A DEAD-box RNA helicase, RH8, is critical for regulation of ABA signalling and the drought stress response via inhibition of PP2CA activity

Abscisic acid (ABA) is major plant hormone involved in regulating abiotic stress responses. Several studies have established that an ABA-signalling transduction pathway-from ABA perception to response-functions in plant cells. The group A PP2Cs constitute core components of ABA signalling, and they negatively regulate ABA signalling and stress responses. Recent studies have identified and functionally analysed regulators of PP2C activity; however, the precise regulatory mechanisms remain unclear. In the present study, we used a yeast 2-hybrid (Y2H) screening analysis to identify the DEAD-box RNA helicase RH8, which interacted with PP2CA in the nucleus. rh8 knockout mutants exhibited ABA hyposensitivity and drought-susceptible phenotypes characterized by high levels of transpirational water loss via reduced stomatal closure and decreased leaf temperatures. However, rh8/pp2ca double mutants showed ABA hypersensitivity and drought-tolerant phenotypes, indicating that RH8 and PP2CA function in the same ABA-signalling pathway in the drought stress response; moreover, RH8 functions upstream of PP2CA. In vitro phosphatase and kinase assays revealed that RH8 inhibits PP2CA phosphatase activity. Our data indicate that RH8 and its interacting partner PP2CA modulate the drought stress response via ABA-dependent signalling.