Plant sulphur metabolism is stimulated by photorespiration

Temperature responses of leaf respiration in light and darkness are similar and modulated by leaf development

Our ability to predict temperature responses of leaf respiration in light and darkness (RL and RDk ) is essential to models of global carbon dynamics. While many models rely on constant thermal sensitivity (characterized by Q10 ), uncertainty remains as to whether Q10 of RL and RDk are actually similar. We measured short-term temperature responses of RL and RDk in immature and mature leaves of two evergreen tree species, Castanopsis carlesii and Ormosia henry in an open field. RL was estimated by the Kok method, the Yin method and a newly developed Kok-iterCc method. When estimated by the Yin and Kok-iterCc methods, RL and RDk had similar Q10 (c. 2.5). The Kok method overestimated both Q10 and the light inhibition of respiration. RL /RDk was not affected by leaf temperature. Acclimation of respiration in summer was associated with a decline in basal respiration but not in Q10 in both species, which was related to changes in leaf nitrogen content between seasons. Q10 of RL and RDk in mature leaves were 40% higher than in immature leaves. Our results suggest similar Q10 values can be used to model RL and RDk while leaf development-associated changes in Q10 require special consideration in future respiration models.

The serine-glycine-one-carbon metabolic network orchestrates changes in nitrogen and sulfur metabolism and shapes plant development

L-serine (Ser) and L-glycine (Gly) are critically important for the overall functioning of primary metabolism. We investigated the interaction of the phosphorylated pathway of Ser biosynthesis (PPSB) with the photorespiration-associated glycolate pathway of Ser biosynthesis (GPSB) using Arabidopsis thaliana PPSB-deficient lines, GPSB-deficient mutants, and crosses of PPSB with GPSB mutants. PPSB-deficient lines mainly showed retarded primary root growth. Mutation of the photorespiratory enzyme Ser-hydroxymethyltransferase 1 (SHMT1) in a PPSB-deficient background resumed primary root growth and induced a change in the plant metabolic pattern between roots and shoots. Grafting experiments demonstrated that metabolic changes in shoots were responsible for the changes in double mutant development. PPSB disruption led to a reduction in nitrogen (N) and sulfur (S) contents in shoots and a general transcriptional response to nutrient deficiency. Disruption of SHMT1 boosted the Gly flux out of the photorespiratory cycle, which increased the levels of the one-carbon (1C) metabolite 5,10-methylene-tetrahydrofolate and S-adenosylmethionine. Furthermore, disrupting SHMT1 reverted the transcriptional response to N and S deprivation and increased N and S contents in shoots of PPSB-deficient lines. Our work provides genetic evidence of the biological relevance of the Ser-Gly-1C metabolic network in N and S metabolism and in interorgan metabolic homeostasis.

Identification and Functional Characterization of cis-Regulatory Elements of Key Photorespiratory Genes in Response to Short-Term Abiotic Stress Conditions

The cis-regulatory elements (CREs) are the short stretches of noncoding DNA upstream of a gene, which play a critical role in fine-tuning gene expression. Photorespiration is a multi-organellar, energy-expensive biochemical process that remains intricately linked to photosynthesis and is conserved in plants. Recently, much focus has been devoted in generating plants with engineered alternative photorespiratory bypasses to enhance photosynthetic efficiency without compromising the beneficial aspect of photorespiration. Varied constitutive or inducible promoters for generating transgenic plants harboring multiple transgenes have been introduced over years; however, most of them suffer from unintended effects. Consequently, a demand for synthetic tunable promoters based on canonical CRE signatures derived from native genes is on the rise. Here, in this chapter, we have provided a detailed method for in silico identification and characterization of CREs associated with photorespiration. In addition to the detailed protocol, we have presented an example of a typical result and explained the significance of the result. Specifically, the method covers how to identify and generate tunable synthetic promoters based on native CREs using three key photorespiratory genes from Arabidopsis and two web-based tools, namely, PlantPAN3.0 and AthaMap. Finally, we have also furnished a protocol on how to test the efficacies of the synthetic promoters harboring predicted CREs using transient tobacco expression coupled with luciferase-based promoter assay in response to ambient conditions and under short-term abiotic stress conditions.

Combining Gas Exchange and Rapid Quenching of Leaf Tissue for Mass Spectrometry and NMR Analysis Using an External Chamber

We describe here a method to study and manipulate photorespiration in intact illuminated leaves. When the CO2/O2 mole fraction ratio changes, instant sampling is critical, to quench leaf metabolism and thus trace rapid metabolic modification due to gaseous conditions. To do so, we combine 13CO2 labeling and gas exchange, using a large custom leaf chamber to facilitate fast sampling by direct liquid nitrogen spraying. Moreover, the use of a high chamber surface area (about 130 cm2) allows one to sample a large amount of leaf material to carry out 13C-nuclear magnetic resonance (NMR) analysis and complementary analyses, such as isotopic analyses by high-resolution mass spectrometry (by both GC and LC-MS). 13C-NMR gives access to absolute 13C amounts at the specific carbon atom position in the labeled molecules and thereby provides an estimate of 13C-flux of photorespiratory intermediates. Since NMR analysis is not very sensitive and can miss minor metabolites, GC or LC-MS analyses are useful to monitor metabolites at low concentrations. Furthermore, 13C-NMR and high-resolution LC-MS allow to estimate isotopologue distribution in response to 13CO2 labeling while modifying photorespiration activity.

