Plants are subjected to fluctuations in light intensity, and this causes unbalanced photosynthetic electron fluxes and overproduction of reactive oxygen species (ROS). Electrons needed for ROS detoxification are drawn, at least partially, from the cellular glutathione (GSH) pool via the ascorbate-glutathione cycle. Here, we explore the dynamics of the chloroplastic glutathione redox potential (chl-EGSH) using high-temporal-resolution monitoring of Arabidopsis (Arabidopsis thaliana) lines expressing the reduction-oxidation sensitive green fluorescent protein 2 (roGFP2in chloroplasts. This was carried out over several days, under dynamic environmental conditions and in correlation with PSII operating efficiency. Peaks in chl-EGSH oxidation during dark-to-light and light-to-dark transitions were observed. Increasing light intensities triggered a binary oxidation response, with a threshold around the light saturating point, suggesting two regulated oxidative states of the chl-EGSH. These patterns were not affected in npq1 plants, which are impaired in nonphotochemical quenching. Oscillations between the two oxidation states were observed under fluctuating light in WT and npq1 plants, but not in pgr5 plants, suggesting a role for PSI photoinhibition in regulating the chl-EGSH dynamics. Remarkably, pgr5 plants showed an increase in chl-EGSH oxidation during the nights following light stresses, linking daytime photoinhibition and nighttime GSH metabolism. This work provides a systematic view of the dynamics of the in vivo chloroplastic glutathione redox state during varying light conditions.

Plants are naturally subjected to fluctuations in light intensity, causing unbalanced photosynthetic electron fluxes and overproduction of reactive oxygen species (ROS). While high rates of ROS production are harmful, moderate levels play a signaling role, coordinating photosynthetic activity and downstream metabolism.Here, we explore the dynamics of light-dependent oxidant production by hightemporal-resolution monitoring of chloroplastic glutathione redox potential (chl-E GSH ) using chloroplast-targeted-roGFP2-expressing Arabidopsis lines, over several days, under dynamic environmental conditions and in correlation with PSII operating efficiency. Peaks in chl-E GSH oxidation during light-darkness transitions, when light harvesting is not balanced with downstream metabolism, were observed. Increasing light intensities triggered a binary oxidation response, with a threshold around the light saturating point, pointing for two regulated oxidative states of the chl-E GSH .These patterns were not affected in npq1 plants which are impaired in nonphotochemical quenching. Frequency-dependent oscillations between the two oxidation states were observed under fluctuating light in WT and npq1 plants, but not in pgr5 plants, suggesting a role for PSI photoinhibition in regulation of oxidant production. Remarkably, pgr5 plants showed an increase in chl-E GSH oxidation during the nights following light stresses, linking between day photoinhibition and night redox metabolism. This work provides a comprehensive view on the balance between photosynthesis-dependent ROS production and antioxidant activity during light acclimation. Asada K (1999) The water-water cycle in chloroplasts: scavenging of active oxygens and dissipation of excess photons. Annu Rev Plant Physiol Plant Mol Biol 50: 601-639 Avenson TJ, Cruz JA, Kanazawa A, Kramer DM (2005) Regulating the proton budget of higher plant photosynthesis. Proc Natl Acad Sci U S A 102: 9709 LP -9713 Awad J, Stotz HU, Fekete A, Krischke M, Engert C, Havaux M, Berger S, Mueller MJ (2015) 2-Cysteine peroxiredoxins and thylakoid ascorbate peroxidase create a waterwater cycle that is essential to protect the photosynthetic apparatus under high light stress conditions. Plant Physiol 167: 1592 LP -1603 Biehler K, Fock H (1996) Evidence for the contribution of the mehler-peroxidase reaction in dissipating excess electrons in drought-stressed wheat. Plant Physiol 112: 265-272 Bratt A, Rosenwasser S, Meyer A, Fluhr R (2016) Organelle redox autonomy during environmental stress. Plant Cell Environ 39: 1909-1919 Buchanan BB, Balmer Y (2005) Redox regulation: a broadening horizon. Annu Rev Plant Biol 56: 187-220 Clough SJ, Bent AF (1998) Floral dip: a simplified method for Agrobacterium -mediated transformation of Arabidopsis thaliana. Plant J 16: 735-743 van Creveld SG, Rosenwasser S, Schatz D, Koren I, Vardi A (2015) Early perturbation in mitochondria redox homeostasis in response to environmental stress predicts cell fate in diatoms. ISME J 9: 385-395 Dangoor I, Peled-Zehavi H, Wittenberg G, Da...

Changes in climate conditions can negatively affect the productivity of crop plants. They can induce chloroplast degradation (senescence), which leads to decreased source capacity, as well as decreased whole-plant carbon/nitrogen assimilation and allocation. The importance, contribution and mechanisms of action regulating source-tissue capacity under stress conditions in tomato (Solanum lycopersicum) are not well understood. We hypothesized that delaying chloroplast degradation by altering the activity of the tomato CHLOROPLAST VESICULATION (CV) under stress would lead to more efficient use of carbon and nitrogen and to higher yields. Tomato CV is upregulated under stress conditions. Specific induction of CV in leaves at the fruit-development stage resulted in stress-induced senescence and negatively affected fruit yield, without any positive effects on fruit quality. CRISPR/CAS9 knockout CV plants, generated using a near-isogenic tomato line with enhanced sink capacity, exhibited stress tolerance at both the vegetative and the reproductive stages, leading to enhanced fruit quantity, quality and harvest index. Detailed metabolic and transcriptomic network analysis of sink tissue revealed that the L-glutamine and L-arginine biosynthesis pathways are associated with stress-response conditions and also identified putative novel genes involved in tomato fruit quality under stress. Our results are the first to demonstrate the feasibility of delayed stress-induced senescence as a stress-tolerance trait in a fleshy fruit crop, to highlight the involvement of the CV pathway in the regulation of source strength under stress, and to identify genes and metabolic pathways involved in increased tomato sink capacity under stress conditions.

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Soil biosolarization (SBS) is an alternative technique for soil pest control to standard techniques such as soil fumigation and soil solarization (SS). By using both solar heating and fermentation of organic amendments, faster and more effective control of soilborne pathogens can be achieved. A circular economy may be created by using the residues of a given crop as organic amendments to biosolarize fields that produce that crop, which is termed circular soil biosolarization (CSBS). In this study, CSBS was employed by biosolarizing soil with amended tomato pomace (TP) residues and examining its impact on tomato cropping under conditions of abiotic stresses, specifically high salinity and nitrogen deficiency. The results showed that in the absence of abiotic stress, CSBS can benefit plant physiological performance, growth and yield relative to SS. Moreover, CSBS significantly mitigated the impacts of abiotic stress conditions. The results also showed that CSBS impacted the soil microbiome and plant metabolome. Mycoplana and Kaistobacter genera were found to be positively correlated with benefits to tomato plants health under abiotic stress conditions. Conversely, the relative abundance of the orders RB41, MND1, and the family Ellin6075 and were negatively correlated with tomato plants health. Moreover, several metabolites were significantly affected in plants grown in SS- and CSBS-treated soils under abiotic stress conditions. The metabolite xylonic acid isomer was found to be significantly negatively correlated with tomato plants health performance across all treatments. These findings improve understanding of the interactions between CSBS, soil ecology, and crop physiology under abiotic stress conditions.