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Annu. Rev. Plant Biol. 2007.58:267-294. Downloaded from www.annualreviews.org by University of Uppsala on 12/13/12.
Primary transcripts (precursor-mRNAs) with introns can undergo alternative splicing to produce multiple transcripts from a single gene by differential use of splice sites, thereby increasing the transcriptome and proteome complexity within and between cells and tissues. Alternative splicing in plants is largely an unexplored area of gene expression, as this phenomenon used to be considered rare. However, recent genome-wide computational analyses have revealed that alternative splicing in ﬂowering plants is far more prevalent than previously thought. Interestingly, pre-mRNAs of many spliceosomal proteins, especially serine/arginine-rich (SR) proteins, are extensively alternatively spliced. Furthermore, stresses have a dramatic effect on alternative splicing of pre-mRNAs including those that encode many spliceosomal proteins. Although the mechanisms that regulate alternative splicing in plants are largely unknown, several reports strongly suggest a key role for SR proteins in spliceosome assembly and regulated splicing. Recent studies suggest that alternative splicing in plants is an important posttranscriptional regulatory mechanism in modulating gene expression and eventually plant form and function.
in the spliceosome, a large multicomponent megadalton complex. Soon after the discovery of exons and introns in adenovirus 2 genes (10, 19), it was proposed that different combinations of exons could be joined together to produce multiple mRNAs from a single gene (32). Although alternative splicing has been studied with some genes as isolated events, the extent of alternative splicing in multicellular eukaryotes was not known until recently. Many recent studies suggest that alternative splicing likely plays an important role in regulating gene expression at the posttranscriptional level. Pre-mRNA splicing imprints information necessary for nucleocytoplasmic transport of spliced mRNA, nonsense mediated mRNA decay, and mRNA localization in the cytoplasm (48). In plants, it is now clear that alternative splicing is prevalent and can generate tremendous transcriptome and proteome complexity (126). As this is an emerging area of study in plants, a critical discussion on whether the alternative splicing postulated from comparisons of genomic and cDNA sequences is real and the relevance of this transcriptome/proteome diversity to plant growth and development is warranted. In this review, I limit my discussion to alternative splicing in ﬂowering plants, how this affects the transcriptome and proteome diversity, and implications of this complexity. The aspects of plant splicing not covered here, such as spliceosome assembly, sequence requirements for splicing of plant pre-mRNAs, localization, and dynamics of spliceosomal proteins are discussed in detail in other comprehensive reviews (16, 29, 39, 87, 88, 104, 110, 117).
Annu. Rev. Plant Biol. 2007.58:267-294. Downloaded from www.annualreviews.org by University of Uppsala on 12/13/12. For personal use only.
This size refers to internal exons only. Information on the number of exon and introns per gene is from http://www.tigr.org/tdb/e2k1/osa1/riceInfo/info.shtml.
Fourteen nucleotides (7 from the exon and 7 from the intron) at the 5 and 3 splice sites of all genes in these organisms were extracted from the annotated sequences and used to construct this ﬁgure. Plant Biol. C. rice.org by University of Uppsala on 12/13/12. 270 Reddy . (b) The sequence pattern at the 3 end of the Arabidopsis introns. The frequencies of A. Bottom: The frequencies of A. Top: Schematic diagram showing two exons and an intron. Rev. For personal use only. C. G.Annu. Figure 1 (a) The sequence patterns at the 5 splice site (left) and 3 splice site (right) of introns in Arabidopsis. The last 70 nucleotides of all introns were retrieved from the annotated sequences and used to generate this ﬁgure.annualreviews. Downloaded from www. G. and T at each position are represented by the height of the corresponding letter. 2007.58:267-294. and T at each position are represented by the height of the corresponding letter. human. and roundworm.
which are recognized by regulatory splicing factors (13).annualreviews. SR. 121). intronic splicing enhancers (ISEs). Studies with animal systems indicate that other exonic and intronic cis-acting regulatory sequences (4 to 18 nucleotides long) that bind trans-acting factors inﬂuence splice site selection during constitutive and alternative splicing. This is probably because of variation in the position of the branch point in different introns. Furthermore. these regulatory sequences are classiﬁed into four groups: exonic splicing enhancers (ESEs). U2 auxillary factor.org by University of Uppsala on 12/13/12. and intronic splicing suppressors (ISSs). SR proteins. U2AF. SPLICE SITE RECOGNITION Although the short consensus sequences at the donor.. Two models termed exon definition and intron deﬁnition have been proposed to explain splice site recognition (see Figure 2) (9). acceptor. For personal use only. In animal systems. respectively. which then recruit U1 snRNP and U2AF to the 5 and 3 splice sites. Although plant exons are rich in GC content. the intronic splicing regulator (ISR) sequences are recognized by the splicing regulators. In plants. little is known about exonic regulatory sequences. discussed below) to these sequences regulates the recruitment of splicing machinery to splice sites. According to the intron deﬁnition model.58:267-294. and branch sites are necessary for splice site recognition. In metazoans. www.g. BP.org • Alternative Splicing in Plants 271 .is mostly U in plants (Figure 1). Binding of proteins (e. respectively. which then recruits U2 snRNP to the branch point. Splicing regulators such as serine/arginine-rich (SR) and other proteins that bind to ESE recruit U1 snRNP to the 5 splice site and U2AF to the 3 splice site. Plant Biol. serine/arginine-rich protein. In organisms such as humans and other vertebrates where large intron sequences separate fairly small exons. these sequences alone are not sufﬁcient. The exon deﬁnition model argues that splicing machinery assembles on the exon by initially recognizing splice sites around an exon. the exon deﬁnition model is favored (22. 2007. These differences between plant and animal intron architecture and composition suggest that the mechanisms involved in splice site recognition likely differ in these organisms. speciﬁc exonic sequences involved in splice Splice sites: nucleotide sequences surrounding the exon-intron boundaries that are critical for pre-mRNA splicing Small nuclear ribonucleoprotein particle (SnRNP): consists of a small nuclear RNA and several proteins Constitutive splicing: generation of only one type of mRNA from intron-containing genes by using the same set of 5 and 3 splice sites Figure 2 Exon and intron deﬁnition models illustrate how splicing machinery accurately recognizes splice sites. exonic splicing silencers (ESSs). branch point. Annu. Downloaded from www. Exons in animal pre-mRNAs contain purinerich ESEs.annualreviews. Recently. it is established that recognition of exons and introns is achieved by these loosely conserved cis-regulatory sequences in exons and introns and their interaction with splicing regulators. it was shown that U-rich elements can function as a splicing signal or a polypyrimidine tract (115). Half circles in exons and introns indicate exonic and intronic splicing enhancers. the branch point sequence (CURAY) is not obvious here. Rev.
