Growth suppression, altered stomatal responses, and augmented induction of heat shock proteins in cytosolic ascorbate peroxidase (Apx1)-deficient Arabidopsis plants

Elevated Temperature Effects on Protein Turnover Dynamics in Arabidopsis thaliana Seedlings Revealed by 15N-Stable Isotope Labeling and ProteinTurnover Algorithm

Global warming poses a threat to plant survival, impacting growth and agricultural yield. Protein turnover, a critical regulatory mechanism balancing protein synthesis and degradation, is crucial for the cellular response to environmental changes. We investigated the effects of elevated temperature on proteome dynamics in Arabidopsis thaliana seedlings using 15N-stable isotope labeling and ultra-performance liquid chromatography-high resolution mass spectrometry, coupled with the ProteinTurnover algorithm. Analyzing different cellular fractions from plants grown under 22 °C and 30 °C growth conditions, we found significant changes in the turnover rates of 571 proteins, with a median 1.4-fold increase, indicating accelerated protein dynamics under thermal stress. Notably, soluble root fraction proteins exhibited smaller turnover changes, suggesting tissue-specific adaptations. Significant turnover alterations occurred with redox signaling, stress response, protein folding, secondary metabolism, and photorespiration, indicating complex responses enhancing plant thermal resilience. Conversely, proteins involved in carbohydrate metabolism and mitochondrial ATP synthesis showed minimal changes, highlighting their stability. This analysis highlights the intricate balance between proteome stability and adaptability, advancing our understanding of plant responses to heat stress and supporting the development of improved thermotolerant crops.

Physiological function and regulation of ascorbate peroxidase isoforms

Ascorbate peroxidase (APX) reduces H2O2 to H2O by utilizing ascorbate as a specific electron donor and constitutes the ascorbate-glutathione cycle in organelles of plants including chloroplasts, cytosol, mitochondria, and peroxisomes. It has been almost 40 years since APX was discovered as an important plant-specific H2O2-scavenging enzyme, during which time many research groups have conducted molecular physiological analyses. It is now clear that APX isoforms function not only just as antioxidant enzymes but also as important factors in intracellular redox regulation through the metabolism of reactive oxygen species. The function of APX isoforms is regulated at multiple steps, from the transcriptional level to post-translational modifications of enzymes, thereby allowing them to respond flexibly to ever-changing environmental factors and physiological phenomena such as cell growth and signal transduction. In this review, we summarize the physiological functions and regulation mechanisms of expression of each APX isoform.

The ascorbate-glutathione cycle coming of age

Concepts regarding the operation of the ascorbate-glutathione cycle and the associated water/water cycle in the processing of metabolically generated hydrogen peroxide and other forms of reactive oxygen species (ROS) are well established in the literature. However, our knowledge of the functions of these cycles and their component enzymes continues to grow and evolve. Recent insights include participation in the intrinsic environmental and developmental signalling pathways that regulate plant growth, development, and defence. In addition to ROS processing, the enzymes of the two cycles not only support the functions of ascorbate and glutathione, they also have 'moonlighting' functions. They are subject to post-translational modifications and have an extensive interactome, particularly with other signalling proteins. In this assessment of current knowledge, we highlight the central position of the ascorbate-glutathione cycle in the network of cellular redox systems that underpin the energy-sensitive communication within the different cellular compartments and integrate plant signalling pathways.

Copper removal efficacy and stress tolerance potential of Leptolyngbya sp. GUEco1015

Cyanobacteria, a group of microalgae are the potent organism having the ability to survive in the copper rich environment and recently gained too much attention for their profuse proliferation in such water bodies. Amongst the members of cyanobacteria, the current study was conducted on Leptolyngbya sp. GUEco1015, collected from hydrocarbon rich water bodies of Assam, India. Morphological images of treated samples showed a remarkable damage in the cell surface as well as the organelles over the control. Biochemical results revealed a significant increase of enzymatic and non-enzymatic antioxidants during oxidative damage of Cu2+. But, ascorbate in 1.2 ppm (p < 0.01), 1.5 ppm (p < 0.001) and catalase content 1.5 ppm (p < 0.05) showed a significant reduction after a certain level. The cells were optimized to evaluate the maximum Cu2+ removal potential by the cells related to growth. Initial metal concentration 0.1 ppm, pH 7.5, temperature 25 °C and shaking rate 100 rpm are the optimized abiotic parameters which showed maximum 83% of Cu2+ removal. FTIR spectroscopy and EDX data has identified a number of notable functional groups that were involved in Cu2+ binding mechanism and revealed a distinctive peak of Cu with 0.41 wt % which makes the species as one of the competent copper adsorbents.

A new method to measure aquaporin-facilitated membrane diffusion of hydrogen peroxide and cations in plant suspension cells

Plant aquaporins (AQPs) facilitate the membrane diffusion of water and small solutes, including hydrogen peroxide (H2 O2 ) and, possibly, cations, essential signalling molecules in many physiological processes. While the determination of the channel activity generally depends on heterologous expression of AQPs in Xenopus oocytes or yeast cells, we established a genetic tool to determine whether they facilitate the diffusion of H2 O2 through the plasma membrane in living plant cells. We designed genetic constructs to co-express the fluorescent H2 O2 sensor HyPer and AQPs, with expression controlled by a heat shock-inducible promoter in Nicotiana tabacum BY-2 suspension cells. After induction of ZmPIP2;5 AQP expression, a HyPer signal was recorded when the cells were incubated with H2 O2 , suggesting that ZmPIP2;5 facilitates H2 O2 transmembrane diffusion; in contrast, the ZmPIP2;5W85A mutated protein was inactive as a water or H2 O2 channel. ZmPIP2;1, ZmPIP2;4 and AtPIP2;1 also facilitated H2 O2 diffusion. Incubation with abscisic acid and the elicitor flg22 peptide induced the intracellular H2 O2 accumulation in BY-2 cells expressing ZmPIP2;5. We also monitored cation channel activity of ZmPIP2;5 using a novel fluorescent photo-switchable Li+ sensor in BY-2 cells. BY-2 suspension cells engineered for inducible expression of AQPs as well as HyPer expression and the use of Li+ sensors constitute a powerful toolkit for evaluating the transport activity and the molecular determinants of PIPs in living plant cells.

Genome-wide characterization of ascorbate peroxidase gene family in pepper (Capsicum annuum L.) in response to multiple abiotic stresses

Pepper is widely grown all over the world, so it faces many abiotic stresses, such as drought, high temperature, low temperature, salt damage, and so on. Stresses causing the accumulation of reactive oxidative species (ROS) in plants are removed by antioxidant defense systems, and ascorbate peroxidase (APX) is an important antioxidant enzyme. Therefore, the present study performed genome-wide identification of the APX gene family in pepper. We identified nine members of the APX gene family in the pepper genome according to the APX proteins' conserved domain in Arabidopsis thaliana. The physicochemical property analysis showed that CaAPX3 had the longest protein sequence and the largest molecular weight of all genes, while CaAPX9 had the shortest protein sequence and the smallest MW. The gene structure analysis showed that CaAPXs were composed of seven to 10 introns. The CaAPX genes were divided into four groups. The APX genes of groups I and IV were localized in the peroxisomes and chloroplasts, respectively; the group II genes were localized in the chloroplasts and mitochondria; and the group III genes were located in the cytoplasm and extracell. The conservative motif analysis showed that all APX genes in the pepper had motif 2, motif 3, and motif 5. The APX gene family members were distributed on five chromosomes (Chr. 2, 4, 6, 8, and 9). The cis-acting element analysis showed that most CaAPX genes contain a variety of cis-elements related to plant hormones and abiotic stress. RNA-seq expression analysis showed that the expression patterns of nine APXs were different in vegetative and reproductive organs at different growth and development stages. In addition, the qRT-PCR analysis of the CaAPX genes revealed significant differential expression in response to high temperature, low temperature, and salinity stresses in leaf tissue. In conclusion, our study identified the APX gene family members in the pepper and predicted the functions of this gene family, which would provide resources for further functional characterization of CaAPX genes.

