Molecular mechanisms of combined heat and drought stress resilience in cereals

Regulation of Proline Accumulation and Protein Secretion in Sorghum under Combined Osmotic and Heat Stress

Plants reprogramme their proteome to alter cellular metabolism for effective stress adaptation. Intracellular proteomic responses have been extensively studied, and the extracellular matrix stands as a key hub where peptide signals are generated/processed to trigger critical adaptive signal transduction cascades inaugurated at the cell surface. Therefore, it is important to study the plant extracellular proteome to understand its role in plant development and stress response. This study examined changes in the soluble extracellular sub-proteome of sorghum cell cultures exposed to a combination of sorbitol-induced osmotic stress and heat at 40 °C. The combined stress significantly reduced metabolic activity and altered protein secretion. While cells treated with osmotic stress alone had elevated proline content, the osmoprotectant in the combined treatment remained unchanged, confirming that sorghum cells exposed to combined stress utilise adaptive processes distinct from those invoked by the single stresses applied separately. Reactive oxygen species (ROS)-metabolising proteins and proteases dominated differentially expressed proteins identified in cells subjected to combined stress. ROS-generating peroxidases were suppressed, while ROS-degrading proteins were upregulated for protection from oxidative damage. Overall, our study provides protein candidates that could be used to develop crops better suited for an increasingly hot and dry climate.

Morpho-Anatomical, Physiological and Biochemical Adjustments in Response to Heat and Drought Co-Stress in Winter Barley

This study aimed to investigate the combined effect of high temperatures 10 °C above the optimum and water withholding during microgametogenesis on vegetative processes and determine the response of winter barley genotypes with contrasting tolerance. For this purpose, two barley varieties were analyzed to compare the effect of heat and drought co-stress on their phenology, morpho-anatomy, physiological and biochemical responses and yield constituents. Genotypic variation was observed in response to heat and drought co-stress, which was attributed to differences in anatomy, ultrastructure and physiological and metabolic processes. The co-stress-induced reduction in relative water content, total soluble protein and carbohydrate contents, photosynthetic pigment contents and photosynthetic efficiency of the sensitive Spinner variety was significantly greater than the tolerant Lambada genotype. Based on these observations, it has been concluded that the heat-and-drought stress-tolerance of the Lambada variety is related to the lower initial chlorophyll content of the leaves, the relative resistance of photosynthetic pigments towards stress-triggered degradation, retained photosynthetic parameters and better-preserved leaf ultrastructure. Understanding the key factors underlying heat and drought co-stress tolerance in barley may enable breeders to create barley varieties with improved yield stability under a changing climate.

Genome-Wide Characterization of the Aux/IAA Gene Family in Orchardgrass and a Functional Analysis of DgIAA21 in Responding to Drought Stress

Drought stress is an important factor that reduces plant biomass production and quality. As one of the most important economic forage grasses, orchardgrass (Dactylis glomerata) has high drought tolerance. Auxin/indole-3-acetic acid (Aux/IAA) is one of the early responsive gene families of auxin and plays a key role in the response to drought stress. However, the characteristics of the Aux/IAA gene family in orchardgrass and their potential function in responding to drought stress remain unclear. Here, 30 Aux/IAA members were identified in orchardgrass. Segmental duplication may be an important driving force in the evolution of the Aux/IAA gene family in orchardgrass. Some Aux/IAA genes were induced by IAA, drought, salt, and temperature stresses, implying that these genes may play important roles in responding to abiotic stresses. Heterologous expression in yeast revealed that DgIAA21 can reduce drought tolerance. Similarly, the overexpression of DgIAA21 also reduced drought tolerance in transgenic Arabidopsis, which was supported by lower total chlorophyll content and relative water content as well as higher relative electrolyte leakage and malondialdehyde content (MDA) than Col-0 plants under drought conditions. The results of this study provided valuable insight into the function of DgIAAs in response to drought stress, which can be further used to improve forage grass breeding programs.

HSF and Hsp Gene Families in sunflower: a comprehensive genome-wide determination survey and expression patterns under abiotic stress conditions

Sunflowers belong to the Asteraceae family, which comprises nutrimental and economic oilseed plants. Heat shock proteins (Hsps) are protein families vital for all organisms' growth and survival. Besides the ordinary conditions, the expression of these proteins ascends during abiotic stress factors such as high temperature, salinity, and drought. Using bioinformatics approaches, the current study identified and analyzed HSF and Hsp gene family members in the sunflower (Helianthus annuus L.) plant. HSF, sHsp, Hsp40, Hsp60, Hsp70, Hsp90, and Hsp100 domains were analyzed in the sunflower genome, and 88, 72, 192, 52, 85, 49, and 148 genes were identified, respectively. The motif structures of the proteins in the same phylogenetic tree were similar, and the α-helical form was dominant in all the protein families except for sHsp. The estimated three-dimensional structure of 28 sHsp proteins was determined as β-sheets. Considering protein-protein interactions, the Hsp60-09 protein (38 interactions) was found to be the most interacting protein. The most orthologous gene pairs (58 genes) were identified between Hsp70 genes and Arabidopsis genes. The expression analysis of selected genes was performed under high temperature, drought, and high temperature-drought combined stress conditions in two sunflower cultivars. In stress conditions, gene expressions were upregulated for almost all genes in the first half and first hours at large. The expressions of HanHSF-45 and HanHsp70-29 genes were raised in two cultivars under high temperature and high temperature-drought combined stress conditions. This study presents a blueprint for subsequent research and delivers comprehensive knowledge of this vital protein domain.

Response Mechanisms of Woody Plants to High-Temperature Stress

High-temperature stress is the main environmental stress that restricts the growth and development of woody plants, and the growth and development of woody plants are affected by high-temperature stress. The influence of high temperature on woody plants varies with the degree and duration of the high temperature and the species of woody plants. Woody plants have the mechanism of adapting to high temperature, and the mechanism for activating tolerance in woody plants mainly counteracts the biochemical and physiological changes induced by stress by regulating osmotic adjustment substances, antioxidant enzyme activities and transcription control factors. Under high-temperature stress, woody plants ability to perceive high-temperature stimuli and initiate the appropriate physiological, biochemical and genomic changes is the key to determining the survival of woody plants. The gene expression induced by high-temperature stress also greatly improves tolerance. Changes in the morphological structure, physiology, biochemistry and genomics of woody plants are usually used as indicators of high-temperature tolerance. In this paper, the effects of high-temperature stress on seed germination, plant morphology and anatomical structure characteristics, physiological and biochemical indicators, genomics and other aspects of woody plants are reviewed, which provides a reference for the study of the heat-tolerance mechanism of woody plants.

Metabolic pathways engineering for drought or/and heat tolerance in cereals

Drought (D) and heat (H) are the two major abiotic stresses hindering cereal crop growth and productivity, either singly or in combination (D/+H), by imposing various negative impacts on plant physiological and biochemical processes. Consequently, this decreases overall cereal crop production and impacts global food availability and human nutrition. To achieve global food and nutrition security vis-a-vis global climate change, deployment of new strategies for enhancing crop D/+H stress tolerance and higher nutritive value in cereals is imperative. This depends on first gaining a mechanistic understanding of the mechanisms underlying D/+H stress response. Meanwhile, functional genomics has revealed several stress-related genes that have been successfully used in target-gene approach to generate stress-tolerant cultivars and sustain crop productivity over the past decades. However, the fast-changing climate, coupled with the complexity and multigenic nature of D/+H tolerance suggest that single-gene/trait targeting may not suffice in improving such traits. Hence, in this review-cum-perspective, we advance that targeted multiple-gene or metabolic pathway manipulation could represent the most effective approach for improving D/+H stress tolerance. First, we highlight the impact of D/+H stress on cereal crops, and the elaborate plant physiological and molecular responses. We then discuss how key primary metabolism- and secondary metabolism-related metabolic pathways, including carbon metabolism, starch metabolism, phenylpropanoid biosynthesis, γ-aminobutyric acid (GABA) biosynthesis, and phytohormone biosynthesis and signaling can be modified using modern molecular biotechnology approaches such as CRISPR-Cas9 system and synthetic biology (Synbio) to enhance D/+H tolerance in cereal crops. Understandably, several bottlenecks hinder metabolic pathway modification, including those related to feedback regulation, gene functional annotation, complex crosstalk between pathways, and metabolomics data and spatiotemporal gene expressions analyses. Nonetheless, recent advances in molecular biotechnology, genome-editing, single-cell metabolomics, and data annotation and analysis approaches, when integrated, offer unprecedented opportunities for pathway engineering for enhancing crop D/+H stress tolerance and improved yield. Especially, Synbio-based strategies will accelerate the development of climate resilient and nutrient-dense cereals, critical for achieving global food security and combating malnutrition.