Integrated transcriptomic and proteomic analyses of two sugarcane (Saccharum officinarum Linn.) varieties differing in their lodging tolerance

BACKGROUND

Lodging seriously affects sugarcane stem growth and sugar accumulation, reduces sugarcane yield and sucrose content, and impedes mechanization. However, the molecular mechanisms underlying sugarcane lodging tolerance remain unclear. In this study, comprehensive transcriptomic and proteomic analyses were performed to explore the differential genetic regulatory mechanisms between upright (GT42) and lodged (GF98-296) sugarcane varieties.

RESULTS

The stain test showed that GT42 had more lignin and vascular bundles in the stem than GF98-296. The gene expression analysis revealed that the genes that were differentially expressed between the two varieties were mainly involved in the phenylpropanoid pathway at the growth stage. The protein expression analysis indicated that the proteins that were differentially expressed between the two varieties were related to the synthesis of secondary metabolites, the process of endocytosis, and the formation of aminoacyl-tRNA. Time-series analysis revealed variations in differential gene expression patterns between the two varieties, whereas significant protein expression trends in the two varieties were largely consistent, except for one profile. The expression of CYP84A, 4CL, and CAD from the key phenylpropanoid biosynthetic pathway was enhanced in GT42 at stage 2 but suppressed in GF98-296 at the growth stage. Furthermore, the expression of SDT1 in the nicotinate and nicotinamide metabolism was enhanced in GT42 cells but suppressed in GF98-296 cells at the growth stage.

CONCLUSION

Our findings provide reference data for mining lodging tolerance-related genes that are expected to facilitate the selective breeding of sugarcane varieties with excellent lodging tolerance.

Growth in fluctuating light buffers plants against photorespiratory perturbations

Photorespiration (PR) is the pathway that detoxifies the product of the oxygenation reaction of Rubisco. It has been hypothesized that in dynamic light environments, PR provides a photoprotective function. To test this hypothesis, we characterized plants with varying PR enzyme activities under fluctuating and non-fluctuating light conditions. Contrasting our expectations, growth of mutants with decreased PR enzyme levels was least affected in fluctuating light compared with wild type. Results for growth, photosynthesis and metabolites combined with thermodynamics-based flux analysis revealed two main causal factors for this unanticipated finding: reduced rates of photosynthesis in fluctuating light and complex re-routing of metabolic fluxes. Only in non-fluctuating light, mutants lacking the glutamate:glyoxylate aminotransferase 1 re-routed glycolate processing to the chloroplast, resulting in photooxidative damage through H2O2 production. Our results reveal that dynamic light environments buffer plant growth and metabolism against photorespiratory perturbations.

Photorespiration: regulation and new insights on the potential role of persulfidation

Photorespiration has been considered a 'futile' cycle in C3 plants, necessary to detoxify and recycle the metabolites generated by the oxygenating activity of Rubisco. However, several reports indicate that this metabolic route plays a fundamental role in plant metabolism and constitutes a very interesting research topic. Many open questions still remain with regard to photorespiration. One of these questions is how the photorespiratory process is regulated in plants and what factors contribute to this regulation. In this review, we summarize recent advances in the regulation of the photorespiratory pathway with a special focus on the transcriptional and post-translational regulation of photorespiration and the interconnections of this process with nitrogen and sulfur metabolism. Recent findings on sulfide signaling and protein persulfidation are also described.

Asymmetric distribution of mineral nutrients aggravates uneven fruit pigmentation driven by sunlight exposure in litchi

