Plastid gene expression is required for singlet oxygen-induced chloroplast degradation and cell death

Summary

Chloroplasts constantly experience photo-oxidative stress while performing photosynthesis. This is particularly true under abiotic stresses that lead to the accumulation of reactive oxygen species (ROS). While ROS leads to the oxidation of DNA, proteins, and lipids, it can also act as a signal to induce acclimation through chloroplast degradation, cell death, and nuclear gene expression. Although the mechanisms behind ROS signaling from chloroplasts remain mostly unknown, several genetic systems have been devised in the model plant Arabidopsis to understand their signaling properties. One system uses the plastid ferrochelatase two (fc2) mutant that conditionally accumulates the ROS singlet oxygen (¹O₂) leading to chloroplast degradation and eventually cell death. Here we have mapped three mutations that suppress chloroplast degradation in the fc2 mutant and demonstrate that they affect two independent loci (PPR30 and mTERF9) encoding chloroplast proteins predicted to be involved in post-transcriptional gene expression. Mutations in either gene were shown to lead to broadly reduced chloroplast gene expression, impaired chloroplast development, and reduced chloroplast stress signaling. In these mutants, however, ¹O₂ levels were uncoupled to chloroplast degradation suggesting that PPR30 and mTERF9 are involved in ROS signaling pathways. In the wild type background, ppr30 and mTERF9 mutants were also observed to be less susceptible to cell death induced by excess light stress. Together these results suggest that plastid gene expression (or the expression of specific plastid genes) is a necessary prerequisite for chloroplasts to activate ¹O₂ signaling pathways to induce chloroplast degradation and/or cell death.