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Abstract The Plastid Terminal Oxidase (PTOX) is a chloroplast localized plastoquinone oxygen oxidoreductase suggested to have the potential to act as a photoprotective safety valve for photosynthesis. However, PTOX overexpression in plants has been unsuccessful at inducing photoprotection, and the factors that control its activity remain elusive. Here, we show that significant PTOX activity is induced in response to high light in the model species Eutrema salsugineum and Arabidopsis thaliana. This activation correlates with structural reorganization of the thylakoid membrane. Over-expression of PTOX in mutants of Arabidopsis thaliana perturbed in thylakoid stacking also results in such activity, in contrast to wild type plants with normal granal structure. Further, PTOX activation protects against photoinhibition of Photosystem II and reduces reactive oxygen production under stress conditions. We conclude that structural re-arrangements of the thylakoid membranes, bringing Photosystem II and PTOX into proximity, are both required and sufficient for PTOX to act as a Photosystem II sink and play a role in photoprotection.

Plant proteases are essential enzymes that play key roles during crucial phases of plant life. Some proteases are mainly involved in general protein turnover and recycle amino acids for protein synthesis. Other proteases are involved in cell signalling, cleave specific substrates and are key players during important genetically controlled molecular processes. Cathepsin B is a cysteine protease that can do both because of its exopeptidase and endopeptidase activities. Animal cathepsin B has been investigated for many years, and much is known about its mode of action and substrate preferences, but much remains to be discovered about this potent protease in plants. Cathepsin B is involved in plant development, germination, senescence, microspore embryogenesis, pathogen defence and responses to abiotic stress, including programmed cell death. This review discusses the structural features, the activity of the enzyme and the differences between the plant and animal forms. We discuss its maturation and subcellular localisation and provide a detailed overview of the involvement of cathepsin B in important plant life processes. A greater understanding of the cell signalling processes involving cathepsin B is needed for applied discoveries in plant biotechnology.

Metacaspases are a class of proteases found in plants that have gained attention in recent years due to their involvement in programmed cell death (PCD) and other essential cellular processes. Although structurally homologous to caspases found in animals, metacaspases have distinct properties and functions. There are nine metacaspase genes in the Arabidopsis thaliana genome; these can be type I or type II, and working out the function of each member of the gene family is challenging. In this study, we report the characterisation of one Arabidopsis type II metacaspase, metacaspase-5 (AtMC5; AtMCA-IIb). We detected the expression of AtMC5 only under specific conditions with a strong upregulation by ER stress and oxidative stress at a narrow 6 h time point. Recombinant AtMC5 was successfully purified from E. coli, with the recombinant AtMC5 working optimally at pH 7, using an optimised reaction buffer containing 10 mM calcium chloride together with 15% sucrose. Like other metacaspases, AtMC5 cleaved after arginine residue and demonstrated a substrate preference towards VRPR. Additionally, AtMC5-RFP was shown to be localised in the cytosol and nucleus of transfected cells. We found no evidence of a strong link between AtMC5 and PCD, and the data provide additional insights into the function of metacaspases in plants and will aid in future research toward further understanding their mode of action.

The sustained increase in atmospheric [CO2] over the past two centuries has brought a series of global challenges in plant-environment interactions. However, genetic mechanisms in botanical adaption and feedback to environmental alteration remain elusive. Here we collected and analysed transcriptome sequencing data from leaf tissues of maize Zea mays conditioned under high [CO2] stress for 0, 7 or 14 d. A total of 1390 genes, either up- or down-regulated, differed significantly in these conditions. Gene Ontology (GO) enrichment terms and KEGG metabolism pathways included protein phosphorylation, protein ubiquitination, oxidation-reduction, plant organ development, cellular response to endogenous response and MAPK signaling. We also identified three densely connected gene clusters: TCP/PYE, WRKYs and MYC/JAZ from significant DEGs. In conclusion, we provided a genetic database for biologists in botany, agronomy and ecology to further investigate the molecular machinery by which C4-plants respond to elevated atmospheric [CO2] and accompanying global climate change.