Ozone-Sensitive Arabidopsis Mutants with Deficiencies in Photorespiratory Enzymes

Consecutive oxidative stress in CATALASE2-deficient Arabidopsis negatively regulates Glycolate Oxidase1 activity through S-nitrosylation

Glycolate oxidase (GOX) is a crucial enzyme of photorespiration involving carbon metabolism and stress responses. It is poorly understood, however, how its activities are modulated in response to oxidative stress elicited by various environmental cues. Analysis of Arabidopsis catalase-defective mutant cat2 revealed that the GOX activities were gradually repressed during the growth, which were accompanied by decreased salicylic acid (SA)-dependent cell death, suggesting photorespiratory H2O2 may entrain negative feedback regulation of GOX in an age-dependent manner. Intriguingly, a loss-of-function mutation in GLYCOLATE OXIDASE1 (GOX1) rather than in GOX2 and GOX3 attenuated the SA responses of cat2. We found that GOX1 is S-nitrosylated at Cys-343 during consecutive oxidative stress in the cat2 mutant. Subsequently, increased GOX1-SNO formations may contribute to progressively decreased GOX activities and then compromised photorespiratory H2O2 flux, which forms a negative feedback loop limiting the amplified activation of SA-dependent defence responses. Together, the data reveal that GOX S-nitrosylation is involved in the crosstalk between photorespiratory H2O2 and NO signalling in the fine-tuning regulation of oxidative stress responses and further highlight that NO-based S-nitrosylation acts as an on-off switch for ROS homeostasis.

Flux Calculation for Primary Metabolism Reveals Changes in Allocation of Nitrogen to Different Amino Acid Families When Photorespiratory Activity Changes

Photorespiration, caused by oxygenation of the enzyme Rubisco, is considered a wasteful process, because it reduces photosynthetic carbon gain, but it also supplies amino acids and is involved in amelioration of stress. Here, we show that a sudden increase in photorespiratory activity not only reduced carbon acquisition and production of sugars and starch, but also affected diurnal dynamics of amino acids not obviously involved in the process. Flux calculations based on diurnal metabolite profiles suggest that export of proline from leaves increases, while aspartate family members accumulate. An immense increase is observed for turnover in the cyclic reaction of glutamine synthetase/glutamine-oxoglutarate aminotransferase (GS/GOGAT), probably because of increased production of ammonium in photorespiration. The hpr1-1 mutant, defective in peroxisomal hydroxypyruvate reductase, shows substantial alterations in flux, leading to a shift from the oxoglutarate to the aspartate family of amino acids. This is coupled to a massive export of asparagine, which may serve in exchange for serine between shoot and root.

Urban Air Pollution and Plant Tolerance: Omics Responses to Ozone, Nitrogen Oxides, and Particulate Matter

Urban air pollution is a crucial global challenge, mainly originating from urbanization and industrial activities, which are continuously increasing. Vegetation serves as a natural air filter for air pollution, but adverse effects on plant health, photosynthesis, and metabolism can occur. Recent omics technologies have revolutionized the study of molecular plant responses to air pollution, overcoming previous limitations. This review synthesizes the latest advancements in molecular plant responses to major air pollutants, emphasizing ozone (O3), nitrogen oxides (NOX), and particulate matter (PM) research. These pollutants induce stress responses common to other abiotic and biotic stresses, including the activation of reactive oxygen species (ROSs)-scavenging enzymes and hormone signaling pathways. New evidence has shown the central role of antioxidant phenolic compound biosynthesis, via the phenylpropanoid pathway, in air pollution stress responses. Transcription factors like WRKY, AP2/ERF, and MYB, which connect hormone signaling to antioxidant biosynthesis, were also affected. To date, research has predominantly focused on laboratory studies analyzing individual pollutants. This review highlights the need for comprehensive field studies and the identification of molecular tolerance traits, which are crucial for the identification of tolerant plant species, aimed at the development of sustainable nature-based solutions (NBSs) to mitigate urban air pollution.

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.