In plants. and both types of introns (U2 and U12) can occur in the same gene. U4atac. 110). The fact that the minor spliceosome is present in vertebrates and plants but not in ancestral eukaryotes indicates that the minor spliceosome evolved multiple times during evolution. 125). and U6atac snRNPs recognizes a small percentage of introns (<1% in Arabidopsis and humans) with noncanonical splice sites. was previously reviewed (59. Rev. 130).58:267-294. The intron deﬁntion mode of splice site recognition is thought to occur in organisms with small introns in their genes (122). SR protein kinases. based on the high frequency of intron retention (56% in Arabidopsis and 53. The minor U12type spliceosome that contains U11. Most animal spliceosomal proteins in major and minor spliceosomes are conserved in plants (84. Members of this family contain one or two RRMs at the N terminus and an RS domain at the C terminus. yeast. Although plant spliceosomes have not been isolated. a large RNA-protein machine. splicing regulators such as SR proteins (discussed below). 93. However.annualreviews. The composition and function of the minor spliceosome in plants appear to be similar to that in humans (84). U4.Annu. Because the ancestral eukaryotes (plasmodium. 87. catalyzes the removal of introns with canonical (GT-AG) splice sites. 116. trypanosomes. analysis of the Arabidopsis genome for genes encoding spliceosomal proteins from nonplant systems has revealed the presence of many of the proteins found in metazoans (125). Downloaded from www. 104). it is not known how many proteins make up the spliceosome. 18. and Caenorhabditis elegans) lack U11/U12. However. Serine/argininerich (SR) proteins: a family of highly conserved phosphoproteins in eukaryotes with modular domain organization. 72. 125). indicating plant-speciﬁc mechanisms in splice site recognition and splicing regulation (125). U5. In plants. A number of in vivo studies on plant premRNA splicing have suggested differences in sequence requirements between plant and animal splice site recognition (reviewed in 16. 2007. and hence is covered only brieﬂy here. which consists of U1. the spliceosome contains many non-snRNP proteins [heterogenous nuclear ribonculeoprotein particles (hnRNPs). 18. some of which are common to all snRNPs. the presence and functionality of U6atac and U12 were ﬁrst reported in Arabidopsis and additional U snRNAs of the U12 spliceosome were identiﬁed using bioinformatics analysis Reddy (114. A total of 74 snRNA genes and 395 genes encoding spliceosomal and spliceosome-associated proteins have been identiﬁed in this model plant. and hnRNP proteins are vastly expanded in plants with many novel proteins. Zhu & Brendel (139) reported a total of 165 U12-type introns in Arabidopsis. There is some evidence to support both of these models in plants (15. or regulation of constitutive and alternative splicing site recognition or regulation of splicing have not been identiﬁed (17. U12. Plant Biol. Apart from U snRNPs. Depending on the combination of alternatively used regions of a gene. suggesting that the composition of plant spliceosomes is most likely similar to that of animals. U2. and SR proteins] (125). They function as essential splicing factors and also regulate alternative splicing Splicing factors: non-snRNP proteins required for spliceosome assembly. 92. All U snRNPs contain one snRNA and several proteins. and U6 snRNPs. consists of 5 snRNAs and nearly 300 proteins. 64. U5. ALTERNATIVE SPLICING IN PLANTS Alternative splicing generates two or more mRNAs from the same pre-mRNA by using different splice sites. In higher eukaryotes there are two types of spliceosomes.org by University of Uppsala on 12/13/12. The major U2-type spliceosome. it has been proposed that splice site recognition in plants occurs predominantly by intron deﬁnition (126). a large number of alternatively 272 . transesteriﬁcation reactions. 69). Proteomic analysis of puriﬁed animal spliceosomes has resulted in the identiﬁcation of about 300 distinct proteins in this complex (59). The assembly of the spliceosome is well understood in animals. DExD/H-box. 110. 104. COMPOSTION OF PLANT SPLICEOSOME The spliceosome.5% in rice as compared with 5% in humans) in alternatively spliced transcripts in plants. For personal use only. it is hypothesized that the minor spliceosome was absent in the “ﬁrst eukaryote” (21).
resulting in different-size transcripts. (d ) Alterative 3 splice site. Plant Biol. Two or more of the above alternative splicing types can occur in a single pre-mRNA to generate multiple mature mRNA from a single gene. Exons are represented by colored boxes and introns by horizontal lines. including mRNA www. Lines above and below the boxes indicate alternative splicing events. Adjacent exons are spliced in such a way that only one of them is included at a time in the mRNA. Yellow or blue boxes or a horizontal line represent sequences that are either included or excluded from the mRNA. Alternative splicing greatly increases transcriptome diversity and the alternatively spliced transcripts may en- code distinct proteins. (c) Alternative 5 splice site. (e) Intron retention. Figure 3 Some common types of alternative splicing. Alternative splicing affects many aspects of RNA metabolism.annualreviews. Rev. (b) Mutually exclusive exons. 2007. The most commonly observed alternative splicing types are shown in Figure 3. Different-size mRNAs are produced depending on the use of a proximal or distal 5 splice site. Different-size mRNAs are produced depending on the usage of a proximal or distal 3 splice site. For personal use only. An intron is either retained or excised in the mRNA.annualreviews. An exon is either included or excluded from the mRNA. spliced mRNAs can be generated.58:267-294.org • Alternative Splicing in Plants 273 . Pre-mRNAs are on the left and spliced mRNAs are on the right.Annu. thus expanding the coding capacity of genes and contributing to the proteome complexity of higher organisms. (a) Cassette exon. Downloaded from www.org by University of Uppsala on 12/13/12.
and translation efﬁciency. Wang & Brendel (126) aligned the gene sequences with EST/cDNA sequences.shtml http://www.albany.tigr. During the past four years.riken.uk/cgi-bin/ atnopdb/home http://pasdb. 140).org/tdb/e2k1/osa1/ expression/alt spliced. Rev. The effects of alternative splicing on proteins include production of protein isoforms with loss or gain of function and altered cellular localization. 56% and 53. are a result of intron retention.php http://www.jp/a splicing/ http://bioinf.org by University of Uppsala on 12/13/12.sari. 44.info.Table 2 Database Database resources on plant spliceosomal proteins and alternative splicing URL http://www. protein stability.org/tdb/e2k1/ath1/ Arabidopsis nonconsensus splice sites. alternative splicing in plants was underappreciated primarily because it was considered rare. More recent analysis using splicingsensitive arrays containing exon junction oli274 Reddy gos revealed that 74% of multiexon genes in humans undergo alternative splicing (57).gsc. 45) Arabidopsis splicing-related genes (ASRG) Alternative splicing in plants (ASIP) TIGR-Arabidopsis splicing variations TIGR-nonconsensus splice sites RARGE-alternative splicing events AtNoPDB-nucleolar protein database Arabidopsis Arabidopsis and human Plants Plants and animals Arabidopsis Rice Arabidopsis (51) (99) (138) (96) (8) Annu. and/or posttranslational modiﬁcations (120). 126. 97.umd. the estimate of alternative splicing in Arabidopsis increased from about 5% to 22% (1. yeast.plantgdb.cbrc. 45) (44.tigr. The alignment of all human cDNAs/ESTs with genome sequences suggests that up to 60% of human genes are alternatively spliced (94). Table 2 provides the URL addresses of publicly available databases on alternative splicing in plants.annualreviews. and animals Arabidopsis. Historically.edu/labs/mount/ 2010-splicing/ Species Arabidopsis.plantgdb. where exon skipping is more prevalent (Table 3).ac.scri.go.5% of all alternative splicing events in Arabidopsis and rice. respectively.jp/ http://www.html http://rice.2% of total genes) undergo alternative splicing.org. This analysis revealed that 4707 Arabidopsis genes (22% of total genes) and 6568 rice genes (21. 51. One outcome of the large-scale sequencing of genomes and expressed sequence tags (ESTs)/cDNAs is that it allowed global analysis of alternative splicing.life.58:267-294. Until 2001. Plant Alternative Splicing database (PASDB) ASTRA Alternative Splicing and TRanscription Archives (ASTRA) mRNA metabolism proteins Rice alternatively spliced genes Exonic splicing enhancers degradation through nonsense mediated decay (NMD) and other mechanisms.shtml http://www. enzyme activity.org/SRGD/ASRG/ index. rice Arabidopsis Arabidopsis Reference (125) (126) (44. 2007. For personal use only. Plant Biol. shtml http://rarge.org/tdb/e2k1/ath1/ altsplicing/splicing variations. studies on alternative splicing in plants were limited to a few genes and the extent of alternative splicing in plants was not known (104). whereas exon skipping accounts for only 8% .org/ASIP/ http://www. Unlike in humans. 95. This is likely to still be an underestimate as there are still relatively few cDNA sequences in plants relative to that available in humans.cn/ http://alterna.tigr. Downloaded from www.edu/faculty/dab/ IPGE HUB. mRNA recruitment to ribosomes.genomics. To analyze the genome-wide alternative splicing events in Arabidopsis and rice.