NtHSP70-8b positively regulates heat tolerance and seed size in Nicotiana tabacum

Heat stress considerably restricts the geographical distribution of crops and affects their growth, development, and productivity. HSP70 plays a critical regulatory role in plant growth response to heat stress. However, the mechanisms of this regulatory remain poorly understood. Here, an HSP70 gene, NtHSP70-8b, which is involved in the heat stress response of tobacco, was cloned and identified. The expression of NtHSP70-8b was induced by exogenous abscisic acid (ABA) treatment and abiotic stress, including heat, drought, and salt. Notably, high NtHSP70-8b expression occurred under heat stress conditions, which was consistent with the β-glucuronidase histochemical analysis. Moreover, NtHSP70-8b overexpression markedly enhanced heat stress tolerance by changing the stomatal conductance and antioxidant capacity in tobacco leaves. qRT-PCR showed that the expression levels of ABA synthesis and response genes (NtNCED3 and NtAREB), stress defence genes (NtERD10C and NtLEA5), and other HSP genes (NtHSP90 and NtHSP26a) in NtHSP70-8b-overexpressing tobacco were high under heat stress. The interaction of NtHSP70-8b with NtHSP26a was further confirmed by a luciferase complementation imaging assay. In contrast, NtHSP70-8b knockout mutants showed significantly reduced antioxidant capacity compared to the wild type (WT) under heat stress conditions, suggesting that NtHSP70-8b acts as a positive regulator of heat stress in tobacco. Moreover, NtHSP70-8b overexpression increased the 1000-seed weight. Taken together, NtHSP70-8b is involved in the heat stress response, and NtHSP70-8b overexpression contributed to enhanced tolerance to heat stress, which is thus an essential gene with potential application value for developing heat stress-tolerant crops.

The Essential Role of H2S-ABA Crosstalk in Maize Thermotolerance through the ROS-Scavenging System

Hydrogen sulfide (H2S) and abscisic acid (ABA), as a signaling molecule and stress hormone, their crosstalk-induced thermotolerance in maize seedlings and its underlying mechanism were elusive. In this paper, H2S and ABA crosstalk as well as the underlying mechanism of crosstalk-induced thermotolerance in maize seedlings were investigated. The data show that endogenous levels of H2S and ABA in maize seedlings could be mutually induced by regulating their metabolic enzyme activity and gene expression under non-heat stress (non-HS) and HS conditions. Furthermore, H2S and ABA alone or in combination significantly increase thermotolerance in maize seedlings by improving the survival rate (SR) and mitigating biomembrane damage. Similarly, the activity of the reactive oxygen species (ROS)-scavenging system, including enzymatic antioxidants catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (POD), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and superoxide dismutase (SOD), as well as the non-enzymatic antioxidants reduced ascorbic acid (AsA), carotenoids (CAR), flavone (FLA), and total phenols (TP), was enhanced by H2S and ABA alone or in combination in maize seedlings. Conversely, the ROS level (mainly hydrogen peroxide and superoxide radical) was weakened by H2S and ABA alone or in combination in maize seedlings under non-HS and HS conditions. These data imply that the ROS-scavenging system played an essential role in H2S-ABA crosstalk-induced thermotolerance in maize seedlings.

Improved abiotic stress tolerance in Arabidopsis by constitutive active form of a banana DREB2 type transcription factor, MaDREB20.CA, than its native form, MaDREB20

Banana is grown as one of the important fruit crops in tropical and subtropical regions of the world. In this study, we report induced expression of a dehydration responsive element binding 2 (DREB2) gene (MaDREB20) under individual heat, drought, and combined drought and heat stress in root of two banana genotypes Grand Nain (GN) and Hill Banana (HB). Motif analysis of MaDREB20 protein demonstrated the presence of a negative regulatory domain (NRD) or PEST motif between 150 and 184 amino acids. Transgenic Arabidopsis overexpressing MaDREB20 gene showed more survival rate, above-ground biomass, seed yield, leaf relative water content, and proline content but less ion leakage and malonaldehyde content, revealing improved tolerance against heat and drought as well as their combination than the wild-type. Overexpression of MaDREB20.CA (constitutive active form of MaDREB20 after removal of PEST region) showed better abiotic stress tolerance in Arabidopsis than its native form (MaDREB20). Transgenic Arabidopsis overexpressing MaDREB20 and MaDREB20.CA genes appeared to be associated with reduced stomatal densities under normal condition, better regulation of stomatal aperture under drought than in wild-type plants, and differential regulation of downstream target (AtTCH4 and AtIAA1) genes under heat, drought, and combined stress. Taken together, our findings revealed important functions of MaDREB20 in abiotic stress responses in transgenic Arabidopsis and could form a basis for CRISPR/Cas9-mediated removal of its NRD to enhance stress tolerance in banana.

Integrated transcriptome and metabolome analysis reveals molecular responses of spider to single and combined high temperature and drought stress

High temperature and drought are abiotic stresses restricting many arthropods' survival and growth. Wolf spiders are poikilothermic arthropods that are vital in managing insects and pests. Nonetheless, investigating changes in spiders under temperature and drought stress are limited, especially at the molecular and gene expression levels. The study found that the combined effects of high temperature and drought stress significantly reduced survival rates and raised superoxide dismutase and malondialdehyde levels in the wolf spider Pardosa pseudoannulata. An integrated transcriptome and metabolome analysis showed that differentially expressed genes and metabolites were highly enriched in pathways involved in the proteolysis and oxidation-reduction process. The gene expression profiles displayed that heat shock protein (HSP) families (i.e., small heat shock protein, HSP70, HSP90, and HSP beta protein) were up-regulated under temperature and/or drought stresses. Additionally, a conjoint analysis revealed that under the combined stress, several important enzymes, including maltase-glucoamylase, glycerol-6-phosphate transporter, alanine-glyoxylate transaminase, and prostaglandin-H2 D-isomerase, were altered, affecting the metabolism of starch, sucrose, amino acids, and arachidonic acid. The protein interaction network further confirmed that under the combined stress, metabolic processes, peptide metabolic processes, and ATP generation from ADP were up-regulated, indicating that spiders could accelerate the metabolism of carbohydrates and proteins to combat stress and maintain homeostasis. Overall, this work showed that exposure to a combination of pressures might cause distinct defensive reactions in spiders and offered novel perspectives to research the molecular underpinnings of spider adaptation to a changing climate.