MAIN CONCLUSION

Sunlight boosts anthocyanin synthesis/accumulation in sunny pericarp of litchi fruit, directly leading to uneven pigmentation. Distribution discrepancy of mineral element aggravates uneven coloration by modulating synthesis/accumulation of anthocyanin and sugar. Uneven coloration, characterized by red pericarp on sunny side and green pericarp on shady side, impacts fruit quality of 'Feizixiao' (cv.) litchi. The mechanisms of this phenomenon were explored by investigating the distribution of chlorophyll, flavonoids, sugars, and mineral elements in both types of pericarp. Transcriptome analysis in pericarp was conducted as well. Sunny pericarp contained higher anthocyanins in an order of magnitude and higher fructose, glucose, co-pigments (flavanols, flavonols, ferulic acid), and mineral elements like Ca, Mg and Mn, along with lower N, P, K, S, Cu, Zn and B (P < 0.01), compared to shady pericarp. Sunlight regulated the expression of genes involved in synthesis/accumulation of flavonoids and sugars and genes functioning in nutrient uptake and transport, leading to asymmetric distribution of these substances. Anthocyanins conferred red color on sunny pericarp, sugars, Ca and Mg promoted synthesis/accumulation of anthocyanins, and co-pigments enhanced color display of anthocyanins. The insufficiencies of anthocyanins, sugars and co-pigments, and inhibition effect of excess K, S, N and P on synthesis/accumulation of anthocyanins and sugars, jointly contributed to green color of shady pericarp. These findings highlight the role of asymmetric distribution of substances, mineral elements in particular, on uneven pigmentation in litchi, and provide insights into coloration improvement via precise fertilization.

Improving photosynthetic efficiency toward food security: Strategies, advances, and perspectives

Photosynthesis in crops and natural vegetation allows light energy to be converted into chemical energy and thus forms the foundation for almost all terrestrial trophic networks on Earth. The efficiency of photosynthetic energy conversion plays a crucial role in determining the portion of incident solar radiation that can be used to generate plant biomass throughout a growth season. Consequently, alongside the factors such as resource availability, crop management, crop selection, maintenance costs, and intrinsic yield potential, photosynthetic energy use efficiency significantly influences crop yield. Photosynthetic efficiency is relevant to sustainability and food security because it affects water use efficiency, nutrient use efficiency, and land use efficiency. This review focuses specifically on the potential for improvements in photosynthetic efficiency to drive a sustainable increase in crop yields. We discuss bypassing photorespiration, enhancing light use efficiency, harnessing natural variation in photosynthetic parameters for breeding purposes, and adopting new-to-nature approaches that show promise for achieving unprecedented gains in photosynthetic efficiency.

Mitogen-activated protein kinase 2 specifically regulates photorespiration in rice

Photorespiration begins with the oxygenation reaction catalyzed by Rubisco and is the second highest metabolic flux in plants after photosynthesis. Although the core biochemical pathway of photorespiration has been well characterized, little is known about the underlying regulatory mechanisms. Some rate-limiting regulation of photorespiration has been suggested to occur at both the transcriptional and posttranslational levels, but experimental evidence is scarce. Here, we found that mitogen-activated protein kinase 2 (MAPK2) interacts with photorespiratory glycolate oxidase and hydroxypyruvate reductase, and the activities of these photorespiratory enzymes were regulated via phosphorylation modifications in rice (Oryza sativa L.). Gas exchange measurements revealed that the photorespiration rate decreased in rice mapk2 mutants under normal growth conditions, without disturbing photosynthesis. Due to decreased photorespiration, the levels of some key photorespiratory metabolites, such as 2-phosphoglycolate, glycine, and glycerate, significantly decreased in mapk2 mutants, but those of photosynthetic metabolites were not altered. Transcriptome assays also revealed that the expression levels of some flux-controlling genes in photorespiration were significantly downregulated in mapk2 mutants. Our findings provide molecular evidence for the association between MAPK2 and photorespiration and suggest that MAPK2 regulates the key enzymes of photorespiration at both the transcriptional and posttranslational phosphorylation levels in rice.

Transcriptome dynamics provides insights into divergences of the photosynthesis pathway between Saccharum officinarum and Saccharum spontaneum

Saccharum spontaneum and Saccharum officinarum contributed to the genetic background of modern sugarcane cultivars. Saccharum spontaneum has shown a higher net photosynthetic rate and lower soluble sugar than S. officinarum. Here, we analyzed 198 RNA-sequencing samples to investigate the molecular mechanisms for the divergences of photosynthesis and sugar accumulation between the two Saccharum species. We constructed gene co-expression networks based on differentially expressed genes (DEGs) both for leaf developmental gradients and diurnal rhythm. Our results suggested that the divergence of sugar accumulation may be attributed to the enrichment of major carbohydrate metabolism and the oxidative pentose phosphate pathway. Compared with S. officinarum, S. spontaneum DEGs showed a high enrichment of photosynthesis and contained more complex regulation of photosynthesis-related genes. Noticeably, S. spontaneum lacked gene interactions with sulfur assimilation stimulated by photorespiration. In S. spontaneum, core genes related to clock and photorespiration displayed a sensitive regulation by the diurnal rhythm and phase-shift. Small subunit of Rubisco (RBCS) displayed higher expression in the source tissues of S. spontaneum. Additionally, it was more sensitive under a diurnal rhythm, and had more complex gene networks than that in S. officinarum. This indicates that the differential regulation of RBCS Rubisco contributed to photosynthesis capacity divergence in both Saccharum species.

