Abstract
Osmoregulation is important for plant growth, development and response to environmental changes. SNF1-related protein kinase 2s (SnRK2s) are quickly activated by osmotic stress and are central components in osmotic stress and abscisic acid (ABA) signaling pathways; however, the upstream components required for SnRK2 activation and early osmotic stress signaling are still unknown. Here, we report a critical role for B2, B3 and B4 subfamilies of Raf-like kinases (RAFs) in early osmotic stress as well as ABA signaling in Arabidopsis thaliana. B2, B3 and B4 RAFs are quickly activated by osmotic stress and are required for phosphorylation and activation of SnRK2s. Analyses of high-order mutants of RAFs reveal critical roles of the RAFs in osmotic stress tolerance and ABA responses as well as in growth and development. Our findings uncover a kinase cascade mediating osmoregulation in higher plants.
Highlights
Osmoregulation is important for plant growth, development and response to environmental changes
In addition to the SnRK2s, we found two groups of protein kinases of approximately 130 and 100 kDa that were strongly activated by osmotic stress but not abscisic acid (ABA) (Fig. 1a)
Activation of the osmotic stress-activated protein kinases (OKs) did not require SnRK2s and was independent of ABA signaling, since OK activation by mannitol treatment was still observed in the snrk2.2/3/6 triple mutant, which is deficient in ABA signaling, and in snrk2.1/4/5/7/8/9/10 septuple a 130 100
Summary
Osmoregulation is important for plant growth, development and response to environmental changes. We report a critical role for B2, B3 and B4 subfamilies of Raflike kinases (RAFs) in early osmotic stress as well as ABA signaling in Arabidopsis thaliana. B2, B3 and B4 RAFs are quickly activated by osmotic stress and are required for phosphorylation and activation of SnRK2s. SNF1-related protein kinase 2s (SnRK2s) are critical for osmotic stress responses[4,5]. Osmotic stress-mediated SnRK2 activation is independent of the ABA signaling pathway[4,5,18,19]. In Arabidopsis, mutants of several members of the B2 and B3 families of Raf-like protein kinases such as ctr[1], raf[10] and raf[11], are insensitive to ABA26,27, and the sis[8] mutant is hypersensitive to salt stress[28].
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