annualreviews.org by University of Uppsala on 12/13/12. most transcripts with a PTC located >50 nucleotides upstream of an exonexon junction are targets of NMD (75. 94) a Annu. A large fraction of splice variants in plants differ only in the noncoding regions [in 5 untranslated region (UTR) and 3 UTR] of the transcripts (1. suggesting that more than one mechanism for mRNA surveillance may exist in plants. UPF2. and UPF3) that are critical components of the NMD pathway exist in plants. In non- plant systems.9)d 5. The percentage of total genes with observed alternative splicing. 47).8)d 58 Intron Retentionc 14. which requires exon junction complexes (EJCs). Plants. have essential components of the NMD pathway and detect and degrade transcripts with PTC (4.6 (53. Plant Biol. 47). In plants.3 (10.2)d 3. In Arabidopsis. 57. the use of whole-genome tiling arrays and/or splicingsensitive chips that contain exon-exon junction probes should reveal the full extent of alternative splicing in plants (57). This estimate is based on alignment of cDNA/ESTs with genome sequence.58:267-294. 126). b of alternative splicing events (Table 3). These alternative splicing events may have a role in mRNA www.1 million in humans) (14.8 (6.2 (11.Table 3 Extent of alternative splicing in plants and humans Total EST/cDNAs used 385.5)d 5 Organism Arabidopsis Rice Humans Reference (126) (126) (14. Alternative splicing due to alternative 5 and 3 splice sites is least prevalent in plants. This conservation of alternative splicing events across phylogenetically diverse dicots and monocots suggests that this process is important. It was recently shown that knockout mutants of UPF1 and UPF3 in Arabidopsis do not degrade mRNAs with PTC (4.3)d 37 Alt. 5 SSc 3. Rev. 78.1)d 14.349 369. These splice variants with PTC may be degraded by NMD.2 (13. d The number in parenthesis indicates the proportion of that event relative to the total number of alternative splicing events. In Arabidopsis. NMD. which is likely to regulate the abundance of different transcripts. 43.4% in 3 UTR).5 (15.9 (3.1)d 3. For personal use only. suggesting that alternative splicing may have functions other than generating protein diversity and/or NMD. 96.000–380. About 50% of alternative splicing events in the coding region have a premature termination codon (PTC) and are the potential targets of nonsense mediated mRNA decay (126).6% of all alternative splicing events occur in the UTRs (15. like nonplant systems. it has been reported that some mRNAs that are degraded due to PTC do not have introns (100). 5 and 3 ASa 21. 126).3 (56.4% of alternative splicing events are in the coding region.annualreviews.7 (21. The homologs of 40% of alternatively spliced Arabidopsis genes in rice are also alternatively spliced.123 million Alt.218 3. Alternative splicing of 22% of genes in Arabidopsis and rice is likely an underestimation as the number of cDNAs/ESTs used in these computational analyses is one tenth of what has been used in humans (∼330. is linked to translation and thought to occur during the “pioneer round” of mRNA translation (90).000 in plants as compared with more than 3. Three proteins (UPF1. c Percentage of total genes that showed the event. The current estimate of alternative splicing in plants will likely increase as more cDNAs/ESTs become available for such analysis.8 21.org • Alternative Splicing in Plants 275 . 2007.3)d Exon Skippingc 1. The differences in the frequencies of alternative splicing events between plants and animals may reﬂect the differences in pre-mRNA splicing between these organisms.1)d SSc 0.2% in 5 UTR and 6. Downloaded from www.8 (8. The estimate in parenthesis is based on splicing-sensitive microarrays that monitor splicing at every exon-exon junction. 118). 3 SSc 6.2 40–60% (74)b Alt. 21.7)d 1. Furthermore.
org by University of Uppsala on 12/13/12. 128). For personal use only. This is primarily because of our inability to obtain individual cell types. stability. 98). the presence of mRNAs on polysomes does not imply that those transcripts are producing protein through multiple rounds of translation. and/or translational regulation as has been reported in animals (62. The fact that the alternative splicing type in many genes including SR genes is conserved across phylogenetically divergent organisms strongly suggests a biological role for alternative splicing (50. Furthermore. a conserved family of splicing regulators. It is interesting that most of the nuclear speckle proteins are either involved in pre-mRNA splicing or other aspects of RNA metabolism. 138). whereas other functional groups are under-represented in both Arabidopsis and rice (126). Annu. It is not known how many of these splice variants are actually recruited to ribosomes for translation. In plants. Analyses of pre-mRNA splicing of 19 Arabidopsis SR genes indicate extensive alternative splicing. The over-represented gene ontology terms are “nuclear speckle” and “light harvesting complex” (126). 2007. Many rice SR genes. A more conclusive approach to demonstrate that the splice variants are translated is to use isoform-speciﬁc antibodies that recognize the predicted proteins. Alternative Splicing of Pre-mRNAs of Splicing Regulators Functional categorization of alternatively spliced genes using gene ontology has revealed that genes in all functional categories undergo alternative splicing. Furthermore. A similar approach with a tagged ribosomal protein expressing in speciﬁc cell types using cellspeciﬁc promoters should allow analysis of splice variant recruitment in individual cell types.annualreviews. This is because mRNAs with PTC that meet the criterion for NMD also go through one round of translation. Downloaded from www. Regulation of Alternative Splicing of Spliceosomal Genes by Stresses Several reports indicate that various biotic (viral and bacterial pathogens) and abiotic . Analysis of alternative splicing in different cell types using splicing-sensitive arrays is expected to provide novel information on cell-speciﬁc alternative splicing and its role in cell fate. alternative splicing of some SR genes is controlled in a developmental and tissue-speciﬁc manner. indicating tight regulation of alternative splicing and leading to differences in the abundance of splice variants in tissues and at developmental stages. However. Because of the modular nature of various domains in SR proteins.58:267-294. However.transport. Remarkably. A transgenic line expressing a tagged ribosomal protein that permits immunoafﬁnity puriﬁcation of polysomes should be useful for analyzing ribosomal recruitment of splice variants (133). 126. For example. 5 UTR may create a new initiation codon that may or may not be inframe with the downstream initiation codon and thereby regulate translation (56). thereby increasing the complexity of the SR gene family transcriptome by sixfold (98). the proteins produced from splice variants may have altered functions. some functional groups are over-represented 276 Reddy (threefold higher than overall alternative splicing). Plant Biol. about 95 transcripts are produced from only 15 genes. Other studies also indicate that genes encoding splicing regulators are extensively alternatively spliced. proteins that lack one of the domains may be localized differently or may still interact with some spliceosomal proteins and function as dominant negative regulators. However. recent advances in ﬂuorescent tagging of all root cell types and isolation of individual cell types by ﬂuorescence-activated cell sorting should permit such an analysis (11). Rev. Sequence analysis of splice variants revealed that predicted proteins from most of these variants either lacked one or more modular domains due to in-frame translation termination codons or contained additional amino acids at the end (55. generate multiple transcripts (55). the extent of cell-type speciﬁc alternative splicing of pre-mRNAs is not known.