Elevated CO2 mitigates the impact of drought stress by upregulating glucosinolate metabolism in Arabidopsis thaliana

Elevated CO₂ (eCO₂ ) reduces the impact of drought, but the mechanisms underlying this effect remain unclear. Therefore, we used a multidisciplinary approach to investigate the interaction of drought and eCO₂ in Arabidopsis thaliana leaves. Transcriptome and subsequent metabolite analyses identified a strong induction of the aliphatic glucosinolate (GL) biosynthesis as a main effect of eCO₂ in drought-stressed leaves. Transcriptome results highlighted the upregulation of ABI5 and downregulation of WRKY63 transcription factors (TF), known to enhance and inhibit the expression of genes regulating aliphatic GL biosynthesis (e.g., MYB28 and 29 TFs), respectively. In addition, eCO₂ positively regulated aliphatic GL biosynthesis by MYB28/29 and increasing the accumulation of GL precursors. To test the role of GLs in the stress-mitigating effect of eCO₂ , we investigated the effect of genetic perturbations of the GL biosynthesis. Overexpression of MYB28, 29 and 76 improved drought tolerance by inducing stomatal closure and maintaining plant turgor, whereas loss of cyp79f genes reduced the stress-mitigating effect of eCO₂ and decreased drought tolerance. Overall, the crucial role of GL metabolism in drought stress mitigation by eCO₂ could be a beneficial trait to overcome future climate challenges.

Regulation of Reactive Oxygen Species during Salt Stress in Plants and Their Crosstalk with Other Signaling Molecules-Current Perspectives and Future Directions

Salt stress is a severe type of environmental stress. It adversely affects agricultural production worldwide. The overproduction of reactive oxygen species (ROS) is the most frequent phenomenon during salt stress. ROS are extremely reactive and, in high amounts, noxious, leading to destructive processes and causing cellular damage. However, at lower concentrations, ROS function as secondary messengers, playing a critical role as signaling molecules, ensuring regulation of growth and adjustment to multifactorial stresses. Plants contain several enzymatic and non-enzymatic antioxidants that can detoxify ROS. The production of ROS and their scavenging are important aspects of the plant's normal response to adverse conditions. Recently, this field has attracted immense attention from plant scientists; however, ROS-induced signaling pathways during salt stress remain largely unknown. In this review, we will discuss the critical role of different antioxidants in salt stress tolerance. We also summarize the recent advances on the detrimental effects of ROS, on the antioxidant machinery scavenging ROS under salt stress, and on the crosstalk between ROS and other various signaling molecules, including nitric oxide, hydrogen sulfide, calcium, and phytohormones. Moreover, the utilization of "-omic" approaches to improve the ROS-regulating antioxidant system during the adaptation process to salt stress is also described.

Crucial role of Arabidopsis glutaredoxin S17 in heat stress response revealed by transcriptome analysis

Heat stress can have detrimental effects on plant growth and development. However, the mechanisms by which the plant is able to perceive changes in ambient temperature, transmit this information, and initiate a temperature-induced response are not fully understood. Previously, we showed that heterologous expression of an Arabidopsis thaliana L. monothiol glutaredoxin AtGRXS17 enhances thermotolerance in various crops, while disruption of AtGRXS17 expression caused hypersensitivity to permissive temperature. In this study, we extend our investigation into the effect of AtGRXS17 and heat stress on plant growth and development. Although atgrxs17 plants were found to exhibit a slight decrease in hypocotyl elongation, shoot meristem development, and root growth compared to wild-type when grown at 22°C, these growth phenotypic differences became more pronounced when growth temperatures were raised to 28°C. Transcriptome analysis revealed significant changes in genome-wide gene expression in atgrxs17 plants compared to wild-type under conditions of heat stress. The expression of genes related to heat stress factors, auxin response, cellular communication, and abiotic stress were altered in atgrxs17 plants in response to heat stress. Overall, our findings indicate that AtGRXS17 plays a critical role in controlling the transcriptional regulation of plant heat stress response pathways.

Chloroplast-localized GUN1 contributes to the acquisition of basal thermotolerance in Arabidopsis thaliana

Heat stress (HS) severely affects different cellular compartments operating in metabolic processes and represents a critical threat to plant growth and yield. Chloroplasts are crucial for heat stress response (HSR), signaling to the nucleus the environmental challenge and adjusting metabolic and biosynthetic functions accordingly. GENOMES UNCOUPLED 1 (GUN1), a chloroplast-localized protein, has been recognized as one of the main players of chloroplast retrograde signaling. Here, we investigate HSR in Arabidopsis wild-type and gun1 plantlets subjected to 2 hours of HS at 45°C. In wild-type plants, Reactive Oxygen Species (ROS) accumulate promptly after HS, contributing to transiently oxidize the cellular environment and acting as signaling molecules. After 3 hours of physiological recovery at growth temperature (22°C), the induction of enzymatic and non-enzymatic antioxidants prevents oxidative damage. On the other hand, gun1 mutants fail to induce the oxidative burst immediately after HS and accumulate ROS and oxidative damage after 3 hours of recovery at 22°C, thus resulting in enhanced sensitivity to HS. These data suggest that GUN1 is required to oxidize the cellular environment, participating in the acquisition of basal thermotolerance through the redox-dependent plastid-to-nucleus communication.

Protein-protein interactions in plant antioxidant defense

The regulation of reactive oxygen species (ROS) levels in plants is ensured by mechanisms preventing their over accumulation, and by diverse antioxidants, including enzymes and nonenzymatic compounds. These are affected by redox conditions, posttranslational modifications, transcriptional and posttranscriptional modifications, Ca²⁺, nitric oxide (NO) and mitogen-activated protein kinase signaling pathways. Recent knowledge about protein-protein interactions (PPIs) of antioxidant enzymes advanced during last decade. The best-known examples are interactions mediated by redox buffering proteins such as thioredoxins and glutaredoxins. This review summarizes interactions of major antioxidant enzymes with regulatory and signaling proteins and their diverse functions. Such interactions are important for stability, degradation and activation of interacting partners. Moreover, PPIs of antioxidant enzymes may connect diverse metabolic processes with ROS scavenging. Proteins like receptor for activated C kinase 1 may ensure coordination of antioxidant enzymes to ensure efficient ROS regulation. Nevertheless, PPIs in antioxidant defense are understudied, and intensive research is required to define their role in complex regulation of ROS scavenging.

Multi-omic characterization of bifunctional peroxidase 4-coumarate 3-hydroxylase knockdown in Brachypodium distachyon provides insights into lignin modification-associated pleiotropic effects

A bifunctional peroxidase enzyme, 4-coumarate 3-hydroxylase (C3H/APX), provides a parallel route to the shikimate shunt pathway for the conversion of 4-coumarate to caffeate in the early steps of lignin biosynthesis. Knockdown of C3H/APX (C3H/APX-KD) expression has been shown to reduce the lignin content in Brachypodium distachyon. However, like many other lignin-modified plants, C3H/APX-KDs show unpredictable pleiotropic phenotypes, including stunted growth, delayed senescence, and reduced seed yield. A system-wide level understanding of altered biological processes in lignin-modified plants can help pinpoint the lignin-modification associated growth defects to benefit future studies aiming to negate the yield penalty. Here, a multi-omic approach was used to characterize molecular changes resulting from C3H/APX-KD associated lignin modification and negative growth phenotype in Brachypodium distachyon. Our findings demonstrate that C3H/APX knockdown in Brachypodium stems substantially alters the abundance of enzymes implicated in the phenylpropanoid biosynthetic pathway and disrupt cellular redox homeostasis. Moreover, it elicits plant defense responses associated with intracellular kinases and phytohormone-based signaling to facilitate growth-defense trade-offs. A deeper understanding along with potential targets to mitigate the pleiotropic phenotypes identified in this study could aid to increase the economic feasibility of lignocellulosic biofuel production.

Reactive Carbonyl Species Inhibit Blue-Light-Dependent Activation of the Plasma Membrane H+-ATPase and Stomatal Opening

Reactive oxygen species (ROS) play a central role in plant responses to biotic and abiotic stresses. ROS stimulate stomatal closure by inhibiting blue light (BL)-dependent stomatal opening under diverse stresses in the daytime. However, the stomatal opening inhibition mechanism by ROS remains unclear. In this study, we aimed to examine the impact of reactive carbonyl species (RCS), lipid peroxidation products generated by ROS, on BL signaling in guard cells. Application of RCS, such as acrolein and 4-hydroxy-(E)-2-nonenal (HNE), inhibited BL-dependent stomatal opening in the epidermis of Arabidopsis thaliana. Acrolein also inhibited H+ pumping and the plasma membrane H+-ATPase phosphorylation in response to BL. However, acrolein did not inhibit BL-dependent autophosphorylation of phototropins and the phosphorylation of BLUE LIGHT SIGNALING1 (BLUS1). Similarly, acrolein affected neither the kinase activity of BLUS1 nor the phosphatase activity of protein phosphatase 1, a positive regulator of BL signaling. However, acrolein inhibited fusicoccin-dependent phosphorylation of H+-ATPase and stomatal opening. Furthermore, carnosine, an RCS scavenger, partially alleviated the abscisic-acid- and hydrogen-peroxide-induced inhibition of BL-dependent stomatal opening. Altogether, these findings suggest that RCS inhibit BL signaling, especially H+-ATPase activation, and play a key role in the crosstalk between BL and ROS signaling pathways in guard cells.

