Role of AGC kinases in plant growth and stress responses

Abstract

AGC kinases are important regulators of cell growth, metabolism, division, and survival in mammalian systems. Mutation or deregulation of members of this family of protein kinases contribute to the pathogenesis of many human diseases, including cancer and diabetes. Although AGC kinases are conserved in the plant kingdom, little is known about their molecular functions and targets. Some of the best-studied plant AGC kinases mediate auxin signaling and are thereby involved in the regulation of growth and morphogenesis. Furthermore, certain members are regulated by lipid-derived signals via the 3-phosphoinositide-dependent kinase 1 (PDK1) and the kinase target of rapamycin (TOR), similar to its animal counterparts. In this review, we discuss recent findings on plant AGC kinases that unravel important roles in the regulation of plant growth, immunity and cell death, and connections to stress-induced mitogen-activated protein kinase signaling cascades.

Fig. 1

Schematic representation of AGC kinase signaling cascades in plant immune responses. a The tomato AGC kinase Adi3 is regulated by the PDK1 and Pto kinases to negatively modulate disease resistance and programmed cell death (PCD). Two parallel kinase pathways mediated by MAPKKKα and Pti1 kinases are activated downstream of Pto and also participate in the modulation of resistance and PCD. b The Arabidopsis AGC kinase OXI1 is activated by PDK1 and H₂O₂ and is required for disease resistance. OXI1 modulates the stress-induced activation of the MAPKs MPK3 and MPK6, putatively through its downstream kinase Pti1-4. c In rice, a phosphorylation cascade including OsPDK1, the AGC kinase OsOXI1 and the downstream kinase OsPti1a was shown to positively regulate disease resistance and cell death in response to H₂O₂ and pathogen recognition. Filled arrows represent direct regulation. Dashed arrows represent processes comprising various or unknown steps. Double-headed arrows represent protein association. PA phosphatidic acid