Transcriptome and metabolome profiling of interspecific CSSLs reveals general and specific mechanisms of drought resistance in cotton

In order to understand the molecular mechanism of cotton's response to drought during the flowering and boll stage, transcriptomics and metabolomics were carried out for two introgression lines (drought-tolerant line: T307; drought-sensitive line: S48) which were screened from Gossypium hirsutum cv. 'Emian22' with some gene fragments imported from Gossypium barbadense acc. 3-79, under drought stress by withdrawing water at flowering and boll stage. Results showed that the basic drought response in cotton included a series of broad-spectrum responses, such as amino acid synthesis, hormone (abscisic acid, ABA) signal transduction, and mitogen-activated protein kinases signal transduction pathway, which activated in both drought-tolerant and drought-sensitive lines. However, the difference of their imported fragments and diminished sequences triggers endoplasmic reticulum (ER) protein processing, photosynthetic-related pathways (in leaves), and membrane solute transport (in roots) in drought-tolerant line T307, while these are missed or not activated in drought-sensitive line S48, reflecting the different drought tolerance of the two genotypes. Virus-induced gene silencing assay of drought-tolerant differentially expressed heat shock protein (HSP) genes (mainly in leaf) and ATP-binding cassette (ABC) transporter genes (mainly in roots) indicated that those genes play important role in cotton drought tolerant. Combined analysis of transcriptomics and metabolomics highlighted the important roles of ER-stress-related HSP genes and root-specific ABC transporter genes in plants drought tolerance. These results provide new insights into the molecular mechanisms underlying the drought stress adaptation in cotton.

Single and Associated Effects of Drought and Heat Stresses on Physiological, Biochemical and Antioxidant Machinery of Four Eggplant Cultivars

The impact of heat and drought stresses, either individually or combined, on physiological and biochemical parameters of four eggplant varieties (Solanum melongena L.) was investigated. The results showed that associated stress generated the highest increment in proline content, MDA concentration, and H2O2 accumulation and generated the lowest increment in RWC. In addition, ‘Bonica’ and ‘Galine’ exhibited higher starch accumulation and lower electrolyte leakage (EL) under combined stress. Moreover, drought and heat stresses applied individually contributed to a substantial decline in Chla, Chlb, total Chl, Chla/b, and carotenoids (p > 0.05) in ‘Adriatica’ and ‘Black Beauty’. The decreasing level of pigments was more substantial under associated drought and heat stresses. The simultaneous application of drought and heat stresses reduced PSII efficiency (Fv/Fm), quantum yield (ΦPSII), and photochemical efficiency (qp) and boosted non-photochemical quenching (NPQ) levels. However, the change recorded in the chlorophyll fluorescence parameters was less pronounced in ‘Bonica’ and ‘Galine’. In addition, the gas exchange parameters, transpiration rate (E), CO2 assimilation rate (A), and net photosynthesis (Pn) were decreased in all varieties under all stress conditions. However, the reduction was more pronounced in ‘Adriatica’ and ‘Black Beauty’. Under associated stress, antioxidant enzymes, SOD, APX, CAT, and GR exhibited a significant increment in all eggplant cultivars. However, the rising was more elevated in ‘Bonica’ and ‘Galine’ (higher than threefold increase) than in ‘Adriatica’ and ‘Black Beauty’ (less than twofold increase). Furthermore, ‘Bonica’ and ‘Galine’ displayed higher non-enzyme scavenging activity (AsA and GSH) compared to ‘Adriatica’ and ‘Black Beauty’ under associated stress. Under stressful conditions, nutrient uptake was affected in all eggplant cultivars; however, the root, stem, and leaf N, P, and K contents, in ‘Adriatica’ and ‘Black Beauty’ were lower than in ‘Bonica’ and ‘Galine’, thereby showing less capacity in accumulating nutrients. The coexistence of drought and heat stresses caused more damage on eggplant varieties than the single appearance of drought or heat stress separately. ‘Bonica’ and ‘Galine’ showed better distinguished performance compared to ‘Adriatica’ and ‘Black Beauty’. The superiority of ‘Bonica’ and ‘Galine’ in terms of tolerance to heat and drought stresses was induced by more effective antioxidant scavenging potential, enhanced osmolyte piling-up, and prominent ability in keeping higher photosynthetic efficiency and nutrient equilibrium compared with ‘Adriatica’ and ‘Black Beauty’.

The Combined Effect of Heat and Osmotic Stress on Suberization of Arabidopsis Roots

The simultaneous occurrence of heat stress and drought is becoming more regular as a consequence of climate change, causing extensive agricultural losses. The application of either heat or osmotic stress increase cell-wall suberization in different tissues, which may play a role in improving plant resilience. In this work, we studied how the suberization process is affected by the combination of drought and heat stress by following the expression of suberin biosynthesis genes, cell-wall suberization and the chemical composition in Arabidopsis roots. The Arabidopsis plants used in this study were at the onset of secondary root development. At this point, one can observe a developmental gradient in the main root, with primary development closer to the root tip and secondary development, confirmed by the suberized phellem, closer to the shoot. Remarkably, we found a differential response depending on the root zone. The combination of drought and heat stress increased cell wall suberization in main root segments undergoing secondary development and in lateral roots (LRs), while the main root zone, at primary development stage, was not particularly affected. We also found differences in the overall chemical composition of the cell walls in both root zones in response to combined stress. The data gathered showed that, under combined drought and heat stress, Arabidopsis roots undergo differential cell wall remodeling depending on developmental stage, with modifications in the biosynthesis and/or assembly of major cell wall components.

Effects of Combined Abiotic Stresses Related to Climate Change on Root Growth in Crops

Climate change is a major threat to crop productivity that negatively affects food security worldwide. Increase in global temperatures are usually accompanied by drought, flooding and changes in soil nutrients composition that dramatically reduced crop yields. Against the backdrop of climate change, human population increase and subsequent rise in food demand, finding new solutions for crop adaptation to environmental stresses is essential. The effects of single abiotic stress on crops have been widely studied, but in the field abiotic stresses tend to occur in combination rather than individually. Physiological, metabolic and molecular responses of crops to combined abiotic stresses seem to be significantly different to individual stresses. Although in recent years an increasing number of studies have addressed the effects of abiotic stress combinations, the information related to the root system response is still scarce. Roots are the underground organs that directly contact with the soil and sense many of these abiotic stresses. Understanding the effects of abiotic stress combinations in the root system would help to find new breeding tools to develop more resilient crops. This review will summarize the current knowledge regarding the effects of combined abiotic stress in the root system in crops. First, we will provide a general overview of root responses to particular abiotic stresses. Then, we will describe how these root responses are integrated when crops are challenged to the combination of different abiotic stress. We will focus on the main changes on root system architecture (RSA) and physiology influencing crop productivity and yield and convey the latest information on the key molecular, hormonal and genetic regulatory pathways underlying root responses to these combinatorial stresses. Finally, we will discuss possible directions for future research and the main challenges needed to be tackled to translate this knowledge into useful tools to enhance crop tolerance.

An overview of sucrose transporter (SUT) genes family in rice

INTRODUCTION

Photosynthesis provides the energy basis for the life activities of plants by producing organic compounds, mainly sugar. As the main energy form of photosynthesis, sugar affects the growth and development of plants. During long-distance transportation, sucrose is the main form of transportation. The rate of sugar transport and the allocation of carbohydrates affect the biomass of crops and are closely related to the reproductive growth of crops.

MAIN TEXT

The transportation of sugar is divided into active transportation and passive transportation. So how does the sucrose transporters (SUT) genes, which are the main carriers of sucrose in active transportation, affect the performance of rice agronomic traits is still to be explored. In this article, we describe the structure of inflorescence and review the transport forms and metabolic processes of sucrose in rice, such as how CO₂ is fixed, carbohydrate assimilation, and transport of organic matter. Sucrose transporters exhibited remarkable effects on the development of reproductive organs in rice.