Evolutionary implications of C2 photosynthesis: how complex biochemical trade-offs may limit C4 evolution

The C2 carbon-concentrating mechanism increases net CO2 assimilation by shuttling photorespiratory CO2 in the form of glycine from mesophyll to bundle sheath cells, where CO2 concentrates and can be re-assimilated. This glycine shuttle also releases NH3 and serine into the bundle sheath, and modelling studies suggest that this influx of NH3 may cause a nitrogen imbalance between the two cell types that selects for the C4 carbon-concentrating mechanism. Here we provide an alternative hypothesis outlining mechanisms by which bundle sheath NH3 and serine play vital roles to not only influence the status of C2 plants along the C3 to C4 evolutionary trajectory, but to also convey stress tolerance to these unique plants. Our hypothesis explains how an optimized bundle sheath nitrogen hub interacts with sulfur and carbon metabolism to mitigate the effects of high photorespiratory conditions. While C2 photosynthesis is typically cited for its intermediary role in C4 photosynthesis evolution, our alternative hypothesis provides a mechanism to explain why some C2 lineages have not made this transition. We propose that stress resilience, coupled with open flux tricarboxylic acid and photorespiration pathways, conveys an advantage to C2 plants in fluctuating environments.

Strategies for manipulating Rubisco and creating photorespiratory bypass to boost C3 photosynthesis: Prospects on modern crop improvement

Photosynthesis is a process that uses solar energy to fix CO₂ in the air and converts it into sugar, and ultimately powers almost all life activities on the earth. C₃ photosynthesis is the most common form of photosynthesis in crops. Current efforts of increasing crop yields in response to growing global food requirement are mostly focused on improving C₃ photosynthesis. In this review, we summarized the strategies of C₃ photosynthesis improvement in terms of Rubisco properties and photorespiratory limitation. Potential engineered targets include Rubisco subunits and their catalytic sites, Rubisco assembly chaperones, and Rubisco activase. In addition, we reviewed multiple photorespiratory bypasses built by strategies of synthetic biology to reduce the release of CO₂ and ammonia and minimize energy consumption by photorespiration. The potential strategies are suggested to enhance C₃ photosynthesis and boost crop production.

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.

Photorespiration - Rubisco's repair crew

The photorespiratory repair pathway (photorespiration in short) was set up from ancient metabolic modules about three billion years ago in cyanobacteria, the later ancestors of chloroplasts. These prokaryotes developed the capacity for oxygenic photosynthesis, i.e. the use of water as a source of electrons and protons (with O₂ as a by-product) for the sunlight-driven synthesis of ATP and NADPH for CO₂ fixation in the Calvin cycle. However, the CO₂-binding enzyme, ribulose 1,5-bisphosphate carboxylase (known under the acronym Rubisco), is not absolutely selective for CO₂ and can also use O₂ in a side reaction. It then produces 2-phosphoglycolate (2PG), the accumulation of which would inhibit and potentially stop the Calvin cycle and subsequently photosynthetic electron transport. Photorespiration removes the 2-PG and in this way prevents oxygenic photosynthesis from poisoning itself. In plants, the core of photorespiration consists of ten enzymes distributed over three different types of organelles, requiring interorganellar transport and interaction with several auxiliary enzymes. It goes together with the release and to some extent loss of freshly fixed CO₂. This disadvantageous feature can be suppressed by CO₂-concentrating mechanisms, such as those that evolved in C₄ plants thirty million years ago, which enhance CO₂ fixation and reduce 2PG synthesis. Photorespiration itself provided a pioneer variant of such mechanisms in the predecessors of C₄ plants, C₃-C₄ intermediate plants. This article is a review and update particularly on the enzyme components of plant photorespiration and their catalytic mechanisms, on the interaction of photorespiration with other metabolism and on its impact on the evolution of photosynthesis. This focus was chosen because a better knowledge of the enzymes involved and how they are embedded in overall plant metabolism can facilitate the targeted use of the now highly advanced methods of metabolic network modelling and flux analysis. Understanding photorespiration more than before as a process that enables, rather than reduces, plant photosynthesis, will help develop rational strategies for crop improvement.

Enhancing crop yields through improvements in the efficiency of photosynthesis and respiration

The rate with which crop yields per hectare increase each year is plateauing at the same time that human population growth and other factors increase food demand. Increasing yield potential (Y p ) of crops is vital to address these challenges. In this review, we explore a component ofY p that has yet to be optimised - that being improvements in the efficiency with which light energy is converted into biomass (ε c ) via modifications to CO2 fixed per unit quantum of light (α), efficiency of respiratory ATP production (ε prod ) and efficiency of ATP use (ε use ). For α, targets include changes in photoprotective machinery, ribulose bisphosphate carboxylase/oxygenase kinetics and photorespiratory pathways. There is also potential forε prod to be increased via targeted changes to the expression of the alternative oxidase and mitochondrial uncoupling pathways. Similarly, there are possibilities to improveε use via changes to the ATP costs of phloem loading, nutrient uptake, futile cycles and/or protein/membrane turnover. Recently developed high-throughput measurements of respiration can serve as a proxy for the cumulative energy cost of these processes. There are thus exciting opportunities to use our growing knowledge of factors influencing the efficiency of photosynthesis and respiration to create a step-change in yield potential of globally important crops.

