The roles of reactive oxygen metabolism in drought: not so cut and dried

Root system architecture, physiological and transcriptional traits of soybean (Glycine max L.) in response to water deficit: A review

Drought stress is the main limiting factor for global soybean growth and production. Genetic improvement for water and nutrient uptake efficiency is critical to advance tolerance and enable more sustainable and resilient production, underpinning yield growth. The identification of quantitative traits and genes related to water and nutrient uptake will enhance our understanding of the mechanisms of drought tolerance in soybean. This review summarizes drought stress in the context of the physiological traits that enable effective acclimation, with a particular focus on roots. Genes controlling root system architecture play an important role in water and nutrient availability, and therefore important targets for breeding strategies to improve drought tolerance. This review highlights the candidate genes that have been identified as regulators of important root traits and responses to water stress. Progress in our understanding of the function of particular genes, including GmACX1, GmMS and GmPEPCK are discussed in the context of developing a system-based platform for genetic improvement of drought tolerance in soybean.

Genome-resolved metagenomics reveals role of iron metabolism in drought-induced rhizosphere microbiome dynamics

Recent studies have demonstrated that drought leads to dramatic, highly conserved shifts in the root microbiome. At present, the molecular mechanisms underlying these responses remain largely uncharacterized. Here we employ genome-resolved metagenomics and comparative genomics to demonstrate that carbohydrate and secondary metabolite transport functionalities are overrepresented within drought-enriched taxa. These data also reveal that bacterial iron transport and metabolism functionality is highly correlated with drought enrichment. Using time-series root RNA-Seq data, we demonstrate that iron homeostasis within the root is impacted by drought stress, and that loss of a plant phytosiderophore iron transporter impacts microbial community composition, leading to significant increases in the drought-enriched lineage, Actinobacteria. Finally, we show that exogenous application of iron disrupts the drought-induced enrichment of Actinobacteria, as well as their improvement in host phenotype during drought stress. Collectively, our findings implicate iron metabolism in the root microbiome's response to drought and may inform efforts to improve plant drought tolerance to increase food security.

A simple method to determine antioxidant status in aromatic plants subjected to drought stress

Drought is a major environmental stress factor that affects the growth and development of plants. All plants have to maintain the reactive oxygen species within certain levels for normal cellular homeostasis by means of their antioxidant systems, which can be classified as enzymatic and non-enzymatic. Plants under drought stress generate an excess production of reactive oxygen species. At high concentrations, this can be detrimental by producing damage to the protein structures and inhibiting enzymes, as well as oxidizing macromolecules, which may eventually lead to cell death. There has been increasing attention paid to the antioxidant capacity of aromatic/medicinal plants, with a high antioxidant content having been reported in some plant extracts, such as in Mentha piperita (peppermint). Peppermint plants cultivated under drought stress also present high levels of phenolic compounds, peroxidase enzyme activity and lipid peroxidation of membranes. A simple and inexpensive laboratory class is proposed for teaching some mechanisms that plants have evolved to avoid reactive oxygen species damage. The series of lab experiments described is aimed at demonstrating the antioxidant status in aromatic plants subjected to drought stress, by measuring total phenolic compound content (non-enzymatic antioxidant compound), peroxidase activity (enzymatic antioxidant) and malondialdehyde, as convenient biomarkers for lipid peroxidation. The proposed class will be carried out by undergraduate students of the advanced biochemistry course, as part of our biology and agronomy studies. The experiment presented is intended to be used as a vehicle to emphasize the concepts that students have learned in their lectures. This lab exercise to be carried out by the students has dual goals: to apply a methodology only learned superficially on previous courses, and also to increase their understanding of how plants developed resistance mechanisms in order to tolerate drought stress.

Lipoxygenase functions in 1O2 production during root responses to osmotic stress

Drought induces osmotic stress in roots, a condition simulated by the application of high-molecular-weight polyethylene glycol. Osmotic stress results in the reduction of Arabidopsis thaliana root growth and production of 1O2 from an unknown non-photosynthetic source. Reduced root growth can be alleviated by application of the 1O2 scavenger histidine (HIS). Here, we examined the possibility that 1O2 production involves Russell reactions occurring among the enzymatic products of lipoxygenases (LOXs), the fatty acid hydroperoxides. LOX activity was measured for purified soybean (Glycine max) LOX1 and in crude Arabidopsis root extracts using linoleic acid as substrate. Formation of the 13(S)-Hydroperoxy-9(Z),11(E)-octadecadienoic acid product was inhibited by salicylhdroxamic acid, which is a LOX inhibitor, but not by HIS, whereas 1O2 production was inhibited by both. D2O, which specifically extends the half-life of 1O2, augmented the LOX-dependent generation of 1O2, as expected from a Russell-type reaction. The addition of linoleic acid to roots stimulated 1O2 production and inhibited growth, suggesting that the availability of LOX substrate is a rate-limiting step. Indeed, water stress rapidly increased linoleic and linolenic acids by 2.5-fold in roots. Mutants with root-specific microRNA repression of LOXs showed downregulation of LOX protein and activity. The lines with downregulated LOX displayed significantly less 1O2 formation, improved root growth in osmotic stress, and an altered transcriptome response compared with wild type. The results show that LOXs can serve as an enzymatic source of "dark" 1O2 during osmotic stress and demonstrate a role for 1O2 in defining the physiological response.

Comparative Phenotypic and Transcriptomic Analysis Reveals Key Responses of Upland Cotton to Salinity Stress During Postgermination

To understand the molecular mechanisms of salinity tolerance during seed germination and post-germination stages, this study characterized phenotypic and transcriptome responses of two cotton cultivars during salinity stress. The two cultivars were salt-tolerant (ST) LMY37 and salt-sensitive (SS) ZM12, with the former exhibiting higher germination rate, growth, and primary-root fresh weight under salinity stress. Transcriptomic comparison revealed that up-regulation of differentially expressed genes (DEGs) was the main characteristic of transcriptional regulation in ST, while SS DEGs were mainly down-regulated. GO and KEGG analyses uncovered both common and specific responses in ST and SS. Common processes, such as reactive oxygen species (ROS) metabolism and cell wall biosynthesis, may be general responses to salinity in cotton. In contrast, DEGs involved in MAPK-signaling pathway activated by ROS, carotenoid biosynthesis pathway and cysteine and methionine metabolism pathway [producing the precursors of stress hormone abscisic acid (ABA) and ethylene (ET), respectively] as well as stress tolerance related transcription factor genes, showed significant expression differences between ST and SS. These differences might be the molecular basis leading to contrasting salinity tolerance. Silencing of GhERF12, an ethylene response factor gene, caused higher salinity sensitivity and increased ROS accumulation after salinity stress. In addition, peroxidase (POD) and superoxide dismutase (SOD) activity obviously declined after silencing GhERF12. These results suggest that GhERF12 is involved in salinity tolerance during early development. This study provides a novel and comprehensive perspective to understand key mechanisms of salinity tolerance and explores candidate genes that may be useful in developing stress-tolerant crops through biotechnology.

Photochemical efficiency correlated with candidate gene expression promote coffee drought tolerance

The aim of this study was to identify the correlation between photochemical efficiency and candidate genes expression to elucidate the drought tolerance mechanisms in coffee progenies (Icatu Vermelho IAC 3851-2 × Catimor UFV 1602-215) previously identified as tolerant in field conditions. Four progenies (2, 5, 12 and 15) were evaluated under water-deficit conditions (water deficit imposed 8 months after transplanting seedlings to the pots) and under irrigated system. Evaluations of physiological parameters and expression of candidate genes for drought tolerance were performed. Progeny 5 showed capacity to maintain water potential, which contributed to lower qP variation between irrigated and deficit conditions. However, the increases of qN and NPQ in response to stress indicate that this progeny is photochemically responsive to small variations of Ψam protecting the photosystem and maintaining qP. Data obtained for progeny 12 indicated a lower water status maintenance capacity, but with increased qN and NPQ providing maintenance of the ɸPSII and ETR parameters. A PCA analysis revealed that the genes coding regulatory proteins, ABA-synthesis, cellular protectors, isoforms of ascorbate peroxidase clearly displayed a major response to drought stress and discriminated the progenies 5 and 12 which showed a better photochemical response. The genes CaMYB1, CaERF017, CaEDR2, CaNCED, CaAPX1, CaAPX5, CaGolS3, CaDHN1 and CaPYL8a were up-regulated in the arabica coffee progenies with greater photochemical efficiency under deficit and therefore contributing to efficiency of the photosynthesis in drought tolerant progenies.

