Abstract
Reversible phosphorylation is a major mechanism for regulating protein function and controls a wide range of cellular functions including responses to external stimuli. The plant-specific SNF1-related protein kinase 2s (SnRK2s) function as central regulators of plant growth and development, as well as tolerance to multiple abiotic stresses. Although the activity of SnRK2s is tightly regulated in a phytohormone abscisic acid (ABA)-dependent manner, recent investigations have revealed that SnRK2s can be activated by group B Raf-like protein kinases independently of ABA. Furthermore, evidence is accumulating that SnRK2s modulate plant growth through regulation of target of rapamycin (TOR) signaling. Here, we summarize recent advances in knowledge of how SnRK2s mediate plant growth and osmotic stress signaling and discuss future challenges in this research field.
Highlights
Throughout their lifecycles, plants are sometimes exposed to water-deficit conditions such as drought, salinity and freezing
P. patens ABIOTIC STRESS-RESPONSIVE RAF-LIKE KINASE (ARK) is rapidly activated after abscisic acid (ABA) treatment [133], while no clear ABA-induced activation of Arabidopsis group B Raf were observed [24,117]
This difference suggests that the regulatory mechanisms upstream of Raf-like kinases may have been altered during evolution
Summary
Throughout their lifecycles, plants are sometimes exposed to water-deficit conditions such as drought, salinity and freezing. When exposed to water-deficit conditions, plants accumulate higher levels of ABA, which in turn induces protective stress responses via ABA-dependent or -independent pathways, both of which trigger activation of multiple SnRK2s [11,12,13] Following their activation, SnRK2s transduce the stress signal through phosphorylation of various downstream substrates, including transcription factors and ion channels. Several additional studies revealed the essential roles of group B Raf-like protein kinases in ABA-independent SnRK2 activation in response to osmotic stress responses in Arabidopsis [21,22,23,24] In addition to these new connections between SnRK2s and Raflike kinases, recent data shows that SnRK2-mediated signaling regulates target of rapamycin (TOR), a master regulator that orchestrates cell proliferation and growth by integrating nutrient, energy, hormone and diverse stress stimuli [25,26,27]. We highlight how SnRK2s function to maintain a balance between growth promotion and stress responses
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