodiments, the plant is a vegetable- or fruit-producing plant. In some embodiments, the plant is a monocot. In some embodiments, the plant is a dicot.

VI. Selecting for Plants with Enhanced Pathogen Resistance

Plants with enhanced pathogen resistance can be selected in many ways. One of ordinary skill in the art will recognize that the following methods are but a few of the possibilities. One method of selecting plants with enhanced pathogen resistance is to determine resistance of a plant to a specific plant pathogen. Possible pathogens include, but are not limited to, viruses, bacteria, nematodes, fungi or insects (see, e.g., Agrios,Plant Pathology(Academic Press, San Diego, Calif.) (1988)). One of skill in the art will recognize that resistance responses of plants vary depending on many factors, including what pathogen, compound, or plant is used. Generally, enhanced resistance is measured by the reduction or elimination of disease symptoms (e.g., reduction in the number or size of lesions or reduction in the amount of fungal biomass on the plant or a part of the plant) when compared to a control plant. In some cases, however, enhanced resistance can also be measured by the production of the hypersensitive response (HR) of the plant (see, e.g., Staskawicz et al. (1995)Science268(5211): 661-7). Plants with enhanced pathogen resistance can produce an enhanced hypersensitive response relative to control plants.

Enhanced pathogen resistance can also be determined by measuring the increased expression of a gene operably linked a defense related promoter. Measurement of such expression can be measured by quantifying the accumulation of RNA or subsequent protein product (e.g., using northern or western blot techniques, respectively (see, e.g., Sambrook et al. and Ausubel et al.).

The following examples are offered to illustrate, but not limit the claimed invention.

Example 1: Fungal Small RNAs Suppress Plant Immunity by Hijacking Host RNA Interference Pathways

Botrytis cinereais a fungal pathogen that infects almost all vegetable and fruit crops and annually causes $10-100 billion losses worldwide. With its broad host range,B. cinereais a useful model for studying the pathogenicity of aggressive fungal pathogens. Many pathogens of plants and animals deliver effectors into host cells to suppress host immunity (H. Ashida et al.,Curr. Opin. Microbiol.14, 16 (2011); M. Rafiqi et al.,Curr. Opin. Plant Biol.15, 477 (2012); T. O. Bozkurt et al.,Curr. Opin. Plant Biol.15, 483 (2012); H. Hilbi, et al.,Traffic13, 1187 (2012)). All the pathogen effectors studied so far are proteins. Here we find that small RNA (sRNA) molecules derived fromB. cinereacan act as effectors to suppress host immunity.

sRNAs induce gene silencing by binding to Argonaute (AGO) proteins and directing the RNA-induced silencing complex (RISC) to genes with complementary sequences. sRNAs from both plant and animal hosts have been recognized as regulators in host-microbial interaction (5-8). Although sRNAs are also present in various fungi and oomycetes, including many pathogens (9-14), it has not been clear whether they regulate host-pathogen interaction.

To explore the role ofB. cinereasRNAs in pathogenicity, we profiled sRNA libraries prepared fromB. cinerea(strain B05.10)-infectedArabidopsis thalianaCol-0 leaves collected at 0, 24, 48, and 72 h post inoculation (hpi) and fromB. cinerea-infectedSolanum lycopersicum(tomato) leaves and fruits at 0, 24, and 72 hpi. sRNA libraries prepared fromB. cinereamycelia, conidiospores and total biomass after 10 days of culture were used as controls. By using 100 normalized reads per millionB. cinereasRNA reads as a cutoff, we identified a total of 832 sRNAs that were present in bothB. cinerea-infectedArabidopsisandS. lycopersicumlibraries and had more reads in these two libraries than in the culturedB. cinerealibraries, with sequences exactly matching theB. cinereaB05.10 genome (15) but notArabidopsisorS. lycopersicumgenomes or cDNA (see,FIGS. 15 and 16and Table 1). The closest sequence matches inArabidopsisorS. lycopersicumcontained a minimum of 2 mismatches. Among them, 27 had predicted microRNA-like precursor structures. A similar number of microRNA-like sRNAs was found inSclerotinia sclerotiorum(9). We found that 73 Bc-sRNAs could target host genes in bothArabidopsisandS. lycopersicumunder stringent target prediction criteria (FIG. 15). Among them, 52 were derived from 6 retrotransposon long terminal repeats (LTR) loci in theB. cinereagenome, 13 were from intergenic regions of 10 loci, and 8 were mapped to 5 protein coding genes.

