Photorespiration Alleviates Photoinhibition of Photosystem I under Fluctuating Light in Tomato

The Kinetics of Mesophyll Conductance and Photorespiration During Light Induction

Mesophyll conductance to CO2 (gm) act as a significant limiting factor influencing the CO2 assimilation rate (AN) during photosynthetic induction. However, the effect of vapor pressure deficit (VPD) on gm kinetics during light induction is not well clarified. We combined gas exchange with chlorophyll fluorescence measurements to assess the induction kinetics of gm during light induction under contrasting vapor pressure deficit (VPD) in two tree species with different stomatal conductance (gs) behavior, Catalpa fargesii and Pterocarya stenoptera. Our results revealed three key findings: (1) the coordination of gm and gs kinetics during light induction occurred in C. fargesii but not in P. stenoptera, and the model of gs kinetics largely determines whether the coordination of gs and gm exist in a given species; (2) a high VPD induced simultaneous changes in gs and gm kinetics in C. fargesii but had separated effects on gs and gm kinetics in P. stenoptera, indicating that the response of gm kinetics during light induction to VPD differs between species; and (3) the relative contribution of photorespiration to total electron flow was flexible in response to the change in relative diffusional and biochemical limitations, pointing out that photorespiration has a significant role in the regulation of photosynthetic electron flow during light induction. These results provide new sight into the species-dependent kinetics of gm and photorespiration during light induction.

Chassis engineering for high light tolerance in microalgae and cyanobacteria

Oxygenic photosynthesis in microalgae and cyanobacteria is considered an important chassis to accelerate energy transition and mitigate global warming. Currently, cultivation systems for photosynthetic microbes for large-scale applications encountered excessive light exposure stress. High light stress can: affect photosynthetic efficiency, reduce productivity, limit cell growth, and even cause cell death. Deciphering photoprotection mechanisms and constructing high-light tolerant chassis have been recent research focuses. In this review, we first briefly introduce the self-protection mechanisms of common microalgae and cyanobacteria in response to high light stress. These mechanisms mainly include: avoiding excess light absorption, dissipating excess excitation energy, quenching excessive high-energy electrons, ROS detoxification, and PSII repair. We focus on the species-specific differences in these mechanisms as well as recent advancements. Then, we review engineering strategies for creating high-light tolerant chassis, such as: reducing the size of the light-harvesting antenna, optimizing non-photochemical quenching, optimizing photosynthetic electron transport, and enhancing PSII repair. Finally, we propose a comprehensive exploration of mechanisms: underlying identified high light tolerant chassis, identification of new genes pertinent to high light tolerance using innovative methodologies, harnessing CRISPR systems and artificial intelligence for chassis engineering modification, and introducing plant photoprotection mechanisms as future research directions.

Aspirin Foliar Spray-Induced Changes in Light Energy Use Efficiency, Chloroplast Ultrastructure, and ROS Generation in Tomato

Aspirin (Asp) is extensively used in human health as an anti-inflammatory, antipyretic, and anti-thrombotic drug. In this study, we investigated if the foliar application of Asp on tomato plants has comparable beneficial effects on photosynthetic function to that of salicylic acid (SA), with which it shares similar physiological characteristics. We assessed the consequences of foliar Asp-spray on the photosystem II (PSII) efficiency of tomato plants, and we estimated the reactive oxygen species (ROS) generation and the chloroplast ultrastructural changes. Asp acted as an osmoregulator by increasing tomato leaf water content and offering antioxidant protection. This protection kept the redox state of plastoquinone (PQ) pull (qp) more oxidized, increasing the fraction of open PSII reaction centers and enhancing PSII photochemistry (ΦPSII). In addition, Asp foliar spray decreased reactive oxygen species (ROS) formation, decreasing the excess excitation energy on PSII. This resulted in a lower singlet oxygen (1O2) generation and a lower quantum yield for heat dissipation (ΦNPQ), indicating the photoprotective effect provided by Asp, especially under excess light illumination. Simultaneously, we observed a decrease in stomatal opening by Asp, which reduced the transpiration. Chloroplast ultrastructural data revealed that Asp, by offering a photoprotective effect, decreased the need for the photorespiration process, which reduces photosynthetic performance. It is concluded that Asp shares similar physiological characteristics with SA, having an equivalent beneficial impact to SA by acting as a biostimulant of the photosynthetic function for an enhanced crop yield.