Arsenic and cadmium induced macronutrient deficiencies trigger contrasting gene expression changes in rice

Arsenic (As) and cadmium (Cd), two major carcinogenic heavy metals, enters into human food chain by the consumption of rice or rice-based food products. Both As and Cd disturb plant-nutrient homeostasis and hence, reduces plant growth and crop productivity. In the present study, As/Cd modulated responses were studied in non-basmati (IR-64) and basmati (PB-1) rice varieties, at physiological, biochemical and transcriptional levels. At the seedling stage, PB-1 was found more sensitive than IR-64, in terms of root biomass; however, their shoot phenotype was comparable under As and Cd stress conditions. The ionomic data revealed significant nutrient deficiencies in As/Cd treated-roots. The principal component analysis identified NH₄⁺ as As-associated key macronutrient; while, NH₄⁺/NO₃^- and K⁺ was majorly associated with Cd mediated response, in both IR-64 and PB-1. Using a panel of 21 transporter gene expression, the extent of nutritional deficiency was ranked in the order of PB-1(As)<IR-64(As)<PB-1(Cd)<IR-64(Cd). A feed-forward model is proposed to explain nutrient deficiency induced de-regulation of gene expression, as observed under Cd-treated IR-64 plants, which was also validated at the level of sulphur metabolism related enzymes. Using urea supplementation, as nitrogen-fertilizer, significant mitigation was observed under As stress, as indicated by 1.018- and 0.794-fold increase in shoot biomass in IR-64 and PB-1, respectively compared to that of control. However, no significant amelioration was observed in response to supplementation of urea under Cd or potassium under As/Cd stress conditions. Thus, the study pinpointed the relative significance of various macronutrients in regulating As- and Cd-tolerance and will help in designing suitable strategies for mitigating As and/or Cd stress conditions.

Improving crop productivity and nitrogen use efficiency using sulfur and zinc-coated urea: A review

Nitrogen (N) is an important macro-nutrient required for crop production and is considered an important commodity for agricultural systems. Urea is a vital source of N that is used widely across the globe to meet crop N requirements. However, N applied in the form of urea is mostly lost in soil, posing serious economic and environmental issues. Therefore, different approaches such as the application of urea coated with different substances are used worldwide to reduce N losses. Urea coating is considered an imperative approach to enhance crop production and reduce the corresponding nitrogen losses along with its impact on the environment. In addition, given the serious food security challenges in meeting the current and future demands for food, the best agricultural management strategy to enhance food production have led to methods that involve coating urea with different nutrients such as sulfur (S) and zinc (Zn). Coated urea has a slow-release mechanism and remains in the soil for a longer period to meet the demand of crop plants and increases nitrogen use efficiency, growth, yield, and grain quality. These nutrient-coated urea reduce nitrogen losses (volatilization, leaching, and N₂O) and save the environment from degradation. Sulfur and zinc-coated urea also reduce nutrient deficiencies and have synergetic effects with other macro and micronutrients in the crop. This study discusses the dynamics of sulfur and zinc-coated urea in soil, their impact on crop production, nitrogen use efficiency (NUE), the residual and toxic effects of coated urea, and the constraints of adopting coated fertilizers. Additionally, we also shed light on agronomic and molecular approaches to enhance NUE for better crop productivity to meet food security challenges.

Wheat grain yield decreased over the past 35 years, but protein content did not change

The extent to which rising atmospheric CO2 concentration has already influenced food production and quality is uncertain. Here, we analyzed annual field trials of autumn-planted common wheat in California from 1985 to 2019, a period during which the global atmospheric CO2 concentration increased 19%. Even after accounting for other major factors (cultivar, location, degree-days, soil temperature, total water applied, nitrogen fertilization, and pathogen infestation), wheat grain yield and protein yield declined 13% over this period, but grain protein content did not change. These results suggest that exposure to gradual CO2 enrichment over the past 35 years has adversely affected wheat grain and protein yield, but not grain protein content.