CONCLUSIONS

Here, the effects of different factors, such as the effects of anthers morphology on starch enrichment of pollen, effects of biotic and abiotic factors on sucrose transporters, effects of changes in trace elements on sucrose transporters, were discussed. Moreover, the regulation of transcription or translation level provides ideas for future research on sucrose transporters.

Interactive effects of hydrogen sulphide and silicon enhance drought and heat tolerance by modulating hormones, antioxidant defence enzymes and redox status in barley (Hordeum vulgare L.)

Recent changes in climate have reduced crop productivity throughout much of the world. Drought and heat stress, particularly in arid and semi-arid regions, have seriously affected barley production. This study explored the separate and interactive effects of silicon (Si) and hydrogen sulphide (H₂ S) on plant growth and mitigation of the adverse effects of heat stress (DS) and drought stress (HS) in a barley pot experiment. The impacts of simultaneous DS + HS were more severe than individual stresses due to increased ROS production, malondialdehyde (MDA) content and higher electrolyte leakage (EL), thereby leading to reduced water, protein and photosynthetic pigment content. Exogenously applied Si and H₂ S alleviated the DS-, HS- and DS + HS-induced effects on barley by reducing ROS production, MDA and EL. A single application of H₂ S or Si + H₂ S increased plant biomass under all stress conditions, which can be ascribed to higher Si accumulation in barley shoots. A single application of Si or H₂ S significantly increased plant biomass. However, Si + H₂ S was the most effective treatment for metabolite accumulation and elevating activity of antioxidant enzymes to prevent toxicity from oxidative stress. This treatment also modulated osmolyte content, enhanced antioxidant activity and regulated the stress signalling-related endogenous hormones, abscisic acid (ABA) and indole acetic acid (IAA). Exogenous treatments regulated endogenous H₂ S and Si and resulted in higher tolerance to individual and combined drought and heat stress in barley.

Concurrent Effects of Drought and Heat Stresses on Physio-Chemical Attributes, Antioxidant Status and Kernel Quality Traits in Maize (Zea mays L.) Hybrids

Maize is one of the most important field crops considering its utilization as food, feed, fodder, and biofuel. However, the sustainability of its production is under serious threat of heat and drought stresses, as these stresses could hamper crop growth, causing a significant loss to kernel yield. The research study was carried out at Maize and Millets Research Institute, Yusafwala-Sahiwal for two consecutive spring seasons (2019-20 and 2020-21) under a split-split plot design. The current study explained the individual and combined effects of drought and heat stresses on morphology, phenology, physiology, reactive oxygen species (stocktickerROS), antioxidant status, and kernel quality traits in four indigenous (YH-5482, YH-5427, YH-5404, and YH-1898) and one multinational maize hybrid (P-1543). Stress treatments, i.e., drought, heat, and drought+heat, were applied ten days before tasseling and lasted for 21 days. The results revealed the incidence of oxidative stress due to overproduction of Hydrogen peroxide; H₂O₂ (control: 1.9, heat+drought: 5.8), and Malondialdehyde; stocktickerMDA (control: 116.5, heat+drought: 193), leading to reduced photosynthetic ability (control: 31.8, heat:16.5), alterations in plant morphology, decrease in kernel yield (control: 10865 kg ha^-1, heat+drought: 5564 kg ha^-1), and quality-related traits. Although all the stress treatments induced the accumulation of stress-responsive osmolytes and enzymatic antioxidants to cope with the negative impact of osmotic stress, the effect of combined drought + heat stress was much higher. The overall performance of indigenous maize hybrid YH-5427 was much more promising than the other hybrids, attributed to its better tolerance of drought and heat stresses. Such stress tolerance was attributed to maintaining photosynthetic activity, a potent antioxidant and osmolyte-based defense mechanisms, and minimum reductions in yield-related traits, which assured the maximum kernel yield under all stress treatments.

Functional characterization of the Pinellia ternata cytoplasmic class II small heat shock protein gene PtsHSP17.2 via promoter analysis and overexpression in tobacco

High temperature is one of the main abiotic factors limiting agricultural production, particularly for heat-sensitive plant species. Small heat-shock proteins contribute substantially to alleviating damage to plants caused by heat stress. In the present study, the heat shock protein gene PtsHSP17.2 from Pinellia ternata was functionally characterized through promoter analysis and its overexpression in tobacco. Respectively, relative expression using real-time RT-PCR and ex situ promoter activity assay indicated that PtsHSP17.2 is strongly inducible under heat stress, and in silico promoter analysis discovered multiple stress-related cis elements including heat shock element. When overexpressing PtsHSP17.2 in tobacco, the thermotolerance of transgenic plants was markedly enhanced. Furthermore, the transgenic tobacco plants exhibited less variation in chlorophyll content, relative electrolyte leakage, and malondialdehyde content under heat stress compared with wild-type (WT) plants. The activities of antioxidant enzymes and content of proline were significantly enhanced under heat stress in transgenic plants relative to WT plants. Transgenic plants also had enhanced water retention and increased antioxidative capacity. Further, the expression levels of genes encoding antioxidant enzymes were more highly induced by heat stress in transgenic lines than WT. These results enrich the current understanding of thermal adaptation of heat-sensitive plant species and encourage further genetic improvement.

'Omics' approaches in developing combined drought and heat tolerance in food crops

Global climate change will significantly increase the intensity and frequency of hot, dry days. The simultaneous occurrence of drought and heat stress is also likely to increase, influencing various agronomic characteristics, such as biomass and other growth traits, phenology, and yield-contributing traits, of various crops. At the same time, vital physiological traits will be seriously disrupted, including leaf water content, canopy temperature depression, membrane stability, photosynthesis, and related attributes such as chlorophyll content, stomatal conductance, and chlorophyll fluorescence. Several metabolic processes contributing to general growth and development will be restricted, along with the production of reactive oxygen species (ROS) that negatively affect cellular homeostasis. Plants have adaptive defense strategies, such as ROS-scavenging mechanisms, osmolyte production, secondary metabolite modulation, and different phytohormones, which can help distinguish tolerant crop genotypes. Understanding plant responses to combined drought/heat stress at various organizational levels is vital for developing stress-resilient crops. Elucidating the genomic, proteomic, and metabolic responses of various crops, particularly tolerant genotypes, to identify tolerance mechanisms will markedly enhance the continuing efforts to introduce combined drought/heat stress tolerance. Besides agronomic management, genetic engineering and molecular breeding approaches have great potential in this direction.

Transcriptome response of maize (Zea mays L.) seedlings to heat stress

Among stresses, heat stress (HS) is a prime factor restricting plant growth and productivity. However, the molecular mechanisms of plants' response to HS need to be further uncovered. Here, the transcriptome response of maize seedlings to HS was dissected using transcriptome data analysis. The data exhibited that a total of 43,221 genes in maize seedlings had been found, 37,534 of which were referred, while 5686 were not. Under HS, comparison with the control without HS, there were 13,607 genes that were differentially expressed (DEGs, 6195 upregulated and 7412 downregulated). In addition, Gene Ontology (GO) enrichment analysis indicated that there were 220, 478, and 1300 terms that were enriched in cellular component, molecular function, and biological process, respectively. Significantly enriched GO terms were involved in 23 cellular components, 27 molecular functions, and 124 biological processes. Also, Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis suggested that there were 2613 DEGs that were assigned to 131 pathways, 14 of which (enriched 1068 DEGs in total) were significantly upregulated. These pathways were mainly related to protein renaturation, biomembrane repair, osmotic adjustment, and redox balance. Among them, protein processing in endoplasmic reticulum was the most significantly upregulated. The transcriptome data decoded that protein renaturation, biomembrane repair, osmotic adjustment, and redox balance played a key role in the response of maize seedlings to HS.

