Calmodulin-dependent Activation Of MAP Kinase In Wound Signaling

Abstract

Rapid recognition and signal transduction of mechanical wounding are necessary early events leading to stress resistance in plants. Accelerated propagation of a wound response operates through various signaling molecules, including Ca²⁺, protein phosphorylation, and ROS. However, the roles of these molecules in early steps of wounding signaling are largely unknown. Here we report a novel function of Arabidopsis mitogen-activated protein kinase 8 (MPK8), which connects protein phosphorylation, Ca²⁺ and ROS in the wound-signaling pathway. MPK8 is activated through mechanical wounding, and this activation requires direct binding of CaMs in a Ca²⁺-dependent manner. MPK8 is also phosphorylated and activated by MKK3 in the prototypic kinase cascade, and full activation of MPK8 needs both CaMs and MKK3 in planta. Loss-of- and gain-of-function analysis revealed that the MPK8 pathway negatively regulates ROS accumulation through controlling expression of the Respiratory burst oxidase homolog D gene. These findings suggest that two major activation modes in eukaryotes, Ca²⁺/CaMs and the MAP kinase phosphorylation cascade, converge at MPK8 to maintain an essential part of ROS homeostasis.