Suppression of photorespiratory metabolism by low O2 and presence of aminooxyacetic acid induces oxidative stress in Arabidopsis thaliana leaves

Photorespiration, an essential component of plant metabolism, was upregulated under abiotic stress conditions, such as high light or drought. One of the signals for such upregulation was the rise in reactive oxygen species (ROS). Photorespiration was expected to mitigate oxidative stress by reducing ROS levels. However, it was unclear if ROS levels would increase when photorespiration was lowered. Our goal was to examine the redox status in leaves when photorespiratory metabolism was restricted under low O2 (medium flushed with N2 gas) or by adding aminooxyacetic acid (AOA), a photorespiratory inhibitor. We examined the impact of low O2 and AOA in leaves of Arabidopsis thaliana under dark, moderate, or high light. Downregulation of typical photorespiratory enzymes, including catalase (CAT), glycolate oxidase (GO), and phosphoglycolate phosphatase (PGLP) under low O2 or with AOA confirmed the lowering of photorespiratory metabolism. A marked increase in ROS levels (superoxide and H2O2) indicated the induction of oxidative stress. Thus, our results demonstrated for the first time that restricted photorespiratory conditions increased the extent of oxidative stress. We propose that photorespiration is essential to sustain normal ROS levels and optimize metabolism in cellular compartments of Arabidopsis leaves.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01388-4.

Phytocyanin-encoding genes confer enhanced ozone tolerance in Arabidopsis thaliana

Ozone is a phytotoxic air pollutant that has various damaging effects on plants, including chlorosis and growth inhibition. Although various physiological and genetic studies have elucidated some of the mechanisms underlying plant ozone sensitivity and lesion development, our understanding of plant response to this gas remains incomplete. Here, we show evidence for the involvement of certain apoplastic proteins called phytocyanins, such as AtUC5, that protect against ozone damage. Two representative ozone-inducible responses, chlorosis and stomatal closure, were suppressed in AtUC5-overexpressing plants. Analysis of transgenic plants expressing a chimeric protein composed of AtUC5 fused to green fluorescent protein indicated that this fusion protein localises to the apoplast of plant cells where it appears to suppress early responses to ozone damage such as generation or signalling of reactive oxygen species. Moreover, yeast two-hybrid analyses suggest that AtUC5 may physically interact with stress-related proteins such as copper amine oxidase and late embryogenesis abundant protein-like protein. In addition to AtUC5, other examined phytocyanins such as AtUC6 and AtSC3 could confer ozone tolerance to plants when overexpressed in A. thaliana, suggesting that these proteins act together to protect plants against oxidative stress factors.

Hydrogen sulfide attenuates intracellular oxidative stress via repressing glycolate oxidase activities in Arabidopsis thaliana

BACKGROUND

Hydrogen sulfide (H₂S) has been proposed to exert anti-oxidative effect under many environmental stresses; however, how it influences oxidative stress remains largely unclear.

RESULTS

Here, we assessed the effects of H₂S on oxidative stress responses such as salicylic acid (SA)-dependent cell death, which triggered by increased H₂O₂ availability in Arabidopsis thaliana catalase-deficient mutants cat2 displaying around 20% wild-type catalase activity. H₂S generation and its producing enzyme _L-cysteine desulfhydrase (LCD/DES) were found to transient increase in response to intracellular oxidative stress. Although introducing the mutation of des1, an important LCD, into the cat2 background produced little effect, H₂S fumigation not only rescued the cell death phenotype of cat2 plant, but also attenuated SA accumulation and oxidation of the glutathione pool. Unexpectedly, the activities of major components of ascorbate-glutathione pathway were less affected by the presence of H₂S treatment, but decreased glycolate oxidase (GOX) in combination with accumulation of glycolate implied H₂S treatment impacts the cellular redox homeostasis by repressing the GOX-catalyzed reaction likely via altering the major GOX transcript levels.

CONCLUSIONS

Our findings reveal a link between H₂S and peroxisomal H₂O₂ production that has implications for the understanding of the multifaceted roles of H₂S in the regulation of oxidative stress responses.

Design and Analysis of Native Photorespiration Gene Motifs of Promoter Untranslated Region Combinations Under Short Term Abiotic Stress Conditions

