Mediator tail module subunits MED16 and MED25 differentially regulate abscisic acid signaling in Arabidopsis

MED25 has been implicated as a negative regulator of the abscisic acid (ABA) signaling pathway. However, it is unclear whether other Mediator subunits could associate with MED25 to participate in the ABA response. Here, we used affinity purification followed by mass spectrometry to uncover Mediator subunits that associate with MED25 in transgenic plants. We found that at least 26 Mediator subunits, belonging to the head, middle, tail, and CDK8 kinase modules, were co-purified with MED25 in vivo. Interestingly, the tail module subunit MED16 was identified to associate with MED25 under both mock and ABA treatments. We further showed that the disruption of MED16 led to reduced ABA sensitivity compared to the wild type. Transcriptomic analysis revealed that the expression of several ABA-responsive genes was significantly lower in med16 than those in wild type. Furthermore, we discovered that MED16 may possibly compete with MED25 to interact with the key transcription factor ABA INSENSITIVE 5 (ABI5) to positively regulate ABA signaling. Consistently, med16 and med25 mutants displayed opposite phenotypes in ABA response, cuticle permeability, and differential ABI5-mediated EM1 and EM6 expression. Together, our data indicate that MED16 and MED25 differentially regulate ABA signaling by antagonistically affecting ABI5-mediated transcription in Arabidopsis.