The soybean root-specific protein kinase GmWNK1 regulates stress-responsive ABA signaling on the root system architecture

In humans, members of the WNK protein kinase family are osmosensitive regulators of cell volume homeostasis and epithelial ion transport, and mutation of these proteins causes a rare inherited form of hypertension due to increased renal NaCI re-absorption. A related class of kinases was recently discovered in plants, but their functions are largely unknown. We have identified a root-specific WNK kinase homolog, GmWNKI, in soybean (Glycine max). GmWNK1 expression was detected in the root, specifically in root cells associated with lateral root formation, and was down-regulated by abscisic acid (ABA), as well as by mannitol, sucrose, polyethylene glycol and NaCI. In vitro and in vivo experiments showed that GmWNKI interacts with another soybean protein, GmCYP707A1, which is a key ABA 8-hydroxylase that functions in ABA catabolism. Furthermore, 35S-GmWNK1 transgenic soybean plants had reduced lateral root number and length compared with wild-type, suggesting a role of GmWNKI in the regulation of root system architecture. We propose that GmWNKI functions to fine-tune ABA-dependent ABA homeostasis, thereby mediating the regulation of the root system architecture by ABA and osmotic signals. The study has revealed a new function of a plant WNK1 gene from the important staple crop soybean, and has identified a new component of a regulatory pathway that is involved not only in ABA signaling, but also in the repression of lateral root formation by an ABA-dependent mechanism distinct from known ABA signaling pathways.