ABA signaling prevents phosphodegradation of the Arabidopsis SR45 splicing factor to negatively autoregulate inhibition of early seedling development

Abstract

ABSTRACTAlternative splicing is a key posttranscriptional mechanism to expand the coding capacity of eukaryotic genomes. Although the functional relevance of this process remains poorly understood in plant systems, major modulators of alternative splicing called serine/arginine-rich (SR) proteins have been implicated in plant stress responses mediated by the abscisic acid (ABA) hormone. Loss of function of the Arabidopsis thaliana SR-like protein SR45, a bona fide splicing factor, has been shown to cause plant hypersensitivity to environmental cues and activation of the ABA pathway. Also, consistent with both animal and plant SR proteins being extensively and reversibly phosphorylated at their C-termini, ABA-induced changes in the phosphorylation status of SR45 have been reported.Here, we show that SR45 overexpression reduces Arabidopsis sensitivity to ABA during early seedling development. Moreover, exposure to ABA dephosphorylates SR45 at multiple amino acid residues and leads to accumulation of the protein via reduction of SR45 ubiquitination and proteasomal degradation. Using phosphomutant and phosphomimetic transgenic Arabidopsis lines, we demonstrate the functional relevance of ABA-mediated dephosphorylation of a single SR45 residue, T264, in antagonizing SR45 ubiquitination and degradation to promote its function as a repressor of seedling ABA sensitivity. Taken together, our results reveal a mechanism in which ABA signaling negatively autoregulates during early plant development via posttranslational control of the SR45 splicing factor.