Some of the predicted plant targets, such as MAPKs, are likely to function in plant immunity. To test whether Bc-sRNAs could indeed suppress host genes during infection, three Bc-sRNAs (Bc-siR3.1, Bc-siR3.2, and Bc-siR5) were selected for further characterization (FIG. 16). These Bc-sRNAs were among the most abundant sRNAs that were 21 nt in length and had potential targets likely to be involved in plant immunity in bothArabidopsisandS. lycopersicum. These sRNAs were also enriched after infection (FIGS. 1A-1B,FIG. 5, and FIG.16), and were the major sRNA products from their encoding loci, LTR retrotransposons (FIG. 5). Bc-siR3.1 and Bc-siR3.2 were derived from the same locus with a four-nucleotide shift in sequence.

To determine whether Bc-sRNAs could trigger silencing of host genes, we examined the transcript levels of the predicted target genes afterB. cinereainfection. The followingArabidopsisgenes were targeted in the coding regions and were suppressed afterB. cinereainfection: mitogen activated protein kinase 2 (MPK2) and MPK1, which are targeted by Bc-siR3.2; an oxidative stress-related gene peroxiredoxin (PRXIIF), which is targeted by Bc-siR3.1; and a putative cell wall-associated kinase gene (WAK), which is targeted by Bc-siR5 (FIG. 1C). In contrast, the plant defense marker genes PDF1.2 and BIK1 (P. Veronese et al.,Plant Cell18, 257 (2006)), which do not contain the Bc-sRNA target sites, were highly induced uponB. cinereainfection (FIG. 1C). We conclude that suppression of some but not all genes is a result of sequence-specific sRNA interaction and not due to cell death within infected lesions. Bc-siR3.2, which silencesArabidopsisMPK1 and MPK2, was enriched also inS. lycopersicumleaves uponB. cinereainfection and was predicted to target another member of the MAPK signaling cascade inS. lycopersicum, MAPKKK4 (FIG. 1B,FIG. 16). Expression of MAPKKK4 was indeed suppressed uponB. cinereainfection (FIG. 1D).

To confirm that the suppression of the targets was indeed triggered by Bc-sRNAs, we performed co-expression assays inNicotiana benthamiana. Expression of HA-epitope tagged MPK2, MPK1, and WAK was reduced when they were co-expressed with the corresponding Bc-sRNAs but not when co-expressed withArabidopsismiR395 that shared no sequence similarity (FIG. 1E). The silencing was abolished, however, when the target genes carried a synonymously mutated version of the relevant Bc-sRNA target sites (FIG. 6A,FIG. 1E). We also observed suppression of YFP-tagged target MPK2 byB. cinereainfection at 24 hpi (FIG. 1FandFIG. 6B); when the Bc-siR3.2 target site of MPK2 was mutated, infection byB. cinereafailed to suppress its expression (FIG. 1F). Thus, Bc-siR3.2 delivered fromB. cinereais sufficient for inducing silencing of wild type MPK2 but cannot silence target site-mutated MPK2. Similarly, of the YFP-sensors with wild type or mutated Bc-siR3.2 target sites (FIG. 6C), only the wild type sensor was suppressed afterB. cinereainfection (FIG. 1G).

To test the effect of Bc-sRNAs on host plant immunity, we generated transgenicArabidopsisplants that ectopically expressed Bc-siR3.1, Bc-siR3.2, or Bc-siR5 using a plant artificial miRNA vector (FIG. 2A) (17). These Bc-sRNA expression (Bc-sRNAox) lines showed normal morphology and development without pathogen challenge when compared to the wild type plants, and expression of the target genes was suppressed (FIG. 2B). With pathogen challenge, all of the Bc-sRNAox lines displayed enhanced susceptibility toB. cinerea(FIG. 2C, 2E). The results indicate that these Bc-sRNAs play a positive role inB. cinereapathogenicity.

Enhanced disease susceptibility of the Bc-sRNAox lines suggests that the target genes of these Bc-sRNAs are likely to be involved in host immunity againstB. cinerea. Plants with mutated target genes showed normal morphology and development without pathogen challenge. TheArabidopsistargets of Bc-siR3.2, MPK1 and MPK2, are homologs that share 87% amino acid identity. These genes are functionally redundant and are co-activated in response to various stress factors (18). The mpk1 mpk2 double mutant exhibited enhanced susceptibility toB. cinerea(FIG. 2D, 2E). A T-DNA knockout mutant of the Bc-siR5 target WAK (SALK_089827) (FIG. 7A) also displayed enhanced susceptibility toB. cinerea(FIG. 2D, 2E). Consistent with this, Bc-siRNAox lines as well as mpk1 mpk2 and wak showed lower induction of the defense marker gene BIK1 (FIG. 7B). These results suggest that the MPK1, MPK2, and WAK genes, all of which are targeted by Bc-sRNAs, participate in the plant's immune response toB. cinerea. To determine whether MAPKKK4 is involved inS. lycopersicumdefense response againstB. cinerea, we applied the virus-induced gene silencing (VIGS) approach to knock down MAPKKK4 inS. lycopersicumusing tobacco rattle virus (TRV) (FIG. 8A) (19). VIGS of TRV-MAPKKK4 caused a dwarf phenotype (FIG. 8B). The MAPKKK4-silenced plants showed enhanced disease susceptibility in response toB. cinereaand contained >15 times more fungal biomass than the control plants (FIG. 2F). We conclude that Bc-sRNAs silence plant genes to suppress host immunity during early infection.

