Review: Specificity determinants of the plant splicing code

Alternative splicing (AS) of precursor mRNAs (pre-mRNAs) constitutes a major means to increase transcriptome complexity in higher eukaryotes and critically contributes to the re-programming of gene expression in response to internal and environmental signals. Technological advances have enabled us to determine transcriptome-wide AS patterns at unprecedented accuracy, depth, and throughput. Furthermore, powerful tools for examining the regulatory mechanisms underlying AS decisions have been successfully established for plants, including methods for profiling the in vivo interaction landscape of splicing regulatory proteins with their target pre-mRNAs. Combining these novel approaches with functional studies of individual AS events identified many critical components of the plant splicing code, consisting of cis-regulatory elements on the pre-mRNA and trans-acting factors, such as splicing regulatory proteins. Their concerted action affects splice site selection by the spliceosome, thereby generating highly dynamic and complex AS outputs. Here, we will review our current knowledge of AS regulation by RNA sequence and structural motifs in cis and networks of trans-acting splicing regulators. We will also discuss how, despite overall low complexity of the target motifs for binding of splicing regulators and their often-redundant functions, high levels of precision and specificity in AS can be achieved.