SnRK2 protein kinases--key regulators of plant response to abiotic stresses

Abstract

The SnRK2 family members are plant-specific serine/threonine kinases involved in plant response to abiotic stresses and abscisic acid (ABA)-dependent plant development. SnRK2s have been classed into three groups; group 1 comprises kinases not activated by ABA, group 2 comprises kinases not activated or activated very weakly by ABA, and group 3 comprises kinases strongly activated by ABA. So far, the ABA-dependent kinases belonging to group 3 have been studied most thoroughly. They are considered major regulators of plant response to ABA. The regulation of the plant response to ABA via SnRK2s pathways occurs by direct phosphorylation of various downstream targets, for example, SLAC1, KAT1, AtRbohF, and transcription factors required for the expression of numerous stress response genes. Members of group 2 share some cellular functions with group 3 kinases; however, their contribution to ABA-related responses is not clear. There are strong indications that they are positive regulators of plant responses to water deficit. Most probably they complement the ABA-dependent kinases in plant defense against environmental stress. So far, data concerning the physiological role of ABA-independent SnRK2s are very limited; it is to be expected they will be studied extensively in the nearest future.

FIG. 1.

Phylogenetic tree of selected higher plant SnRK2s. Group 1— ABA-independent kinases, group 2—kinases not dependent or weakly dependent on ABA, group 3—ABA-dependent kinases. The unrooted UPGMA (unweighted pair group method with arithmetic mean) tree was built using ClustalW2 and presented using TreeView.

FIG. 2.

Structure of Arabidopsis thaliana SnRK2s. (A) Model of SnRK2 structure. Ser/Thr kinase domain—kinase catalytic domain, I—domain involved in ABA-independent activation in response to osmotic stress, characteristic for all SnRK2, II—domain needed for ABA-dependent activation of SnRK2s. (B) Alignment of Arabidopsis thaliana SnRK2s with characteristic domains marked: activation loop (green box), domain I (violet box), and domain II (blue box). Gaps introduced to maximize alignment are marked with dashes.

FIG. 3.

Simplified model of SnRK2s pathways involved in plant response to osmotic stress. Model was done based on published and unpublished data described in the main text. ABA, abscisic acid; NO, nitric oxide; PA, phosphatidic acid; ROS, reactive oxygen species; unknown SnRK2K(s), an SnRK2 upstream kinase(s)— still unknown; unknown PP, an unknown phosphoprotein phosphatase dephosphorylating and inactivating ABA-independent SnRK2s; clade A PP2C, clade A PP2C phosphoprotein phosphatase; SCS, SnRK2-interacting calcium sensor; PYR/PYL/RCAR, soluble ABA receptor; Rboh, plasma membrane NADPH oxidase; SLAC1, Slow Anion Channel 1; KAT1, inward-rectifying potassium channel; TFs, transcription factors. Connections represent positive (arrow) and negative (block) regulation. Bold line—direct regulation/interaction, dashed line—indirect regulation. Question marks indicate currently unknown or not confirmed regulation.