These fungal sRNAs hijack the plant's own gene silencing mechanism. 63 of the 73 Bc-sRNAs that had predictedArabidopsisandS. lycopersicumtargets were 20-22 nucleotides in length with a 5′ terminal U (see Table 1). This sRNA structure is favored for binding to AGO1 inArabidopsis(S. J. Mi et al.,Cell133, 116 (2008); T. A. Montgomery et al.,Cell133, 128 (2008)). In order to determine whether Bc-sRNAs act throughArabidopsisAGO1, we immunoprecipitated AGO1 fromB. cinerea-infectedArabidopsiscollected at 24, 32 and 48 hpi and analyzed the AGO1-associated sRNAs. Bc-siR3.1, Bc-siR3.2 and Bc-siR5 were clearly detected in the AGO1-associated fraction pulled down from the infected plant samples but hardly in the control (FIG. 3A) or in the AGO2- and AGO4-associated sRNA fractions (FIG. 9). The sRNAs that had no predicted plant targets or had predicted targets that were not down-regulated byB. cinereainfection were not found in the AGO1-associated fractions (FIG. 10).

If AGO1 plays an essential role in Bc-sRNA-mediated host gene silencing, we would expect to see reduced disease susceptibility in the ago1 mutant since these Bc-sRNAs could no longer suppress host immunity genes. For plants carrying the ago1-27 mutant allele (J. B. Morel et al.,Plant Cell14, 629 (2002)) and were inoculated withB. cinerea, the disease level was significantly less than on the wild type (FIG. 3BandFIG. 11A). Consistent with this, BIK1 induction was increased compared to wild type (FIG. 11B). Furthermore, the expression of Bc-siR3.2 targets MPK2 and MPK1, Bc-siR3.1 target PRXIIF, and Bc-siR5 target WAK in ago1-27 was not suppressed compared to wild type infected plants afterB. cinereainfection (FIG. 3C). On the contrary,ArabidopsismiRNA biogenesis mutant dicer-like (dcl) 1-7 that shows similar morphological defects to ago1-27 exhibited an enhanced disease level toB. cinerea(FIG. 3D). These results suggest that the increased resistance phenotype we observed in ago1-27 is not caused by any reduced vigor or pleiotropic phenotype, but due to the function of the Bc-siRNAs, and thatArabidopsisDCL1 is not required for the function of Bc-siRNAs. Thus,B. cinereaBc-sRNAs evidently hijacked host RNAi machinery by loading into AGO1; the complex in turn suppressed host immunity genes.

