The role of ion disequilibrium in induction of root cell death and autophagy by environmental stresses.

Environmental stresses such as salinity, drought, oxidants, heavy metals, hypoxia, extreme temperatures and others can induce autophagy and necrosis-type programmed cell death (PCD) in plant roots. These reactions are accompanied by the generation of reactive oxygen species (ROS) and ion disequilibrium, which is induced by electrolyte/K+ leakage through ROS-activated ion channels, such as the outwardly-rectifying K+ channel GORK and non-selective cation channels. Here, we discuss mechanisms of the stress-induced ion disequilibrium and relate it with ROS generation and onset of morphological, biochemical and genetic symptoms of autophagy and PCD in roots. Based on our own data and that in the literature, we propose a hypothesis on the induction of autophagy and PCD in roots by loss of cytosolic K+. To support this, we present data showing that in conditions of salt stress-induced autophagy, gork1-1 plants lacking root K+ efflux channel have fewer autophagosomes compared with the wild type. Overall, literature analyses and presented data strongly suggest that stress-induced root autophagy and PCD are controlled by the level of cytosolic potassium and ROS.