Development of a Genome-Edited Tomato With High Ascorbate Content During Later Stage of Fruit Ripening Through Mutation of SlAPX4

Ascorbate is an essential antioxidant substance for humans. Due to the lack of ascorbate biosynthetic enzyme, a human must intake ascorbate from the food source. Tomato is one of the most widely consumed fruits, thus elevation of ascorbate content in tomato fruits will improve their nutritional value. Here we characterized Solanum lycopersicum ASCORBATE PEROXIDASE 4 (SlAPX4) as a gene specifically induced during fruit ripening. In tomatoes, ascorbate accumulates in the yellow stage of fruits, then decreases during later stages of fruit ripening. To investigate whether SlAPX is involved in the decrease of ascorbate, the expression of SlAPXs was analyzed during fruit maturation. Among nine SlAPXs, SlAPX4 is the only gene whose expression was induced during fruit ripening. Mutation of SlAPX4 by the CRISPR/Cas9 system increased ascorbate content in ripened tomato fruits, while ascorbate content in leaves was not significantly changed by mutation of SlAPX4. Phenotype analysis revealed that mutation of SlAPX4 did not induce an adverse effect on the growth of tomato plants. Collectively, we suggest that SlAPX4 mediates a decrease of ascorbate content during the later stage of fruit ripening, and mutation of SlAPX4 can be used for the development of genome-edited tomatoes with elevated ascorbate content in fruits.

Reactive Oxygen Species, Antioxidant Responses and Implications from a Microbial Modulation Perspective

Simple Summary

Environmental conditions are subject to unprecedented changes due to recent progressive anthropogenic activities on our planet. Plants, as the frontline of food security, are susceptible to these changes, resulting in the generation of unavoidable byproducts of metabolism (ROS), which eventually affect their productivity. The response of plants to these unfavorable conditions is highly intricate and depends on several factors, among them are the species/genotype tolerance level, intensity, and duration of stress factors. Defensive mechanisms in plant systems, by nature, are concerned primarily with generating enzymatic and non-enzymatic antioxidants. In addition to this, plant-microbe interactions have been found to improve immune systems in plants suffering from drought and salinity stress.

Abstract

Plants are exposed to various environmental stresses in their lifespan that threaten their survival. Reactive oxygen species (ROS), the byproducts of aerobic metabolism, are essential signalling molecules in regulating multiple plant developmental processes as well as in reinforcing plant tolerance to biotic and abiotic stimuli. However, intensified environmental challenges such as salinity, drought, UV irradiation, and heavy metals usually interfere with natural ROS metabolism and homeostasis, thus aggravating ROS generation excessively and ultimately resulting in oxidative stress. Cellular damage is confined to the degradation of biomolecular structures, including carbohydrates, proteins, lipids, pigments, and DNA. The nature of the double-edged function of ROS as a secondary messenger or harmful oxidant has been attributed to the degree of existing balance between cellular ROS production and ROS removal machinery. The activities of enzyme-based antioxidants, catalase (CAT, EC 1.11.1.6), monodehydroascorbate reductase (MDHAR, E.C.1.6.5.4), dehydroascorbate reductase (DHAR, EC 1.8.5.1), superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.6.4.2), and guaiacol peroxidase (GPX, EC 1.11.1.7); and non-enzyme based antioxidant molecules, ascorbate (AA), glutathione (GSH), carotenoids, α-tocopherol, prolines, flavonoids, and phenolics, are indeed parts of the defensive strategies developed by plants to scavenge excess ROS and to maintain cellular redox homeostasis during oxidative stress. This review briefly summarises current knowledge on enzymatic and non-enzymatic antioxidant machinery in plants. Moreover, additional information about the beneficial impact of the microbiome on countering abiotic/biotic stresses in association with roots and plant tissues has also been provided.

Improving oxidative stress resilience in plants

Originally conceived as harmful metabolic byproducts, reactive oxygen species (ROS) are now recognized as an integral part of numerous cellular programs. Thanks to their diverse physicochemical properties, compartmentalized production, and tight control exerted by the antioxidant machinery they activate signaling pathways that govern plant growth, development, and defense. Excessive ROS levels are often driven by adverse changes in environmental conditions, ultimately causing oxidative stress. The associated negative impact on cellular constituents have been a major focus of decade-long research efforts to improve the oxidative stress resilience by boosting the antioxidant machinery in model and crop species. We highlight the role of enzymatic and non-enzymatic antioxidants as integral factors of multiple signaling cascades beyond their mere function to prevent oxidative damage under adverse abiotic stress conditions.

Phenotyping stomatal closure by thermal imaging for GWAS and TWAS of water use efficiency-related genes

Stomata allow CO2 uptake by leaves for photosynthetic assimilation at the cost of water vapor loss to the atmosphere. The opening and closing of stomata in response to fluctuations in light intensity regulate CO2 and water fluxes and are essential for maintaining water-use efficiency (WUE). However, a little is known about the genetic basis for natural variation in stomatal movement, especially in C4 crops. This is partly because the stomatal response to a change in light intensity is difficult to measure at the scale required for association studies. Here, we used high-throughput thermal imaging to bypass the phenotyping bottleneck and assess 10 traits describing stomatal conductance (gs) before, during and after a stepwise decrease in light intensity for a diversity panel of 659 sorghum (Sorghum bicolor) accessions. Results from thermal imaging significantly correlated with photosynthetic gas exchange measurements. gs traits varied substantially across the population and were moderately heritable (h2 up to 0.72). An integrated genome-wide and transcriptome-wide association study identified candidate genes putatively driving variation in stomatal conductance traits. Of the 239 unique candidate genes identified with the greatest confidence, 77 were putative orthologs of Arabidopsis (Arabidopsis thaliana) genes related to functions implicated in WUE, including stomatal opening/closing (24 genes), stomatal/epidermal cell development (35 genes), leaf/vasculature development (12 genes), or chlorophyll metabolism/photosynthesis (8 genes). These findings demonstrate an approach to finding genotype-to-phenotype relationships for a challenging trait as well as candidate genes for further investigation of the genetic basis of WUE in a model C4 grass for bioenergy, food, and forage production.

Nuclear transport factor GmNTF2B-1 enhances soybean drought tolerance by interacting with oxidoreductase GmOXR17 to reduce reactive oxygen species content

Drought is a critical abiotic stressor that modulates soybean yield. Drought stress drastically enhances reactive oxygen species (ROS) formation, and maintaining ROS content above a cytostatic level but below a cytotoxic level is essential for normal biology processes in plants. At present, most of the known ROS-scavenging systems are in the cytoplasm, and the mechanism of ROS regulation in the nucleus remains unclear. GmNTF2B-1 is a member of the IV subgroup in the nucleus transporter family. Its expression is localized to the roots and is stimulated by drought stress. In this study, the overexpression of GmNTF2B-1 was found to improve the drought tolerance of transgenic soybean by influencing the ROS content in plants. An oxidoreductase, GmOXR17, was identified to interact with GmNTF2B-1 in the nucleus through the yeast two-hybrid, coimmunoprecipitation and bimolecular fluorescence complementation assays. The drought tolerance of GmOXR17 transgenic soybean was similar to that of GmNTF2B-1. GmNTF2B-1 was expressed in both cytoplasm and nucleus, and GmOXR17 transferred from the cytoplasm to the nucleus under drought stress. The overexpression of GmNTF2B-1 enhanced the nuclear entry of GmOXR17, and the overexpression of GmNTF2B-1 or GmOXR17 could decrease the H2 O2 content and oxidation level in the nucleus. In conclusion, the interaction between GmNTF2B-1 and GmOXR17 may enhance the nuclear entry of GmOXR17, thereby enhancing nuclear ROS scavenging to improve the drought resistance of soybean.