Comparative Study of Drought Stress Effects on Traditional and Modern Apple Cultivars

Genotype-dependent responses of apples to drought stress were evaluated between commercial and traditional apple cultivars. The results indicate different mechanisms of tolerance to investigated drought stress conditions. Chlorophyll fluorescence induction (OJIP) parameters, chlorophyll and carotenoid content, malondialdehyde (MDA), hydrogen peroxide (H2O2), proline, phenols and leaf water content (WC) were measured. The traditional cultivar "Crvenka" confirmed the best tolerance to a drought stress condition, presenting higher photosynthetic efficiency, higher leaf water content, higher levels of chlorophyll content and lower lipid peroxidation with greater membrane stability. The commercial cultivar "Golden Delicious Reinders" showed decreased water content in leaves, increased lipid peroxidation levels and photoinhibition. Considering all results, the commercial cultivar "Golden Delicious Reinders" was adversely affected by drought, while traditional cultivars exhibited better tolerance to drought stress.

MxMPK6-2-bHLH104 interaction is involved in reactive oxygen species signaling in response to iron deficiency in apple rootstock

Iron (Fe) is a trace element necessary for plant growth. Many land plants have evolved a set of mechanisms associated with the Fe absorption process to deal with the problem of insufficient Fe supply in the soil. During Fe absorption, reactive oxygen species (ROS) can be used as a signal to initiate a response to stress caused by Fe deficiency. However, the molecular mechanisms underlying the involvement of ROS in the Fe deficiency stress response remains unclear. In this study, we have identified a kinase, MxMPK6-2, from Malus xiaojinensis, an apple rootstock that is highly efficient at Fe absorption. MxMPK6-2 has been shown to be responsive to ROS signals during Fe deficiency, and MxMPK6-2 overexpression in apple calli enhanced its tolerance to Fe deficiency. We further screened for proteins in the Fe absorption pathway and identified MxbHLH104, a transcription factor which interacts with MxMPK6-2. MxbHLH104 can be phosphorylated by MxMPK6-2 in vivo, and we confirmed that its phosphorylation increased Fe absorption in apple calli under Fe deficiency, with the presence of ROS promoting this process. Overall, we have demonstrated that MxMPK6-2 is responsive to ROS signaling during Fe deficiency, and is able to control its response by regulating MxbHLH104.

Physiological and Biochemical Dissection Reveals a Trade-off Between Antioxidant Capacity and Heat Tolerance in Bread Wheat (Triticum aestivum L.)

Heat stress alters photosynthetic components and the antioxidant scavenging system, negatively affecting plant growth and development. Plants overcome heat stress damage through an integrated network involving enzymatic and non-enzymatic antioxidants. This study aimed to assess physiological and biochemical responses in contrasting thermo-tolerant wheat varieties exposed to 25 °C (control) and 35 °C (heat stress), during the seedling stage. Our results revealed a substantial decrease in the photosynthetic pigments, carotenoids, anthocyanin content, and increased membrane injury index, malondialdehyde, methylglyoxal (MG), H₂O₂ contents and lipoxygenase activity compared to non-stress wheat seedlings. The heat-tolerant variety BARI Gom 26 ("BG26") maintained higher cellular homeostasis compared to the heat susceptible variety Pavon 76 ("Pavon"), perpetuated by higher accumulation of proline, glycine betaine, ascorbate-glutathione cycle associated enzymes, reduced glutathione and ascorbate concentration in plant cells. Significantly lower levels of MG detoxification and antioxidant activities and ascorbate-glutathione cycle-related enzymatic activities lead to increased susceptibility in variety "Pavon". Hierarchical clustering and principal component analysis revealed that variety "BG26" possess a combination of biochemical responses tailoring antioxidant activities that induced a higher level of tolerance. Taken together, our results provide a pipeline for establishing a trade-off between antioxidant capacity and heat tolerance to facilitate functional genomics and translational research to unravel underlying mechanisms to better adapt wheat to heat stress.

Priming with a Seaweed Extract Strongly Improves Drought Tolerance in Arabidopsis

Drought represents a major threat to plants in natural ecosystems and agricultural settings. The biostimulant Super Fifty (SF), produced from the brown alga Ascophyllum nodosum, enables ecologically friendly stress mitigation. We investigated the physiological and whole-genome transcriptome responses of Arabidopsis thaliana to drought stress after a treatment with SF. SF strongly decreased drought-induced damage. Accumulation of reactive oxygen species (ROS), which typically stifle plant growth during drought, was reduced in SF-primed plants. Relative water content remained high in SF-treated plants, whilst ion leakage, a measure of cell damage, was reduced compared to controls. Plant growth requires a functional shoot apical meristem (SAM). Expression of a stress-responsive negative growth regulator, RESPONSIVE TO DESICCATION 26 (RD26), was repressed by SF treatment at the SAM, consistent with the model that SF priming maintains the function of the SAM during drought stress. Accordingly, expression of the cell cycle marker gene HISTONE H4 (HIS4) was maintained at the SAMs of SF-primed plants, revealing active cell cycle progression after SF priming during drought. In accordance with this, CYCP2;1, which promotes meristem cell division, was repressed by drought but enhanced by SF. SF also positively affected stomatal behavior to support the tolerance to drought stress. Collectively, our data show that SF priming mitigates multiple cellular processes that otherwise impair plant growth under drought stress, thereby providing a knowledge basis for future research on crops.

Exogenous γ-aminobutyric acid (GABA)-induced signaling events and field performance associated with mitigation of drought stress in Phaseolus vulgaris L

Not much information is available to substantiate the possible role of γ -aminobutyric acid (GABA) signaling in mitigating water-deficit stress in snap bean (Phaseolus vulgaris L.) plants under semiarid conditions. Present work aims to investigate the role of exogenous GABA (foliar application; 0.5, 1 and 2 mM) in amelioration of drought stress and improvement of field performance on snap bean plants raised under two drip irrigation regimes (100% and 70% of water requirements). Water stress led to significant reduction in plant growth, leaf relative water content (RWC), cell membrane stability index (CMSI), nutrient uptake (N, P, K, Ca, Fe and Zn), pod yield and its content from protein and total soluble solids (TSS). Meanwhile, lipid peroxidation (malondialdehyde content- MDA), osmolyte content (free amino acids- FAA, proline, soluble sugars) antioxidative defense (activity of superoxide dismutase- SOD, catalase- CAT, peroxidase- POX and ascorbate peroxidase- APX) and the pod fiber content exhibited significantly increase due to water stress. Exogenous GABA application (especially at 2 mM) revealed partial normalization of the effects of drought stress in snap bean plants. GABA-induced mitigation of drought stress was manifested by improvement in growth, water status, membrane integrity, osmotic adjustment, antioxidant defense and nutrient acquisition. Furthermore, GABA application during water stress in snap bean plants resulted in improvement of field performance being manifested by increased pod yield and its quality attributes. To sum up, exogenous GABA appears to function as an effective priming molecule to alleviate drought stress in snap bean plants under semiarid conditions.

Mechanisms Regulating the Dynamics of Photosynthesis Under Abiotic Stresses

Photosynthesis sustains plant life on earth and is indispensable for plant growth and development. Factors such as unfavorable environmental conditions, stress regulatory networks, and plant biochemical processes limits the photosynthetic efficiency of plants and thereby threaten food security worldwide. Although numerous physiological approaches have been used to assess the performance of key photosynthetic components and their stress responses, though, these approaches are not extensive enough and do not favor strategic improvement of photosynthesis under abiotic stresses. The decline in photosynthetic capacity of plants due to these stresses is directly associated with reduction in yield. Therefore, a detailed information of the plant responses and better understanding of the photosynthetic machinery could help in developing new crop plants with higher yield even under stressed environments. Interestingly, cracking of signaling and metabolic pathways, identification of some key regulatory elements, characterization of potential genes, and phytohormone responses to abiotic factors have advanced our knowledge related to photosynthesis. However, our understanding of dynamic modulation of photosynthesis under dramatically fluctuating natural environments remains limited. Here, we provide a detailed overview of the research conducted on photosynthesis to date, and highlight the abiotic stress factors (heat, salinity, drought, high light, and heavy metal) that limit the performance of the photosynthetic machinery. Further, we reviewed the role of transcription factor genes and various enzymes involved in the process of photosynthesis under abiotic stresses. Finally, we discussed the recent progress in the field of biodegradable compounds, such as chitosan and humic acid, and the effect of melatonin (bio-stimulant) on photosynthetic activity. Based on our gathered researched data set, the logical concept of photosynthetic regulation under abiotic stresses along with improvement strategies will expand and surely accelerate the development of stress tolerance mechanisms, wider adaptability, higher survival rate, and yield potential of plant species.

