Pohsphorylation of MAP4K1 and MAP4K2 by OST1 is required for drought tolerance

Protein phosphorylation has pivotal roles in ABA and dehydration stress signaling in higher plants. However, little is known about MITOGEN-ACTIVATED PROTEIN KINASE KINASE KINASE KINASE (MAP4K) in dehydration tolerance. Here, we report MAP4K1 and MAP4K2 are involved in dehydration tolerance. Mutations in MAP4K1 or MAP4K2 did not show obvious phenotype whereas map4k1 map4k2 double mutant exhibited the dwarf phenotype and low dehydration tolerance with hyposensitivity to exogenous abscisic acid. MAP4K1 and MAP4K2 interacted with SnRK2.2, SnRK2.3, and OST1 in vitro and in vivo. OST1, but not SnRK2.2 or SnRK2.3, phosphorylated MAP4K1 and MAP4K2. Mass spectrometry and phosphorylation activity assay revealed that OST1 phosphorylated MAP4K1 on serine 487 and MAP4K2 on serine 488, respectively. MAP4K1 and MAP4K2 as well as OST1 act in the same dehydration stress pathway, with OST1 acting downstream of MAP4K1 and MAP4K2. Together, we identified MAP4K1 and MAP4K2 as new substrates of OST1, and MAP4K1 and MAP4K2 are redundant in dehydration tolerance.