129). There are several examples of autoregulation of alternative splicing of spliceosomal genes (SR30. 49. These results suggest that stresses alter SR protein expression. SR1.annualreviews. However. Downloaded from www. The mere presence of a splice variant in tissues does not mean that it has a biological function. 98. Interestingly. In several cases. hnRNPs. For personal use only. There is also evidence for autoregulation of alternative splicing of SR genes as well as regulation by other SR proteins. Heat stress inhibits splicing of pre-mRNAs of maize polyubiquitin and hsp70 (20. 79). which in turn alters the splicing of other pre-mRNAs including SR pre-mRNAs. proteins kinases. reporting that cold and other stresses affect alternative splicing proﬁles of Arabidopsis genes (51). Stress may regulate alternative splicing by altering the (a) ratio of splicing factors by changing splicing pattern.org by University of Uppsala on 12/13/12. Feedback positive and negative autoregulation or regulation of SR pre-mRNAs by other SR proteins may be involved in stress-regulated alternative splicing. 58. tors (SRs. which encodes a stress-induced nuclear protein similar to the human 102 kD U5 snRNP-associated protein. some splice variants either increased or decreased by abiotic stresses. 63. it is not known if the splice variants are genuine and whether they represent products of regulated splicing with functional implications. mRNA export. intron-containing transcripts (91). and rice waxy gene (68). It has also been shown that manipulating the expression of SR protein alters the splicing of its own pre-mRNA and other SR genes (60.). 104. GRP7) as well as regulation of alternative splicing of other spliceosomal proteins (U1-70K. One reason for this extensive alternative splicing might be that the splice variants have important functions in increasing proteome size and/or regulating the abundance of functional transcripts by regulated unproductive splicing and translation (RUST) (118). Arabidopsis hsp81. is necessary for pre-mRNA splicing of a cold-induced gene and turnover of unstable mRNAs (73). Proteins implicated in other aspects of mRNA metabolism (e.org • Alternative Splicing in Plants 277 . STABLIZED 1 (STA1). Overexpression of the PRP38-like protein conferred salt tolerance in Arabidopsis plants (30). Stress regulation of alternative splicing of splicing regulators may allow plants to quickly regulate splicing and gene expression of many unrelated genes. The combined effect of these is altered constitutive and alternative splicing of other genes. temperature stress (cold and heat) dramatically altered alternative splicing of premRNAs of several SR genes (98). (b) subcelluar redistribution of splicing regula- Annu. 88. etc. cold. Do Splicing Errors Contribute to Enhanced Transcriptome Complexity? It is now clear that splice variants are abundant in plants.annualreviews. 76. Exposure of maize seedlings to cadmium increased the level of unspliced. An alternative explanation for the observed transcriptome complexity is that pre-mRNA splicing in plants is www. 38. heavy metals) inﬂuence alternative splicing of pre-mRNAs (25. mRNA capping) were also reported to be important in ABA signaling and plant responses to abiotic stresses (30. SRZ33. This enables plants to rapidly alter their transcriptome posttranscriptionally in response to changing environmental conditions. 136). resulting in altered transcriptome in response to these signals (Figure 4). Together these results indicate that pre-mRNA splicing and mRNA metabolism play important roles in stress responses. and/or (d ) expression of SR genes. (c) phosphorylation/dephosphorylation status of splicing regulators. Accumulating evidence suggests that stresses change the alternative splicing of splicing regulators.. new splice variants appeared in response to stress. In addition.58:267-294. AtGRP8) by other SR proteins. Plant Biol. 2007. Expression of one of the U1 snRNP-speciﬁc proteins (U1A) or the PRP38-like spliceosomal protein increased salt tolerance in yeast. A recent study supports this. How stress alters the alternative splicing pattern of plant SR genes remains to be elucidated. Rev.g.stresses (heat. 46).
in a few cases where biological functions of splice variants were investigated. including the intronretained forms.. Only detailed functional studies with each splice variant of a gene can resolve this. 55. SR proteins. splice isoforms with introns are recruited for translation (97). Although there are no documented cases of splicing errors by the spliceosome (41). Several observations suggest that splice variants may have a biological role. First. Plant Biol. Finally. Third. Sixth. 98. it is a daunting task to address 278 Reddy . a biological role for these cannot be excluded. rice. resulting in the generation of splice variants. However. was necessary for gene functioning (see below).annualreviews. Figure 4 Schematic diagram illustrating how stresses and developmental cues can change the transcriptome by altering the splicing pattern of splicing regulators. However. if the generation of splice variants was due to splicing errors then one would expect that most intron-containing genes should produce splice variants. 60. Hence.58:267-294. in many cases alternative splicing occurs in genes that encode multidomain proteins where splice variants encode proteins that differ in their domain organization and hence are likely to differ in function. hnRNPs) Altered levels or new isoforms of splicing regulators Annu. Fourth. Feedback positive regulation (self and/or nonself) of AS of splicing regulators Feedback negative regulation (self and/or nonself) of AS of splicing regulators Alteration of CS and AS of other genes Altered transcriptome inherently inefﬁcient and the multiple spliced variants are generated due to splicing errors. and most importantly. 126). Fifth. Downloaded from www. numerous studies have provided evidence that alternative splicing in plants is regulated by tissue-speciﬁc. it is likely that splicing machinery (like replication machinery) makes occasional splicing errors. For personal use only. However. and ferns) (50. all splice variants deserve close scrutiny to determine if they have a regulatory role before they are ignored as artifacts. 113. Rev. Second. because of the vast number of splice variants.org by University of Uppsala on 12/13/12. alternative splicing is predominant in some gene families whereas other gene families with similar size and number of introns do not produce splice variants. 131).Stresses/developmental cues Change in AS of splicing regulators (e. the position of alternatively spliced introns is conserved across evolutionarily distant plants (Arabidopsis. 79. many mRNAs with retained introns are not removed by RNA surveillance mechanisms in the cytoplasm.g. the presence of splice variants. developmental cues and stresses (34. 2007.