Impact of Single and Combined Salinity and High-Temperature Stresses on Agro-Physiological, Biochemical, and Transcriptional Responses in Rice and Stress-Release

Here, for the first time, we aimed to identify in rice the key mechanisms and processes underlying tolerance to high-temperature (HT) or salt stress (SS) alone, the co-occurrence of both stresses, and recovery using physiological and biochemical measurements and gene expression analysis. We also investigated whether recovery from the two stressors depended on the relative intensities/relief of each stressor. Wild type ('Yukinkomai') rice plants were found to be more susceptible to salinity or heat applied individually. SS leads to a depletion of cellular water content, higher accumulation of Na⁺, and alterations in photosynthetic pigments. The stress-tolerant cultivar 'YNU31-2-4' (YNU) displayed a lower Na⁺/K⁺ ratio, higher water content in cells and improved photosynthetic traits, antioxidant system, and expression of defence genes. Strikingly, the SS + HT combination provided a significant level of protection to rice plants from the effects of SS alone. The expression pattern of a selected set of genes showed a specific response and dedicated pathways in plants subjected to each of the different stresses, while other genes were explicitly activated when the stresses were combined. Aquaporin genes were activated by SS, while stress-related (P5CS, MSD1, HSPs, and ions transporters) genes were shaped by HT. Hierarchical clustering and principal component analyses showed that several traits exhibited a gradually aggravating effect as plants were exposed to the combined stresses and identified heat as a mitigating factor, clearly separating heat + salt-stressed from salt-non-heat-stressed plants. Furthermore, seedling recovery was far more dependent on the relative intensities of stressors and cultivars, demonstrating the influence of one stressor over another upon stress-release. Taken together, our data show the uniqueness and complexity of the physiological and molecular network modules used by rice plants to respond to single and combined stresses and recovery.

Xylella fastidiosa and Drought Stress in Olive Trees: A Complex Relationship Mediated by Soluble Sugars

Simple Summary

Carbohydrates play important roles in tolerance to both biotic and abiotic stressors. Xylella fastidiosa, the causal agent of “Olive Quick Decline Syndrome”, is a quarantine pathogen that induces drought stress in the host, aggravated by eventual water shortage, which is a frequent environmental condition in Mediterranean olive groves. At present, the resistance mechanisms shown by few resistant olive cultivars (e.g., cv Leccino) are not completely known; therefore, the aim of this research is to understand whether sugar metabolism is involved in the cross-talk mechanisms of biotic and abiotic responses. The results show that drought stress response induces effects beneficial to resistance of Xylella fastidiosa in cv Leccino. In the current context of global climate change, this study supports the importance of investigating the complex drought–disease interaction to detect resistance traits and thus find ways to counter the threat of this pathogen in the future.

Abstract

Xylella fastidiosa (Xf) subsp. pauca “De Donno” is the etiological agent of “Olive Quick Decline Syndrome” (OQDS) on olive trees (Olea europaea L.); the presence of the bacterium causes xylem vessel occlusions inducing a drought stress and the development of leaf scorch symptoms, which may be worsened by water shortage in summer. In order to evaluate how the two stress factors overlap each other, the carbohydrate content and the expression patterns of genes related to carbohydrate metabolism have been evaluated in two olive cvs trees (Cellina di Nardò, susceptible to Xf, and Leccino, resistant to Xf) reporting transcriptional dynamics elicited by Xf infection, drought, or combined stress (drought/Xf). In the Xf-susceptible Cellina di Nardò plants, Xf and its combination with drought significantly decrease total sugars compared to control (−27.0% and −25.7%, respectively). In contrast, the Xf-resistant Leccino plants show a more limited reduction in sugar content in Xf-positive conditions (−20.1%) and combined stresses (−11.1%). Furthermore, while the amount of glucose decreases significantly in stressed Cellina di Nardò plants (≈18%), an increase was observed in Leccino plants under drought/Xf combined stresses (+11.2%). An opposite behavior among cvs was also observed for sucrose, as an accumulation of the disaccharide was recorded in stressed Leccino plants (≈37%). The different response to combined stress by Xf-resistant plants was confirmed considering genes coding for the sucrose or monosaccharide transporter (OeSUT1, OeMST2), the cell wall or vacuolar invertase (OeINV-CW, OeINV-V), the granule-bound starch synthase I (OeGBSSI) and sucrose synthase (OeSUSY), with a higher expression than at least one single stress (e.g., ≈1-fold higher or more than Xf for OeMST2, OeINV-CW, OeINV-V, OeGBSSI). It is probable that the pathways involved in drought stress response induce positive effects useful for pathogen resistance in cv Leccino, confirming the importance of investigating the mechanisms of cross-talk of biotic and abiotic responses.

GhWRKY1-like, a WRKY transcription factor, mediates drought tolerance in Arabidopsis via modulating ABA biosynthesis

BACKGROUND

Drought stress has great negative effects on the plant growth and development. The tolerance of plants to such abiotic stress is triggered by complicated and multilayered signaling pathways to restore cellular homeostasis and to promote survival. The WRKY family is one of the largest transcription factor families in higher plants, and has been well recognized for the roles in regulating plants tolerance to abiotic and biotic stress. However, little is known about how the WRKY genes regulate drought resistance in cotton.

RESULTS

In this work, we identified the WRKY transcription factor GhWRKY1-like from upland cotton as a positive regulator of tolerance to drought that directly manipulates abscisic acid (ABA) biosynthesis. Overexpression of GhWRKY1-like in Arabidopsis constitutively activated ABA biosynthesis genes, signaling genes, responsive genes and drought related maker genes, and led to enhanced tolerance to drought. Further analysis has shown that GhWRKY1-like can interact with "W-box" cis-elements of the promoters of AtNCED2, AtNCED5, AtNCED6 and AtNCED9 which are essential enzymes for ABA biosynthesis, and promotes the expression of those target genes.

CONCLUSIONS

In summary, our findings suggest that GhWRKY1-like may act as a positive regulator in Arabidopsis tolerance to drought via directly interacting with the promoters of AtNCED2, AtNCED5, AtNCED6 and AtNCED9 to promote ABA biosynthesis.

An update on redox signals in plant responses to biotic and abiotic stress crosstalk: insights from cadmium and fungal pathogen interactions

Complex signalling pathways are involved in plant protection against single and combined stresses. Plants are able to coordinate genome-wide transcriptional reprogramming and display a unique programme of transcriptional responses to a combination of stresses that differs from the response to single stresses. However, a significant overlap between pathways and some defence genes in the form of shared and general stress-responsive genes appears to be commonly involved in responses to multiple biotic and abiotic stresses. Reactive oxygen and nitrogen species, as well as redox signals, are key molecules involved at the crossroads of the perception of different stress factors and the regulation of both specific and general plant responses to biotic and abiotic stresses. In this review, we focus on crosstalk between plant responses to biotic and abiotic stresses, in addition to possible plant protection against pathogens caused by previous abiotic stress. Bioinformatic analyses of transcriptome data from cadmium- and fungal pathogen-treated plants focusing on redox gene ontology categories were carried out to gain a better understanding of common plant responses to abiotic and biotic stresses. The role of reactive oxygen and nitrogen species in the complex network involved in plant responses to changes in their environment is also discussed.

