Abstract
The phytohormone abscisic acid (ABA) is crucial for plant responses to environmental challenges. The SNF1-regulated protein kinase 2s (SnRK2s) are key components in ABA-receptor coupled core signaling, and are rapidly phosphorylated and activated by ABA. Recent studies have suggested that Raf-like protein kinases (RAFs) participate in ABA-triggered SnRK2 activation. In vitro kinase assays also suggest the existence of autophosphorylation of SnRK2s. Thus, how SnRK2 kinases are quickly activated during ABA signaling still needs to be clarified. Here, we show that both B2 and B3 RAFs directly phosphorylate SnRK2.6 in the kinase activation loop. This transphosphorylation by RAFs is essential for SnRK2 activation. The activated SnRK2s then intermolecularly trans-phosphorylate other SnRK2s that are not yet activated to amplify the response. High-order Arabidopsis mutants lacking multiple B2 and B3 RAFs show ABA hyposensitivity. Our findings reveal a unique initiation and amplification mechanism of SnRK2 activation in ABA signaling in higher plants.
Highlights
The phytohormone abscisic acid (ABA) is crucial for plant responses to environmental challenges
We find that ABA does not activate B2 and B3 Raf-like protein kinases (RAFs); instead, the basal level of RAF kinase activity is essential for SNF1-related protein kinase 2 s (SnRK2s) activation and initiation of ABA signaling
Subgroup B RAFs belong to the MITOGEN-ACTIVATED PROTEIN KINASE KINASE KINASE (MAPKKK) family due to their similarity with animal B-Raf protein kinases[37,38,39]
Summary
The phytohormone abscisic acid (ABA) is crucial for plant responses to environmental challenges. The SNF1-regulated protein kinase 2s (SnRK2s) are key components in ABAreceptor coupled core signaling, and are rapidly phosphorylated and activated by ABA. Recent studies have suggested that Raf-like protein kinases (RAFs) participate in ABAtriggered SnRK2 activation. How SnRK2 kinases are quickly activated during ABA signaling still needs to be clarified. We show that both B2 and B3 RAFs directly phosphorylate SnRK2.6 in the kinase activation loop. This transphosphorylation by RAFs is essential for SnRK2 activation. 1234567890():,; Environmental challenges like drought, cold, and high salinity induce the accumulation of abscisic acid (ABA), a major stress phytohormone that triggers multiple stress responses in plants[1,2,3,4,5,6].
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