The RRM recognizes cisacting sequences in pre-mRNA. the known interaction of ﬁve SR proteins with U1-70K and 11 SRs with U11-35K strongly suggests their involvement in recruiting corresponding snRNPs to the 5 splice site and/or connecting these snRNPs to other members of the spliceosome. three different types of protein kinases (Clk/LAMMERtype. and U1-70K).annualreviews.Annu. and cylcophilins. In addition.org • Alternative Splicing in Plants 279 . protein-protein interactions. For personal use only. 78. U1 and U11 snRNP play a key role in 5 splice site recognition in major and minor spliceosomes. The SR proteins are essential splicing factors required for both constitutive and alternative splicing (13). CypRS64) is modulated by the phosphorylation status of SR proteins (103). Alternative splicing events that are not evolutionarily conserved are not necessarily unimportant as they may be speciﬁc to one organism and reﬂect the biology of that organism and/or might have evolved more recently and contributed to the diversiﬁcation of species. they interacted with three other groups of proteins. and MAP kinases) phosphorylate one or more SR proteins (28. Interactions with protein kinases www. Hence. Interaction studies revealed a complex network of direct interactions among SR proteins and SR protein interactions with other spliceosomal proteins (35. and caused development defects (108). ﬂowering plants have the highest number of SR proteins with a total of 24 in rice and 19 in Arabidopsis (50. and the RS domain functions as a protein-protein interaction module to recruit other proteins of the splicing machinery and contacts the premRNA branch point (111).g. The interaction of plant SR proteins with SRs and other proteins (e. the interaction of several plant-speciﬁc SR proteins with U1-70K and U11-35K indicates that early stages of spliceosome formation and/or splice site selection in plants may differ from animals. 55. SR1/SR34. RS domains of SR proteins are extensively phosphorylated in vivo and this is thought to inﬂuence their RNA binding activity and subcellular localization. They bind pre-mRNAs and function as activators or repressors of both constitutive and alternative splicing.annualreviews. Ectopic expression of a LAMMER-type kinase in Arabidopsis altered the splicing pattern of spliceosomal genes (SR30. The presence of novel plant SR proteins and expansion of this family of proteins in plants coupled with increased transcriptome complexity of SR genes and complex interactions with spliceosomal proteins (see below) indicate that these proteins play key roles in plant pre-mRNA splicing. AFC2. contain one or two RNA recognition motifs (RRMs) at the N terminus and a C-terminal domain with many repeating arginines and serines [the arginine/serine-rich (RS) domain]. 2007. as well as the expression of numerous other genes. SRp34/SR1. 80). Furthermore. Rev. SERINE/ARGININE-RICH PROTEINS SR proteins. 104). 36. The differences between plants and animals in intron/exon architecture and plant-speciﬁc cis-elements involved in pre-mRNA splicing could partly account for this expansion. RNA-protein interactions. 85) (Figure 5). 112). These include U1 and U11 snRNP-speciﬁc proteins (70K/35K). One way to deal with this initially is to narrow the candidates using the conservation of the alternative splicing event in phylogenetically diverse organisms. 109). 37. functions of all isoforms. 28). Plant SR proteins. Phosphoproteomics of Arabidopsis proteins has revealed that 13 of the 19 SR proteins are phosphorylated in vivo (24. 84. like animal SR proteins. In plants. Plant Biol. Downloaded from www.org by University of Uppsala on 12/13/12. 36. 42. Among eukaryotes. there is a caveat in assuming that only conserved alternative splicing types are meaningful. In addition to interactions with other SR proteins. a family of conserved splicing factors.58:267-294.. protein kinases. SRPKs. are phosphoproteins (79. and splicing activities (13. respectively (34. In metazoans. 103). However. RS domains harbor signals for nuclear and subnuclear localization and nucleocytoplasmic shuttling (13).
org by University of Uppsala on 12/13/12. 85.58:267-294. 2007. Each interaction in the network. 84. RS31 and RSZ33 do not interact (indicated with a cross). is based on published reports (35. Black double-headed arrows indicate all other interactions. SR33/SCL33 interacts with itself (shown with an arrow turning on itself ).Annu. AFC2. all interactions with Cyp59 are shown with red double-headed arrows. SR proteins are shown in cyan. a LAMMER-type kinase from tobacco. Rev. (SRPK. cyclophilin-like proteins in yellow. As described above. For clarity. Asterisks indicate that SR33/SCL33 and RS31 interact only with SRZ21/RSZ21. Plant Biol. 36. SRZ21/RSZ21 and SRZ22/RSZ22 are two different proteins with similar interactions with other proteins. phos280 Reddy phorylation of some SR proteins by these kinases has been shown in vitro. Cyp59 interacts with 11 SR proteins and also with the C-terminal domain of the largest subunit . protein kinases in dark blue. a LAMMER-type protein kinase. and RNA polymerase II in pink. indicated by a double-headed arrow. Downloaded from www. PK12. 108). cyclophilin with multiple domains. For personal use only. U1 and U11 small nuclear ribonucleoprotein (snRNP) proteins in red. AFC) that phosphorylate SR proteins suggest regulation of activity/function of these proteins by phosphorylation and dephosphorylation. CypRS. cyclophilin-like protein with RS domain. interactions with CypRS92 with cyan double-headed arrows. Cyp59. Figure 5 A complex network of interactions among serine/arginine-rich (SR) proteins and between SR proteins and other proteins. 42. The interaction of SRZ22/RSZ22 was not tested with Cyp59. interactions with CypRS64 with orange double-headed arrows. and all SR interactions with U11-35K with dark blue double-headed arrows. 78.annualreviews.
Overexpression of another SR protein (RSZ33) altered the splicing pattern of two other SR genes (SR30 and SR1/SR34) whereas the splicing of SR30 was enhanced and correct splicing of the tenth intron of SR1/SR34 was promoted (60). In summary. Most SR proteins showed distinct as well as overlapping expression patterns. There is no in vitro splicing system derived from plant cells.org/resources/ microarray/AtGenExpress/). gene knockout lines are beginning to be used for functional studies. 81).weigelworld. Overexpression of maize ASF/SF2-like genes (ZmSRp31 or ZmSRp32) in cultured cells enhances premRNA splicing of the wheat dwarf virus replication initiator protein (31). reverse transcription–polymerase chain reaction (RT-PCR) and promoter-reporter fusions have shown differential expression of SR proteins in different tissues and cell types (27. RSZ33 overexpression showed many developmental abnormalities including twin embryos. 123). including some plant-speciﬁc ones. 60. 106. (55) overexpressed nine rice SR proteins in protoplasts and analyzed the splicing of the Waxy b -GUS fusion gene. 82. SR34. overexpression of maize ASF/SF2-like proteins in cultured cells also altered 5 splice site selection (31). In addition to splicing changes. Finally. Hence. Isshiki et al. Annu. 70. REGULATION OF CONSTITUTIVE AND ALTERNATIVE SPLICING Expression studies using Northerns. suggesting that it may be involved in and/or connecting both of these processes. Heat caused dramatic reorganization of speckles (2). Similarly. They demonstrated that two SR proteins (RSp29 and RSZp23) enhance splicing and alter the 5 splice site selection. 36. it was shown that the RS domain is essential for splicing and that the RRM1 is required for the enhanced splicing and splice site selection. U1-70K). Second.58:267-294.annualreviews. 79.or ﬁve-branched trichomes. 79–81). plant biologists have relied on three approaches to analyze SR function in pre-mRNA splicing. enhanced cell expansion and changes in cell polarity.org • Alternative Splicing in Plants 281 . 2007. it has been shown that several plant SR proteins. First. 83. whereas the RRM2 was dispensable. reduced apical dominance. Using the ﬁrst approach. A T-DNA insertion mutant of SR45. abnormal ﬂowers and trichomes. showed pleiotropic developmental www. Expression data from microarrays indicate that all SR genes are expressed in different tissues at different levels ( http://www. Sixteen of the 19 SRs are localized to the nucleus in a diffuse nucleoplasmic pool and in speckles (2. 26. Plant Biol. For personal use only. these complex interactions of SR proteins indicate their importance in splice site recognition and spliceosome assembly. It was also shown that RSZ33 regulates its own expression. 35. More recently. four. an animal in vitro splicing system was used to demonstrate whether plant SRs function as essential splicing factors.org by University of Uppsala on 12/13/12. Overexpression of two other rice SR proteins (RSZ36 or SRp33b) in transgenic rice altered the alternative splicing of other SR protein pre-mRNAs as well as their own.of RNA polymerase II (42). and reduced pollen germination (60). Ectopic expression of SR30 greatly reduced the amount of SR1/SR34 full-length mRNA and showed pleiotropic changes in morphology and development. These data also indicate that SR genes show relatively low expression. Using domain deletion and swapping experiments. proteins are constitutively overexpressed either stably or transiently and analyzed for alternative splicing of genes. suggesting that redistribution caused by heat may affect splicing. a plant-speciﬁc SR protein. impaired bilateral symmetry during embryogenesis. including larger rosette leaves and ﬂowers. can complement the splicing-deﬁcient HeLa cell S100 extract and promote splice site switching in vitro (71. Downloaded from www. and delayed transition from the vegetative to the ﬂowering stage (79). Rev.annualreviews. Overexpression of SR30 inﬂuenced splice site choice and changed the alternative splicing pattern of its own pre-mRNA and that of several other endogenous plant genes (RS31.