To delete the siR3 and siR5 loci from theB. cinereagenome by homologous recombination would be an ideal way to confirm their function; however, it is not feasible because siR3 is from a LTR with 3 copies and siR5 is from a LTR with 13 copies. To better understand the function and biogenesis of the Bc-sRNAs, we chose to knock out theB. cinerea DCL genes, which encode the core sRNA processing enzymes. B. cinereastrain B05.10 possesses two Dicer-like genes (Bc-DCL1 and Bc-DCL2) (FIG. 12). We generated dcl1 and dcl2 single and dcl1 dcl2 double knockout mutant strains through homologous recombination (FIG. 13A-13B). We found that dcl1 and dcl2 single mutants showed reduced growth and delayed sporulation (FIG. 13C). The dcl1 dcl2 double mutant displayed a more obvious phenotype than each of the single mutants, suggesting partial functional redundancy between DCL1 and DCL2 inB. cinerea. Bc-siR3.1, Bc-siR3.2, and Bc-siR5 could not be detected in the dcl1 dcl2 double mutant (FIG. 4A), indicating that they were DCL-dependent, while two other Bc-siRNAs, Bc-milR2 and Bc-siR1498, could still be detected in dcl1 dcl2 double mutant (FIG. 13D). Fungi have diverse sRNA biogenesis pathways, and not all sRNAs are DCL-dependent (H. C. Lee et al.,Mol. Cell38, 803 (2010)). The dcl1 dcl2 double mutant caused significantly smaller lesions than the wild type or dcl1 and dcl2 single mutants on bothArabidopsisandS. lycopersicumleaves (FIG. 4B-4C), in consistence with the significantly reduced fungal biomass at 72 hpi inArabidopsisand 48 hpi inS. lycopersicum(FIG. 14), which indicates that the virulence of the dcl1 dcl2 mutant was greatly reduced. These results further support the conclusion that Bc-siRNAs, particularly Bc-siR3.1, Bc-siR3.2 and Bc-siR5 that depend on DCL function, contribute to the pathogenicity ofB. cinerea. Mutation of dcl1 or dcl2 inB. cinereacaused delayed growth and sporulation (FIG. 13C) but had no effect on pathogenicity (FIG. 4B-4C). Furthermore, expression of the YFP sensor carrying the Bc-siR3.2 target site inN. benthamianawas silenced when infected with wild typeB. cinerea. The suppression was abolished when inoculated with the dcl1 dcl2 strain (FIG. 4D), indicating that the dcl1 dcl2 double mutant was unable to generate Bc-siR3.2 to suppress the target. We also confirmed the inability of dcl1 dcl2 to suppress Bc-siR3.1 and Bc-siR3.2 target genes MPK2, MPK1, and PRXIIF inArabidopsisand MAPKKK4 in tomato upon infection (FIG. 4E). Consistent with this, the dcl1 dcl2 virulence was partially restored when infected onArabidopsisBc-siR3.1ox and Bc-siR3.2ox plants as well as in tomato TRV-MAPKKK4 silenced plants (FIG. 4F-4G).

Animal and plant pathogens have evolved virulence or effector proteins to counteract host immune responses. Various protein effectors have been predicted or discovered in fungal or oomycete pathogens from whole-genome sequencing and secretome analysis (M. Rafiqi et al.,Curr. Opin. Plant Biol.15, 477 (2012); T. O. Bozkurt et al.,Curr. Opin. Plant Biol.15, 483 (2012)), although delivery mechanisms are still under active investigation (D. Kale et al.,Cell142, 284 (2010); S. Wawra et al.,Curr. Opin. Microbiol.15, 685 (2012); M. Rafiqi et al.,Plant Cell22, 2017 (2010); S. Schornack et al.,Proc. Natl. Acad. Sci. USA107, 17421 (2010); S. Wawra et al.,Proc. Natl. Acad. Sci. USA109, 2096 (2012)). Here, we show that sRNAs as well can act as effectors through a mechanism that silences host genes in order to debilitate plant immunity and achieve infection. The sRNAs fromB. cinereahijack the plant RNAi machinery by binding to AGO proteins which in turn direct host gene silencing. Another fungal plant pathogen,Verticllium(V.)dahliae, also depends on AGO1 function for its pathogenicity (U. Ellendorff, et al.,J. Exp. Bot.60, 591 (2009)). The implications of these findings suggest an extra mechanism underlying pathogenesis promoted by sophisticated pathogens with the capability to generate and deliver small regulatory RNAs into hosts to suppress host immunity.

Material and Methods

Generation of dcl1, dcl2 Single and Double Mutants ofB. cinerea

Plant Materials and Protocols

Pathogen Assay

Four-week-old plants were inoculated by applying a single 20 μl droplet per leaf or by spray-inoculating the entire plant, using 2×105spores/ml forArabidopsisand 1×104spores/ml forS. lycopersicumandN. benthamiana. Disease was assessed by measuring lesion size (ImageJ software) and/or by quantifyingB. cinereabiomass using quantitative PCR withB. cinerea-specific ITS primers (FIG. 8).

Confocal Microscopy

YFP-tagged protein expression inN. benthamianawas quantified using the confocal microscopy system Leica SP2. Z-series images (10 images in a distance of 0.7 μM) were merged to gain average signal intensity. Merged images were exported as TIFF files and YFP quantity was measured using the ImageJ software.

ArabidopsisAGO IP (X. Zhang et al.,Mol. Cell42, 356-366 (2011)) was conducted with 5 g fresh leaves collected at 24, 32 and 48 h after spray inoculation withB. cinerea. Uninfected leaves mixed with at least double amount ofB. cinereabiomass as in 48 hpi samples were used as a control. AGO1 was purified with a peptide-specific antibody. AGO2 and AGO4 IPs were conducted using native promoter-driven transgenic epitope HA-tagged and c-MYC-tagged lines, respectively and commercial HA and c-MYC antibodies.