Quantitative traits are rarely controlled by a single gene, thereby making multi-gene transformation an indispensable component of modern synthetic biology approaches. However, the shortage of unique gene regulatory elements (GREs) for the robust simultaneous expression of multiple nuclear transgenes is a major bottleneck that impedes the engineering of complex pathways in plants. In this study, we compared the transcriptional efficacies of a comprehensive list of well-documented promoter and untranslated region (UTR) sequences side by side. The strength of GREs was examined by a dual-luciferase assay in conjunction with transient expression in tobacco. In addition, we created suites of new GREs with higher transcriptional efficacies by combining the best performing promoter-UTR sequences. We also tested the impact of elevated temperature and high irradiance on the effectiveness of these GREs. While constitutive promoters ensure robust expression of transgenes, they lack spatiotemporal regulations exhibited by native promoters. Here, we present a proof-of-principle study on the characterization of synthetic promoters based on cis-regulatory elements of three key photorespiratory genes. This conserved biochemical process normally increases under elevated temperature, low CO2, and high irradiance stress conditions and results in ∼25% loss in fixed CO2. To select stress-responsive cis-regulatory elements involved in photorespiration, we analyzed promoters of two chloroplast transporters (AtPLGG1 and AtBASS6) and a key plastidial enzyme, AtPGLP using PlantPAN3.0 and AthaMap. Our results suggest that these motifs play a critical role for PLGG1, BASS6, and PGLP in mediating response to elevated temperature and high-intensity light stress. These findings will not only enable the advancement of metabolic and genetic engineering of photorespiration but will also be instrumental in related synthetic biology approaches.

Glutathione-dependent denitrosation of GSNOR1 promotes oxidative signalling downstream of H2 O2

Photorespiratory hydrogen peroxide (H₂ O₂ ) plays key roles in pathogenesis responses by triggering the salicylic acid (SA) pathway in Arabidopsis. However, factors linking intracellular H₂ O₂ to activation of the SA pathway remain elusive. In this work, the catalase-deficient Arabidopsis mutant, cat2, was exploited to elucidate the impact of S-nitrosoglutathione reductase 1 (GSNOR1) on H₂ O₂ -dependent signalling pathways. Introducing the gsnor1-3 mutation into the cat2 background increased S-nitrosothiol levels and abolished cat2-triggered cell death, SA accumulation, and associated gene expression but had little additional effect on the major components of the ascorbate-glutathione system or glycolate oxidase activities. Differential transcriptome profiles between gsnor1-3 and cat2 gsnor1-3 together with damped ROS-triggered gene expression in cat2 gsnor1-3 further indicated that GSNOR1 acts to mediate the SA pathway downstream of H₂ O₂ . Up-regulation of GSNOR activity was compromised in cat2 cad2 and cat2 pad2 mutants in which glutathione accumulation was genetically prevented. Experiments with purified recombinant GSNOR revealed that the enzyme is posttranslationally regulated by direct denitrosation in a glutathione-dependent manner. Together, our findings identify GSNOR1-controlled nitrosation as a key factor in activation of the SA pathway by H₂ O₂ and reveal that glutathione is required to maintain this biological function.

Enzymatic Properties of Recombinant Phospho-Mimetic Photorespiratory Glycolate Oxidases from Arabidopsis thaliana and Zea mays

In photosynthetic organisms, the photorespiratory cycle is an essential pathway leading to the recycling of 2-phosphoglycolate, produced by the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase, to 3-phosphoglycerate. Although photorespiration is a widely studied process, its regulation remains poorly understood. In this context, phosphoproteomics studies have detected six phosphorylation sites associated with photorespiratory glycolate oxidases from Arabidopsis thaliana (AtGOX1 and AtGOX2). Phosphorylation sites at T4, T158, S212 and T265 were selected and studied using Arabidopsis and maize recombinant glycolate oxidase (GOX) proteins mutated to produce either phospho-dead or phospho-mimetic enzymes in order to compare their kinetic parameters. Phospho-mimetic mutations (T4D, T158D and T265D) led to a severe inhibition of GOX activity without altering the K_M glycolate. In two cases (T4D and T158D), this was associated with the loss of the cofactor, flavin mononucleotide. Phospho-dead versions exhibited different modifications according to the phospho-site and/or the GOX mutated. Indeed, all T4V and T265A enzymes had kinetic parameters similar to wild-type GOX and all T158V proteins showed low activities while S212A and S212D mutations had no effect on AtGOX1 activity and AtGOX2/ZmGO1 activities were 50% reduced. Taken together, our results suggest that GOX phosphorylation has the potential to modulate GOX activity.