bind oligouridylates. ESEs/ESSs) (104). There are a large number of hnRNPs in plants. and/or other potential signals (ISEs/ISSs. The importance of the GC content in exons in efﬁcient splicing has also been shown in plants (17. It is likely that some SR proteins have redundant functions and the lack of one is compensated for by other related SR proteins. For personal use only.annualreviews. Generating double and triple knockout may be necessary to unravel their functions. Deleting this element reduced the splicing efﬁciency of intron 12. Downloaded from www. Yoshimura et al. Currently. the functions of many hnRNP proteins are unknown. It is speculated that SR proteins and/or other RNA binding proteins bind to these elements and promote spliceosome assembly. conﬁrming the importance of this element in splicing. the identity of this protein is not known. (131) identiﬁed a putative cis-element (with a high AU content). including several closely related ones. elegans. . Overexpression of one of these proteins [U-rich binding protein 1 (UBP1)] enhanced the splicing of poorly spliced U2 introns. which is highly conserved in this gene in various plants. Both of these proteins. 86). like UBP. In C. The exon upstream of the U12 intron in Luminidepends (LD) contains two regions (potential ESEs) that increase splicing (74). However. These studies indicate the role of SR proteins as important regulators of constitutive and alternative splicing of pre-mRNAs. protein function. functional redundancy may pose a problem in analyzing their function(s) using a single gene knockout approach. It was proposed that these proteins function as a complex and stabilize mRNA by binding to U-rich sequences. Some RNA binding proteins that interact speciﬁcally with U-rich sequences were identiﬁed in plants (66). or cellular localization of a protein (104). 69). composition of exons. speciﬁc cis-elements that bind to plant SR proteins have not been identiﬁed. an NSF 2010 project aimed at identifying splicing signals in plants using computational tools and experimental Annu. Using gel shift assays it was shown that a nuclear protein binds to this cis-element. However.defects and resulted in dramatic changes in the alternative splicing pattern of pre-mRNAs of other SR genes (3). However. inactivation of some single SRs showed no phenotypes whereas inactivation of two or more SR proteins simultaneously caused lethality or developmental defects (77). Plant Biol. However. suggesting that UBP binding to the U-rich sequences in introns recruits splicing machinery to pre-mRNAs. overexpression of several rice SR proteins in rice plants showed no obvious phenotypes in plant growth and development (55). Each of these proteins belongs to a small gene family consisting of three members each. Overexpression of one of these RNA binding proteins (AtGRP7) altered the alternative splicing pattern of its own pre-mRNAs and those of another RNA binding protein (AtGRP8) (119). However. Overexpression of these pro282 Reddy teins enhanced steady-state levels of reporter mRNA either in a promoter-dependent (in the case of UBA1a) or promoter-independent (in the case of UBA2a) manner (65. UBP1 interacts with two plant-speciﬁc proteins (UBA1a and UBA2a) that contain one (UBA1) or two (UBA2) RRMs and an acidic domain. Because plants have a large number of SR proteins. There are other examples in plants where alternative splicing regulates enzyme activity. overexpression of UBP1 did not affect the splicing activity of U12 introns (74). upstream of the acceptor site in intron 12.org by University of Uppsala on 12/13/12. A mutant of rice RSZ36 also showed no growth or developmental defects. suggesting differences between U2 and U12 intron splicing. including some plantspeciﬁc ones that might be important for splicing regulation (125). 2007. An AG-rich exonic element that is capable of promoting downstream 5 splice site selection was reported (93). Rev. Figure 6 shows the potential roles of plant SR and other RNA binding proteins during the early stages of spliceosome formation.58:267-294. Studies on splicing of U2 and U12 introns in plant cells have shown that the splicing efﬁciency of plant pre-mRNAs depends on both intronic elements (U or UA content).
Arrows indicate SR protein-mediated interactions. 126). It is likely that at least some cis-elements involved in alternative splicing of plant pre-mRNAs are conserved during evolution. life. Downloaded from www. 131). heterogeneous nuclear ribonucleoprotein particle proteins. Colored lines in introns indicate intronic splicing regulators (ISRs) such as a U-rich region. SR. White boxes indicate exons and the horizontal lines between and on either side of each exon indicate introns. In animals. they can bridge the spliceosomal components at 5 and 3 splice sites. Trans-factors (SR and other RNA binding proteins) that are likely to interact with exonic and intronic elements are shown. it should be possible to use bioinformatics tools to identify the putative exonic and intronic cis-regulatory sequences involved in constitutive and regulated splic- ing.edu/labs/mount/2010-splicing/). Some interactions shown here have experimental evidence (e. SR proteins can promote the recognition of 5 and 3 splice sites by recruiting U1 snRNP. 2007. interaction among SR proteins. BIOLOGICAL ROLES OF ALTERNATIVE SPLICING Although several thousand plant genes are now known to produce multiple transcripts (104. A model illustrating the possible functions of plant SR and other RNA binding proteins during the early stages of spliceosome assembly. U2 auxillary factor small subunit. Plant Biol. Based on Figure 1b. Systematic evolution of ligands by exponential enrichment (SELEX) and genomic SELEX have been successfully used to identify target sequences for the RRMs of metazoan SR proteins and other RNA binding proteins (13. U2AF35 . such analyses are already providing some useful insights into regulated splicing (40).annualreviews. 132). In only a few of the following examples proteins encoded by splice variants have been veriﬁed using isoform-speciﬁc antibodies or amino acid sequence analysis (127. interactions of several SR proteins with U1-70K and U11-35K.Figure 6 Annu. 52.58:267-294. Functional studies conducted with a few splice variants indicate important functions of alternative splicing. U2AF65 . Consensus sequences at 5 and 3 splice sites in plants are from Figure 1a. Colored boxes in exons indicate putative exonic splicing regulators (ESRs).g. With the availability of the complete genome sequence of several phylogenetically diverse plants and the expected completion of genomes of other crop plants together with ESTs/cDNAs (33. 61). the precise functions of most of the splice variants and their encoded proteins are not known. etc. serine/arginine-rich protein. hnRNP. veriﬁcation of the predicted ESE using splicing reporters is underway (http://www. For personal use only.umd. The activity of such predicted elements can then be tested experimentally.).org by University of Uppsala on 12/13/12. Using genomic SELEX to identify natural binding sequences of plant SR proteins should also be useful in identifying regulatory cis-elements.org • Alternative Splicing in Plants 283 . and others are hypothetical interactions.. and other spliceosomal proteins.annualreviews. In addition. the U or U/A region is shown in place of the animal polypyrimidine tract. UBP binding to U-rich sequences. U2AF. Rev. U2 auxillary factor large subunit. Photosynthesis It is interesting that one gene of the overrepresented class of alternative splicing genes www.