Photorespiratory glycine contributes to photosynthetic induction during low to high light transition

Leaves experience near-constant light fluctuations daily. Past studies have identified many limiting factors of slow photosynthetic induction when leaves transition from low light to high light. However, the contribution of photorespiration in influencing photosynthesis during transient light conditions is largely unknown. This study employs dynamic measurements of gas exchange and metabolic responses to examine the contribution of photorespiration in constraining net rates of carbon assimilation during light induction. This work indicates that photorespiratory glycine accumulation during the early light induction contributes 5-7% to the additional carbon fixed relative to the low light conditions. Mutants with large glycine pools under photorespiratory conditions (5-formyl THF cycloligase and hydroxypyruvate reductase 1) showed a transient spike in net CO2 assimilation during light induction, with glycine buildup accounting for 22-36% of the extra carbon assimilated. Interestingly, levels of many C3 cycle intermediates remained relatively constant in both mutants and wild-type throughout the light induction period where glycine accumulated, indicating that recycling of carbon into the C3 cycle via photorespiration is not needed to maintain C3 cycle activity under transient conditions. Furthermore, our data show that oxygen transient experiments can be used as a proxy to identify the photorespiratory component of light-induced photosynthetic changes.

Ecophysiological and nutritional characterisation of two morphotypes of Cakile maritima subsp. maritima Scop. from Puglia region, Southern Italy

Cakile maritima subsp. maritima Scop. (sea rocket) is a succulent halophyte with significant potential as a nutritious food source, being rich in essential nutrients such as vitamins, minerals, and antioxidants. This annual species exhibits two distinct leaf morphotypes: entire lamina (EL) and pinnatifid lamina (PL). Our understanding of their ecophysiological and nutritional profiles is still limited. The present study investigated the wild EL and PL sea rocket plants from southern Italy during their vegetative stage. The bio-morphological traits (leaf mass area-LMA, dry matter and chlorophyll concentrations), main inorganic ions, key antioxidants (carotenoids, anthocyanins, phenols, flavonoids, glucosinolates, vitamin C as ascorbic and dehydroascorbic acid), and antioxidant activity (by FRAP, DPPH, ABTS assays) were analyzed. Additionally, photosynthetic gas exchange and chlorophyll fluorescence were measured. PL plants showed thicker leaves (higher LMA) and greater accumulation of photo-protective pigments (carotenoids and anthocyanins), despite similar chlorophyll levels. The PL plants also demonstrated higher photosynthetic activity, transpiration rates, and stomatal conductance, with reduced non-photochemical quenching. The EL morphotype had higher cation (K, Mg, Ca, Na) and vitamin C (135.3 mg 100 g-1 FW) concentrations, while no significant disparities were observed between the morphotypes in phenolic concentration (208.5 mg g.a.e. 100 g-1 FW), flavonoids (71.5 mg q.e. 100 g-1 FW), or glucosinolates (61 mg g-1 FW). Interestingly, while the EL type had higher vitamin C, the PL morphotype showed superior antioxidant activity (FRAP, DPPH) and seems to be better adapted to water/nutrient scarcity typical of southern Italy. Both morphotypes offer potential as high-nutritional foods, however, future research should investigate the genotype-specific production of antioxidant compounds in EL and PL plants in response to environmental stresses, including salinity for potential exploitation as a new crop.