Genome-wide analysis and expression profiling of H-type Trx family in Phaseolus vulgaris revealed distinctive isoforms associated with symbiotic N2-fixing performance and abiotic stress response

Thioredoxins (Trxs) are implicated in plant development and stress tolerance through redox regulation of target proteins. Trxs of Type h (Trxhs) constitute the largest and the most complicated cluster in the Trx family because of their unknown individual functions. Here, we identified and characterized the Phaseolus vulgaris Trxh family during development, mutualistic interactions and in response to abiotic stress. P. vulgaris (common bean) Trxh gene family (PvTrxh) encompasses 12 isoforms (PvTrxh1-h12), subdivided into 3 groups according to their amino acid sequence features. In silico RNA-seq -based expression analysis showed a differential expression of PvTrxh genes during development. RT-qPCR analysis of PvTrxh genes during nodule organogenesis revealed their highest expression in the nodule primordium (NP). Interestingly, in response to symbiosis, specific PvTrxh isoforms (PvTrxh3 and h5) were found to be highly upregulated compared to mock-inoculated plants. In addition, their expression patterns in the NP positively correlated with the symbiotic N₂-fixing efficiency of the Rhizobium strain, as revealed by a number of symbiotic efficiency parameters (ARA, leghemoglobin content, biomass, and total soluble proteins), concomitantly with increased amounts of hydrogen peroxide (H₂O₂). On the other hand, distinctive PvTrxh isoforms were found to be upregulated in plant leaves, where H₂O₂ amounts were elevated, in response to both salt and drought constraints. When exogenously applied, H₂O₂ upregulated specific PvTrxh isoforms in plant leaves and roots. These findings point to a specific, rather than redundant, function for Trxh proteins in common bean beside the association of distinctive Trxh isoforms with symbiosis and abiotic stress response.

Comparative transcriptome analyses revealed different heat stress responses in pigeonpea (Cajanus cajan) and its crop wild relatives

Comparative transcriptome analyses accompanied by biochemical assays revealed high variability in heat stress response in Cajanus species. Among the studied species, C. scarabaeoides was the most thermotolerant followed by C. cajanifolius, C. cajan, and C. acutifolius. Pigeonpea is one of the climate-resilient grain legumes. Though the optimum temperature for cultivated pigeonpea is ~ 25-35 °C, its wild relatives grow in temperatures ranging between 18 and 45 °C. To gain insight into molecular mechanisms responsible for the heat stress tolerance in pigeonpea, we conducted time-series transcriptome analysis of one pigeonpea cultivar (Cajanus cajan) and two wild relatives, Cajanus acutifolius, and Cajanus scarabaeoides subjected to heat stress at 42 ± 2 ºC for 30 min and 3 h. A total of 9521, 12,447, and 5282 identified transcripts were differentially expressed in C. cajan, C. acutifolius, and C. scarabaeoides, respectively. In this study, we observed that a significant number of genes undergo alternative splicing in a species-specific pattern during heat stress. Gene expression profiling analysis, histochemical assay, chlorophyll content, and electrolyte leakage assay showed that C. scarabaeoides has adaptive features for heat stress tolerance. The gene set enrichment analyses of differentially expressed genes in these Cajanus species during heat stress revealed that oxidoreductase activity, transcription factor activity, oxygen-evolving complex, photosystem-II, thylakoid, phenylpropanoid biosynthetic process, secondary metabolic process, and flavonoid biosynthetic process were highly affected. The histochemical assay showed more lipid peroxidation in C. acutifolius compared to other Cajanus species inferring the presence of higher quantities of polyunsaturated fatty acids in the plasma membrane which might have led to severe damage of membrane-bound organelles like chloroplast, and high electrolyte leakage during heat stress. This study paves the way for the identification of candidate genes, which can be useful for the development of thermo-tolerant pigeonpea cultivars.

Physiological and Gene Expression Changes of Clematis crassifolia and Clematis cadmia in Response to Heat Stress

Clematis is a superior perennial ornamental vine known for varied colors and shapes of its flowers. Clematis crassifolia is sensitive to high temperature, whereas Clematis cadmia has a certain temperature adaptability. Here we analyzed the potential regulatory mechanisms of C. crassifolia and C. cadmia in response to heat stress by studying the photosynthesis, antioxidant parameters, amino acids, and gene expression patterns under three temperature treatments. Heat stress caused the fading of leaves; decreased net photosynthetic rate, stomatal conductance, superoxide dismutase, and catalase activity; increased 13 kinds of amino acids content; and up-regulated the expression of seven genes, including C194329_G3, C194434_G1, and C188817_g1, etc., in C. crassifolia plants. Under the treatments of heat stress, the leaf tips of C. cadmia were wilted, and the net photosynthetic rate and soluble protein content decreased, with the increase of 12 amino acids content and the expression of c194329_g3, c194434_g1, and c195983_g1. Our results showed that C. crassifolia and C. cadmia had different physiological and molecular response mechanisms to heat stress during the ecological adaptation.

Biosynthesis and Cellular Functions of Tartaric Acid in Grapevines

Tartaric acid (TA) is an obscure end point to the catabolism of ascorbic acid (Asc). Here, it is proposed as a "specialized primary metabolite", originating from carbohydrate metabolism but with restricted distribution within the plant kingdom and lack of known function in primary metabolic pathways. Grapes fall into the list of high TA-accumulators, with biosynthesis occurring in both leaf and berry. Very little is known of the TA biosynthetic pathway enzymes in any plant species, although recently some progress has been made in this space. New technologies in grapevine research such as the development of global co-expression network analysis tools and genome-wide association studies, should enable more rapid progress. There is also a lack of information regarding roles for this organic acid in plant metabolism. Therefore this review aims to briefly summarize current knowledge about the key intermediates and enzymes of TA biosynthesis in grapes and the regulation of its precursor, ascorbate, followed by speculative discussion around the potential roles of TA based on current knowledge of Asc metabolism, TA biosynthetic enzymes and other aspects of fruit metabolism.

Identification and Characterization of the APX Gene Family and Its Expression Pattern under Phytohormone Treatment and Abiotic Stress in Populus trichocarpa

Ascorbate peroxidase (APX) is a member of class I of the heme-containing peroxidase family. The enzyme plays important roles in scavenging reactive oxygen species for protection against oxidative damage and maintaining normal plant growth and development, as well as in biotic stress responses. In this study, we identified 11 APX genes in the Populus trichocarpa genome using bioinformatic methods. Phylogenetic analysis revealed that the PtrAPX proteins were classifiable into three clades and the members of each clade shared similar gene structures and motifs. The PtrAPX genes were distributed on six chromosomes and four segmental-duplicated gene pairs were identified. Promoter cis-elements analysis showed that the majority of PtrAPX genes contained a variety of phytohormone- and abiotic stress-related cis-elements. Tissue-specific expression profiles indicated that the PtrAPX genes primarily function in roots and leaves. Real-time quantitative PCR (RT-qPCR) analysis indicated that PtrAPX transcription was induced in response to drought, salinity, high ammonium concentration, and exogenous abscisic acid treatment. These results provide important information on the phylogenetic relationships and functions of the APX gene family in P. trichocarpa.

Abiotic Stress and Reactive Oxygen Species: Generation, Signaling, and Defense Mechanisms

Climate change is an invisible, silent killer with calamitous effects on living organisms. As the sessile organism, plants experience a diverse array of abiotic stresses during ontogenesis. The relentless climatic changes amplify the intensity and duration of stresses, making plants dwindle to survive. Plants convert 1-2% of consumed oxygen into reactive oxygen species (ROS), in particular, singlet oxygen (1O2), superoxide radical (O2•-), hydrogen peroxide (H2O2), hydroxyl radical (•OH), etc. as a byproduct of aerobic metabolism in different cell organelles such as chloroplast, mitochondria, etc. The regulatory network comprising enzymatic and non-enzymatic antioxidant systems tends to keep the magnitude of ROS within plant cells to a non-damaging level. However, under stress conditions, the production rate of ROS increases exponentially, exceeding the potential of antioxidant scavengers instigating oxidative burst, which affects biomolecules and disturbs cellular redox homeostasis. ROS are similar to a double-edged sword; and, when present below the threshold level, mediate redox signaling pathways that actuate plant growth, development, and acclimatization against stresses. The production of ROS in plant cells displays both detrimental and beneficial effects. However, exact pathways of ROS mediated stress alleviation are yet to be fully elucidated. Therefore, the review deposits information about the status of known sites of production, signaling mechanisms/pathways, effects, and management of ROS within plant cells under stress. In addition, the role played by advancement in modern techniques such as molecular priming, systems biology, phenomics, and crop modeling in preventing oxidative stress, as well as diverting ROS into signaling pathways has been canvassed.