which limits the production of full-length FCA mRNA. was one of the ﬁrst genes to show alternative splicing in dicots and monocots and to generate two proteins that differ only at the carboxyl terminus (107. an alternative exon in intron 3. produces four transcripts (α. premature cleavage and polyadenylation within intron 3 yields transcript β. retention of intron 2 and 3 or only intron 3 generates two other transcripts with an in-frame stop codon. Interestingly. Alternative splicing of the FCA transcript limits the amount of FCA protein. β. 135). The NS encodes the fulllength protein (1144 amino acids) whereas the inclusion of the alternative exon in the NL shifts the reading frame. (101) demonstrated that FCA negatively regulates its own expression by promoting cleavage and polyadenylation within intron 3. and removal of intron 3 generates transcript γ. Flowering Alternative processing of FCA pre-mRNA controls the developmental switch from the vegetative to the reproductive phase. β. Rubisco activase. a mechanistic understanding of how these splice variants function is unknown. TMV infection regulates the splicing pattern of the N gene in such a way that NL becomes more abundant after 4–8 h of infection (25). resulting in excision of a large intron. suggesting that alternative splicing is also important in the transition to ﬂowering in other species. In tobacco. The other three encode truncated proteins.belongs to photosynthesis. 137). FCA. Defense Responses Some reports indicate that alternative splicing of pre-mRNAs of resistance (R) genes plays an important role in plant defense responses. In transcript α. which complements the late ﬂowering phenotype in fca.annualreviews. The N gene produces two transcripts [a short (NS ) and a long (NL ) transcript] that lack or contain. and γ) of these are generated by differential processing of intron 3. confers resistance to tobacco mosaic virus (TMV). Three (α. suggesting the importance of regulated alternative splicing of the N gene in disease resistance (25). respectively.5-bisphosphate carboxylase/oxygenase (Rubisco). the N gene. Alternative splicing of intron 13. Alternative processing of intron 3 of FCA is conserved between Arabidopsis and Brassica napus (89). Only transcript γ encodes the full-length protein. transgenic lines expressing NS cDNA containing intron 3 and the 3 sequence of the N gene or a genomic region capable of producing both NS and NL transcripts showed complete resistance. 127. resulting in a truncated protein (652 amino acids) that lacks 13 of the 14 LRR regions. Zhang & Gassmann (136) demonstrated that the presence of constitutively as well as alternatively spliced transcripts is necessary for RPS4 function. For personal use only. and this autoregulation prevents precocious ﬂowering. both spatially and temporally. 124. Plant Biol. 102).org by University of Uppsala on 12/13/12. which encodes a nuclear ABA receptor with an RNA binding domain. only the large isoform is redox regulated (134. This generates a truncated inactive transcript. Rev. Quesada et al. Downloaded from www. Alternative splicing was also observed with pre-mRNAs of several other plant disease R genes encoding the TIR-NBS-LRR family of proteins (58). a member of a class of R genes that encode proteins with Toll/interleukin 1 receptor (TIR)— nucleotide binding site (NBS)–leucine-rich repeat (LRR) (TIR-NBS-LRR) domains.58:267-294. intron 3 is retained. 2007. and δ) by alternative processing (101. Arabidopsis RPS4. a nuclear-encoded chloroplast protein required for the light activation of ribulose 1. γ. another disease R gene that encodes a member of the TIR284 Reddy NBS-LRR family. Although both forms activate Rubisco. Although these reports established the requirement of alternative transcripts. Annu. which in turn controls ﬂowering. yields transcript δ. produces three transcripts by alternative splicing. Transgenic plants expressing NS transcript did not show complete resistance to TMV. Thermal induction of ﬂowering upregulates genes involved . In addition to a constitutively spliced transcript.
A high amylose quantity in rice grains is not desirable. In contrast. the budding yeast has introns in about 4% of its genes (∼250 out of the 6000 genes) and only six of these have two introns (7). Rev. However. The Waxy (Wx) gene encodes a granulebound starch synthase that is necessary for the synthesis of amylose in endosperm. mutations in a constitutive splice site result in a suboptimal or weaker site.in RNA splicing. Are the splice variants real? Do they encode protein isoforms that are functionally different? How much of this alternative splicing www. Plant Biol. How did alternative splicing evolve? Comparing splice sites in unicellular eukaryotes that show no alternative splicing with multicellular eukaryotes that show extensive alternative splicing reveals a high degree of plasticity in the 5 splice sites in multicellular organisms (5). which have less amylose content in the endosperm.annualreviews. thereby allowing the splicing machinery to skip the splicing site. the Wx b has a mutation (G to T) in the 5 splice site in intron 1. it is also possible that the rarity of alternative splicing in unicellular organisms could be a derived state owing to the loss of alternative splicing machinery. These two theories are not mutually exclusive and both may have contributed to the evolution of alternative splicing. which might result in an increase or decrease in the binding of the splicing machinery to constitutive sites. implying a role for regulated splicing in ﬂowering. This is expected to release the selective pressure from the splice site. suggesting that DU-1 and DU-2 encode regulators of alternative splicing (54). The cultivated rice species. CONCLUDING REMARKS The unexpected prevalence of alternative splicing in plants raises myriad questions. with up to 74% of intron-containing genes undergoing alternative splicing.58:267-294.annualreviews. It was later shown that this mutation reduces the splicing efﬁciency of Wx b pre-mRNA and promotes alternative splicing at cryptic sites in exon 1. including 6 SR proteins (6). Two theories have been proposed to explain the evolution of alternative splicing (5). are derived from wild varieties that have high amylose content. 2007. For personal use only. 80% of genes have introns. suggesting that alternative splicing may have evolved more recently. According to one theory that is sequence based. 41).org by University of Uppsala on 12/13/12. Alternative splicing in unicellular eukaryotes such as budding yeast is rare or nonexistent (7. 23. The second transfactor-based model invokes the evolution of splicing regulatory factors. A single mutation at the 5 splice site of one gene (Waxy) that affects the alternative splicing of its premRNA results in the reduced levels of amylose. Annu.org • Alternative Splicing in Plants 285 . Overexpression of SR30 that regulates alternative splicing of other SR pre-mRNAs results in delayed ﬂowering (79). thereby allowing mutations that weaken the splice site. EVOLUTION OF ALTERNATIVE SPLICING Although pre-mRNA splicing occurs in all eukaryotes. alternative splicing is more prevalent in highly evolved multicellular eukaryotes as compared with ancestral unicellular eukaryotes. the amylose quantity affects the seed quality. Grain Quality in Rice In rice. In one of the two naturally occurring alleles of the Wx gene (Wx a and Wx b ). Downloaded from www. Two mutations (du-1 and du-2) that reduce the splicing of Wx b without affecting the splicing of Wx a have been isolated. resulting in reduced content of amylose in varieties that contain this allele (53). causing a tenfold reduction in its expression as compared with Wx a . The number of intron-containing genes and the extent of alternative splicing have increased considerably from ancestral unicellular eukaryotes to complex multicellular organisms. The vast expansion of splicing regulators such as hnRNPs and SR proteins in complex organisms such as humans and plants may have partly contributed to an increase in alternative splicing in these organisms. In plants and animals.