Mesophyll conductance limits photosynthesis in fluctuating light under combined drought and heat stresses

Drought and heat stresses usually occur concomitantly in nature, with increasing frequency and intensity of both stresses expected due to climate change. The synergistic agricultural impacts of these compound climate extremes are much greater than those of the individual stresses. However, the mechanisms by which drought and heat stresses separately and concomitantly affect dynamic photosynthesis have not been thoroughly assessed. To elucidate this, we used tomato (Solanum lycopersicum) seedlings to measure dynamic photosynthesis under individual and compound stresses of drought and heat. Individual drought and heat stresses limited dynamic photosynthesis at the stages of diffusional conductance to CO2 and biochemistry, respectively. However, the primary limiting factor for photosynthesis shifted to mesophyll conductance under the compound stresses. Compared with the control, photosynthetic carbon gain in fluctuating light decreased by 38%, 73%, and 114% under the individual drought, heat, and compound stresses, respectively. Therefore, compound stresses caused a greater reduction in photosynthetic carbon gain in fluctuating light conditions than individual stress. These findings highlight the importance of mitigating the effects of compound climate extremes on crop productivity by targeting mesophyll conductance and improving dynamic photosynthesis.

Fluctuating light induces a significant photoinhibition of photosystem I in maize

In nature, light intensity usually fluctuates and a sudden shade-sun transition can induce photodamage to photosystem I (PSI) in many angiosperms. Photosynthetic regulation in fluctuating light (FL) has been studied extensively in C3 plants; however, little is known about how C4 plants cope FL to prevent PSI photoinhibition. We here compared photosynthetic responses to FL between maize (Zea mays, C4) and tomato (Solanum lycopersicum, C3) grown under full sunlight. Maize leaves had significantly higher cyclic electron flow (CEF) activity and lower photorespiration activity than tomato. Upon a sudden shade-sun transition, maize showed a significant stronger transient PSI over-reduction than tomato, resulting in a significant greater PSI photoinhibition in maize after FL treatment. During the first seconds upon shade-sun transition, CEF was stimulated in maize at a much higher extent than tomato, favoring the rapid formation of trans-thylakoid proton gradient (ΔpH), which was helped by a transient down-regulation of chloroplast ATP synthase activity. Therefore, modulation of ΔpH by regulation of CEF and chloroplast ATP synthase adjusted PSI redox state at donor side, which partially compensated for the deficiency of photorespiration. We propose that C4 plants use different photosynthetic strategies for coping with FL as compared with C3 plants.

Measurement of Photorespiratory Cycle Enzyme Activities in Leaves Exposed to Abiotic Stress

Photorespiration, an essential metabolic component, is a classic example of interactions between the intracellular compartments of a plant cell: the chloroplast, peroxisome, mitochondria, and cytoplasm. The photorespiratory pathway is often modulated by abiotic stress and is considered an adaptive response. Monitoring the patterns of key enzymes located in different subcellular components would be an ideal approach to assessing the modulation of the photorespiratory metabolism under abiotic stress. This chapter describes the procedures for assaying several individual enzyme activities of key photorespiratory enzymes and evaluating their response to oxidative/photooxidative stress. It is essential to ascertain the presence of stress in the experimental material. Therefore, procedures for typical abiotic stress induction in leaves by highlighting without or with menadione (an oxidant that targets mitochondria) are also included.

Overexpression of chloroplastic Zea mays NADP-malic enzyme (ZmNADP-ME) confers tolerance to salt stress in Arabidopsis thaliana

The C4 plants photosynthesize better than C3 plants especially in arid environment. As an attempt to genetically convert C3 plant to C4, the cDNA of decarboxylating C4 type NADP-malic enzyme from Zea mays (ZmNADP-ME) that has lower Km for malate and NADP than its C3 isoforms, was overexpressed in Arabidopsis thaliana under the control of 35S promoter. Due to increased activity of NADP-ME in the transgenics the malate decarboxylation increased that resulted in loss of carbon skeletons needed for amino acid and protein synthesis. Consequently, amino acid and protein content of the transgenics declined. Therefore, the Chl content, photosynthetic efficiency (Fv/Fm), electron transport rate (ETR), the quantum yield of photosynthetic CO2 assimilation, rosette diameter, and biomass were lower in the transgenics. However, in salt stress (150 mM NaCl), the overexpressers had higher Chl, protein content, Fv/Fm, ETR, and biomass than the vector control. NADPH generated in the transgenics due to increased malate decarboxylation, contributed to augmented synthesis of proline, the osmoprotectant required to alleviate the reactive oxygen species-mediated membrane damage and oxidative stress. Consequently, the glutathione peroxidase activity increased and H2O2 content decreased in the salt-stressed transgenics. The reduced membrane lipid peroxidation and lower malondialdehyde production resulted in better preservation, of thylakoid integrity and membrane architecture in the transgenics under saline environment. Our results clearly demonstrate that overexpression of C4 chloroplastic ZmNADP-ME in the C3 Arabidopsis thaliana, although decrease their photosynthetic efficiency, protects the transgenics from salinity stress.