The impact of photorespiration on plant primary metabolism through metabolic and redox regulation

Photorespiration is an inevitable trait of all oxygenic phototrophs, being the only known metabolic route that converts the inhibitory side-product of Rubisco's oxygenase activity 2-phosphoglycolate (2PG) back into the Calvin-Benson (CB) cycle's intermediate 3-phosphoglycerate (3PGA). Through this function of metabolite repair, photorespiration is able to protect photosynthetic carbon assimilation from the metabolite intoxication that would occur in the present-day oxygen-rich atmosphere. In recent years, much plant research has provided compelling evidence that photorespiration safeguards photosynthesis and engages in cross-talk with a number of subcellular processes. Moreover, the potential of manipulating photorespiration to increase the photosynthetic yield potential has been demonstrated in several plant species. Considering this multifaceted role, it is tempting to presume photorespiration itself is subject to a suite of regulation mechanisms to eventually exert a regulatory impact on other processes, and vice versa. The identification of potential pathway interactions and underlying regulatory aspects has been facilitated via analysis of the photorespiratory mutant phenotype, accompanied by the emergence of advanced omics' techniques and biochemical approaches. In this mini-review, I focus on the identification of enzymatic steps which control the photorespiratory flux, as well as levels of transcriptional, posttranslational, and metabolic regulation. Most importantly, glycine decarboxylase (GDC) and 2PG are identified as being key photorespiratory determinants capable of controlling photorespiratory flux and communicating with other branches of plant primary metabolism.

Sulfur Regulates the Trade-Off Between Growth and Andrographolide Accumulation via Nitrogen Metabolism in Andrographis paniculata

Nitrogen (N) and sulfur (S) are essential mineral nutrients for plant growth and metabolism. Here, we investigated their interaction in plant growth and andrographolide accumulation in medicinal plant Andrographis paniculata grown at different N (4 and 8 mmol·L^-1) and S concentration levels (0.1 and 2.4 mmol L^-1). We found that increasing the S application rate enhanced the accumulation of andrographolide compounds (AGCs) in A. paniculata. Simultaneously, salicylic acid (SA) and gibberellic acid 4 (GA₄) concentrations were increased but trehalose/trehalose 6-phosphate (Tre/Tre6P) concentrations were decreased by high S, suggesting that they were involved in the S-mediated accumulation of AGCs. However, S affected plant growth differentially at different N levels. Metabolite analysis revealed that high S induced increases in the tricarboxylic acid (TCA) cycle and photorespiration under low N conditions, which promoted N assimilation and S metabolism, and simultaneously increased carbohydrate consumption and inhibited plant growth. In contrast, high S reduced N and S concentrations in plants and promoted plant growth under high N conditions. Taken together, the results indicated that increasing the S application rate is an effective strategy to improve AGC accumulation in A. paniculata. Nevertheless, the interaction of N and S affected the trade-off between plant growth and AGC accumulation, in which N metabolism plays a key role.

Metabolite Profiling in Arabidopsisthaliana with Moderately Impaired Photorespiration Reveals Novel Metabolic Links and Compensatory Mechanisms of Photorespiration

Photorespiration is an integral component of plant primary metabolism. Accordingly, it has been often observed that impairing the photorespiratory flux negatively impacts other cellular processes. In this study, the metabolic acclimation of the Arabidopsisthaliana wild type was compared with the hydroxypyruvate reductase 1 (HPR1; hpr1) mutant, displaying only a moderately reduced photorespiratory flux. Plants were analyzed during development and under varying photoperiods with a combination of non-targeted and targeted metabolome analysis, as well as ¹³C- and ¹⁴C-labeling approaches. The results showed that HPR1 deficiency is more critical for photorespiration during the vegetative compared to the regenerative growth phase. A shorter photoperiod seems to slowdown the photorespiratory metabolite conversion mostly at the glycerate kinase and glycine decarboxylase steps compared to long days. It is demonstrated that even a moderate impairment of photorespiration severely reduces the leaf-carbohydrate status and impacts on sulfur metabolism. Isotope labeling approaches revealed an increased CO₂ release from hpr1 leaves, most likely occurring from enhanced non-enzymatic 3-hydroxypyruvate decarboxylation and a higher flux from serine towards ethanolamine through serine decarboxylase. Collectively, the study provides evidence that the moderate hpr1 mutant is an excellent tool to unravel the underlying mechanisms governing the regulation of metabolic linkages of photorespiration with plant primary metabolism.

Photorespiration: The Futile Cycle?

Photorespiration, or C₂ photosynthesis, is generally considered a futile cycle that potentially decreases photosynthetic carbon fixation by more than 25%. Nonetheless, many essential processes, such as nitrogen assimilation, C₁ metabolism, and sulfur assimilation, depend on photorespiration. Most studies of photosynthetic and photorespiratory reactions are conducted with magnesium as the sole metal cofactor despite many of the enzymes involved in these reactions readily associating with manganese. Indeed, when manganese is present, the energy efficiency of these reactions may improve. This review summarizes some commonly used methods to quantify photorespiration, outlines the influence of metal cofactors on photorespiratory enzymes, and discusses why photorespiration may not be as wasteful as previously believed.