Natural Variation among Arabidopsis Accessions in the Regulation of Flavonoid Metabolism and Stress Gene Expression by Combined UV Radiation and Cold

Plants are constantly exposed to stressful environmental conditions. Plant stress reactions were mainly investigated for single stress factors. However, under natural conditions plants may be simultaneously exposed to different stresses. Responses to combined stresses cannot be predicted from the reactions to the single stresses. Flavonoids accumulate in Arabidopsis thaliana during exposure to UV-A, UV-B or cold, but the interactions of these factors on flavonoid biosynthesis were unknown. We therefore investigated the interaction of UV radiation and cold in regulating the expression of well-characterized stress-regulated genes, and on transcripts and metabolites of the flavonoid biosynthetic pathway in 52 natural Arabidopsis accessions that differ widely in their freezing tolerance. The data revealed interactions of cold and UV on the regulation of stress-related and flavonoid biosynthesis genes, and on flavonoid composition. In many cases, plant reactions to a combination of cold and UV were unique under combined stress and not predictable from the responses to the single stresses. Strikingly, all correlations between expression levels of flavonoid biosynthesis genes and flavonol levels were abolished by UV-B exposure. Similarly, correlations between transcript levels of flavonoid biosynthesis genes or flavonoid contents, and freezing tolerance were lost in the presence of UV radiation, while correlations with the expression levels of cold-regulated genes largely persisted. This may indicate different molecular cold acclimation responses in the presence or absence of UV radiation.

Comparison of Transcriptional Response of C3 and C4 Plants to Drought Stress Using Meta-Analysis and Systems Biology Approach

Drought stress affects a range of plant processes. It is still not well-known how C3 and C4 plants respond to drought. Here, we used a combination of meta-analysis and network analysis to compare the transcriptional responses of Oryza sativa (rice), a C3 plant, and Zea mays (maize), a C4 plant, to drought stress. The findings showed that drought stress changes the expression of genes and affects different mechanisms in the C3 and C4 plants. We identified several genes that were differentially expressed genes (DEGs) under stress conditions in both species, most of which are associated with photosynthesis, molecule metabolic process, and response to stress. Additionally, we observed that many DEGs physically located within the quantitative trait locus regions are associated with C isotope signature (d13C), photosynthetic gas exchange, and root characteristics traits. Through the gene co-expression and differential co-expression network methods, we identified sets of genes with similar and different behaviors among C3 and C4 plants during drought stress. This result indicates that mitogen-activated protein kinases (MAPK) signaling pathway plays an important part in the differences between the C3 and C4 species. The present study provides a better understanding of the mechanisms underlying the response of C3 and C4 plants to drought stress, which may useful for engineering drought tolerance in plants.

Global Warming, Climate Change, and Environmental Pollution: Recipe for a Multifactorial Stress Combination Disaster

Global warming, climate change, and environmental pollution present plants with unique combinations of different abiotic and biotic stresses. Although much is known about how plants acclimate to each of these individual stresses, little is known about how they respond to a combination of many of these stress factors occurring together, namely a multifactorial stress combination. Recent studies revealed that increasing the number of different co-occurring multifactorial stress factors causes a severe decline in plant growth and survival, as well as in the microbiome biodiversity that plants depend upon. This effect should serve as a dire warning to our society and prompt us to decisively act to reduce pollutants, fight global warming, and augment the tolerance of crops to multifactorial stress combinations.

The impact of multifactorial stress combination on plant growth and survival

Climate change-driven extreme weather events, combined with increasing temperatures, harsh soil conditions, low water availability and quality, and the introduction of many man-made pollutants, pose a unique challenge to plants. Although our knowledge of the response of plants to each of these individual conditions is vast, we know very little about how a combination of many of these factors, occurring simultaneously, that is multifactorial stress combination, impacts plants. Seedlings of wild-type and different mutants of Arabidopsis thaliana plants were subjected to a multifactorial stress combination of six different stresses, each applied at a low level, and their survival, physiological and molecular responses determined. Our findings reveal that, while each of the different stresses, applied individually, had a negligible effect on plant growth and survival, the accumulated impact of multifactorial stress combination on plants was detrimental. We further show that the response of plants to multifactorial stress combination is unique and that specific pathways and processes play a critical role in the acclimation of plants to multifactorial stress combination. Taken together our findings reveal that further polluting our environment could result in higher complexities of multifactorial stress combinations that in turn could drive a critical decline in plant growth and survival.

The impact of water deficit and heat stress combination on the molecular response, physiology, and seed production of soybean

A combination of drought and heat stress, occurring at the vegetative or reproductive growth phase of many different crops can have a devastating impact on yield. In soybean (Glycine max), a considerable effort has been made to develop genotypes with enhanced yield production under conditions of drought or heat stress. However, how these genotypes perform in terms of growth, physiological responses, and most importantly seed production, under conditions of drought and heat combination is mostly unknown. Here, we studied the impact of water deficit and heat stress combination on the physiology, seed production, and yield per plant of two soybean genotypes, Magellan and Plant Introduction (PI) 548313, that differ in their reproductive responses to heat stress. Our findings reveal that although PI 548313 produced more seeds than Magellan under conditions of heat stress, under conditions of water deficit, and heat stress combination its seed production decreased. Because the number of flowers and pollen germination of PI 548313 remained high under heat or water deficit and heat combination, the reduced seed production exhibited by PI 548313 under the stress combination could be a result of processes that occur at the stigma, ovaries and/or other parts of the flower following pollen germination.

The impact of water deficit and heat stress combination on the molecular response, physiology, and seed production of soybean

A combination of drought and heat stress, occurring at the vegetative or reproductive growth phase of many different crops can have a devastating impact on yield. In soybean (Glycine max), a considerable effort has been made to develop genotypes with enhanced yield production under conditions of drought or heat stress. However, how these genotypes perform in terms of growth, physiological responses, and most importantly seed production, under conditions of drought and heat combination is mostly unknown. Here, we studied the impact of water deficit and heat stress combination on the physiology, seed production, and yield per plant of two soybean genotypes, Magellan and Plant Introduction (PI) 548313, that differ in their reproductive responses to heat stress. Our findings reveal that although PI 548313 produced more seeds than Magellan under conditions of heat stress, under conditions of water deficit, and heat stress combination its seed production decreased. Because the number of flowers and pollen germination of PI 548313 remained high under heat or water deficit and heat combination, the reduced seed production exhibited by PI 548313 under the stress combination could be a result of processes that occur at the stigma, ovaries and/or other parts of the flower following pollen germination.

Membrane dynamics during individual and combined abiotic stresses in plants and tools to study the same

The plasma membrane (PM) is possibly the most diverse biological membrane of plant cells; it separates and guards the cell against its external environment. It has an extremely complex structure comprising a mosaic of lipids and proteins. The PM lipids are responsible for maintaining fluidity, permeability and integrity of the membrane and also influence the functioning of membrane proteins. However, the PM is the primary target of environmental stress, which affects its composition, conformation and properties, thereby disturbing the cellular homeostasis. Maintenance of integrity and fluidity of the PM is a prerequisite for ensuring the survival of plants during adverse environmental conditions. The ability of plants to remodel membrane lipid and protein composition plays a crucial role in adaptation towards varying abiotic environmental cues, including high or low temperature, drought, salinity and heavy metals stress. The dynamic changes in lipid composition affect the functioning of membrane transporters and ultimately regulate the physical properties of the membrane. Plant membrane-transport systems play a significant role in stress adaptation by cooperating with the membrane lipidome to maintain the membrane integrity under stressful conditions. The present review provides a holistic view of stress responses and adaptations in plants, especially the changes in the lipidome and proteome of PM under individual or combined abiotic stresses, which cause alterations in the activity of membrane transporters and modifies the fluidity of the PM. The tools to study the varying lipidome and proteome of the PM are also discussed.

