The G-protein β subunit AGB1 represses abscisic acid signaling via attenuation of the MPK3-VIP1 phosphorylation cascade in Arabidopsis.

Heterotrimeric G proteins play key roles in cellular processes. Although phenotypic analyses of Gβ (AGB1) mutants have implicated G proteins in abscisic acid (ABA) signaling, the AGB1-mediated modules for ABA signaling remain unclear. We found that an AGB1 portion was localized to nucleus where it interacted with ABA-activated VirE2-interacting protein 1 (VIP1) and mitogen-activated protein kinase 3 (MPK3). AGB1 acts as an upstream negative regulator of VIP1 activity by initiating responses to ABA and drought stress treatment, and VIP1 regulates the ABA signaling pathway in an MPK3-dependent manner in Arabidopsis. AGB1 outcompeted VIP1 for interaction with MPK3 C-terminus and prevented VIP1 phosphorylation by MPK3. Importantly, ABA treatment reduced AGB1 expression in the wild-type but increased it in vip1 and mpk3 mutants. VIP1 is associated with ABA response elements present in the AGB1 promoter, thus forming a negative feedback regulatory loop. Thus, our study defines a new mechanism for fine-tuning ABA signaling through the interplay between AGB1 and MPK3-VIP1. Furthermore, it suggests a common G protein strategy to receive and transduce signals from the external environments.