Light activates the translational regulatory GCN2 kinase via reactive oxygen species emanating from the chloroplast

Abstract

Cytosolic mRNA translation is subject to global and mRNA-specific controls. Phosphorylation of translation initiation factor eIF2 anchors a reversible switch that represses translation globally. The stress-responsive GCN2 kinase is the only known kinase for eIF2 in Arabidopsis. Here we show that conditions that generate reactive oxygen species (ROS) in the chloroplast, such as dark-light transitions, high light, and the herbicide methyl viologen all rapidly activated the GCN2 kinase, whereas mitochondrial and ER stress did not. In addition, GCN2 activation was light dependent and mitigated by photosynthesis inhibitors and ROS quenchers. Accordingly, seedling growth of multiple gcn2 mutant alleles was retarded under conditions of excess light, implicating the GCN2-eIF2 pathway in responses to light and associated ROS. Once activated, the GCN2 kinase preferentially suppressed the ribosome loading of mRNAs for functions such as mitochondrial ATP synthesis, the chloroplast thylakoids, vesicle trafficking, and translation. The transcriptome of gcn2 mutants was sensitized to abiotic stress, including oxidative stress, as well as innate immune responses. Accordingly, gcn2 displayed defects in immune priming by the fungal elicitor, chitin. In conclusion, we provide evidence that reactive oxygen species produced by the photosynthetic apparatus help to activate the highly conserved GCN2 kinase, leading to eIF2 phosphorylation and thus affecting the status of the cytosolic protein synthesis apparatus.