13C Isotope Labelling to Follow the Flux of Photorespiratory Intermediates

Measuring the carbon flux through metabolic pathways in intact illuminated leaves remains challenging because of, e.g., isotopic dilution by endogenous metabolites, the impossibility to reach isotopic steady state, and the occurrence of multiple pools. In the case of photorespiratory intermediates, our knowledge of the partitioning between photorespiratory recycling, storage, and utilization by other pathways is thus rather limited. There has been some controversy as to whether photorespiratory glycine and serine may not be recycled, thus changing the apparent stoichiometric coefficient between photorespiratory O₂ fixation and CO₂ release. We describe here an isotopic method to trace the fates of glycine, serine and glycerate, taking advantage of positional ¹³C content with NMR and isotopic analyses by LC–MS. This technique is well-adapted to show that the proportion of glycerate, serine and glycine molecules escaping photorespiratory recycling is very small.

The Versatile Roles of Sulfur-Containing Biomolecules in Plant Defense-A Road to Disease Resistance

Sulfur (S) is an essential plant macronutrient and the pivotal role of sulfur compounds in plant disease resistance has become obvious in recent decades. This review attempts to recapitulate results on the various functions of sulfur-containing defense compounds (SDCs) in plant defense responses to pathogens. These compounds include sulfur containing amino acids such as cysteine and methionine, the tripeptide glutathione, thionins and defensins, glucosinolates and phytoalexins and, last but not least, reactive sulfur species and hydrogen sulfide. SDCs play versatile roles both in pathogen perception and initiating signal transduction pathways that are interconnected with various defense processes regulated by plant hormones (salicylic acid, jasmonic acid and ethylene) and reactive oxygen species (ROS). Importantly, ROS-mediated reversible oxidation of cysteine residues on plant proteins have profound effects on protein functions like signal transduction of plant defense responses during pathogen infections. Indeed, the multifaceted plant defense responses initiated by SDCs should provide novel tools for plant breeding to endow crops with efficient defense responses to invading pathogens.

Role of sulfur in combating arsenic stress through upregulation of important proteins, and in-silico analysis to study the interaction between phosphate transporter (PHO1), arsenic and phosphate in spinach

An adequate amount of Sulfur (S) is essential for proper plant growth and defence against abiotic stresses including metals and metalloids. Arsenic (As) contamination is increasing in agricultural soils rapidly due to anthropogenic activities. Sulfur deficiency and arsenic stress could be more harmful than these individual stresses alone. To understand the impact of S-deficiency and arsenic (31 ppm Na₃AsO₄ of soil) on ecophysiology, growth, inorganic phosphate level, and proteomic profile of spinach, the present study was conducted. Interaction of arsenic with phosphate transporters, phytochelatins, and glutathione was also analyzed in silico. Comparative 2D MS/MS proteomics helped in the identification of important proteins which might be the key players under S-deficiency and As stress. Upregulation and downregulation of 36 and 21 proteins under As stress; 19 and 36 proteins under S-deficiency; 38 and 31 proteins under combined stress, respectively was observed. A total, 87 proteins subjected to identification via MS/MS ion search were found to be associated with important plant functions. PHO1 abundance was highly influenced by As stress; hence an in-silico homology modeling based molecular docking was performed which indicated high interaction between PHO1 and As/phosphate. Varied proximity of arsenic with phosphate transporters, phytochelatin, and glutathione revealed these components as a potential target of As toxicity/detoxification in Spinach, reflecting sulfur as an important criterion for arsenic tolerance.

Sulphur fertilization enhanced yield, its uptake, use efficiency and economic returns of Aloe vera L

Sulphur plays a vital role in the formation and biosynthesis of protein, chlorophyll, and few amino acids. To investigate the effect of sulphur fertilizer on leaf biomass yield, critical sulphur concentration, sulphur requirement and uptake by Aloe vera L., a pot experiment was carried out following completely randomized design with six levels of sulphur viz., 0, 15, 30, 45, 60 and 80 kg ha-1 with three replications. The results of the study revealed that the growth attributes, leaf and gel yield, and sulphur uptake significantly improved with sulphur application and the best results were obtained from the application of 45 kg sulphur ha-1. On average, addition of sulphur enhanced the leaf biomass yield by 47.5% and sulphur use efficiency by 38% compared to control. The effect of sulphur on the growth parameters and their significant and positive correlations with yield signifies the importance of sulphur on the yield and quality of A. vera. The calculated minimum amount of sulphur for 80% leaf biomass production was 21.1 kg sulphur ha-1 with a critical leaf sulphur concentration of 0.23% in A. vera. Moreover, sulphur addition to soil substantially enhanced the economic returns of A. vera. Therefore, addition of 45 kg sulphur ha-1 could be a better option for obtaining higher yield and economic return of A. vera.