Development of a robust hydroponic method for screening of sunflower (Helianthus annuus L.) accessions for tolerance to heat and osmotic stress

Hydroponic systems are known to provide a platform for uniform growth conditions until the reproductive stage. However, many plant species, including sunflower, show poor growth and survivability under conventional hydroponic systems due to poor nutrient availability, hypoxia and algal contamination. Thus, we tested various hydroponic systems to select a hydroponic system suitable for screening of sunflower germplasm. Sunflower accessions showed better growth and leaf gas exchange in newly-designed over conventional hydroponic systems. Selected hydroponic systems were further engaged in sunflower accession screening under heat and osmotic stress in a two-pan system (210 cm × 60 cm). Heat stress treatment was applied by growing sunflower germplasm at 42 °C and osmotic stress by adding polyethylene glycol 8000 which decreased the osmotic potential to - 0.6 MPa. There was significant variability among the sunflower accessions for their ability to survive under stress. Accessions such as C-2721 (43%), C-291 (46%) and D-14 (43%) had lower cell membrane injury percentage under osmotic stress and high seedling survivability (60‒80%) under heat stress when compared with susceptible accessions. Moreover, resistant accessions exhibited greater cuticular waxes and root length but lower transpiration losses. The newly designed hydroponic platform proved reliable for the selection of resistant sunflower accessions. Selected parental lines were validated by assessing their hybrids under field trials across two seasons under water and temperature stress during the reproductive phase (autumn). Hybrid H3 obtained by crossing drought and heat resistant parents had the highest seed yield and water use efficiency.

Cyclic Nucleotide-Gated Ion Channel 6 Mediates Thermotolerance in Arabidopsis Seedlings by Regulating Hydrogen Peroxide Production via Cytosolic Calcium Ions

We previously reported the involvement of cyclic nucleotide-gated ion channel 6 (CNGC6) and hydrogen peroxide (H₂O₂) in plant responses to heat shock (HS). To demonstrate their relationship with plant thermotolerance, we assessed the effect of HS on several groups of Arabidopsis (Arabidopsis thaliana) seedlings: wild-type, cngc6 mutant, and its complementation line. Under exposure to HS, the level of H₂O₂ was lower in the cngc6 mutant seedlings than in the wild-type (WT) seedlings but obviously increased in the complementation line. The treatment of Arabidopsis seeds with calcium ions (Ca²⁺) increased the H₂O₂ levels in the seedlings under HS treatment, whereas treatment with a Ca²⁺ chelator (EGTA) inhibited it, indicating that CNGC6 may stimulate the accumulation of H₂O₂ in a manner dependent on an increase in cytosolic Ca²⁺ ([Ca²⁺]_cyt). This point was verified by phenotypic observations and thermotolerance testing with transgenic plants overexpressing AtRbohB and AtRbohD (two genes involved in HS-responsive H₂O₂ production), respectively, in a cngc6 background. Real-time reverse transcription-polymerase chain reactions and Western blotting suggested that CNGC6 enhanced the gene transcription of HS factors (HSFs) and the accumulation of HS proteins (HSPs) via H₂O₂. These upon results indicate that H₂O₂ acts downstream of CNGC6 in the HS signaling pathway, increasing our understanding of the initiation of plants responses to high temperatures.

Protein phosphorylation associated with drought priming-enhanced heat tolerance in a temperate grass species

Protein phosphorylation is known to play crucial roles in plant tolerance to individual stresses, but how protein phosphorylation is associated with cross-stress tolerance, particularly drought priming-enhanced heat tolerance is largely unknown. The objectives of the present study were to identify phosphorylated proteins and phosphorylation sites that were responsive to drought priming and to determine whether drought priming-enhanced heat tolerance in temperate grass species involves changes in protein phosphorylation. Comparative analysis of phosphoproteomic profiles was performed on leaves of tall fescue (Festuca arundinacea) exposed to heat stress (38/33 °C, day/night) with or without drought priming. A total of 569 differentially regulated phosphoproteins (DRPs) with 1098 phosphorylation sites were identified in response to drought priming or heat stress individually or sequentially. Most DRPs were nuclear-localized and cytosolic proteins. Motif analysis detected [GS], [DSD], and [S..E] as major phosphorylation sites in casein kinase-II and mitogen-activated protein kinases regulated by drought priming and heat stress. Functional annotation and gene ontology analysis demonstrated that DRPs in response to drought priming and in drought-primed plants subsequently exposed to heat stress were mostly enriched in four major biological processes, including RNA splicing, transcription control, stress protection/defense, and stress perception/signaling. These results suggest the involvement of post-translational regulation of the aforementioned biological processes and signaling pathways in drought priming memory and cross-tolerance with heat stress in a temperate grass species.

High temperatures modify plant responses to abiotic stress conditions

Climate change is altering environments in which plants and different crops grow and survive. We already experienced an increase in worldwide average earth surface temperatures, as well as frequency and extent of damaging heat waves. These conditions collide in the field with other abiotic stresses such as water deficit, high salinity, increased light irradiation, and so on, generating complex harmful conditions that destabilize agricultural systems. The conditions generated during these episodes of stress combination greatly differ from those occurring in the field when different stress factors occur individually; conditions that have been the focus of study for decades. Fortunately, knowledge of physiological and molecular responses to stress combinations and the cost they inflict on plant growth and yield has been exponentially increasing in the past several years. Understanding plant performance under multiple stress combinations will allow breeding crops capable of maintaining yield production under the new climatic conditions. Here, after reviewing recent data on physiological, hormonal and transcriptional responses to different stress combinations, we highlight the importance of photodamage avoidance, abscisic and jasmonic acid signaling, and the upregulation of genes involved in oxidation-reduction processes, photosynthesis and protein metabolism, for plant acclimation to conditions of high temperatures, in combination with other common abiotic stress factors such as drought or salinity. Finally, we propose new approaches to investigate the response of plants to stress combinations and discuss strategies for improving crop resilience to stress combination.

Regulative role of calcium signaling on methylglyoxal-improved heat tolerance in maize (Zea mays L) seedlings

Nowadays, calcium (Ca2+) and methylglyoxal (MG) are all deemed to be second messengers in plants, which participate in various physiological processes, such as seed germination, seedling establishment, plant growth and development, as well as response to environmental stress. However, the Ca2+-MG interaction in the development of thermotolerance in maize seedlings remains unclear. Here, using maize seedlings as materials, the crosstalk between Ca2+ and MG signaling in the acquisition of thermotolerance was explored. The results showed that root-irrigation with Ca2+ and MG alone or in combination increased the survival rate of maize seedlings under heat stress, mitigated the decrease in the tissue vitality, and reduced the membrane lipid peroxidation (in term of the content of malondialdehyde), indicating that Ca2+ and MG could improve the thermotolerance in maize seedlings. In addition, MG-improved thermotolerance was impaired by ethylene glycol-bis(b-aminoethylether)-N,N,N΄,N΄-tetraacetic acid (a Ca2+ chelator), La3+ (plasma membrane Ca2+ channel blocker), ruthenium red (a mitochondrial Ca2+ channel blocker), neomycin (vacuole Ca2+ channel blocker), caffeine (an endoplasmic reticulum Ca2+ channel blocker), and calmodulin antagonists (chlorpromazine and trifluoperazine), respectively. Also, MG scavengers (N-acetyl-cysteine, aminoguanidine, and vitamin B6) had no significant effect on Ca2+-triggered thermotolerance (in terms of survival rate, malondialdehyde, and tissue vitality) of maize seedlings. The data illustrated that calcium signaling regulated MG-improved thermotolerance in maize seedlings by mobilizing intracellular and extracellular Ca2+ pools.