Highlighting reactive oxygen species as multitaskers in root development

Reactive oxygen species (ROS) are naturally produced by several redox reactions during plant regular metabolism such as photosynthesis and respiration. Due to their chemical properties and high reactivity, ROS were initially described as detrimental for cells during oxidative stress. However, they have been further recognized as key players in numerous developmental and physiological processes throughout the plant life cycle. Recent studies report the important role of ROS as growth regulators during plant root developmental processes such as in meristem maintenance, in root elongation, and in lateral root, root hair, endodermis, and vascular tissue differentiation. All involve multifaceted interplays between steady-state levels of ROS with transcriptional regulators, phytohormones, and nutrients. In this review, we attempt to summarize recent findings about how ROS are involved in multiple stages of plant root development during cell proliferation, elongation, and differentiation.

Effects of exogenous silicon on maize seed germination and seedling growth

As the global population continues to increase, global food production needs to double by 2050 to meet the demand. Given the current status of the not expansion of cultivated land area, agronomic seedlings are complete, well-formed and strong, which is the basis of high crop yields. The aim of this experiment was to study the effects of seed germination and seedling growth in response to silicon (from water-soluble Si fertilizer). The effects of Si on the maize germination, seedling growth, chlorophyll contents, osmoprotectant contents, antioxidant enzyme activities, non-enzymatic antioxidant contents and stomatal characteristics were studied by soaking Xianyu 335 in solutions of different concentrations of Si (0, 5, 10, 15, 20, and 25 g·L^-1). In this study, Si treatments significantly increased the seed germination and per-plant dry weight of seedlings (P < 0.05), and the optimal concentration was 15 g·L^-1. As a result of the Si treatment of the seeds, the chlorophyll content, osmotic material accumulation and antioxidant defence system activity increased, reducing membrane system damage, reactive oxygen species contents, and stomatal aperture. The results suggested that 15 g·L^-1 Si significantly stimulated seed germination and promoted the growth of maize seedlings, laying a solid foundation for subsequent maize growth.

Targeting salt stress coping mechanisms for stress tolerance in Brassica: A research perspective

Brassica genus comprises numerous cultivated brassica species with various economic importance. Salt stress is an overwhelming problem causing serious losses in Brassica species (e.g. B. napus, B. rapa, B. oleracea, B. juncea) growth and grain yield production by inducing ionic and ROS toxicity. Given that a significant variation exists in salt tolerance level in Brassica genus, Brassica species exhibited numerous salt tolerance mechanisms which were either overlooked or given less importance to improve and understand innate salt stress tolerance mechanism in Brassica species. In this review, we tried to highlight the importance and recent findings relating to some overlooked and potential mechanisms such as role of neurotransmitters, and role of cytosolic Ca²⁺ and ROS as signaling elements to enhance salt stress tolerance. Studies revealed that salt tolerant brassica species retained more K⁺ in leaf mesophyll which confers overall salinity tolerance in salt tolerance brassica species. Neurotransmitter such as melatonin, dopamiane and eATP regulates K⁺ and Ca²⁺ permeable ion channels and plays a very crucial role in ionic homeostasis under salinity stress in brassica. At the end, the numerous possible salt stress agronomic strategies were also discussed to mitigate the severity of the salt stress in Brassica species.

Physiological and metabolomic responses of bermudagrass (Cynodon dactylon) to alkali stress

Bermudagrass (Cynodon dactylon) is a widely used warm-season turfgrass species with superior stress tolerance except for cold. In this study, a comparative analysis of the responses to alkali stress in bermudagrass at the physiological and metabolomic levels were performed. Mild alkali with relatively low pH slightly inhibited growth of bermudagrass as evidenced by lower electrolyte leakage, more rapid growth and higher survival rate when compared to moderate and severe alkali treatments. Moreover, the amount of 37 metabolites including amino acids, organic acids, sugars and sugar alcohols were modulated by the alkali treatments. Among them, 15 metabolites were involved in carbon and amino acid metabolic pathways. Under mild alkali stress, bermudagrass possibly slowed down metabolisms to maintain basic growth. However, moderate and severe alkali-stressed plants accumulated significantly higher amount of carbohydrates which might result in carbon starvation. Taken together, alkali stress had severely inhibitory effect partially due to combined ionic stress and high pH stress. These results suggested that bermudagrass employed different strategies in response to alkali stresses with different pH and ionic values.

Regulatory Network of Cotton Genes in Response to Salt, Drought and Wilt Diseases (Verticillium and Fusarium): Progress and Perspective

In environmental conditions, crop plants are extremely affected by multiple abiotic stresses including salinity, drought, heat, and cold, as well as several biotic stresses such as pests and pathogens. However, salinity, drought, and wilt diseases (e.g., Fusarium and Verticillium) are considered the most destructive environmental stresses to cotton plants. These cause severe growth interruption and yield loss of cotton. Since cotton crops are central contributors to total worldwide fiber production, and also important for oilseed crops, it is essential to improve stress tolerant cultivars to secure future sustainable crop production under adverse environments. Plants have evolved complex mechanisms to respond and acclimate to adverse stress conditions at both physiological and molecular levels. Recent progresses in molecular genetics have delivered new insights into the regulatory network system of plant genes, which generally includes defense of cell membranes and proteins, signaling cascades and transcriptional control, and ion uptake and transport and their relevant biochemical pathways and signal factors. In this review, we mainly summarize recent progress concerning several resistance-related genes of cotton plants in response to abiotic (salt and drought) and biotic (Fusarium and Verticillium wilt) stresses and classify them according to their molecular functions to better understand the genetic network. Moreover, this review proposes that studies of stress related genes will advance the security of cotton yield and production under a changing climate and that these genes should be incorporated in the development of cotton tolerant to salt, drought, and fungal wilt diseases (Verticillium and Fusarium).

Effects of Exogenous Salicylic Acid on Drought Response and Characterization of Dehydrins in Impatiens walleriana

Impatiens walleriana is a valued ornamental plant sensitive to drought stress. We investigated whether the foliar application of 2mM salicylic acid (SA) can protect potted I. walleriana plants from drought stress. The plants were divided into: watered plants, drought-stressed plants, watered plants treated with SA and drought-stressed plants treated with SA. The number of flowers and flower buds, relative water content (RWC), contents of malondialdehyde (MDA) and proline (Pro) and the activities of superoxide dismutases, catalases and peroxidases were recorded at different time points. Three dehydrin sequences were identified in de novo assembled leaf transcriptome: IwDhn1, IwDhn2.1 and IwDhn2.2. Drought stress caused wilting, floral abortion, reduction of RWC and increased MDA-an indicator of lipid peroxidation. In response to drought, Impatiens accumulated Pro and induced chloroplastic Cu/ZnSOD and two peroxidase isoforms. The most remarkable drought response was strong induction of IwDhn2.1 and IwDhn2.2. Rehydration restored RWC, Pro level, Cu/ZnSOD activity and dehydrins expression in drought-stressed plants approximately to the values of watered plants.SA had ameliorating effects on plants exposed to drought, including prevention of wilting, preservation of RWC, increased Pro accumulation, modulation of antioxidative activities and remarkable decrease of lipid peroxidation, but without effects on flowers' preservation.

Glutathione Enhances Auxin Sensitivity in Arabidopsis Roots

Root development is regulated by the tripeptide glutathione (GSH), a strong non-enzymatic antioxidant found in plants but with a poorly understood function in roots. Here, Arabidopsis mutants deficient in GSH biosynthesis (cad2, rax1, and rml1) and plants treated with the GSH biosynthesis inhibitor buthionine sulfoximine (BSO) showed root growth inhibition, significant alterations in the root apical meristem (RAM) structure (length and cell division), and defects in lateral root formation. Investigation of the molecular mechanisms of GSH action showed that GSH deficiency modulated total ubiquitination of proteins and inhibited the auxin-related, ubiquitination-dependent degradation of Aux/IAA proteins and the transcriptional activation of early auxin-responsive genes. However, the DR5 auxin transcriptional response differed in root apical meristem (RAM) and pericycle cells. The RAM DR5 signal was increased due to the up-regulation of the auxin biosynthesis TAA1 protein and down-regulation of PIN4 and PIN2, which can act as auxin sinks in the root tip. The transcription auxin response (the DR5 signal and expression of auxin responsive genes) in isolated roots, induced by a low (0.1 µM) auxin concentration, was blocked following GSH depletion of the roots by BSO treatment. A higher auxin concentration (0.5 µM) offset this GSH deficiency effect on DR5 expression, indicating that GSH deficiency does not completely block the transcriptional auxin response, but decreases its sensitivity. The ROS regulation of GSH, the active GSH role in cell proliferation, and GSH cross-talk with auxin assume a potential role for GSH in the modulation of root architecture under stress conditions.