RNA was extracted fromB. cinerea-infected plant tissue or the AGO pull-down fraction using the Trizol method. Purified RNA was treated with DNase I and then used in RT-PCR (E. Varkonyi-Gasic et al.,Plant Methods3, 12 (2007)) to detect Bc-sRNAs. 35-40 cycles were used for detecting Bc-sRNAs, 22-28 cycles were used for detecting actin genes fromArabidopsis, S. lycopersicumandB. cinerea. Primers used for reverse transcription and amplification of Bc-siRNAs are listed in Table 2.

sRNA Cloning and Illumina HiSeq Data Analysis

sRNAs (18-28 nucleotides) were isolated by 15% PAGE and libraries were constructed using the miRCat cloning system and deep sequencing was performed on an Illumina HiSeq 2000. The sequence datasets of sRNA libraries fromB. cinerea(GSE45320),B. cinerea-infectedArabidopsis(GSE45323) andB. cinerea-infectedS. lycopersicum(GSE45321) are available at the NCBI database. The sRNA sequencing reads were preprocessed with the procedure of quality control and adapter trimming by using fastx-toolkit (http://hannonlab.cshl.edu/fastx_toolkit/index.html). Following adapter trimming, sequences were mapped toB. cinereaB05.10,Arabidopsis(TAIR10), orS. lycopersicum(ITAG_SL2.40) genomes and only the reads that matched perfectly to each genome were used for further analysis. The read number for each distinct sRNA was normalized to the totalB. cinereamapped reads inB. cinerea-infectedA. thalianaandS. lycopersicumlibraries. The ratio of totalB. cinereamapped reads ofA. thalianaandS. lycopersicumlibraries is 2.5:1, so we divide the normalized siRNA read number ofS. lycopersicumby 2.5.

The sRNAs we selected have satisfied the following conditions: 1) it must be present in bothB. cinerea-infectedA. thalianaandS. lycopersicumlibraries; 2) its normalized read number was larger than 100 inA. thalianaorS. lycopersicumlibraries; 3) its normalized reads must be higher than that in culturedB. cinerealibraries and 4) it has predicted targets in bothA. thalianaandS. lycopersicum.

Target gene prediction for Bc-sRNA was performed using TAPIR1.1 (E. Bonnet et al.,Bioinformatics26, 1566-1568 (2010)) with more stringent requirement than described in (E. Bonnet et al.,Bioinformatics26, 1566-1568 (2010)). No gap or bulge within the alignment between the sRNA and the target was allowed, and the 10th nucleotide of the sRNA must perfectly match its target. At most one mismatch or two wobbles was allowed from position 2 to 12. A maximum of two continuous mismatches was allowed and a score of 4.5 was used as a cutoff. If a sRNA has predicted targets in bothA. thalianaandS. lycopersicum, it was selected. The sRNAs were grouped if their 5′ end position and 3′ end position were within 3 nucleotides on the genomic loci. We presented the selected sRNAs with targets in bothA. thalianaandS. lycopersicumin Table 1.

Example 2: Sequences of Promoters and sRNA-Resistant Targets

Example 3: Sequences of sRNA Targets and Mutations for Making sRNA-Resistant Targets

Example 4: STTM Primers for Blocking the Function of Pathogen sRNAs

STTM primer sequences were designed against 30BotyritissRNAs (“Bc-sRNAs”) from Table 1 that were identified as having targets in bothArabidopsisand tomato. The designed STTM sequences can be used in other species which are also targeted by the Bc-sRNAs. The STTM primer sequences (forward primers and reverse primers) for generating STTM constructs, and the Bc-sRNAs targeted by each set of primers, are shown in Table 2.

STTM sequences can be expressed in plants according to the methods described in Yan et al.,Plant Cell24:415-427 (2012). Briefly, the STTM modules are inserted in a vector (e.g., the pOT2 vector) between the promoter and terminator. Insertion of the STTM modules is accomplished by PCR amplification of the vector with a proofreading Taq polymerase and a pair of long primers covering the entire STTM sequences (to minimize errors in STTM regions during the PCR reaction). The PCR product is and transformed into cells, e.g., XL1-blue. Single colonies are propagated for plasmid isolation and the recombinant constructs are verified, e.g., by linearization of the plasmids by a restriction enzyme. The recombinant plasmids are further amplified, and the PCR products containing the STTM and a selection marker (e.g., chloramphenicol) are introduced into a binary vector. Recombinant binary plasmids are selected on Luria-Bertani plates containing the appropriate selection antibiotics (e.g., chloramphenicol and kanamycin). The final constructs are verified by DNA sequencing before being used for plant transformation.