G. JK, Makarla U, Ramu VS. Stress-Induced Detoxification Enzymes in Rice Have Broad Substrate Affinity

Reactive carbonyl compounds (RCCs) such as hydroxynonenol, malondialdehyde, acrolein, crotonaldehyde, methylglyoxal, and glyoxal accumulate at higher levels under stress in plants and damage the cell metabolic activities. Plants have evolved several detoxifying enzymes such as aldo-keto reductases (AKRs), aldehyde/alcohol dehydrogenases (ALDH/ADH), and glyoxalases. We report the phylogenetic relationship of these proteins and in silico analysis of rice-detoxifying protein structures and their substrate affinity with cofactors using docking and molecular simulation studies. Molecular simulations with nicotinamide adenine dinucleotide phosphate or glutathione cofactor docking with commonly known reactive substrates suggests that the AKRs, ALDH, and ADH proteins attain maximum conformational changes, whereas glyoxalase has fewer conformational changes with cofactor binding. Several AKRs showed a significant binding affinity with many RCCs. The rice microarray studies showed enhanced expression of many AKRs in resistant genotypes, which also showed higher affinity to RCCs, signifying their importance in managing carbonyl stress. The higher expression of AKRs is regulated by stress-responsive transcription factors (TFs) as we identified stress-specific cis-elements in their promoters. The study reports the stress-responsive nature of AKRs, their regulatory TFs, and their best RCC targets, which may be used for crop improvement programs.

Signaling Toward Reactive Oxygen Species-Scavenging Enzymes in Plants

Reactive oxygen species (ROS) are signaling molecules essential for plant responses to abiotic and biotic stimuli as well as for multiple developmental processes. They are produced as byproducts of aerobic metabolism and are affected by adverse environmental conditions. The ROS content is controlled on the side of their production but also by scavenging machinery. Antioxidant enzymes represent a major ROS-scavenging force and are crucial for stress tolerance in plants. Enzymatic antioxidant defense occurs as a series of redox reactions for ROS elimination. Therefore, the deregulation of the antioxidant machinery may lead to the overaccumulation of ROS in plants, with negative consequences both in terms of plant development and resistance to environmental challenges. The transcriptional activation of antioxidant enzymes accompanies the long-term exposure of plants to unfavorable environmental conditions. Fast ROS production requires the immediate mobilization of the antioxidant defense system, which may occur via retrograde signaling, redox-based modifications, and the phosphorylation of ROS detoxifying enzymes. This review aimed to summarize the current knowledge on signaling processes regulating the enzymatic antioxidant capacity of plants.

Heat Stress Responses and Thermotolerance in Maize

High temperatures causing heat stress disturb cellular homeostasis and impede growth and development in plants. Extensive agricultural losses are attributed to heat stress, often in combination with other stresses. Plants have evolved a variety of responses to heat stress to minimize damage and to protect themselves from further stress. A narrow temperature window separates growth from heat stress, and the range of temperatures conferring optimal growth often overlap with those producing heat stress. Heat stress induces a cytoplasmic heat stress response (HSR) in which heat shock transcription factors (HSFs) activate a constellation of genes encoding heat shock proteins (HSPs). Heat stress also induces the endoplasmic reticulum (ER)-localized unfolded protein response (UPR), which activates transcription factors that upregulate a different family of stress response genes. Heat stress also activates hormone responses and alternative RNA splicing, all of which may contribute to thermotolerance. Heat stress is often studied by subjecting plants to step increases in temperatures; however, more recent studies have demonstrated that heat shock responses occur under simulated field conditions in which temperatures are slowly ramped up to more moderate temperatures. Heat stress responses, assessed at a molecular level, could be used as traits for plant breeders to select for thermotolerance.

Heat-Induced Protein and Superoxide Dismutase Changes in Wild Tetraena propinqua ssp. Migahidii Seedlings

&lt;b&gt;Background and Objective:&lt;/b&gt; Heat stress is known as a raise of 5°C or more than the optimal temperature. In this study, we analyzed the effect of heat stress on protein content, protein electrophoretic pattern and Superoxide Dismutase (SOD) profile in three populations of &lt;i&gt;Tetraena propinqua&lt;/i&gt; subspecies migahidii. &lt;b&gt;Materials and Methods:&lt;/b&gt; Populations of &lt;i&gt;Tetraena propinqua&lt;/i&gt; ssp. migahidii were studied. The seeds were subjected to 25 (control), 30, 35 and 40°C for 4, 24 and 48 hrs and 10 days. &lt;b&gt;Results:&lt;/b&gt; Heat stress (35 and 40°C) elicited total soluble protein in populations 1 and 2 however reduced in population 3 with increasing exposure time to 10 days. New polypeptides of 23 KD at 4 hrs in population 3 below 35°C and population 2 below 40°C and 28 KD at 48 hrs in population one below 30°C however 20 KD altogether populations below 40°C. The expression of most polypeptides diminished for 4 hrs however induced for 24, 48 hrs and 10 days with increase heat temperature to 40°C relative to their expression among the management seedlings. SOD1 and SOD2 have detected altogether most of the genotypes, however, heat stress (35, 40°C) induced the expression of SOD2 and SOD1 and was altogether genotyped for 10 days as compared with the control. &lt;b&gt;Conclusion:&lt;/b&gt; The heat stress caused protein degradation and conjointly induced expression of new synthesized HSPs throughout heat acclimatization may be related to heat injury and the improved thermotolerance in early hours of germination and additional studies are required for its protein identification.

PRC2-mediated H3K27me3 modulates shoot iron homeostasis in Arabidopsis thaliana

Plants use intricate mechanisms to adapt to changing iron conditions because iron is essential and also one of the most limiting nutrients for plant growth. Furthermore, iron is potentially toxic in excess and must be tightly regulated. Previously, we showed that chromatin remodeling via histone 3 lysine 27 trimethylation (H3K27me3) modulates the expression of FIT-dependent genes under iron deficiency in roots. This study builds on our previous findings, showing that H3K27me3 also modulates iron regulation in shoots. In the clf mutant, which lacks the predominant H3K27 tri-methyltransferase, we detected increased iron translocation to shoots under iron deficiency as compared to wild type. Transcriptomic analysis of shoots also revealed differential expression of genes consistent with higher iron levels in clf shoots than wild type shoots under iron-deficient conditions. In addition, we verify that YSL1 and IMA1, two genes involved in signaling iron status from shoots to roots, are direct targets of H3K27me3 and reveal iron-dependent deposition of H3K27me3 on these loci. This study contributes to a better understanding of the molecular mechanisms behind iron regulation in plants, as the effect of PRC2-mediated H3K27me3 on iron homeostasis genes expressed in the shoots has not been previously reported to our knowledge.

Molecular characterization of ascorbate peroxidase (APX) and APX-related (APX-R) genes in Triticum aestivum L

Ascorbate peroxidases (APXs) are heme-dependent H₂O₂ scavenging enzymes involved in myriad biological processes. Herein, a total of 21 TaAPX and six TaAPX-R genes were identified from the A, B and D sub-genomes of Triticum aestivum L. The occurrence of three paralogous gene pairs with unequal evolutionary rate suggested functional divergence. The phylogenetic analysis formed four distinct clades having conserved gene and protein architecture, and sub-cellular localization. The tertiary structure analysis revealed the presence of helices and coils and residues involved in ligand binding. Transcriptional profiling of each TaAPX and TaAPX-R gene suggested their specific role during development and stress response. Modulated transcript expression and APX enzyme activity during various stress conditions indicated their role in stress response. Interaction analyses suggested their association with other genes, miRNAs and various legends. The present study reported numerous features of these genes, and may provide a platform for their detailed functional characterization in future studies.

Cytosolic Ascorbate Peroxidases Plays a Critical Role in Photosynthesis by Modulating Reactive Oxygen Species Level in Stomatal Guard Cell

Photosynthetic rate is one of the key factors limiting yield of cotton. Reactive oxygen species (ROS) generated by abiotic stress imposes numerous detrimental effects and causes tremendous loss of yield. It is worth to study whether ROS scavenging enzymes could affect yield through regulating photosynthetic rate in cotton. In this study, we created transgenic cotton with changes of endogenous ROS by overexpressing or suppressing the expression of cytosolic ascorbate peroxidases (APXs), which are hydrogen peroxide (H₂O₂) scavenging enzymes in plants. The suppression of cytosolic APXs by RNAi brings about a great influence on plant growth and development. Plant height and leaf size declined, and yield-related traits including single boll weight, seed weight, seed size, and lint weight dropped significantly, in IAO lines (cytosolic APX-suppressed lines). The stunted plant growth was due to the decrease of plant photosynthetic rate. The evidences showed that increased ROS level in guard cells inhibited stomatal opening and suppressed the absorption of CO₂ and H₂O in IAO line. The decrease of water content and the increase of water loss rate in leaf exacerbated the decline of photosynthetic rate in cytosolic APX-suppressed lines. Based on these results, it implies that cytosolic APXs as a whole play an important role in maintaining REDOX balance to regulate photosynthetic rate and yield in cotton.

