The rice MAPKK-MAPK interactome: the biological significance of MAPK components in hormone signal transduction

Mitogen-activated protein kinase (MAPK) signaling cascades are evolutionarily conserved fundamental signal transduction pathways. A MAPK cascade consists of many distinct MAPKKK-MAPKK-MAPK modules linked to various upstream receptors and downstream targets through sequential phosphorylation and activation of the cascade components. These cascades collaborate in transmitting a variety of extracellular signals and in controlling cellular responses and processes such as growth, differentiation, cell death, hormonal signaling, and stress responses. Although MAPK proteins play central roles in signal transduction pathways, our knowledge of MAPK signaling in hormonal responses in rice has been limited to a small subset of specific upstream and downstream interacting targets. However, recent studies revealing direct MAPK and MAPKK interactions have provided the basis for elucidating interaction specificities, functional divergence, and functional modulation during hormonal responses. In this review, we highlight current insights into MAPKK-MAPK interaction patterns in rice, with emphasis on the biological significance of these interacting pairs in SA (salicylic acid), JA (jasmonic acid), ET (ethylene), and ABA (abscisic acid) responses, and discuss the challenges in understanding functional signal transduction networks mediated by these hormones.