Roles for chloroplast-localized PPR Protein 30 and the “Mitochondrial” Transcription Termination Factor 9 in chloroplast quality control

Abstract

Chloroplasts constantly experience photo-oxidative stress while performing photosynthesis. This is particu- larly true under abiotic stresses that lead to the accumulation of reactive oxygen species (ROS) which oxi- dize DNA, proteins and lipids. Reactive oxygen species can also act as signals to induce acclimation through chloroplast degradation, cell death and nuclear gene expression. To better understand the mechanisms behind ROS signaling from chloroplasts, we have used the Arabidopsis thaliana mutant plastid fer- rochelatase two (fc2) that conditionally accumulates the ROS singlet oxygen (1O2) leading to chloroplast degradation and eventually cell death. Here we have mapped mutations that suppress chloroplast degrada- tion 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. These mutants exhib- ited broadly reduced chloroplast gene expression, impaired chloroplast development and reduced chloro- plast stress signaling. Levels of 1O2, however, could be uncoupled from 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. While broad inhibition of plastid transcription with rifampicin was also able to suppress cell death in fc2 mutants, specific reductions in plastid gene expression using other mutations was not always sufficient. Together these results suggest that plastid gene expression, or the expression of specific plastid genes by PPR30 and mTERF0, is a necessary prerequisite for chloroplasts to activate the 1O2 signaling pathways to induce chloroplast quality control pathways and/or cell death.