Nuclear pre-mRNA splicing in plants

The coding regions of about 80% of plant nuclear genes contain one or more noncoding intervening sequences (introns). The transcription of these genes results in a precursor mRNA (pre-mRNA) with coding sequences (exons) and introns. The noncoding intervening sequences are then accurately removed and the coding regions are joined in the nucleus to generate functional mRNAs by a process called pre-mRNA splicing. In addition to basic/constitutive splicing, many plant pre-mRNAs, like metazoan pre-mRNAs, undergo alternative splicing, thereby contributing to proteomic complexity. The splicing of pre-mRNAs takes place in a large RNA-protein complex named the spliceosome, which is made up of several small nuclear ribonucleoprotein (snRNP) particles and other associated proteins. Until recently, it was thought that there is only one type of spliceosome in all eukaryotes. However, it is now established that most metazoans have a second (minor) type of spliceosome that is compositionally different from the widely studied major spliceosome and functions in splicing of some rare introns. Based on the conservation of many components of major and minor spliceosomes in plants, it is likely that plants contain both types of spliceosomes and the basic mechanisms involved in spliceosome formation and intron removal are likely to be similar between plants and animals. However, a number of reports using in vivo splicing assays have shown that the cis-elements that are necessary for intron recognition and proper splicing of plant introns differ considerably from yeast and animals. These studies suggest that the mechanisms of intron recognition in plants are likely to differ from yeast and animals and involve novel proteins that recognize the plant-specific cis-elements. In recent years, several proteins that are implicated in plant pre-mRNA splicing have been characterized, including some novel ones that are not present in metazoans. The recent completion of the Arabidopsis and other eukaryotic genomes sequence should facilitate the identification of plant orthologs of various animal spliceosomal. proteins in the near future. However, identification and functional analysis of the splicing proteins that are specific to plants will demand novel approaches.