Qihua Ling

Summary: Photosynthesis generates reactive oxygen species that damage photosynthetic machinery. Singlet oxygen ( ¹ O ₂ ) produced at photosystem II (PSII) damages PSII proteins, with D1 as a primary target due to its proximity to the site of ¹ O ₂ generation. During PSII repair, D1 is degraded by the FtsH metalloprotease complex, followed by resynthesis and PSII reassembly. Oxidation of a conserved tryptophan residue (Trp14) in N‐terminal D1 enhances its recognition and degradation by FtsH, serving as a regulatory signal under photooxidative stress. The FtsH complex also undergoes turnover, but the mechanism remains unclear. In Arabidopsis thaliana , the FtsH complex comprises four isomers (FtsH1/2/5/8), grouped into types A and B. We identified a conserved, oxidation‐prone Trp residue in type B isomers (FtsH2/8). Using transgenic plants expressing FtsH2 variants with Trp substitutions, we found that Trp oxidation affects FtsH2 stability and its ability to degrade D1. In ftsh2 ftsh8 double mutants, expressing FtsH2 with a Trp‐to‐phenylalanine substitution (oxidation‐mimicking) failed to sustain growth and did not survive to reproduction, whereas a Trp‐to‐leucine substitution (oxidation‐insensitive) enhanced PSII repair and improved growth under conditions tested. These findings demonstrate that oxidative modification influences FtsH2 function and provide mechanistic insight into how redox‐sensitive residues affect PSII repair and chloroplast protein homeostasis.

Dear Editor,

Macroautophagy/autophagy is a conserved process in eukaryotes responsible for degrading unwanted or damaged macromolecules and organelles through the lysosome or vacuole for recycling and reutilization. Our previous studies revealed the degradation of chloroplast proteins through a pathway dependent on the ubiquitin proteasome system, known as CHLORAD. Recently, we demonstrated a role for selective autophagy in regulating chloroplast protein import and enhancing stress tolerance in plants. Specifically, we found that K63-ubiquitination of TOC components at the chloroplast outer envelope membrane is recognized by the selective autophagy adaptor NBR1, leading to the degradation of TOC proteins under UV-B irradiation and heat stresses in Arabidopsis. This process was shown to control chloroplast protein import and influence photosynthetic activity. Based on our results, we have, for the first time, demonstrated that selective autophagy plays a vital role in chloroplast protein degradation, specifically in response to certain abiotic stresses.

Chloroplasts are plant organelles responsible for photosynthesis and environmental sensing. Most chloroplast proteins are imported from the cytosol through the translocon at the outer envelope membrane of chloroplasts (TOC). Previous work has shown that TOC components are regulated by the ubiquitin-proteasome system (UPS) to control the chloroplast proteome, which is crucial for the organelle's function and plant development. Here, we demonstrate that the TOC apparatus is also subject to K63-linked polyubiquitination and regulation by selective autophagy, potentially promoting plant stress tolerance. We identify NBR1 as a selective autophagy adaptor targeting TOC components, and mediating their relocation into vacuoles for autophagic degradation. Such selective autophagy is shown to control TOC protein levels and chloroplast protein import and to influence photosynthetic activity as well as tolerance to UV-B irradiation and heat stress in Arabidopsis plants. These findings uncover the vital role of selective autophagy in the proteolytic regulation of specific chloroplast proteins, and how dynamic control of chloroplast protein import is critically important for plants to cope with challenging environments.

Leaf color mutants are ideal materials for studying the regulatory mechanism of chloroplast development and photosynthesis. We isolated a cucumis melo spontaneous mutant (MT), which showed yellow-green leaf phenotype in the whole growing period and could be inherited stably. We compared its leaves with the wild type (WT) in terms of cytology, physiology, transcriptome and metabolism. The results showed that the thylakoid grana lamellae of MT were loosely arranged and fewer in number than WT. Physiological experiments also showed that MT had less chlorophyll content and more accumulation of reactive oxygen species (ROS) than WT. Furthermore, the activity of several key enzymes in C₄ photosynthetic carbon assimilation pathway was more enhanced in MT than WT. Transcriptomic and metabolomic analyses showed that differential expression genes and differentially accumulated metabolites in MT were mainly co-enriched in the pathways related to photosystem-antenna proteins, central carbon metabolism, glutathione metabolism, phenylpropanoid biosynthesis and flavonoid metabolism. We also analyzed several key proteins in photosynthesis and chloroplast transport by Western blot. In summary, the results may provide a new insight into the understanding of how plants respond to the impaired photosynthesis by regulating chloroplast development and photosynthetic carbon assimilation pathways.

Photosynthesis is the energetic basis for most life on Earth, and in plants it operates inside double membrane–bound organelles called chloroplasts. The photosynthetic apparatus comprises numerous proteins encoded by the nuclear and organellar genomes. Maintenance of this apparatus requires the action of internal chloroplast proteases, but a role for the nucleocytosolic ubiquitin-proteasome system (UPS) was not expected, owing to the barrier presented by the double-membrane envelope. Here, we show that photosynthesis proteins (including those encoded internally by chloroplast genes) are ubiquitinated and processed via the CHLORAD pathway: They are degraded by the 26 S proteasome following CDC48-dependent retrotranslocation to the cytosol. This demonstrates that the reach of the UPS extends to the interior of endosymbiotically derived chloroplasts, where it acts to regulate photosynthesis, arguably the most fundamental process of life.

Chloroplasts are defining organelles in plant and algae, which carried out various critical metabolic processes, including photosynthesis. Roles of chloroplast protein homeostasis in plant development and stress adaptation were clearly demonstrated in previous studies, and its maintenance requires internal proteases originated from the prokaryotic ancestor. Recently, increasing evidence revealed that eukaryotic proteolytic pathways, ubiquitin-proteasome system (UPS) and autophagy, are also involved in the turnover of chloroplast proteins, in response to developmental and environmental cues. Notably, chloroplasts can be regulated via the selective degradation of chloroplast materials in a process called chlorophagy. Yet, understandings of the mechanism of chlorophagy are still rudimentary, particularly regarding its initiation and operation. Here we provide an updated overview of autophagy pathways for chloroplast protein degradation and discuss their importance for plant physiology. In addition, recent advance in analogous mitophagy in yeast and mammals will also be discussed, which provides clues for further elucidating the mechanism of chlorophagy.

Photosynthesis is the energetic basis for most life on Earth, and in plants it operates inside double-membrane-bound organelles called chloroplasts. The photosynthetic apparatus comprises numerous proteins encoded by the nuclear and organellar genomes. Maintenance of this apparatus requires the action of internal chloroplast proteases, but a role for the nucleocytosolic ubiquitin-proteasome system (UPS) was not expected owing to the barrier presented by the double-membrane envelope. Here, we show that photosynthesis proteins (including those encoded internally by chloroplast genes) are ubiquitinated, and processed via the CHLORAD pathway: they are degraded by the 26S proteasome following CDC48-dependent retrotranslocation to the cytosol. This demonstrates that the reach of the UPS extends to the interior of endosymbiotically-derived chloroplasts, where it acts to regulate one of the most fundamental processes of life.