The Arabidopsis Mediator Complex Subunit MED19a is Involved in ABI5-mediated ABA Responses

Abstract

Arabidopsis Mediator complex subunit 19a (MED19a), which mediates interactions between transcriptional regulators and RNA polymerase II, plays a critical role in plant response to infection by pathogens. However, the roles of MED19a in other signaling pathways are unknown. Here, we report that MED19a plays an important role in regulation of abscisic acid (ABA)-mediated transcriptional regulation in Arabidopsis. Plants deficient in MED19a showed reduced sensitivity to ABA inhibition of seed germination, cotyledon greening, root growth, and stomatal opening. MED19a-deficient mutants also had reduced resistance to drought stress, evidenced by high water-loss rates and low survival rates. Molecular genetic analysis revealed that MED19a mutants had down-regulated ABA-induced genes, including Em1, Em6, and RD29B, and MED19a could occupy the promoters of Em1 and Em6 in an ABA-dependent manner. Furthermore, MED19a interacted with the transcription factor ABA-insensitive 5 (ABI5) in split-luciferase complementation assays and co-immunoprecipitation assays. An analysis of double mutants (med19a-2 and abi5-7) suggested that the action of MED19a in ABA signaling was dependent upon ABI5. Furthermore, MED19a and ABI5 influenced each other in recruiting the promoters of the target genes Em1 and Em6, which are involved in embryonic development. Altogether, these results indicate that MED19a acts as a positive regulator in ABI5-mediated ABA responses.