Faster Removal of 2-Phosphoglycolate through Photorespiration Improves Abiotic Stress Tolerance of Arabidopsis

Photorespiration metabolizes 2-phosphoglyolate (2-PG) to avoid inhibition of carbon assimilation and allocation. In addition to 2-PG removal, photorespiration has been shown to play a role in stress protection. Here, we studied the impact of faster 2-PG degradation through overexpression of 2-PG phosphatase (PGLP) on the abiotic stress-response of Arabidopsis thaliana (Arabidopsis). Two transgenic lines and the wild type were subjected to short-time high light and elevated temperature stress during gas exchange measurements. Furthermore, the same lines were exposed to long-term water shortage and elevated temperature stresses. Faster 2-PG degradation allowed maintenance of photosynthesis at combined light and temperatures stress and under water-limiting conditions. The PGLP-overexpressing lines also showed higher photosynthesis compared to the wild type if grown in high temperatures, which also led to increased starch accumulation and shifts in soluble sugar contents. However, only minor effects were detected on amino and organic acid levels. The wild type responded to elevated temperatures with elevated mRNA and protein levels of photorespiratory enzymes, while the transgenic lines displayed only minor changes. Collectively, these results strengthen our previous hypothesis that a faster photorespiratory metabolism improves tolerance against unfavorable environmental conditions, such as high light intensity and temperature as well as drought. In case of PGLP, the likely mechanism is alleviation of inhibitory feedback of 2-PG onto the Calvin–Benson cycle, facilitating carbon assimilation and accumulation of transitory starch.

Heterologous Expression of the Transcription Factor EsNAC1 in Arabidopsis Enhances Abiotic Stress Resistance and Retards Growth by Regulating the Expression of Different Target Genes

Heterologous expression of a transcription factor (TF) gene in a related species is a useful method for crop breeding and the identification of gene function. The differences in phenotype and target gene expression between HE lines (with the heterologous expression of an ortholog) and OX lines (with an overexpressed native gene) must be understood. EsNAC1, encoding a NAC protein and the ortholog of RD26 in Arabidopsis, was cloned from Eutrema salsugineum and introduced into Arabidopsis. The heterologous expression of EsNAC1 retarded the vegetative growth of Arabidopsis, and the transgenic plants (HE lines) showed much greater resistance to salt and oxidative stress than the wild type, Col-0. The HE lines accumulated 2.8-fold (8-h light) of starch, 1.42-fold of Chlorophyll a and 1.31-fold of Chlorophyll b than Col-0 during the light period, with obvious differences compared to the RD26OX line. A genome-wide ChIP (chromatin immunoprecipitation analysis)-on-chip assay revealed that EsNAC1 targeted promoters of different genes compared to RD26. In HE lines, EsNAC1 could specifically upregulate the expression level of TF genes NAC DOMAIN CONTAINING PROTEIN 62 (ANAC062), INTEGRASE-TYPE DNA-BINDING PROTEIN (TINY2), and MYB HYPOCOTYL ELONGATION-RELATED (MYBH) to show more effective abiotic stress resistance than RD26OX lines. Moreover, DELTA1-PYRROLINE-5-CARBOXYLATE SYNTHASE 1 (P5CS1), TRYPTOPHAN BIOSYNTHESIS 2 (TRP2) or GALACTINOL SYNTHASE 2 (GOLS2), was also specifically regulated by EsNAC1 to retard the vegetative growth of HE lines, but not the brassinosteroid singling pathway in RD26OX lines. These differences in phenotypes and metabolism between the HE lines and the RD26OX line implied that the differential features could be produced from the diversity of target genes in the transgenic plants when the ortholog was introduced.

Arabidopsis MLO2 is a negative regulator of sensitivity to extracellular reactive oxygen species

The atmospheric pollutant ozone (O₃ ) is a strong oxidant that causes extracellular reactive oxygen species (ROS) formation, has significant ecological relevance, and is used here as a non-invasive ROS inducer to study plant signalling. Previous genetic screens identified several mutants exhibiting enhanced O₃ sensitivity, but few with enhanced tolerance. We found that loss-of-function mutants in Arabidopsis MLO2, a gene implicated in susceptibility to powdery mildew disease, exhibit enhanced dose-dependent tolerance to O₃ and extracellular ROS, but a normal response to intracellular ROS. This phenotype is increased in a mlo2 mlo6 mlo12 triple mutant, reminiscent of the genetic redundancy of MLO genes in powdery mildew resistance. Stomatal assays revealed that enhanced O₃ tolerance in mlo2 mutants is not caused by altered stomatal conductance. We explored modulation of the mlo2-associated O₃ tolerance, powdery mildew resistance, and early senescence phenotypes by genetic epistasis analysis, involving mutants with known effects on ROS sensitivity or antifungal defence. Mining of publicly accessible microarray data suggests that these MLO proteins regulate accumulation of abiotic stress response transcripts, and transcript accumulation of MLO2 itself is O₃ responsive. In summary, our data reveal MLO2 as a novel negative regulator in plant ROS responses, which links biotic and abiotic stress response pathways.