Overexpression Levels of LbDREB6 Differentially Affect Growth, Drought, and Disease Tolerance in Poplar

The application of drought stress-regulating transcription factors (TFs) offers a credible way to improve drought tolerance in plants. However, many drought resistant TFs always showed unintended adverse effects on plant growth or other traits. Few studies have been conducted in trees to evaluate and overcome the pleiotropic effects of drought tolerance TFs. Here, we report the dose-dependent effect of the Limonium bicolor LbDREB6 gene on its overexpression in Populus ussurensis. High- and moderate-level overexpression of LbDREB6 significantly increased drought tolerance in a dose-dependent manner. However, the OE18 plants showed stunted growth under normal conditions, but they were also more sensitive to Marssonina brunnea infection than wild type (WT) and OE14 plants. While, OE14 showed normal growth, the pathogen tolerance of them was not significantly different from WT. Many stress-responsive genes were up-regulated in OE18 and OE14 compared to WT, especially for OE18 plants. Meanwhile, more pathogen tolerance related genes were down-regulated in OE18 compared to OE14 and WT plants. We achieved improved drought tolerance by adjusting the increased levels of exogenous DREB genes to avoid the occurrence of growth reduction and reduced disease tolerance.

Physiological and Biochemical Changes in Sugar Beet Seedlings to Confer Stress Adaptability under Drought Condition

The present study was conducted to examine the adaptability of 11 sugar beet cultivars grown under drought stress in the controlled glasshouse. The treatment was initiated on 30-day-old sugar beet plants where drought stress was made withholding water supply for consecutive 10 days while control was done with providing water as per requirement. It was observed that drought stress expressively reduced plant growth, photosynthetic pigments, and photosynthetic quantum yield in all the cultivars but comparative better results were observed in S1 (MAXIMELLA), S2 (HELENIKA), S6 (RECODDINA), S8 (SV2347), and S11 (BSRI Sugarbeet 2) cultivars. Besides, osmolytes like proline, glycine betaine, total soluble carbohydrate, total soluble sugar, total polyphenol, total flavonoid, and DPPH free radical scavenging activity were remarkably increased under drought condition in MAXIMELLA, HELENIKA, TERRANOVA, GREGOIA, SV2348, and BSRI Sugar beet 2 cultivars. In contrast, activities of enzymes like superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) were significantly decreased in all, while the cultivars SV2347, BSRI Sugar beet 1 and BSRI Sugar beet 2 were found with increased ascorbate peroxidase (APX) activity under drought condition. In parallel, polyphenol oxidase (PPO) was increased in all cultivars except HELENIKA. Overall, the cultivars HELENIKA, RECODDINA, GREGOIA, SV2347, SV2348, BSRI Sugar beet 1, and BSRI Sugar beet 2 were found best fitted to the given drought condition. These findings would help further for the improvement of stress adaptive sugar beet cultivars development in the breeding program for drought-prone regions.

Loss of rice PARAQUAT TOLERANCE 3 confers enhanced resistance to abiotic stresses and increases grain yield in field

Plants frequently suffer from environmental stresses in nature and have evolved sophisticated and efficient mechanisms to cope with the stresses. To balance between growth and stress response, plants are equipped with efficient means to switch off the activated stress responses when stresses diminish. We previously revealed such an off-switch mechanism conferred by Arabidopsis PARAQUAT TOLERANCE 3 (AtPQT3) encoding an E3 ubiquitin ligase, knockout of which significantly enhances resistance to abiotic stresses. To explore whether the rice homologue OsPQT3 is functionally conserved, we generated three knockout mutants with CRISPR-Cas9 technology. The OsPQT3 knockout mutants (ospqt3) display enhanced resistance to oxidative and salt stress with elevated expression of OsGPX1, OsAPX1 and OsSOD1. More importantly, the ospqt3 mutants show significantly enhanced agronomic performance with higher yield compared with the wild type under salt stress in greenhouse as well as in field conditions. We further showed that OsPQT3 expression rapidly decreased in response to oxidative and other abiotic stresses as AtPQT3 does. Taken together, these results show that OsPQT3 is functionally well conserved in rice as an off-switch in stress response as AtPQT3 in Arabidopsis. Therefore, PQT3 locus provides a promising candidate for crop improvement with enhanced stress resistance by gene editing technology.

Individual and combined effects of amoxicillin, enrofloxacin, and oxytetracycline on Lemna minor physiology

We investigated individual and combined effects of environmentally representative concentrations of amoxicillin (AMX; 2 μg l^-1), enrofloxacin (ENR; 2 μg l^-1), and oxytetracycline (OXY; 1 μg l^-1) on the aquatic macrophyte Lemna minor. While the concentrations of AMX and ENR tested were not toxic, OXY decreased plant growth and cell division. OXY induced hydrogen peroxide (H₂O₂) accumulation and related oxidative stress through its interference with the activities of mitochondria electron transport chain enzymes, although those deleterious effects could be ameliorated by the presence of AMX and/or ENR, which prevented the overaccumulation of ROS by increasing catalase enzyme activity. L. minor plants accumulated significant quantities of AMX, ENR and OXY from the media, although competitive uptakes were observed when plants were submitted to binary or tertiary mixtures of those antibiotics. Our results therefore indicate L. minor as a candidate for phytoremediation of service waters contaminated by AMX, ENR, and/or OXY.

Hydrogen sulfide promotes rice drought tolerance via reestablishing redox homeostasis and activation of ABA biosynthesis and signaling

Hydrogen sulfide (H₂S) has been explored as the third biologically gasotransmitter regulating plant adaptation response, however, its possible mechanisms on drought tolerance are not completely clear yet. Here, we discovered that during dehydration treatment, the activities of L-cysteine desulfhydrase (LCD), the important synthetic enzymes of H₂S in rice, was enhanced in rice seedling leaves, further leading to continuous increasing of endogenous H₂S production. Pretreatment with NaHS, a well-known H₂S donor, significantly improved rice performance under drought stress. The beneficial roles of NaHS were confirmed by the alleviation of lipid peroxidation, and the activation of antioxidant defence system. Importantly, besides repressing its degradation pathway, NaHS pretreatment promoted ABA de-novo synthesis as well. This resulted in an increase of ABA accumulation and the expression of downstream ABA-responsive genes in rice seedling upon drought stress. Together, the present study illustrated that H₂S improve drought tolerance via reestablishing redox homeostasis and triggering ABA signaling.

Regulation of the size of photosystem II light harvesting antenna represents a universal mechanism of higher plant acclimation to stress conditions

We investigated acclimatory responses of Arabidopsis plants to drought and salinity conditions before the appearance of obvious signs of damage caused by these factors. We detected changes indicating an increase in the reduction level of the chloroplast plastoquinone pool (PQ pool) 5-7 days after introduction of the stress factors. After 10-14 days, a decrease in the size of PSII light harvesting antenna was observed in plants under conditions of drought and salinity. This was confirmed by a decrease in content of PSII antenna proteins and by downregulation of gene expression levels of these proteins under the stress conditions. No changes in values of performance index and maximum quantum yield of PSII were detected. Under drought and salinity, the content of hydrogen peroxide in leaves was higher than in control leaves. Thus, we propose that reduction of the size of PSII antenna represents one of the universal mechanisms of acclimation of higher plants to stress factors and the downsizing already begins to manifest under mild stress conditions. Both the PQ pool reduction state and the hydrogen peroxide content are important factors needed for the observed rearrangement.

Comparative Transcriptomics and Co-Expression Networks Reveal Tissue- and Genotype-Specific Responses of qDTYs to Reproductive-Stage Drought Stress in Rice (Oryza sativa L.)