Dynamic response of photorespiration in fluctuating light environments

Photorespiration is a dynamic process that is intimately linked to photosynthetic carbon assimilation. There is a growing interest in understanding carbon assimilation during dynamic conditions, but the role of photorespiration under such conditions is unclear. In this review, we discuss recent work relevant to the function of photorespiration under dynamic conditions, with a special focus on light transients. This work reveals that photorespiration is a fundamental component of the light induction of assimilation where variable diffusive processes limit CO2 exchange with the atmosphere. Additionally, metabolic interactions between photorespiration and the C3 cycle may help balance fluxes under dynamic light conditions. We further discuss how the energy demands of photorespiration present special challenges to energy balancing during dynamic conditions. We finish the review with an overview of why regulation of photorespiration may be important under dynamic conditions to maintain appropriate fluxes through metabolic pathways related to photorespiration such as nitrogen and one-carbon metabolism.

Photoinhibition of Photosystem I Induced by Different Intensities of Fluctuating Light Is Determined by the Kinetics of ∆pH Formation Rather Than Linear Electron Flow

Fluctuating light (FL) can cause the selective photoinhibition of photosystem I (PSI) in angiosperms. In nature, leaves usually experience FL conditions with the same low light and different high light intensities, but the effects of different FL conditions on PSI redox state and PSI photoinhibition are not well known. In this study, we found that PSI was highly reduced within the first 10 s after transition from 59 to 1809 μmol photons m^-2 s^-1 in tomato (Solanum lycopersicum). However, such transient PSI over-reduction was not observed by transitioning from 59 to 501 or 923 μmol photons m^-2 s^-1. Consequently, FL (59-1809) induced a significantly stronger PSI photoinhibition than FL (59-501) and FL (59-923). Compared with the proton gradient (∆pH) level after transition to high light for 60 s, tomato leaves almost formed a sufficient ∆pH after light transition for 10 s in FL (59-501) but did not in FL (59-923) or FL (59-1809). The difference in ∆pH between 10 s and 60 s was tightly correlated to the extent of PSI over-reduction and PSI photoinhibition induced by FL. Furthermore, the difference in PSI photoinhibition between (59-923) and FL (59-1809) was accompanied by the same level of linear electron flow. Therefore, PSI photoinhibition induced by different intensities of FL is more related to the kinetics of ∆pH formation rather than linear electron flow.

ROS production and signalling in chloroplasts: cornerstones and evolving concepts

Reactive oxygen species (ROS) such as singlet oxygen, superoxide (O₂^●- ) and hydrogen peroxide (H₂ O₂ ) are the markers of living cells. Oxygenic photosynthesis produces ROS in abundance, which act as a readout of a functional electron transport system and metabolism. The concept that photosynthetic ROS production is a major driving force in chloroplast to nucleus retrograde signalling is embedded in the literature, as is the role of chloroplasts as environmental sensors. The different complexes and components of the photosynthetic electron transport chain (PETC) regulate O₂^●- production in relation to light energy availability and the redox state of the stromal Cys-based redox systems. All of the ROS generated in chloroplasts have the potential to act as signals and there are many sulphhydryl-containing proteins and peptides in chloroplasts that have the potential to act as H₂ O₂ sensors and function in signal transduction. While ROS may directly move out of the chloroplasts to other cellular compartments, ROS signalling pathways can only be triggered if appropriate ROS-sensing proteins are present at or near the site of ROS production. Chloroplast antioxidant systems serve either to propagate these signals or to remove excess ROS that cannot effectively be harnessed in signalling. The key challenge is to understand how regulated ROS delivery from the PETC to the Cys-based redox machinery is organised to transmit redox signals from the environment to the nucleus. Redox changes associated with stromal carbohydrate metabolism also play a key role in chloroplast signalling pathways.