Appraisal of H2S metabolism in Arabidopsis thaliana: In silico analysis at the subcellular level

Hydrogen sulfide (H₂S) has become a new signal molecule in higher plants which seems to be involved in almost all physiological processes from seed germination, root and plant growth until flowering and fruit ripening. Moreover, H₂S also participates in the mechanism of response against adverse environmental stresses. However, its basic biochemistry in plant cells can be considered in a nascent stage. Using the available information of the model plant Arabidopsis thaliana, the goal of the present study is to provide a broad overview of H₂S metabolism and to display an in silico analysis of the 26 enzymatic components involved in the metabolism of H₂S and their subcellular compartmentation (cytosol, chloroplast and mitochondrion) thus providing a wide picture of the cross-talk inside the organelles and amongst them and, consequently, to get a better understanding of the cellular and tissue implications of H₂S. This information will be also relevant for other crop species, especially those whose whole genome is not yet available.

Ribulose 1,5-bisphosphate carboxylase/oxygenase activates O2 by electron transfer

Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the cornerstone of atmospheric CO2 fixation by the biosphere. It catalyzes the addition of CO2 onto enolized ribulose 1,5-bisphosphate (RuBP), producing 3-phosphoglycerate which is then converted to sugars. The major problem of this reaction is competitive O2 addition, which forms a phosphorylated product (2-phosphoglycolate) that must be recycled by a series of biochemical reactions (photorespiratory metabolism). However, the way the enzyme activates O2 is still unknown. Here, we used isotope effects (with 2H, 25Mg, and 18O) to monitor O2 activation and assess the influence of outer sphere atoms, in two Rubisco forms of contrasted O2/CO2 selectivity. Neither the Rubisco form nor the use of solvent D2O and deuterated RuBP changed the 16O/18O isotope effect of O2 addition, in clear contrast with the 12C/13C isotope effect of CO2 addition. Furthermore, substitution of light magnesium (24Mg) by heavy, nuclear magnetic 25Mg had no effect on O2 addition. Therefore, outer sphere protons have no influence on the reaction and direct radical chemistry (intersystem crossing with triplet O2) does not seem to be involved in O2 activation. Computations indicate that the reduction potential of enolized RuBP (near 0.49 V) is compatible with superoxide (O2•-) production, must be insensitive to deuteration, and yields a predicted 16O/18O isotope effect and energy barrier close to observed values. Overall, O2 undergoes single electron transfer to form short-lived superoxide, which then recombines to form a peroxide intermediate.

Growth performance and nitrogen allocation within leaves of two poplar clones after exponential and conventional nitrogen applications

Populus species are fast growing with high N requirements; an optimum level of fertilization is necessary for high seedling quality and subsequent plantation productivity. In this study, the morphological and physiological responses of two poplar clones (XH and BL3) to exponential and conventional N dosages were investigated, with a specific focus on leaf traits, the photorespiratory N cycle, and the interconversion of amino acids within leaves. Results show that shoot height and leaf number exponentially increased with plant growth. Leaf area, chlorophyll concentration, and net photosynthetic rate significantly increased for both clones during N fertilization, with a significant difference only in leaf area of clone XH between exponential and conventional dosages. Leaf concentrations of free amino acids and soluble sugars were not different but soluble proteins and fatty acids were significantly different for clone XH between N dosages; the amino acids glutamate, alanine, and aspartic acid concentrations increased in exponentially fertilized seedlings compared to controls. Amino acids, including the composition concentration and activity of glutamic-oxalacetic and -pyruvic transaminase, and soluble sugars were significantly higher for clone BL3 in fertilized seedlings. Photorespiration (glycine and glycolate oxidase) and glutathione redox (oxidized glutathione) were affected by fertilization. The activities of key enzymes (glycolate oxidase, catalase, and γ-glutamate cysteine ligase) involved in photorespiration and glutathione metabolism were lower for clone XH with exponential fertilization. Phenylalanine catabolism was influenced by fertilization and the interaction, clone × fertilization, showing accumulation of phenylalanine and tyrosine but decreases in phenylalanine ammonialyase activity and flavonoid concentrations in leaves of fertilized seedlings. The results indicate that leaf area and the interconversion of amino acids through deamidation/transamination are key regulatory hubs in poplar acclimation to soil N availability.

Photorespiration in the context of Rubisco biochemistry, CO2 diffusion and metabolism

Photorespiratory metabolism is essential for plants to maintain functional photosynthesis in an oxygen-containing environment. Because the oxygenation reaction of Rubisco is followed by the loss of previously fixed carbon, photorespiration is often considered a wasteful process and considerable efforts are aimed at minimizing the negative impact of photorespiration on the plant's carbon uptake. However, the photorespiratory pathway has also many positive aspects, as it is well integrated within other metabolic processes, such as nitrogen assimilation and C₁ metabolism, and it is important for maintaining the redox balance of the plant. The overall effect of photorespiratory carbon loss on the net CO₂ fixation of the plant is also strongly influenced by the physiology of the leaf related to CO₂ diffusion. This review outlines the distinction between Rubisco oxygenation and photorespiratory CO₂ release as a basis to evaluate the costs and benefits of photorespiration.