Rice (Oryza sativa L.) is more sensitive to drought stress than other cereals. To dissect molecular mechanisms underlying drought-tolerant yield in rice, we applied differential expression and co-expression network approaches to transcriptomes from flag-leaf and emerging panicle tissues of a drought-tolerant yield introgression line, DTY-IL, and the recurrent parent Swarna, under moderate reproductive-stage drought stress. Protein turnover and efficient reactive oxygen species scavenging were found to be the driving factors in both tissues. In the flag-leaf, the responses further included maintenance of photosynthesis and cell wall reorganization, while in the panicle biosynthesis of secondary metabolites was found to play additional roles. Hub genes of importance in differential drought responses included an expansin in the flag-leaf and two peroxidases in the panicle. Overlaying differential expression data with allelic variation in DTY-IL quantitative trait loci allowed for the prioritization of candidate genes. They included a differentially regulated auxin-responsive protein, with DTY-IL-specific amino acid changes in conserved domains, as well as a protein kinase with a DTY-IL-specific frameshift in the C-terminal region. The approach highlights how the integration of differential expression and allelic variation can aid in the discovery of mechanism and putative causal contribution underlying quantitative trait loci for drought-tolerant yield.

LED lights increase an antioxidant capacity of Arabidopsis thaliana under wound-induced stresses

A comparison among four light emitting diode (LED) lights including red LED (R), blue LED (B), red (70%) + blue (30%) LED (RB) and white LED (W) as well as fluorescent (F) light was made on antioxidative capacity of Arabidopsis thaliana (L.) Heynh. in response to wounding. Under wound-stress condition, LED-exposed plants, especially RB-irradiated plants, maintained significantly higher shoot dry weight and antioxidant enzymes activities compared with those irradiated with fluorescent lights. The highest amounts of both chlorophyll a and b were observed in the leaves treated with B light. Also, the concentration of H2O2 was higher under the condition of RB and B lights compared with the other light environments. The highest amount of malondialdehyde was measured in plants exposed to F and B lights. Similarly, wounded leaves under F and B light conditions showed the maximum lipoxygenase activity, whereas R-exposed leaves had the lowest lipoxygenase activity. In contrast, the highest level of phenolic compounds was found in R and RB exposed leaves in response to wounding. Among the five light treatments, RB and B lights were more effective in stimulating anthocyanin synthesis; however, RB-exposed plants were more efficient in the late-induction of the PAL gene (phenylalanine ammonia lyase catalyses the first step of the general phenylpropanoid pathway). Collectively, we reasoned that RB light condition gives a superior capacity to Arabidopsis thaliana to tolerate wound-stress. Also, we propose the probable signalling role of ROS in light-stimulated wound responses in Arabidopsis.

ROS Homeostasis in Abiotic Stress Tolerance in Plants

Climate change-induced abiotic stress results in crop yield and production losses. These stresses result in changes at the physiological and molecular level that affect the development and growth of the plant. Reactive oxygen species (ROS) is formed at high levels due to abiotic stress within different organelles, leading to cellular damage. Plants have evolved mechanisms to control the production and scavenging of ROS through enzymatic and non-enzymatic antioxidative processes. However, ROS has a dual function in abiotic stresses where, at high levels, they are toxic to cells while the same molecule can function as a signal transducer that activates a local and systemic plant defense response against stress. The effects, perception, signaling, and activation of ROS and their antioxidative responses are elaborated in this review. This review aims to provide a purview of processes involved in ROS homeostasis in plants and to identify genes that are triggered in response to abiotic-induced oxidative stress. This review articulates the importance of these genes and pathways in understanding the mechanism of resistance in plants and the importance of this information in breeding and genetically developing crops for resistance against abiotic stress in plants.

Glycine Betaine Accumulation, Significance and Interests for Heavy Metal Tolerance in Plants

Unexpected biomagnifications and bioaccumulation of heavy metals (HMs) in the surrounding environment has become a predicament for all living organisms together with plants. Excessive release of HMs from industrial discharge and other anthropogenic activities has threatened sustainable agricultural practices and limited the overall profitable yield of different plants species. Heavy metals at toxic levels interact with cellular molecules, leading towards the unnecessary generation of reactive oxygen species (ROS), restricting productivity and growth of the plants. The application of various osmoprotectants is a renowned approach to mitigate the harmful effects of HMs on plants. In this review, the effective role of glycine betaine (GB) in alleviation of HM stress is summarized. Glycine betaine is very important osmoregulator, and its level varies considerably among different plants. Application of GB on plants under HMs stress successfully improves growth, photosynthesis, antioxidant enzymes activities, nutrients uptake, and minimizes excessive heavy metal uptake and oxidative stress. Moreover, GB activates the adjustment of glutathione reductase (GR), ascorbic acid (AsA) and glutathione (GSH) contents in plants under HM stress. Excessive accumulation of GB through the utilization of a genetic engineering approach can successfully enhance tolerance against stress, which is considered an important feature that needs to be investigated in depth.

Knockout of Pi21 by CRISPR/Cas9 and iTRAQ-Based Proteomic Analysis of Mutants Revealed New Insights into M. oryzae Resistance in Elite Rice Line

Rice blast (Magnaporthe oryzae) is a devastating disease affecting rice production globally. The development of cultivars with host resistance has been proved to be the best strategy for disease management. Several rice-resistance genes (R) have been recognized which induce resistance to blast in rice but R gene-mediated mechanisms resulting in defense response still need to be elucidated. Here, mutant lines generated through CRISPR/Cas9 based targeted mutagenesis to investigate the role of Pi21 against blast resistance and 17 mutant plants were obtained in T0 generation with the mutation rate of 66% including 26% bi-allelic, 22% homozygous, 12% heterozygous, and 3% chimeric and 17 T-DNA-free lines in T1 generation. The homozygous mutant lines revealed enhanced resistance to blast without affecting the major agronomic traits. Furthermore, comparative proteome profiling was adopted to study the succeeding proteomic regulations, using iTRAQ-based proteomic analysis. We identified 372 DEPs, among them 149 up and 223 were down-regulated, respectively. GO analysis revealed that the proteins related to response to stimulus, photosynthesis, carbohydrate metabolic process, and small molecule metabolic process were up-regulated. The most of DEPs were involved in metabolic, ribosomal, secondary metabolites biosynthesis, and carbon metabolism pathways. 40S ribosomal protein S15 (P31674), 50S ribosomal protein L4, L5, L6 (Q10NM5, Q9ZST0, Q10L93), 30S ribosomal protein S5, S9 (Q6YU81, Q850W6, Q9XJ28), and succinate dehydrogenase (Q9S827) were hub-proteins. The expression level of genes related to defense mechanism, involved in signaling pathways of jasmonic acid (JA), salicylic acid (SA), and ethylene metabolisms were up-regulated in mutant line after the inoculation of the physiological races of M. oryzae as compared to WT. Our results revealed the fundamental value of genome editing and expand knowledge about fungal infection avoidance in rice.

Ecophysiological Traits of Invasive C3 Species Calotropis procera to Maintain High Photosynthetic Performance Under High VPD and Low Soil Water Balance in Semi-Arid and Seacoast Zones

The evergreen C3 plant Calotropis procera is native to arid environments. Thus, it grows under high vapor pressure deficit (VPD), intense light, and severe drought conditions. We measured several ecophysiological traits in C. procera plants growing in semi-arid and seacoast environments to assess the attributes that support its photosynthetic performance under these contrasting conditions. Gas exchange analysis, primary metabolism content, nutrients, the antioxidant system, and leaf anatomy traits were measured under field conditions. In the semi-arid environment, C. procera was exposed to a prolonged drought season with a negative soil water balance during the 2 years of the study. Calotropis procera plants were exposed to a positive soil water balance only in the rainy season in the seacoast environment. The leaves of C. procera showed the same photosynthetic rate under high or low VPD, even in dry seasons with a negative soil water balance. Photosynthetic pigments, leaf sugar content, and the activity of antioxidant enzymes were increased in both places in the dry season. However, the anatomical adjustments were contrasting: while, in the semi-arid environment, mesophyll thickness increased in the driest year, in the seacoast environment, the cuticle thickness and trichome density were increased. The ability to maintain photosynthetic performance through the seasons would be supported by new leaves with different morpho-anatomical traits, with contrasting changes between semi-arid and seacoast environments. Furthermore, our results suggest that an efficient antioxidative system and leaf sugar dynamics can contribute to protecting the photosynthetic machinery even under severe drought.

