Chloroplast inner envelope protein FtsH11 is involved in the adjustment of assembly of chloroplast ATP synthase under heat stress.

The maintenance of a proton gradient across the thylakoid membrane is an integral aspect of photosynthesis that is mainly established by the splitting of water molecules in photosystem II and plastoquinol oxidation at the cytochrome complex, and it is necessary for the generation of ATP in the last step of photophosphorylation. Although environmental stresses, such as high temperatures, are known to disrupt this fundamental process, only a few studies have explored the molecular mechanisms underlying proton gradient regulation during stress. The present study identified a heat-sensitive mutant that displays aberrant photosynthesis at high temperatures. This mutation was mapped to AtFtsH11, which encodes an ATP-dependent AAA-family metalloprotease. We showed that AtFtsH11 localizes to the chloroplast inner envelope membrane and is capable of degrading the ATP synthase assembly factor BFA3 under heat stress. We posit that this function limits the amount of ATP synthase integrated into the thylakoid membrane to regulate proton efflux from the lumen to the stroma. Our data also suggest that AtFtsH11 is critical in stabilizing photosystem II and cytochrome complexes at high temperatures, and additional studies can further elucidate the specific molecular functions of this critical regulator of photosynthetic thermotolerance.

Plants response to light stress.

Plants require solar energy to grow through oxygenic photosynthesis; however, when light intensity exceeds the optimal range for photosynthesis, it causes abiotic stress and physiological damage in plants. In response to high light stress, plants initiate a series of signal transduction from chloroplasts to whole cells and from locally stressed tissues to the rest of the plant body. These signals trigger a variety of physiological and biochemical reactions intended to mitigate the deleterious effects of high light intensity, such as photodamage and photoinhibition. Light stress protection mechanisms include chloroplastic Reactive oxygen species (ROS) scavenging, chloroplast and stomatal movement, and anthocyanin production. Photosynthetic apparatuses, being the direct targets of photodamage, have also developed various acclimation processes such as thermal energy dissipation through nonphotochemical quenching (NPQ), photorepair of Photosystem II (PSII), and transcriptional regulation of photosynthetic proteins. Fluctuating light is another mild but persistent type of light stress in nature, which unfortunately has been poorly investigated. Current studies, however, suggest that state transitions and cyclic electron transport are the main adaptive mechanisms for mediating fluctuating light stress in plants. Here, we review the current breadth of knowledge regarding physiological and biochemical responses to both high light stress and fluctuating light stress.

Arabidopsis Seedling Lethal 1 Interacting With Plastid-Encoded RNA Polymerase Complex Proteins Is Essential for Chloroplast Development.

Mitochondrial transcription termination factors (mTERFs) are highly conserved proteins in metazoans. Plants have many more mTERF proteins than animals. The functions and the underlying mechanisms of plants' mTERFs remain largely unknown. In plants, mTERF family proteins are present in both mitochondria and plastids and are involved in gene expression in these organelles through different mechanisms. In this study, we screened Arabidopsis mutants with pigment-defective phenotypes and isolated a T-DNA insertion mutant exhibiting seedling-lethal and albino phenotypes [seedling lethal 1 (sl1)]. The SL1 gene encodes an mTERF protein localized in the chloroplast stroma. The sl1 mutant showed severe defects in chloroplast development, photosystem assembly, and the accumulation of photosynthetic proteins. Furthermore, the transcript levels of some plastid-encoded proteins were significantly reduced in the mutant, suggesting that SL1/mTERF3 may function in the chloroplast gene expression. Indeed, SL1/mTERF3 interacted with PAP12/PTAC7, PAP5/PTAC12, and PAP7/PTAC14 in the subgroup of DNA/RNA metabolism in the plastid-encoded RNA polymerase (PEP) complex. Taken together, the characterization of the plant chloroplast mTERF protein, SL1/mTERF3, that associated with PEP complex proteins provided new insights into RNA transcription in the chloroplast.