Vitamin C in Plants: Novel Concepts, New Perspectives, and Outstanding Issues

Significance: The concept that vitamin C (l-ascorbic acid) is at the heart of the peroxide processing and redox signaling hub in plants is well established, but our knowledge of the precise mechanisms involved remains patchy at best. Recent Advances: Ascorbate participates in the multifaceted signaling pathways initiated by both reactive oxygen species (ROS) and reactive nitrogen species. Crucially, the apoplastic ascorbate/dehydroascorbate (DHA) ratio that is regulated by ascorbate oxidase (AO) sculpts the apoplastic ROS (apoROS) signal that controls polarized cell growth, biotic and abiotic defences, and cell to cell signaling, as well as exerting control over the light-dependent regulation of photosynthesis. Critical Issues: Here we re-evaluate the roles of ascorbate in photosynthesis and other processes, addressing the question of how much we really know about the regulation of ascorbate homeostasis and its functions in plants, or how AO is regulated to modulate apoROS signals. Future Directions: The role of microRNAs in the regulation of AO activity in relation to stress perception and signaling must be resolved. Similarly, the molecular characterization of ascorbate transporters and mechanistic links between photosynthetic and respiratory electron transport and ascorbate synthesis/homeostasis are a prerequisite to understanding ascorbate homeostasis and function. Similarly, there is little in vivo evidence for ascorbate functions as an enzyme cofactor.

Label-Free Comparative Proteomic Analysis Combined with Laser-Capture Microdissection Suggests Important Roles of Stress Responses in the Black Layer of Maize Kernels

The black layer (BL) is traditionally used as an indicator for kernel harvesting in maize, as it turns visibly dark when the kernel reaches physiological maturity. However, the molecular roles of BL in kernel development have not been fully elucidated. In this work, microscopy images showed that BL began to appear at a growth stage earlier than 10 days after pollination (DAP), and its color gradually deepened to become dark as the development period progressed. Scanning electron microscopy observations revealed that BL is a tissue structure composed of several layers of cells that are gradually squeezed and compressed during kernel development. Laser-capture microdissection (LCM) was used to sample BL and its neighboring inner tissue, basal endosperm transfer layer (BETL), and outer tissue, inner epidermis (IEP), from 20 DAP of kernels. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry profiling (MALDI-TOF MS profiling) detected 41, 104, and 120 proteins from LCM-sampled BL, BETL, and IEP, respectively. Gene ontology (GO) analysis indicated that the 41 BL proteins were primarily involved in the response to stress and stimuli. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis found that the BL proteins were enriched in several defense pathways, such as the ascorbate and aldarate metabolic pathways. Among the 41 BL proteins, six were BL-specific proteins that were only detected from BL. Annotations of five BL-specific proteins were related to stress responses. During kernel development, transcriptional expression of most BL proteins showed an increase, followed by a decrease, and reached a maximum zero to 20 DAP. These results suggest a role for BL in stress responses for protecting filial tissue against threats from maternal sides, which helps to elucidate the biological functions of BL.

The R2R3-MYB transcription factor BnaMYB111L from rapeseed modulates reactive oxygen species accumulation and hypersensitive-like cell death

As one of the largest families of transcription factors in plants, the R2R3-MYB proteins play important roles in diverse biological processes including growth and development, primary and secondary metabolism such as flavonoid and anthocyanin biosynthesis as well as abiotic and biotic stress responses. However, functions of R2R3-MYB genes in rapeseed (Brassica napus L.) remain elusive. Here, we characterized BnaMYB111L, which is homologous to Arabidopsis MYB111 and encodes an R2R3-MYB protein in rapeseed. BnaMYB111L is responsive to abscisic acid (ABA), heat, cold, hydrogen peroxide and fungal pathogen Sclerotinia scelerotiorum treatments through quantitative RT-PCR assay. BnaMYB111L encodes a transcriptional activator and is localized exclusively to nuclei. Interestingly, overexpression of BnaMYB111L in tobacco (Nicotiana benthamiana) and rapeseed protoplasts promoted reactive oxygen species (ROS) production and hypersensitive response-like cell death, accumulation of malondialdehyde (MDA) as well as degradation of chlorophyll. Furthermore, BnaMYB111L expression evoked the alterations of transcript levels of genes encoding ROS-producing enzyme, vacuolar processing enzymes and genes implicated in defense responses. A further dual luciferase reporter assay indicated that BnaMYB111L activated the expression of RbohB, MC4 and ACRE132, which are involved in ROS generation, cell death as well as defense responses. Taken together, this study characterized the function of rapeseed MYB111L and identified its putative target genes involved in ROS production and cell death.

Lack of FIBRILLIN6 in Arabidopsis thaliana affects light acclimation and sulfate metabolism

Arabidopsis thaliana contains 13 fibrillins (FBNs), which are all localized to chloroplasts. FBN1 and FBN2 are involved in photoprotection of photosystem II, and FBN4 and FBN5 are thought to be involved in plastoquinone transport and biosynthesis, respectively. The functions of the other FBNs remain largely unknown. To gain insight into the function of FBN6, we performed coexpression and Western analyses, conducted fluorescence and transmission electron microscopy, stained reactive oxygen species (ROS), measured photosynthetic parameters and glutathione levels, and applied transcriptomics and metabolomics. Using coexpression analyses, FBN6 was identified as a photosynthesis-associated gene. FBN6 is localized to thylakoid and envelope membranes, and its knockout results in stunted plants. The delayed-growth phenotype cannot be attributed to altered basic photosynthesis parameters or a reduced CO₂ assimilation rate. Under moderate light stress, primary leaves of fbn6 plants begin to bleach and contain enlarged plastoglobules. RNA sequencing and metabolomics analyses point to an alteration in sulfate reduction in fbn6. Indeed, glutathione content is higher in fbn6, which in turn confers cadmium tolerance of fbn6 seedlings. We conclude that loss of FBN6 leads to perturbation of ROS homeostasis. FBN6 enables plants to cope with moderate light stress and affects cadmium tolerance.

The Role of ROS Homeostasis in ABA-Induced Guard Cell Signaling

The hormonal and environmental regulation of stomatal aperture is mediated by a complex signaling pathway found within the guard cells that surround stomata. Abscisic acid (ABA) induces stomatal closure in response to drought stress by binding to its guard cell localized receptor, initiating a signaling cascade that includes synthesis of reactive oxygen species (ROS). Genetic evidence in Arabidopsis indicates that ROS produced by plasma membrane respiratory burst oxidase homolog (RBOH) enzymes RBOHD and RBOHF modulate guard cell signaling and stomatal closure. However, ABA-induced ROS accumulates in many locations such as the cytoplasm, chloroplasts, nucleus, and endomembranes, some of which do not coincide with plasma membrane localized RBOHs. ABA-induced guard cell ROS accumulation has distinct spatial and temporal patterns that drive stomatal closure. Productive ROS signaling requires both rapid increases in ROS, as well as the ability of cells to prevent ROS from reaching damaging levels through synthesis of antioxidants, including flavonols. The relationship between locations of ROS accumulation and ABA signaling and the role of enzymatic and small molecule ROS scavengers in maintaining ROS homeostasis in guard cells are summarized in this review. Understanding the mechanisms of ROS production and homeostasis and the role of ROS in guard cell signaling can provide a better understanding of plant response to stress and could provide an avenue for the development of crop plants with increased stress tolerance.

PtomtAPX, a mitochondrial ascorbate peroxidase, plays an important role in maintaining the redox balance of Populus tomentosa Carr

Plant mitochondria are important energy-producing structure and ROS are generated as byproducts. APX is one enzyme of the AsA-GSH cycle to reduces H₂O₂ to water. We identified both PtomtAPX and PtosAPX are located in mitochondria of Populus tomentosa Carr. PtomtAPX is specifically targeted to mitochondria, while PtosAPX is dual targeted to both chloroplast and mitochondria. The expression of PtomtAPX in mitochondria was 60-fold that of PtosAPX by ELISA and qPCR analysis. Under high light stress, the expression levels of PtosAPX increased, while that of PtomtAPX only slightly changed. Compared to the WT, the antisense transgenic PtomtAPX cell lines showed slowed growth, smaller cells impaired mitochondria in MS medium under normal growth. RNA-seq results showed 3121 genes significantly altered expression in the antisense cells, and most of them are important for mitochondrial function, particularly in oxidative phosphorylation. Our findings demonstrates a mitochondrial location for one APX isoform, and provide valuable insight into the mechanism which ROS balance is modulated by AsA-GSH cycle in mitochondria.