Ethylene: A Master Regulator of Salinity Stress Tolerance in Plants

Salinity stress is one of the major threats to agricultural productivity across the globe. Research in the past three decades, therefore, has focused on analyzing the effects of salinity stress on the plants. Evidence gathered over the years supports the role of ethylene as a key regulator of salinity stress tolerance in plants. This gaseous plant hormone regulates many vital cellular processes starting from seed germination to photosynthesis for maintaining the plants' growth and yield under salinity stress. Ethylene modulates salinity stress responses largely via maintaining the homeostasis of Na+/K+, nutrients, and reactive oxygen species (ROS) by inducing antioxidant defense in addition to elevating the assimilation of nitrates and sulfates. Moreover, a cross-talk of ethylene signaling with other phytohormones has also been observed, which collectively regulate the salinity stress responses in plants. The present review provides a comprehensive update on the prospects of ethylene signaling and its cross-talk with other phytohormones to regulate salinity stress tolerance in plants.

Reciprocal regulation between nicotinamide adenine dinucleotide metabolism and abscisic acid and stress response pathways in Arabidopsis

Nicotinamide adenine dinucleotide (NAD) is an essential coenzyme that has emerged as a central hub linking redox equilibrium and signal transduction in living organisms. The homeostasis of NAD is required for plant growth, development, and adaption to environmental cues. In this study, we isolated a chilling hypersensitive Arabidopsis thaliana mutant named qs-2 and identified the causal mutation in the gene encoding quinolinate synthase (QS) critical for NAD biosynthesis. The qs-2 mutant is also hypersensitive to salt stress and abscisic acid (ABA) but resistant to drought stress. The qs-2 mutant accumulates a reduced level of NAD and over-accumulates reactive oxygen species (ROS). The ABA-hypersensitivity of qs-2 can be rescued by supplementation of NAD precursors and by mutations in the ABA signaling components SnRK2s or RBOHF. Furthermore, ABA-induced over-accumulation of ROS in the qs-2 mutant is dependent on the SnRK2s and RBOHF. The expression of QS gene is repressed directly by ABI4, a transcription factor in the ABA response pathway. Together, our findings reveal an unexpected interplay between NAD biosynthesis and ABA and stress signaling, which is critical for our understanding of the regulation of plant growth and stress responses.

What is the Difference between the Response of Grass Pea (Lathyrus sativus L.) to Salinity and Drought Stress?—A Physiological Study

Understanding the mechanisms of plant tolerance to osmotic and chemical stress is fundamental to maintaining high crop productivity. Soil drought often occurs in combination with physiological drought, which causes chemical stress due to high concentrations of ions. Hence, it is often assumed that the acclimatization of plants to salinity and drought follows the same mechanisms. Grass pea (Lathyrus sativus L.) is a legume plant with extraordinary tolerance to severe drought and moderate salinity. The aim of the presented study was to compare acclimatization strategies of grass pea seedlings to osmotic (PEG) and chemical (NaCl) stress on a physiological level. Concentrations of NaCl and PEG were adjusted to create an osmotic potential of a medium at the level of 0.0, −0.45 and −0.65 MPa. The seedlings on the media with PEG were much smaller than those growing in the presence of NaCl, but had a significantly higher content percentage of dry weight. Moreover, the stressors triggered different accumulation patterns of phenolic compounds, soluble and insoluble sugars, proline and β-N-oxalyl-L-α,β-diamino propionic acid, as well as peroxidase and catalase activity. Our results showed that drought stress induced a resistance mechanism consisting of growth rate limitation in favor of osmotic adjustment, while salinity stress induced primarily the mechanisms of efficient compartmentation of harmful ions in the roots and shoots. Furthermore, our results indicated that grass pea plants differed in their response to drought and salinity from the very beginning of stress occurrence.

Primed acclimation: A physiological process offers a strategy for more resilient and irrigation-efficient crop production

Optimizing plant physiological function is essential to maintaining crop yields under water scarcity and in developing more water-efficient production practices. However, the most common strategies in addressing water conservation in agricultural production have focused on water-efficient technologies aimed at managing water application or on improving crop water-use efficiency through breeding. Few management strategies explicitly consider the management or manipulation of plant physiological processes, but one which does is termed primed acclimation (PA). The PA strategy uses the physiological processes involved in priming to pre-acclimate plants to water deficits while reducing irrigation. It has been shown to evoke multi-mechanistic responses across numerous crop species. A combination of existing literature and emerging studies find that mechanisms for pre-acclimating plants to water deficit stress include changes in root:shoot partitioning, root architecture, water use, photosynthetic characteristics, osmotic adjustment and anti-oxidant production. In many cases, PA reduces agricultural water use by improving plant access to existing soil water. Implementing PA in seasonally water-limited environments can mitigate yield losses to drought. Genotypic variation in PA responses offers the potential to screen for crop varieties with the greatest potential for beneficial priming responses and to identify specific priming and acclimation mechanisms. In this review we: 1) summarize the concept of priming within the context of plant stress physiology; 2) review the development of a PA management system that utilizes priming for water conservation in agroecosystems; and 3) address the future of PA, how it should be evaluated across crop species, and its utility in managing crop stress tolerance.

Silicon and plant growth promoting rhizobacteria differentially regulate AgNP-induced toxicity in Brassica juncea: Implication of nitric oxide

An emerging stress of nanomaterials in soil and water is of great concern as it limits crop productivity and affects humans as well. Therefore, it is required to manage this problem. Silicon and plant growth promoting rhizobacteria has gained the engaging role in agriculture as (bio-)fertilizers. However, their role against silver nanoparticles (AgNPs) is still not known. Hence, present study was envisaged to investigate role of Si, PGPR and phytohormone indole acetic acid (IAA) in regulating AgNP stress in Brassica juncea seedlings. The study highlighted the impact of various treatments with respect to overproduction of reactive oxygen species, signaling molecule nitric oxide, oxidative markers like antioxidant enzymes and nonenzymatic components of ascorbate-glutathione pathway. Interestingly, silicon when present with AgNPs enhanced toxicity by reducing growth and mechanistic properties of B. juncea. Moreover, the results highlight the role of PGPR and IAA towards reduction in toxicity by promoting the plant growth under stressed conditions. Treatments AgNP + Si + PGPR/IAA were observed to significantly reduce the stress and enhance plant growth against treatment AgNPs alone. This reversal in toxicity by PGPR and IAA along with Si suggests the idea to formulate and utilize their combination as biofertilizers for eradicating the stress in near future.

A multiscale approach reveals regulatory players of water stress responses in seeds during germination

Seed germination is regulated by environmental factors, particularly water availability. Water deficits at the time of sowing impair the establishment of crop plants. Transcriptome and proteome profiling was used to document the responses of sunflower (Helianthus annuus) seeds to moderate water stress during germination in two hybrids that are nominally classed as drought sensitive and drought tolerant. Differences in the water stress-dependent accumulation reactive oxygen species and antioxidant enzymes activities were observed between the hybrids. A pathway-based analysis of the hybrid transcriptomes demonstrated that the water stress-dependent responses of seed metabolism were similar to those of the plant, with a decreased abundance of transcripts encoding proteins associated with metabolism and cell expansion. Moreover, germination under water stress conditions was associated with increased levels of transcripts encoding heat shock proteins. Exposure of germinating seeds to water stress specifically affected the abundance of a small number of proteins, including heat shock proteins. Taken together, these data not only identify factors that are likely to play a key role in drought tolerance during seed germination, but they also demonstrate the importance of the female parent in the transmission of water stress tolerance.

Photosynthesis, respiration and growth: A carbon and energy balancing act for alternative oxidase

This review summarizes knowledge of alternative oxidase, a mitochondrial electron transport chain component that lowers the ATP yield of plant respiration. Analysis of mutant and transgenic plants has established that alternative oxidase activity supports leaf photosynthesis. The interaction of alternative oxidase respiration with chloroplast metabolism is important under conditions that challenge energy and/or carbon balance in the photosynthetic cell. Under such conditions, alternative oxidase provides an extra-chloroplastic means to optimize the status of chloroplast energy pools (ATP, NADPH) and to manage cellular carbohydrate pools in response to changing rates of carbon fixation and carbon demand for growth and maintenance. Transcriptional and post-translational mechanisms ensure that alternative oxidase can respond effectively when carbon and energy balance are being challenged. This function appears particularly significant under abiotic stress conditions such as water deficit, high salinity, or temperature extremes. Under such conditions, alternative oxidase respiration positively affects growth and stress tolerance, despite it lowering the energy yield and carbon use efficiency of respiration. In part, this beneficial effect relates to the ability of alternative oxidase respiration to prevent excessive reactive oxygen species generation in both mitochondria and chloroplasts. Recent evidence suggests that alternative oxidase respiration is an interesting target for crop improvement.