6. 2007. The large number of SR proteins in ﬂowering plants. plant introns differ from animals in their size. analysis of alternative splicing across phylogenetically divergent as well as closely related species may also help us understand if transcriptome diversity generated by alternative splicing contributed to speciation. including several plant-speciﬁc members. Recent analysis of expression data of all Arabidopsis genes reveals that highly expressed genes have four or more introns (1). but it is not prevalent in animals. This type of feedback regulation is likely important in controlling the levels of SR proteins and splicing of other pre-mRNAs.58:267-294. with pre-mRNAs of more than 21% of all genes producing multiple transcripts generating considerable transcriptome complexity and likely proteome diversity. Pre-mRNAs of SR proteins undergo extensive alternative splicing. Because SR proteins are modular and are involved in multiple stages of splicing. It appears that alternative splicing in plants will take center stage and emerge as an important posttranscriptional regulatory mechanism in ﬁne-tuning gene expression. analysis of the completed sequences of the Arabidopsis genome for the presence of known spliceosomal proteins in nonplant systems indicates that the core spliceosomal machinery is conserved between plants and animals. I hope that answers to some of these questions are forthcoming in the next decade. contributes to proteome diversity? What is the role of splice variants in controlling cellular processes and ultimately plant growth and development? Do splice variants function as regulatory RNAs to control gene expression by different means? What proteins and mechanisms regulate alternative splicing? Answering these questions is a major challenge facing plant biologists. which supports earlier reports that introns enhance gene expression. These differences indicate that some of the initial events involved in splice site recognition are likely unique to plants. are greatly expanded in plants and some plant splicing proteins do not have homologs in animal systems.org by University of Uppsala on 12/13/12. Thus. Furthermore. Intron retention accounts for more than 50% of all alternative splicing events in plants. 3. nucleotide composition. localization.annualreviews. a better understanding of pre-mRNA splicing will have implications in optimizing transgene expression in plants. suggesting the existence of plant-speciﬁc splice site recognition and splicing regulatory mechanisms. SUMMARY POINTS 1. 5. For personal use only. speciﬁc families. However. Downloaded from www. and complex network of interactions among themselves and with other spliceosomal proteins strongly indicate an important role for these proteins in the early stages of spliceosome assembly and regulated splicing. and their structural features. it is likely that the splice variants and the encoded proteins are functionally signiﬁcant. Plant and animal introns have similar 5 and 3 splice sites. branch point sequence. This offers a quick way 286 Reddy . Although plant spliceosomes have not been isolated. and polypyrimdine tract. Alternative splicing of plant SR pre-mRNAs is either autoregulated and/or modulated by other SR proteins. especially splicing regulators.Annu. Plant Biol. It is well established that introns enhance gene expression (reviewed in 104). 4. However. Alternative splicing in ﬂowering plants is much more prevalent than previously thought. 2. Rev.
Plant gene expression in the age of systems biology: Integrating transcriptional and post-transcriptional events. Belostotsky DA. Lejeune F. Reddy ASN. Brover VV. Wang JY. Hirst JA. Brett D. Spliced segments at the 5 terminus of adenovirus 2 late mRNA. 2003. Black DL. Reddy ASN. Justin Anderson for preparing the ﬁgures. Mol. Natl.org • Alternative Splicing in Plants 287 . Broad speciﬁcity of SR (serine/arginine) proteins in the regulation of alternative splicing of premessenger RNA. Birnbaum K. 2005. Plant J. Lempe J. Methods 2:615–19 12. Sci. and Prasad for their contributions to splicing research in my laboratory. Nat.annualreviews. Sureshkumar S. Proc. 2004. Plant Biol. Features of Arabidopsis genes and genome discovered using full-length cDNAs. 2003. USA 74:3171–75 11. How did alternative splicing evolve? Nat. Moore C. ACKNOWLEDGMENTS Annu.58:267-294. Downloaded from www. 60:69–85 2. 2004. Wootton L. 2005. Dale Richardson for preparing the splice site logos. Reich J. J. Biol. 30:29–30 www. Ali GS. UPF1 is required for nonsensemediated mRNA decay (NMD) and RNAi in Arabidopsis. Stresses have a dramatic effect on alternative splicing of pre-mRNA SR proteins. Kieffer M. Palusa SG. Lambert GM. Hanumappa. Prasad J. and Gul Shad Ali. Arciga-Reyes L. Prog. Valcarcel J. Acad. Berget SM. 2006. 47:480–89 5. LITERATURE CITED 1. Golovkin. Berget SM. Rev.S. Pospisil H. Nuclear localization and in vivo dynamics of a plant-speciﬁc serine/arginine-rich protein. Barrass JD. Ast G. Goud. Davies B. Sci. 2:e106 7. Ali GS. Work on pre-mRNA splicing research in my laboratory is supported by a grant from the U. Nat. Genet. I thank Drs. Golovkin M. Rev. Rose AB. Chem. Annu. Nucleic Acids Res. Stevenin J. 19:295–98 8. 2006. A plant-speciﬁc splicing factor regulates multiple developmental processes. 1977. Weigel D. Trends Plant. Cell type-speciﬁc expression proﬁling in plants via cell sorting of protoplasts from ﬂuorescent reporter lines. Biol. In review 4. It is likely that part of the known stress-induced changes in the transcriptome could be due to changes in pre-mRNA splicing of the affected genes. 7. 270:2411–14 10. Troukhan ME. Feldmann KA. Tatarinova T. Manley JL. Bourgeois CF. For personal use only. 5:773–82 6. 2003. 10:347– 53 9. Exon recognition in vertebrate splicing. et al. Flavell RB. 2002. 1995. Plant Mol.to modulate constitutive and alternative splicing of pre-mRNAs of nonspliceososmal genes in response to signals and thereby alter the transcriptome. Alexandrov NN. Jung JW.org by University of Uppsala on 12/13/12. which in turn could regulate the splicing of their own and/or other SR pre-mRNAs. Irene Day. Potent induction of Arabidopsis thaliana ﬂowering by elevated growth temperature. 36:883–93 3. Balasubramanian S. Ali. and Saiprasad Goud for constructive comments on the manuscript. 2006. Rev. Trends Genet. 78:37–88 14. Genet. Splicing goes global. Sharp PA. Biochem. Beggs JD. Bork P. Mechanisms of alternative premessenger RNA splicing. PLoS Genet.annualreviews. Alternative splicing and genome complexity. Biol. Golovkin M. Department of Energy (DE-FG02-04ER15556). 2006. 72:291–336 13. 2007. Plant J.
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Hyo Jung Kim. Jianping Yu. Posewitz. Richard Sibout. For personal use only. Schachtman and Ryoung Shin p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 47 Hydrogenases and Hydrogen Photoproduction in Oxygenic Photosynthetic Organisms Maria L. Assmann. 2007 Annu. and Hong Gil Nam p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 115 The Biology of Arabinogalactan Proteins Georg J. and Toshinori Kinoshita p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 219 Volume 58. Osmont. Rev. Masatoshi Nakajima. Ghirardi. Seifert and Keith Roberts p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 137 Stomatal Development Dominique C. Hardtke p p p p p p p p p p p p p p p p p p p p p p p p p p 93 Leaf Senescence Pyung Ok Lim. Pin-Ching Maness. Downloaded from www. and Makoto Matsuoka p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 183 Cyclic Electron Transport Around Photosystem I: Genetic Approaches Toshiharu Shikanai p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 199 Light Regulation of Stomatal Movement Ken-ichiro Shimazaki. Sack p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 163 Gibberellin Receptor and Its Role in Gibberellin Signaling in Plants Miyako Ueguchi-Tanaka.annualreviews.58:267-294. 2007. and Christian S. Plant Biol.org by University of Uppsala on 12/13/12. Bergmann and Fred D. v . Matthew C.Annual Review of Plant Biology Contents Frontispiece Diter von Wettstein p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p pxii From Analysis of Mutants to Genetic Engineering Diter von Wettstein p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 1 Phototropin Blue-Light Receptors John M. Alexandra Dubini. Michio Doi. Ashikari Motoyuki. Christie p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 21 Nutrient Sensing and Signaling: NPKS Daniel P. and Michael Seibert p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 71 Hidden Branches: Developments in Root System Architecture Karen S. Sarah M.
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