Autophagy controls reactive oxygen species homeostasis in guard cells that is essential for stomatal opening

Reactive oxygen species (ROS) function as key signaling molecules to inhibit stomatal opening and promote stomatal closure in response to diverse environmental stresses. However, how guard cells maintain basal intracellular ROS levels is not yet known. This study aimed to determine the role of autophagy in the maintenance of basal ROS levels in guard cells. We isolated the Arabidopsis autophagy-related 2 (atg2) mutant, which is impaired in stomatal opening in response to light and low CO₂ concentrations. Disruption of other autophagy genes, including ATG5, ATG7, ATG10, and ATG12, also caused similar stomatal defects. The atg mutants constitutively accumulated high levels of ROS in guard cells, and antioxidants such as ascorbate and glutathione rescued ROS accumulation and stomatal opening. Furthermore, the atg mutations increased the number and aggregation of peroxisomes in guard cells, and these peroxisomes exhibited reduced activity of the ROS scavenger catalase and elevated hydrogen peroxide (H₂O₂) as visualized using the peroxisome-targeted H₂O₂ sensor HyPer. Moreover, such ROS accumulation decreased by the application of 2-hydroxy-3-butynoate, an inhibitor of peroxisomal H₂O₂-producing glycolate oxidase. Our results showed that autophagy controls guard cell ROS homeostasis by eliminating oxidized peroxisomes, thereby allowing stomatal opening.

Overexpression of Solanum habrochaites microRNA319d (sha-miR319d) confers chilling and heat stress tolerance in tomato (S. lycopersicum)

BACKGROUND

MicroRNA319 (miR319) acts as an essential regulator of gene expression during plant development and under stress conditions. Although the role of miR319a in regulating leaf development has been well studied in tomato (Solanum lycopersicum), the function of the recently discovered wild tomato Solanum habrochaites miRNA319d (sha-miR319d) remains poorly understood. In this study, we overexpressed sha-miR319d in cultivated tomato 'Micro-Tom' to further investigate its role in tomato temperature stress responses.

RESULTS

Under chilling or heat stress, sha-miR319d-overexpressing plants showed enhanced stress tolerance, including lower relative electrolyte leakage (REL), malondialdehyde (MDA) concentration, O₂^- generation and H₂O₂ concentration and higher chlorophyll contents and Fv/Fm values than wild-type (WT) plants. Overexpression of sha-miR319d enhanced the activities of superoxide dismutase (SOD) and catalase (CAT), with possible correlation with elevated expression levels of the genes FeSOD, CuZnSOD and CAT. Moreover, different expression levels of key genes involved in chilling (MYB83 and CBF1), heat (HsfA1a, HsfA1b and Hsp90), and reactive oxygen species (ROS) (ZAT12 and ZAT10) signaling in transgenic plants and WT were determined, suggesting a role for sha-miR319d in regulating tomato temperature stress via chilling, heat and ROS signaling. Silencing GAMYB-like1 increased tomato chilling tolerance as well as the expression levels of CBF1, CuZnSOD, CAT, APX1, APX2, ZAT12 and ZAT10. Additionally, overexpression of sha-miR319d in tomato caused plant leaf crinkling and reduced height.

CONCLUSIONS

Overexpression of sha-miR319d confers chilling and heat stress tolerance in tomato. Sha-miR319d regulates tomato chilling tolerance, possibly by inhibiting expression of GAMYB-like1 and further alters chilling, heat and ROS signal transduction. Our research provides insight for further study of the role of sha-miR319d in tomato growth and stress regulation and lays a foundation for the genetic improvement of tomato.

4-Coumarate 3-hydroxylase in the lignin biosynthesis pathway is a cytosolic ascorbate peroxidase

Lignin biosynthesis is evolutionarily conserved among higher plants and features a critical 3-hydroxylation reaction involving phenolic esters. However, increasing evidence questions the involvement of a single pathway to lignin formation in vascular plants. Here we describe an enzyme catalyzing the direct 3-hydroxylation of 4-coumarate to caffeate in lignin biosynthesis as a bifunctional peroxidase that oxidizes both ascorbate and 4-coumarate at comparable rates. A combination of biochemical and genetic evidence in the model plants Brachypodium distachyon and Arabidopsis thaliana supports a role for this coumarate 3-hydroxylase (C3H) in the early steps of lignin biosynthesis. The subsequent efficient O-methylation of caffeate to ferulate in grasses is substantiated by in vivo biochemical assays. Our results identify C3H as the only non-membrane bound hydroxylase in the lignin pathway and revise the currently accepted models of lignin biosynthesis, suggesting new gene targets to improve forage and bioenergy crops.

Lignin biosynthesis in higher plants relies upon a 3-hydroxylation reaction that can occur via shikimate esters of 4-coumarate. Here, Barros et al. define an alternative biosynthetic pathway via cytosolic ascorbate peroxidase that can catalyze direct 3-hydroxylation of 4-coumarate.

Analyzing the Function of Catalase and the Ascorbate-Glutathione Pathway in H2O2 Processing: Insights from an Experimentally Constrained Kinetic Model

SIGNIFICANCE

Plant stress involves redox signaling linked to reactive oxygen species such as hydrogen peroxide (H₂O₂), which can be generated at high rates in photosynthetic cells. The systems that process H₂O₂ include catalase (CAT) and the ascorbate-glutathione pathway, but interactions between them remain unclear. Modeling can aid interpretation and pinpoint areas for investigation. Recent Advances: Based on emerging data and concepts, we introduce a new experimentally constrained kinetic model to analyze interactions between H₂O₂, CAT, ascorbate, glutathione, and NADPH. The sensitivity points required for accurate simulation of experimental observations are analyzed, and the implications for H₂O₂-linked redox signaling are discussed.

CRITICAL ISSUES

We discuss several implications of the modeled results, in particular the following. (i) CAT and ascorbate peroxidase can share the load in H₂O₂ processing even in optimal conditions. (ii) Intracellular H₂O₂ concentrations more than the low μM range may rarely occur. (iii) Ascorbate redox turnover is largely independent of glutathione until ascorbate peroxidation exceeds a certain value. (iv) NADPH availability may determine glutathione redox status through its influence on monodehydroascorbate reduction. (v) The sensitivity of glutathione status to oxidative stress emphasizes its potential suitability as a sensor of increased H₂O₂.

FUTURE DIRECTIONS

Important future questions include the roles of other antioxidative systems in interacting with CAT and the ascorbate-glutathione pathway as well as the nature and significance of processes that achieve redox exchange between different subcellular compartments. Progress in these areas is likely to be favored by integrating kinetic modeling analyses into experimentally based programs, allowing each approach to inform the other.

Photosynthetic and antioxidant response of wheat to di(2-ethylhexyl) phthalate (DEHP) contamination in the soil

Di(2-ethylhexyl) phthalate (DEHP) is a commonly used, artificially-synthesized, industrial chemical that can be released into the soil. However, to date, there is no comprehensive study on the effects of DEHP on photosynthesis, induction of reactive oxygen species, and response of the antioxidant defense system in wheat plants growing in DEHP contaminated soil. This study was conducted to address this gap in knowledge. Our results showed that after application of 10, 20, and 40 mg/kg DEHP, photosynthetic parameters, fluorescence parameters, and chlorophyll content of wheat leaves at seedling, jointing, and booting stages decreased, while the intercellular carbon dioxide concentration increased. This indicates that the observed decrease in net photosynthetic rate in wheat leaves was due to a non-stomatal limitation, wherein DEHP seems to have hindered the photoelectron transfer process. Both superoxide anion (O₂^-) and hydrogen peroxide (H₂O₂) content increased in the roots, stems, and leaves in plant under DEHP treatment compared with those in the control plants. Antioxidant enzyme activity increased with increasing DEHP stress, except under the 40 mg/kg treatment at the seedling stage. The antioxidant system had a certain protective effect on wheat, but DEHP still caused peroxidation of cell membrane lipids. The extent of DEHP damage to the roots, stems, and leaves was concentration dependent. Furthermore, enzymatic activity tolerance increased with metabolism, and long-term effects of DEHP gradually decreased with plant growth. Finally, the toxic effects of DEHP on root tissues were more serious at the seedling and jointing stages.

β-Lactam Antibiotics Modify Root Architecture and Indole Glucosinolate Metabolism in Arabidopsis thaliana

The presence of antibiotics in soils could be due to natural production by soil microorganisms or to the effect of anthropogenic activities. However, the impact of these compounds on plant physiology has not been thoroughly investigated. To evaluate the effect of β-lactam antibiotics (carbenicillin and penicillin) on the growth and development of Arabidopsis thaliana roots, plants were grown in the presence of different amounts and we found a reduction in root size, an increase in the size of root hairs as well as an abnormal position closer to the tip of the roots. Those phenomena were dependent on the accumulation of both antibiotics inside root tissues and also correlated with a decrease in size of the root apical meristem not related to an alteration in cell division but to a decrease in cell expansion. Using an RNA sequencing analysis, we detected an increase in the expression of genes related to the response to oxidative stress, which would explain the increase in the levels of endogenous reactive oxygen species found in the presence of those antibiotics. Moreover, some auxin-responsive genes were misregulated, especially an induction of CYP79B3, possibly explaining the increase in auxin levels in the presence of carbenicillin and the decrease in the amount of indole glucosinolates, involved in the control of fungal infections. Accordingly, penicillin-treated plants were hypersensitive to the endophyte fungus Colletotrichum tofieldiae. These results underscore the risks for plant growth of β-lactam antibiotics in agricultural soils, and suggest a possible function for these compounds as fungus-produced signaling molecules to modify plant behavior.