Bacillus licheniformis FMCH001 Increases Water Use Efficiency via Growth Stimulation in Both Normal and Drought Conditions

Increasing agricultural losses due to biotic and abiotic stresses caused by climate change challenge food security worldwide. A promising strategy to sustain crop productivity under conditions of limited water availability is the use of plant growth promoting rhizobacteria (PGPR). Here, the effects of spore forming Bacillus licheniformis (FMCH001) on growth and physiology of maize (Zea mays L. cv. Ronaldinho) under well-watered and drought stressed conditions were investigated. Pot experiments were conducted in the automated high-throughput phenotyping platform PhenoLab and under greenhouse conditions. Results of the PhenoLab experiments showed that plants inoculated with B. licheniformis FMCH001 exhibited increased root dry weight (DW) and plant water use efficiency (WUE) compared to uninoculated plants. In greenhouse experiments, root and shoot DW significantly increased by more than 15% in inoculated plants compared to uninoculated control plants. Also, the WUE increased in FMCH001 plants up to 46% in both well-watered and drought stressed plants. Root and shoot activities of 11 carbohydrate and eight antioxidative enzymes were characterized in response to FMCH001 treatments. This showed a higher antioxidant activity of catalase (CAT) in roots of FMCH001 treated plants compared to uninoculated plants. The higher CAT activity was observed irrespective of the water regime. These findings show that seed coating with Gram positive spore forming B. licheniformis could be used as biostimulants for enhancing plant WUE under both normal and drought stress conditions.

Microbial inoculation in rice regulates antioxidative reactions and defense related genes to mitigate drought stress

Microbial inoculation in drought challenged rice triggered multipronged steps at enzymatic, non-enzymatic and gene expression level. These multifarious modulations in plants were related to stress tolerance mechanisms. Drought suppressed growth of rice plants but inoculation with Trichoderma, Pseudomonas and their combination minimized the impact of watering regime. Induced PAL gene expression and enzyme activity due to microbial inoculation led to increased accumulation of polyphenolics in plants. Enhanced antioxidant concentration of polyphenolics from microbe inoculated and drought challenged plants showed substantially high values of DPPH, ABTS, Fe-ion reducing power and Fe-ion chelation activity, which established the role of polyphenolic extract as free radical scavengers. Activation of superoxide dismutase that catalyzes superoxide (O2-) and leads to the accumulation of H2O2 was linked with the hypersensitive cell death response in leaves. Microbial inoculation in plants enhanced activity of peroxidase, ascorbate peroxidase, glutathione peroxidase and glutathione reductase enzymes. This has further contributed in reducing ROS burden in plants. Genes of key metabolic pathways including phenylpropanoid (PAL), superoxide dismutation (SODs), H2O2 peroxidation (APX, PO) and oxidative defense response (CAT) were over-expressed due to microbial inoculation. Enhanced expression of OSPiP linked to less-water permeability, drought-adaptation gene DHN and dehydration related stress inducible DREB gene in rice inoculated with microbial inoculants after drought challenge was also reported. The impact of Pseudomonas on gene expression was consistently remained the most prominent. These findings suggested that microbial inoculation directly caused over-expression of genes linked with defense processes in plants challenged with drought stress. Enhanced enzymatic and non-enzymatic antioxidant reactions that helped in minimizing antioxidative load, were the repercussions of enhanced gene expression in microbe inoculated plants. These mechanisms contributed strongly towards stress mitigation. The study demonstrated that microbial inoculants were successful in improving intrinsic biochemical and molecular capabilities of rice plants under stress. Results encouraged us to advocate that the practice of growing plants with microbial inoculants may find strategic place in raising crops under abiotic stressed environments.

Genome-wide expression analysis suggests glutaredoxin genes response to various stresses in cotton

Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species (ROS) and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Glutaredoxins (GRXs) are ubiquitous oxidoreductase enzymes involved in diverse cellular processes and play a key role in oxidative stress responsive mechanisms. This study was aimed to explore the structure-function relationship and to provide a framework for functional validation and biochemical characterization of various GRX members. In this study, our analysis revealed the presence of 127 genes encoding GRX proteins in G. hirsutum. A total of 758 genes from two typical monocot and nine dicot species were naturally divided into four classes based on phylogenetic analysis. The classification was supported with organization of conserved protein motifs and sequence logos comparison between cotton, rice and Arabidopsis. Cotton GRX gene family has underwent strong purifying selection with limited functional divergence. A good collinearity was observed in the synteny analysis of four Gossypium species. Majority of cotton GRXs were influenced by various phytohormones and abiotic stress conditions during expression analysis, suggesting an important role of GRX proteins in response to oxidative stress. Cis-regulatory elements, gene enrichments and co-expression network analysis also support their predicted role against various abiotic stresses. Whole genome and segmental duplication were determined to be the two major impetuses for the expansion of gene numbers during the evolution. The identification of GRX genes showing differential expression in specific tissues or in response to environmental stimuli provides a new avenue for in-depth characterization of selected genes of importance. This study will further broaden our insights into the evolution and functional elucidation of GRX gene family in cotton.

Manipulation of Ascorbate Biosynthetic, Recycling, and Regulatory Pathways for Improved Abiotic Stress Tolerance in Plants

Abiotic stresses, such as drought, salinity, and extreme temperatures, are major limiting factors in global crop productivity and are predicted to be exacerbated by climate change. The overproduction of reactive oxygen species (ROS) is a common consequence of many abiotic stresses. Ascorbate, also known as vitamin C, is the most abundant water-soluble antioxidant in plant cells and can combat oxidative stress directly as a ROS scavenger, or through the ascorbate-glutathione cycle-a major antioxidant system in plant cells. Engineering crops with enhanced ascorbate concentrations therefore has the potential to promote broad abiotic stress tolerance. Three distinct strategies have been utilized to increase ascorbate concentrations in plants: (i) increased biosynthesis, (ii) enhanced recycling, or (iii) modulating regulatory factors. Here, we review the genetic pathways underlying ascorbate biosynthesis, recycling, and regulation in plants, including a summary of all metabolic engineering strategies utilized to date to increase ascorbate concentrations in model and crop species. We then highlight transgene-free strategies utilizing genome editing tools to increase ascorbate concentrations in crops, such as editing the highly conserved upstream open reading frame that controls translation of the GDP-L-galactose phosphorylase gene.

Biomarkers for grain yield stability in rice under drought stress

Crop yield stability requires an attenuation of the reduction of yield losses caused by environmental stresses such as drought. Using a combination of metabolomics and high-throughput colorimetric assays, we analysed central metabolism and oxidative stress status in the flag leaf of 292 indica rice (Oryza sativa) accessions. Plants were grown in the field and were, at the reproductive stage, exposed to either well-watered or drought conditions to identify the metabolic processes associated with drought-induced grain yield loss. Photorespiration, protein degradation, and nitrogen recycling were the main processes involved in the drought-induced leaf metabolic reprogramming. Molecular markers of drought tolerance and sensitivity in terms of grain yield were identified using a multivariate model based on the values of the metabolites and enzyme activities across the population. The model highlights the central role of the ascorbate-glutathione cycle, particularly dehydroascorbate reductase, in minimizing drought-induced grain yield loss. In contrast, malondialdehyde was an accurate biomarker for grain yield loss, suggesting that drought-induced lipid peroxidation is the major constraint under these conditions. These findings highlight new breeding targets for improved rice grain yield stability under drought.

You Want it Sweeter: How Glycosylation Affects Plant Response to Oxidative Stress

Oxidative stress is a cellular threat which puts at risk the productivity of most of crops valorized by humankind in terms of food, feed, biomaterial, or bioenergy. It is therefore of crucial importance to understand the mechanisms by which plants mitigate the deleterious effects of oxidizing agents. Glycosylation of antioxidant molecules and phytohormones modifies their chemical properties as well as their cellular and histological repartition. This review emphasizes the mechanisms and the outcomes of this conjugation reaction on plant ability to face growing conditions favoring oxidative stress, in mirror with the activity of deglycosylating enzymes. Pioneer evidence bridging flavonoid, glycosylation, and redox homeostasis paved the way for numerous functional analyses of UDP-glycosyltransferases (UGTs), such as the identification of their substrates and their role to circumvent oxidative stress resulting from various environmental challenges. (De)glycosylation appears as a simple chemical reaction regulating the biosynthesis and/or the activity of a myriad of specialized metabolites partaking in response to pathogen and abiotic stresses. This outcome underlies the possibility to valorize UGTs potential to upgrade plant adaptation and fitness in a rising context of sub-optimal growing conditions subsequent to climate change.

