Chlorophyllase (Chlase, CLH) is one of the earliest discovered enzymes present in plants and green algae. It was long considered to be the first enzyme involved in chlorophyll (Chl) degradation, while strong evidence showed that it is not involved in Chl breakdown during leaf senescence. On the other hand, it is possible that CLH is involved in Chl breakdown during fruit ripening. Recently, it was discovered that Arabidopsis CLH1 is located in developing chloroplasts but not in mature chloroplasts, and it plays a role in protecting young leaves from long-term photodamage by catalysing Chl turnover in the photosystem II (PSII) repair cycle. However, there remain other important questions related to CLH. In this article, we briefly reviewed the research progress on CLH and listed the main unanswered questions related to CLH for further study.

The J-proteins, also called DNAJ-proteins or heat shock protein 40 (HSP40), are one of the famous molecular chaperones. J-proteins, HSP70s and other chaperones work together as constitute ubiquitous types of molecular chaperone complex, which function in a wide variety of physiological processes. J-proteins are widely distributed in major cellular compartments. In the chloroplast of higher plants, around 18 J-proteins and multiple J-like proteins are present; however, the functions of most of them remain unclear. During the last few years, important progress has been made in the research on their roles in plants. There is increasing evidence that the chloroplast J-proteins play essential roles in chloroplast development, photosynthesis, seed germination and stress response. Here, we summarize recent research advances on the roles of J-proteins in the chloroplast, and discuss the open questions that remain in this field.

Group II introns are large catalytic RNAs (ribozymes) in the bacteria and organelle genomes of several lower eukaryotes. Many critical photosynthesis-related genes in the plant chloroplast genome also contain group II introns, and their splicing is critical for chloroplast biogenesis and photosynthesis processes. The structure of chloroplast group II introns was altered during evolution, resulting in the loss of intron self-splicing. Therefore, the assistance of protein factors was required for their splicing processes. As an increasing number of studies focus on the mechanism of chloroplast intron splicing; many new nuclear-encoded splicing factors that are involved in the chloroplast intron splicing process have been reported. This report reviewed the research progress of the updated splicing factors found to be involved in the splicing of chloroplast group II introns. We discuss the main problems that remain in this research field and suggest future research directions.

Water shortages caused by climate change seriously threaten the survival and production of plants and are also one of the major environmental pressures faced by plants. DORN1 was the first identified purinoceptor for the plant response to extracellular ATP. It has been established that DORN1 could play key roles in a series of biological activities in plants. However, the biological roles of DORN1 and the mechanism remain unclear under drought stress conditions in plants. Here, DORN1 was targeted for knockout by using the CRISPR/Cas 9 system. It was found that the loss function of DORN1 resulted in a significant decrease in the effective quantum yield of PSII [Y(II)], the photochemical quenching coefficient (qP), and the rate of photosynthetic electron transport through PSII (ETR), which reflected plants’ photochemical efficiency. Whereas Y_(NO) values showed obvious enhancement under drought stress conditions. Further experimental results showed that the Y_(II), q_P, and ETR, which reflect plants’ photochemical efficiency, increased significantly with CaCl₂ treatment. These results indicated that the drought tolerance of the mutant was decreased, and the exogenous application of calcium ions could effectively promote the drought tolerance of the dorn1 mutant. Transpiration loss controlled by stomata is closely related to drought tolerance, further, we examined the transpirational water loss in dorn1 and found that it was greater than wild-type (WT). Besides, the dorn1 mutant’s stomatal aperture significantly increased compared with the WT and the stomata of dorn1 mutant plants tend to close after CaCl₂ treatment. Taken together, our results show that DORN1 plays a key role in drought stress tolerance in plants, which may depend on calcium and calcium-related signaling pathways.

Chloroplast iron–sulfur (Fe–S) clusters, essential cofactors for Fe–S proteins, are assembled by the sulfur utilization factor (SUF) BC2D complex, but regulation of their biogenesis remains unclear. We show that during leaf senescence, SUFB transcription rapidly increases while SUFC/SUFD transcripts and the abundance of all three SUF proteins remain unchanged. Leaf necrosis with green color occurs solely when SUFB RNA interference (RNAi) is induced, and the decrease of SUFs is obviously faster in SUFB-RNAi than SUFC-RNAi lines. Furthermore, SUFB exhibits a higher turnover rate than SUFC/SUFD, which accelerates during senescence. Overexpressing SUFB elevates SUFC/SUFD/SUFBC2D levels, revealing that SUFB stabilizes SUFC/SUFD via SUFBC2D complex formation, a function SUFC lacks. Under iron deficiency, SUFB transcript level declines, whereas SUFC/SUFD transcript levels remain unchanged. Nevertheless, abundance of SUFs decreases, suggesting that the reduced amount of SUFBC2D is mediated by the down-regulation of SUFB transcription. We subsequently found that SUFs accumulated in a caseinolytic protease (CLP)-impaired mutant and exhibited resistance to degradation when cytoplasmic translation was inhibited by cycloheximide, revealing that their turnover is mediated by CLP. Moreover, SUFB, SUFC, and SUFD physically interact with CLPS1, a substrate adaptor of the CLP complex. Collectively, SUFB expression and CLP coordinately regulate Fe–S biogenesis by controlling SUFBC2D abundance, providing critical insights into plant adaption to leaf senescence and iron deficiency.