Schiff bases containing a furoxan moiety as potential nitric oxide donors in plant tissues

Stable Schiff bases containing a furoxan moiety are synthesized as single regioisomers by the reaction of 3-methyl-2-oxy-furazan-4-carbaldehydewith various amino compounds at room temperature. The structures of synthesized compounds were fully characterized by multinuclear NMR spectroscopy and X-ray crystallography. The effect of synthesized Schiff bases containing a furoxan moiety on biological generation of reactive oxygen species and nitric oxide in plant tissues was investigated for the first time by fluorescence microscopy and the released NO identified as nitrite with Griess reagent. There is a good correlation between the biological generation of NO determined by fluorescence microscopy and with Griess reagent. Some of the synthesized compounds exhibited both nitric oxide and reactive oxygen species generation abilities and represent potential NO donors in plant tissues.

Functional switching of ascorbate peroxidase 2 of rice (OsAPX2) between peroxidase and molecular chaperone

Ascorbate peroxidase (APX) is a class I haem-containing peroxidase, which catalyses the conversion of H₂O₂ to H₂O and O₂ using ascorbate as the specific electron donor. APX plays a central role in the elimination of intracellular reactive oxygen species (ROS) and protects plants from the oxidative damage that can occur as a result of biotic and abiotic stresses. At present, the only known function of APX is as a peroxidase. However, in this study, we demonstrate that Oryza sativa APX2 also operates as a molecular chaperone in rice. The different functions of OsAPX2 correlate strongly with its structural conformation. The high-molecular-weight (HMW) complexes had chaperone activity, whereas the low-molecular-weight (LMW) forms displayed predominantly APX activity. The APX activity was effectively inhibited by sodium azide, which is an inhibitor of haem-containing enzymes, but this did not affect the protein’s activity as a chaperone. Additionally, the OsAPX2 conformational changes could be regulated by salt and heat stresses and these stimulated OsAPX2 dissociation and association, respectively. Our results provide new insight into the roles of APXs.

Cytosolic APX2 is a pleiotropic protein involved in H2O2 homeostasis, chloroplast protection, plant architecture and fertility maintenance

Rice cytoplasmic APX2 is a pleiotropic protein, densely distributed around chloroplasts. It plays key roles in H₂O₂ homeostasis and chloroplast protection, and is related to plant architecture and fertility regulation. Ascorbate peroxidases (APXs) catalyze the conversion of H₂O₂ into H₂O. In this report, we systematically investigated the function of cytosolic APX2 using a T-DNA knockout mutant. Loss of OsAPX2 altered rice architecture including shoot height and leaf inclination, resulting in shoot dwarfing, leaf dispersion and fertility decline. Sixty-five differentially expressed proteins were identified in flag leaves of the milk-ripe stage, mainly involved in photosynthesis, glycolysis and TCA cycle, redox homeostasis, and defense. The absence of APX2 severely impacted the stability of chloroplast proteins, and dramatically reduced their expression levels. Subcellular localization showed that APX2 was enriched around each chloroplast to form a high concentration sphere, highlighting chloroplasts as key targets protected by the protein. Accumulation of H₂O₂ was suppressed in the KO-APX2 mutant, which may benefit from increased CAT activity and functional complementation of APX family members. Unexpectedly, the accumulation of soluble sugar, especially sucrose increased significantly, suggesting that APX2 was involved in regulation of sugar metabolism. Obviously, roles of the cytosolic APX2 are very profound and complex in rice. It can be concluded that the cytosolic APX2 is a pleiotropic protein and an important regulator in ROS homeostasis, chloroplast protection, carbohydrate metabolism as well as plant architecture and fertility maintenance.

Phytohormones and induction of plant-stress tolerance and defense genes by seed and foliar inoculation with Azospirillum brasilense cells and metabolites promote maize growth

Azospirillum spp. are plant-growth-promoting bacteria used worldwide as inoculants for a variety of crops. Among the beneficial mechanisms associated with Azospirillum inoculation, emphasis has been given to the biological nitrogen fixation process and to the synthesis of phytohormones. In Brazil, the application of inoculants containing A. brasilense strains Ab-V5 and Ab-V6 to cereals is exponentially growing and in this study we investigated the effects of maize inoculation with these two strains applied on seeds or by leaf spray at the V2.5 stage growth—a strategy to relieve incompatibility with pesticides used for seed treatment. We also investigate the effects of spraying the metabolites of these two strains at V2.5. Maize growth was promoted by the inoculation of bacteria and their metabolites. When applied via foliar spray, although A. brasilense survival on leaves was confirmed by confocal microscopy and cell recovery, few cells were detected after 24 h, indicating that the effects of bacterial leaf spray might also be related to their metabolites. The major molecules detected in the supernatants of both strains were indole-3-acetic acid, indole-3-ethanol, indole-3-lactic acid and salicylic acid. RT-PCR of genes related to oxidative stress (APX1, APX2, CAT1, SOD2, SOD4) and plant defense (pathogenesis-related PR1, prp2 and prp4) was evaluated on maize leaves and roots. Differences were observed according to the gene, plant tissue, strain and method of application, but, in general, inoculation with Azospirillum resulted in up-regulation of oxidative stress genes in leaves and down-regulation in roots; contrarily, in general, PR genes were down-regulated in leaves and up-regulated in roots. Emphasis should be given to the application of metabolites, especially of Ab-V5 + Ab-V6 that in general resulted in the highest up-regulation of oxidative-stress and PR genes both in leaves and in roots. We hypothesize that the benefits of inoculation of Azospirillum on seeds or by leaf spray, as well as of leaf spraying of Azospirillum metabolites, are strongly correlated with the synthesis of phytohormones and by eliciting genes related to plant-stress tolerance and defense against pathogens.

Coordination between bZIP28 and HSFA2 in the regulation of heat response signals in Arabidopsis

Heat stress can have detrimental effects on yield production worldwide. Although bZIP28 and HSFA2 were identified as putative heat sensors in plants, coordination between them has not been uncovered. In this study, the deficiency in bZIP28 did not affect heat tolerance in plants. However, the plants lacking bZIP28 showed enhanced activation of APXs-, MBF1c-and HSPs-dependent pathways as well as higher level of HsfA2 transcripts and H₂O₂ accumulation, suggesting that these pathways might compensate for the deficiency in bZIP28 during heat stress. In addition, requirement of HSFA2 for the activation of APXs-dependent pathway during heat stress was supported by the analyses of plants lacking HSFA2. Our study demonstrated the flexible mode of heat response pathways involving bZIP28, HSFA2 and ROS-dependent signals.

Effect of lead on physiological and antioxidant responses in two Vigna unguiculata cultivars differing in Pb-accumulation

Lead (Pb) is one of the most toxic anthropogenic pollutants, occurring widely in both terrestrial and aquatic ecosystems, where it impairs plant growth and development. In this work, the effect of 0.5 mM EDTA-Pb was evaluated in two Vigna unguiculata cultivars (SV and SET), with the aim of detecting genotype/cultivar dependent changes in the physiological and anti-oxidant responses (CAT and APX) of a leguminous plant. The data showed that SV accumulated more Pb in roots while SET accumulated more in leaves, indicating differential regulation in Pb-translocation/accumulation. Lead affected the growth of SV less severely than SET, mainly associated with reduced inhibition in photosynthetic parameters. Furthermore, CAT and APX activities increased or were sustained at elevated levels in both cultivars in response to lead. However, gene expression analyses revealed that CAT1 was the main lead responsive gene in SET while CAT2 was more responsive in SV. APX1 was higher expressed in tissues with higher Pb-accumulation while APX2 was ubiquitously responsive to lead in both cultivars. Taken together, these results reveal differential ability of V. unguiculata cultivars in Pb-accumulation in different tissues affecting distinctly physiological and anti-oxidant responses. In addition, the existence of cultivars with predominant Pb-accumulation in aerial tissues invokes a need for studies to identify pollution-safe cultivars of leguminous plants to ensure food safety.