MicroRNA528, a hub regulator modulating ROS homeostasis via targeting of a diverse set of genes encoding copper-containing proteins in monocots

Plant microRNAs (miRNAs) regulate vital cellular processes, including responses to extreme temperatures with which reactive oxygen species (ROS) are often closely associated. In the present study, it was found that aberrant temperatures caused extensive changes in abundance to numerous miRNAs in banana fruit, especially the copper (Cu)-associated miRNAs. Among them, miR528 was significantly downregulated under cold stress and it was found to target genes encoding polyphenol oxidase (PPO), different from those identified in rice and maize. Expression of PPO genes was upregulated by > 100-fold in cold conditions, leading to ROS surge and subsequent peel browning of banana fruit. Extensive comparative genomic analyses revealed that the monocot-specific miR528 can potentially target a large collection of genes encoding Cu-containing proteins. Most of them are actively involved in cellular ROS metabolism, including not only ROS generating oxidases, but also ROS scavenging enzymes. It also was demonstrated that miR528 has evolved a distinct preference of target genes in different monocots, with its target site varying in position among/within gene families, implying a highly dynamic process of target gene diversification. Its broad capacity to target genes encoding Cu-containing protein implicates miR528 as a key regulator for modulating the cellular ROS homeostasis in monocots.

A class B heat shock factor selected for during soybean domestication contributes to salt tolerance by promoting flavonoid biosynthesis

Soybean (Glycine max) production is severely affected in unfavorable environments. Identification of the regulatory factors conferring stress tolerance would facilitate soybean breeding. In this study, through coexpression network analysis of salt-tolerant wild soybeans, together with molecular and genetic approaches, we revealed a previously unidentified function of a class B heat shock factor, HSFB2b, in soybean salt stress response. We showed that HSFB2b improves salt tolerance through the promotion of flavonoid accumulation by activating one subset of flavonoid biosynthesis-related genes and by inhibiting the repressor gene GmNAC2 to release another subset of genes in the flavonoid biosynthesis pathway. Moreover, four promoter haplotypes of HSFB2b were identified from wild and cultivated soybeans. Promoter haplotype II from salt-tolerant wild soybean Y20, with high promoter activity under salt stress, is probably selected for during domestication. Another promoter haplotype, III, from salt-tolerant wild soybean Y55, had the highest promoter activity under salt stress, had a low distribution frequency and may be subjected to the next wave of selection. Together, our results revealed the mechanism of HSFB2b in soybean salt stress tolerance. Its promoter variations were identified, and the haplotype with high activity may be adopted for breeding better soybean cultivars that are adapted to stress conditions.

Physiological and Biochemical Mechanisms and Cytology of Cold Tolerance in Brassica napus

Cold damage has negatively impacted the yield, growth and quality of the edible cooking oil in Northern China and Brassica napus L.(rapeseed) planting areas decreased because of cold damage. In the present study we analyzed two Brassica napus cultivars of 16NTS309 (highly resistant to cold damage) and Tianyou2238 (cold sensitive) from Gansu Province, China using physiological, biochemical and cytological methods to investigate the plant's response to cold stress. The results showed that cold stress caused seedling dehydration, and the contents of malondialdehyde (MDA), relative electrolyte leakage and O₂ ^- and H₂O₂ were increased in Tianyou2238 than 16NTS309 under cold stress at 4°C for 48 h, as well as the proline, soluble protein and soluble sugars markedly accumulated, and antioxidant enzymes of peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT) were higher in 16NTS309 compared with in Tianyou2238, which play key roles in prevention of cell damage. After exposure to cold stress, the accumulation of the blue formazan precipitate and reddish brown precipitate indicated that O₂ ^- and H₂O₂, respectively, were produced in the root, stem, and leaf were higher than under non-cold conditions. Contents of O₂ ^- and H₂O₂ in cultivar Tianyou2238 were higher than 16NTS309, this is consistent with the phenotypic result. To understand the specific distribution of O₂ ^- in the sub-cellular, we found that in both cultivars O₂ ^- signals were distributed mainly in cambium tissue, meristematic cells, mesophyll cytoplasm, and surrounding the cell walls of root, stem, leaves, and leaf vein by morphoanatomical analysis, but the quantities varied. Cold stress also triggered obvious ultrastructural alterations in leaf mesophyll of Tianyou2238 including the damage of membrane system, destruction of chloroplast and swelling of mitochondria. This study are useful to provide new insights about the physiological and biochemical mechanisms and cytology associated with the response of B. napus to cold stress for use in breeding cold-resistant varieties.

Exogenous melatonin reduces the inhibitory effect of osmotic stress on photosynthesis in soybean

Understanding the relationship between exogenous melatonin and water deficit stress is crucial for achieving high yields and alleviating the effects of water deficit stress on soybean (Glycine max (L.) Merrill) plants in agriculture. This study investigated the effects of exogenous melatonin on soybean photosynthetic capacity under water deficit stress induced by polyethylene glycol (PEG) 6000. We conducted a potting experiment in 2018 using the soybean (Glycine max L. Merrill) cultivar Suinong 26. We identified the impacts of a concentration of PEG 6000 simulating drought (15%, w/v) and an appropriate melatonin concentration (100 μmol/L) on the growth of soybean seedlings and flowering stages in a preliminary test. We applied exogenous melatonin by foliar spraying and root application to determine the effects on leaf photosynthesis during water deficit stress. Our results indicated that 15% PEG 6000 had an obvious inhibitory effect on the growth of soybean seedlings and flowering stages, causing oxidative stress and damage due to reactive oxygen species (ROS) (H2O2 and O2·-) accumulation and potentially reducing air exchange parameters and photosystem II (PSII) efficiency. The application of exogenous melatonin significantly relieved the inhibitory effects of PEG 6000 stress on seedlings and flowering growth, and gas exchange parameters, potentially improved PSII efficiency, improved the leaf area index (LAI) and the accumulation of dry matter, slowed down oxidative stress and damage to leaves by increasing the activity of antioxidant enzymes (SOD, POD, and CAT), reduced the content of malondialdehyde (MDA), and ultimately improved soybean yield. Overall, the results of this study demonstrated that application of exogenous melatonin at the seedlings and flowering stages of soybean is effective in alleviating plant damage caused by water deficit stress and improving the drought resistance of soybean plants. In addition, the results showed that application of exogenous melatonin by root is superior to foliar spraying.

Dehydrin ERD14 activates glutathione transferase Phi9 in Arabidopsis thaliana under osmotic stress

BACKGROUND

Fully intrinsically disordered plant dehydrin ERD14 can protect enzymes via its chaperone-like activity, but it was not formally linked with enzymes of the plant redox system yet. This is of particular interest, as the level of H₂O₂ in Arabidopsis plants increases during osmotic stress, which can be counteracted by overexpression of ERD14.

METHODS

The proteomic mass-spectrometry analysis of stressed plants was performed to find the candidates affected by ERD14. With cross-linking, microscale thermophoresis, and active-site titration kinetics, the interaction and influence of ERD14 on the function of two target proteins: glutathione transferase Phi9 and catalase was examined.

RESULTS

Under osmotic stress, redox enzymes, specifically the glutathione transferase Phi enzymes, are upregulated. Using microscale thermophoresis, we showed that ERD14 directly interacts with GSTF9 with a K_D of ~25 μM. ERD14 activates the inactive GSTF9 molecules, protects GSTF9 from oxidation, and can also increases the activity of the enzyme. Aside from GSTF9, we found that ERD14 can also interact with catalase, an important cellular H₂O₂ scavenging enzyme, with a K_D of ~0.13 μM, and protects it from dehydration-induced loss of activity.

CONCLUSIONS

We propose that fully intrinsically disordered dehydrin ERD14 might protect and even activate redox enzymes, helping plants to survive oxidative stress under dehydration conditions.

GENERAL SIGNIFICANCE

ERD14 has a direct effect on the activity of redox enzymes.