The M3Ks, a missing component discovered in the early ABA core signaling module (Nature Comms)

About a decade ago, the ABA receptors [PYR-ABACTIN RESISTANCE (PYR/PYL)/REGULATORY COMPONENT OF ABA RECEPTOR (RCAR)] were discovered, establishing the PYL-PP2C-SnRK2s core signalling module, in which ABA-binding to PYR/PYL receptors inhibits PP2C phosphatases, leading to activation of SnRK2 kinases, yet it remained unclear whether there are upstream protein kinases leading to SnRK2s activation via phosphorylation. Here, Takahashi et al. and Lin et al. have identified a family of MAP kinase kinase kinases (M3Ks), specifically the B2, B3 & B4 subfamilies of Raf-like protein kinases, that are required to (re)activate SnRK2s protein kinases to mediating osmotic and ABA responses. Takahashi and colleagues show that autophosphorylation of OST1 (SnRK2.6) is surprisingly insufficient for reactivation after dephosphorylation by PP2C, but the reactivation of OST1 could be rescued by M3Ks phosphorylation in vitro. The triple mutant of M3Ks (subgroup B3 members) has ABA and osmotic insensitive phenotypes in seed germination. Interestingly, this triple mutant shows stronger impairment in rapid osmotic stress activation of SnRK2 compared to ABA application, indicating that this M3Ks-SnRK2 cascade could also be required in ABA-independent osmotic stress signal transduction. Lin et al. generated higher order mutants of M3Ks subgroup B2/B3/B4, i.e., OK¹³⁰-weak (B4 subgroup weak alleles), OK¹³⁰-null (B4 subgroup mutant), OK-quatdec and -quindec mutants (B2, B4 and B4 subgroup mutant). OK¹³⁰-null and OK-quatdec mutants are hypersensitive to osmotic stress and insensitive to ABA seed germination, respectively. Both studies uncover the M3Ks as the upstream kinases to trigger the SnRKs activation in ABA and osmotic stress responses. Further understanding this mechanism in crop plants will help to produce crops resistant to abiotic stresses.  (Summary by Min May Wong) Nature Communication 10.1038/s41467-019-13875-y & 10.1038/s41467-020-14477-9