The EAR motif-containing adaptor protein ECAP enhances ABI5-mediated ABA signaling to inhibit seed germination in Arabidopsis

The phytohormone abscisic acid (ABA) regulates diverse biological processes, including seed germination, in which the transcription factor abscisic acid insensitive 5 (ABI5) plays an essential role. Here, we show that the EAR motif-containing adaptor protein (ECAP) functions as a positive regulator of ABI5 to inhibit seed germination in Arabidopsis. Like ABI5, ECAP is required for the inhibition of seed germination by ABA. Transcriptome analysis reveals that ABI5 and ECAP regulate the expression of a common set of ABA-responsive genes. Biochemically, ECAP enhances the expression of ABI5, interacts with ABI5 to promote its transcriptional activity, and binds to the promoters of ABA-responsive genes in an ABI5-dependent manner. Genetically, loss of ABI5 compromises the enhanced ABA sensitivity of the ECAP overexpression line, and overexpression of ABI5 partially rescues the reduced ABA sensitivity of the ecap mutant. Together, our results suggest that ECAP and ABI5 function coordinately to fine-tune the inhibition of seed germination by ABA.