An Arabidopsis SR protein relieving ABA inhibition of seedling establishment represses ABA-responsive alternative splicing

The phytohormone abscisic acid (ABA) inhibits postgerminative growth under unfavorable conditions to delay the transition to the autotrophic stage and promote plant survival. While stress-induced ABA accumulation is well established to trigger extensive transcriptional changes, it is becoming clear that it also relies on alternative splicing to enhance stress tolerance. However, the upstream components modulating posttranscriptional regulation of the ABA response remain largely unknown. Here, we show that loss of function of the Arabidopsis SR34a protein enhances sensitivity to ABA during seedling establishment. Individual-nucleotide crosslinking and immunoprecipitation (iCLIP) combined with RNA-sequencing revealed that SR34a is an alternative splicing regulator that binds predominantly GCU-rich exonic sequences near splice sites. We find that SR34a targets all alternative splicing event types, including in RNAs encoding known determinants of ABA sensitivity, to prevent ABA-responsive splicing in germinated seeds. Our study sheds mechanistic light on how plant SR proteins regulate alternative splicing and counteract ABA inhibition of early plant growth.