Chloroplast proteostasis: A story of birth, life and death

Protein homeostasis (proteostasis) is a dynamic balance of protein synthesis and degradation. Because of the endosymbiotic origin of the chloroplasts and the massive transfer of their genetic information to the nucleus of the host cell, many protein complexes in the chloroplasts are constituted by subunits encoded by both genomes. Hence, the proper function of chloroplasts relies on the coordinated expression of chloroplast- and nucleus-encoded genes. The biogenesis and maintenance of chloroplast proteostasis are dependent on the synthesis of chloroplast-encoded proteins, import of nucleus-encoded chloroplast proteins from the cytosol, and clearance of damaged or otherwise undesired "old" proteins. This review focuses on the regulation of chloroplast proteostasis, the interaction with the proteostasis of the cytosol and its retrograde control to the nuclear gene expression. We also discuss the significant issues and perspectives for the future studies, and the potential applications to improve the photosynthetic performance and stress tolerance of crops.