SNF1-related protein kinases 2 (SnRK2s) are key regulators governing the plant adaptive responses to osmotic stresses, such as drought and high salinity. Subclass III SnRK2s function as central regulators of abscisic acid (ABA) signalling and orchestrate ABA-regulated adaptive responses to osmotic stresses. Seed plants have acquired other types of osmotic stress-activated but ABA-unresponsive subclass I SnRK2s that regulate mRNA decay and promote plant growth under osmotic stresses. In contrast to subclass III SnRK2s, the regulatory mechanisms underlying the rapid activation of subclass I SnRK2s in response to osmotic stress remain elusive. Here, we report that three B4 Raf-like MAP kinase kinase kinases (MAPKKKs) phosphorylate and activate subclass I SnRK2s under osmotic stress. Transcriptome analyses reveal that genes downstream of these MAPKKKs largely overlap with subclass I SnRK2-regulated genes under osmotic stress, which indicates that these MAPKKKs are upstream factors of subclass I SnRK2 and are directly activated by osmotic stress.

SNF1相关蛋白激酶2（SnRK2s）是调控植物对诸如干旱和高盐度等渗透胁迫的适应性反应的关键调控因子。子类III SnRK2s充当脱落酸（ABA）信号的中央调节剂，并协调ABA调节的对渗透胁迫的适应性反应。种子植物已获得其他类型的渗透胁迫激活但ABA无反应的亚类I SnRK2，它们调节mRNA衰变并在渗透胁迫下促进植物生长。与III类SnRK2s相比，响应渗透压而快速激活I类SnRK2s的调节机制仍然难以捉摸。在这里，我们报道了三种B4 Raf样的MAP激酶激酶激酶（MAPKKKs）在渗透胁迫下磷酸化并激活亚类I SnRK2s。