OsMAP4K5 is an Upstream Activator of OsMKKK28 in Abscisic Acid Signalling in Rice

MAPK cascades (MAPKKK-MAPKK-MAPK) have been demonstrated to play a pivotal role in plant responses to various environmental stresses and in abscisic acid (ABA) signalling. The activation of MAPKKK is a prerequisite for the activation of the entire MAPK cascade. It was discovered as early as 30 years ago in yeast and mammalian cells that MAPK cascades can be activated by their upstream MAPK kinase kinase kinases (MAP4Ks). However, no evidence to date has shown that plant MAP4Ks can interact with and phosphorylate MAPKKKs. Here, we show that OsMAP4K5 directly interacts with and phosphorylates OsMKKK28 both in vitro and in vivo. We found that OsMAP4K5 directly phosphorylates the Ser135 residue of OsMKKK28, and that OsMAP4K5-mediated Ser135 phosphorylation of OsMKKK28 is essential for the activation of OsMKKK28 and its downstream kinase OsMKK1 in ABA signalling. Genetic evidence reveals that OsMAP4K5 is a positive regulator of ABA responses in rice, and its role in regulating rice ABA responses is achieved, at least partially, through OsMKKK28. In summary, our findings reveal a new mechanism for regulating the activation of MAPK cascade in plant cells, which directly links MAP4K to MAPKKK in ABA signalling, thus resolving a long-standing question in plant biology.