Chloroplast Autophagy and Ubiquitination Combine to Manage Oxidative Damage and Starvation Responses

Autophagy and the ubiquitin-proteasome system are the major degradation processes for intracellular components in eukaryotes. Although ubiquitination acts as a signal inducing organelle-targeting autophagy, the interaction between ubiquitination and autophagy in chloroplast turnover has not been addressed. In this study, we found that two chloroplast-associated E3 enzymes, SUPPRESSOR OF PPI1 LOCUS1 and PLANT U-BOX4 (PUB4), are not necessary for the induction of either piecemeal autophagy of chloroplast stroma or chlorophagy of whole damaged chloroplasts in Arabidopsis (Arabidopsis thaliana). Double mutations of an autophagy gene and PUB4 caused synergistic phenotypes relative to single mutations. The double mutants developed accelerated leaf chlorosis linked to the overaccumulation of reactive oxygen species during senescence and had reduced seed production. Biochemical detection of ubiquitinated proteins indicated that both autophagy and PUB4-associated ubiquitination contributed to protein degradation in the senescing leaves. Furthermore, the double mutants had enhanced susceptibility to carbon or nitrogen starvation relative to single mutants. Together, these results indicate that autophagy and chloroplast-associated E3s cooperate for protein turnover, management of reactive oxygen species accumulation, and adaptation to starvation.