The Secret Life of Chloroplast Precursor Proteins in the Cytosol

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 chloroplasts and the massive transfer of their genetic information to the nucleus of the host cell, many protein complexes in the chloroplasts are constituted from 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 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, its interaction with proteostasis of the cytosol, and its retrograde control over nuclear gene expression. We also discuss significant issues and perspectives for future studies and potential applications for improving the photosynthetic performance and stress tolerance of crops.

New Insights into the Chloroplast Outer Membrane Proteome and Associated Targeting Pathways

Plastids are a dynamic class of organelle in plant cells that arose from an ancient cyanobacterial endosymbiont. Over the course of evolution, most genes encoding plastid proteins were transferred to the nuclear genome. In parallel, eukaryotic cells evolved a series of targeting pathways and complex proteinaceous machinery at the plastid surface to direct these proteins back to their target organelle. Chloroplasts are the most well-characterized plastids, responsible for photosynthesis and other important metabolic functions. The biogenesis and function of chloroplasts rely heavily on the fidelity of intracellular protein trafficking pathways. Therefore, understanding these pathways and their regulation is essential. Furthermore, the chloroplast outer membrane proteome remains relatively uncharted territory in our understanding of protein targeting. Many key players in the cytosol, receptors at the organelle surface, and insertases that facilitate insertion into the chloroplast outer membrane remain elusive for this group of proteins. In this review, we summarize recent advances in the understanding of well-characterized chloroplast outer membrane protein targeting pathways as well as provide new insights into novel targeting signals and pathways more recently identified using a bioinformatic approach. As a result of our analyses, we expand the known number of chloroplast outer membrane proteins from 117 to 138.

The SnRK2.10 kinase mitigates the adverse effects of salinity by protecting photosynthetic machinery

SNF1-Related protein kinases Type 2 (SnRK2) are plant-specific enzymes widely distributed across the plant kingdom. They are key players controlling abscisic acid (ABA)-dependent and ABA-independent signaling pathways in the plant response to osmotic stress. Here we established that SnRK2.4 and SnRK2.10, ABA-nonactivated kinases, are activated in Arabidopsis thaliana rosettes during the early response to salt stress and contribute to leaf growth retardation under prolonged salinity but act by maintaining different salt-triggered mechanisms. Under salinity, snrk2.10 insertion mutants were impaired in the reconstruction and rearrangement of damaged core and antenna protein complexes in photosystem II (PSII), which led to stronger non-photochemical quenching, lower maximal quantum yield of PSII, and lower adaptation of the photosynthetic apparatus to high light intensity. The observed effects were likely caused by disturbed accumulation and phosphorylation status of the main PSII core and antenna proteins. Finally, we found a higher accumulation of reactive oxygen species (ROS) in the snrk2.10 mutant leaves under a few-day-long exposure to salinity which also could contribute to the stronger damage of the photosynthetic apparatus and cause other deleterious effects affecting plant growth. We found that the snrk2.4 mutant plants did not display substantial changes in photosynthesis. Overall, our results indicate that SnRK2.10 is activated in leaves shortly after plant exposure to salinity and contributes to salt stress tolerance by maintaining efficient photosynthesis and preventing oxidative damage.

Engineered Accumulation of Bicarbonate in Plant Chloroplasts: Known Knowns and Known Unknowns

Heterologous synthesis of a biophysical CO2-concentrating mechanism (CCM) in plant chloroplasts offers significant potential to improve the photosynthetic efficiency of C3 plants and could translate into substantial increases in crop yield. In organisms utilizing a biophysical CCM, this mechanism efficiently surrounds a high turnover rate Rubisco with elevated CO2 concentrations to maximize carboxylation rates. A critical feature of both native biophysical CCMs and one engineered into a C3 plant chloroplast is functional bicarbonate (HCO3 -) transporters and vectorial CO2-to-HCO3 - converters. Engineering strategies aim to locate these transporters and conversion systems to the C3 chloroplast, enabling elevation of HCO3 - concentrations within the chloroplast stroma. Several CCM components have been identified in proteobacteria, cyanobacteria, and microalgae as likely candidates for this approach, yet their successful functional expression in C3 plant chloroplasts remains elusive. Here, we discuss the challenges in expressing and regulating functional HCO3 - transporter, and CO2-to-HCO3 - converter candidates in chloroplast membranes as an essential step in engineering a biophysical CCM within plant chloroplasts. We highlight the broad technical and physiological concerns which must be considered in proposed engineering strategies, and present our current status of both knowledge and knowledge-gaps which will affect successful engineering outcomes.