Interplay between hydrogen sulfide and methylglyoxal initiates thermotolerance in maize seedlings by modulating reactive oxidative species and osmolyte metabolism

Hydrogen sulfide (H₂S) and methylglyoxal (MG) were supposed to be novel signaling molecules in plants. However, whether interplay between H₂S and MG can initiate thermotolerance in maize seedlings and in relation to metabolism of reactive oxygen species (ROS) and osmolytes is little known. In this study, watering with MG and NaHS (H₂S donor) alone or in combination elevated survival and tissue vigor of maize seedlings under heat stress and coped with an increase in the biomembrane injury (as indicated in membrane lipid peroxidation and electrolyte leakage). The above-mentioned effects were separately weakened by MG scavengers (N-acetyl cysteine: NAC; aminoguanidine: AG) and H₂S inhibitor (DL-propargylglycine, PAG) and scavenger (hypotaurine, HT). These suggested that the interplay between H₂S and MG initiated the thermotolerance in maize seedlings. The further data indicated that, under non-heat stress and heat stress conditions, MG and NaHS alone or in combination modulated ROS metabolism by regulating the activities of antioxidant enzymes (catalase, ascorbate peroxidase, guaiacol peroxidase, glutathione reductase, monodehydroascorbate reductase, and dehydroascorbate reductase) and the contents of non-enzymatic antioxidants (ascorbic acid, glutathione, flavonoids, and carotenoids) in maize seedlings. In addition, MG and NaHS alone or in combination also separately modulated the metabolism of osmolytes (proline, trehalose, glycine betaine, and total soluble sugar), H₂S (L-cysteine desulfhydrase and O-acetylserine (thione) lyase), and MG (glyoxalase I, glyoxalase II, and MG reductase). These physiological effects also were separately impaired by NAC, AG, PAG, and HT. The current data illustrated that the interplay between H₂S and MG initiated the thermotolerance in maize seedlings by modulating ROS, osmolyte, H₂S, and MG metabolism.

Transcriptional Basis for Differential Thermosensitivity of Seedlings of Various Tomato Genotypes

Transcriptional reprograming after the exposure of plants to elevated temperatures is a hallmark of stress response which is required for the manifestation of thermotolerance. Central transcription factors regulate the stress survival and recovery mechanisms and many of the core responses controlled by these factors are well described. In turn, pathways and specific genes contributing to variations in the thermotolerance capacity even among closely related plant genotypes are not well defined. A seedling-based assay was developed to directly compare the growth and transcriptome response to heat stress in four tomato genotypes with contrasting thermotolerance. The conserved and the genotype-specific alterations of mRNA abundance in response to heat stress were monitored after exposure to three different temperatures. The transcripts of the majority of genes behave similarly in all genotypes, including the majority of heat stress transcription factors and heat shock proteins, but also genes involved in photosynthesis and mitochondrial ATP production. In turn, genes involved in hormone and RNA-based regulation, such as auxin- and ethylene-related genes, or transcription factors like HsfA6b, show a differential regulation that associates with the thermotolerance pattern. Our results provide an inventory of genes likely involved in core and genotype-dependent heat stress response mechanisms with putative role in thermotolerance in tomato seedlings.

Signal transduction networks during stress combination

Episodes of heat waves combined with drought can have a devastating impact on agricultural production worldwide. These conditions, as well as many other types of stress combinations, impose unique physiological and developmental demands on plants and require the activation of dedicated pathways. Here, we review recent RNA sequencing studies of stress combination in plants, and conduct a meta-analysis of the transcriptome response of plants to different types of stress combination. Our analysis reveals that each different stress combination is accompanied by its own set of stress combination-specific transcripts, and that the response of different transcription factor families is unique to each stress combination. The alarming rate of increase in global temperatures, coupled with the predicted increase in future episodes of extreme weather, highlight an urgent need to develop crop plants with enhanced tolerance to stress combination. The uniqueness and complexity of the physiological and molecular response of plants to each different stress combination, highlighted here, demonstrate the daunting challenge we face in accomplishing this goal. Dedicated efforts combining field experimentation, omics, and network analyses, coupled with advanced phenotyping and breeding methods, will be needed to address specific crops and particular stress combinations relevant to maintaining our future food chain secured.

Flowers and climate change: a metabolic perspective

Adverse climatic conditions at the time of flowering severely hinder crop yields and threaten the interactions between plants and their pollinators. These features depend on a common trait: the metabolism of flowers. In this Viewpoint article, we aim to provide insight into the metabolic changes that occur in flowers in response to changes in climate and emphasize that these changes severely impact the fitness of autogamous and allogamous species, plant-pollinator interactions, and overall ecosystem health. We review the biochemical processes that lead to failure of gamete development and to alterations of color, scent and nectar secretion. Then, making use of open access expression data, we examine the expression of genes that may drive these changes in response to heat and drought. Finally, we present measurements of metabolites from flowers exposed to a heat wave and discuss how the results of this short-term experiment may give rise to misleading conclusions regarding the positive effect of heat on flower fitness. We hope this article draws attention to this often-neglected dynamic and its important consequences.

Cyclic nucleotide-gated ion channel 6 mediates thermotolerance in Arabidopsis seedlings by regulating nitric oxide production via cytosolic calcium ions

BACKGROUND

We previously reported the involvement of nitric oxide (NO) and cyclic nucleotide-gated ion channel 6 (CNGC6) in the responses of plants to heat shock (HS) exposure. To elucidate their relationship with heat tolerance in Arabidopsis thaliana, we examined the effects of HS on several groups of seedlings: wild type, cngc6, and cngc6 complementation and overexpression lines.

RESULTS

After HS exposure, the level of NO was lower in cngc6 seedlings than in wild-type seedlings but significantly elevated in the transgenic lines depending on CNGC6 expression level. The treatment of seeds with calcium ions (Ca²⁺) enhanced the NO level in Arabidopsis seedlings under HS conditions, whereas treatment with EGTA (a Ca²⁺ chelator) reduced it, implicating that CNGC6 stimulates the accumulation of NO depending on an increase in cytosolic Ca²⁺ ([Ca²⁺]_cyt). This idea was proved by phenotypic observations and thermotolerance testing of transgenic plants overexpressing NIA2 and NOA1, respectively, in a cngc6 background. Western blotting indicated that CNGC6 stimulated the accumulation of HS proteins via NO.

CONCLUSION

These data indicate that CNGC6 acts upstream of NO in the HS pathway, which improves our insufficient knowledge of the initiation of plant responses to high temerature.

Metabolic responses of rice source and sink organs during recovery from combined drought and heat stress in the field

BACKGROUND

Drought and heat stress effects on rice have been extensively studied, in particular during the sensitive flowering and grain-filling stages. However, in the field these stresses usually occur together because reduced transpirational cooling under drought conditions results in increased plant tissue temperature. In addition, environmental stresses are usually transient and the ability to efficiently recover from stress may be at least as important for overall stress tolerance as the direct stress response itself. Nevertheless, nothing is known about recovery mechanisms after drought and heat stress in rice under field conditions.

RESULTS

We have used gas chromatography-mass spectrometry-based metabolomics to elucidate the metabolic responses of flag leaves, flowering spikelets, and developing seeds from 3 rice cultivars differing in their drought and heat tolerance to rewatering after stress in the field. Within 60 hours after rewatering, many stress-responsive metabolites returned to their control levels, although recovery was not complete. In addition, control plants showed developmental differences that were revealed by metabolite profiles during 60 hours of post-stress sampling, in particular in developing seeds. Correlation analysis identified several metabolites as marker candidates for the stability of grain yield or quality under conditions of combined drought and heat stress.

CONCLUSIONS

The rewatering responses of stressed plants seemed to be a combination of the reversal of stress effects and reinitiation of development after stress relief. The identified potential markers can be useful in efforts to breed stress-tolerant rice germplasm to ensure food availability under changing climate conditions.

Metabolic responses of rice cultivars with different tolerance to combined drought and heat stress under field conditions

BACKGROUND

Rice is susceptible to both drought and heat stress, in particular during flowering and grain filling, when both grain yield and quality may be severely compromised. However, under field conditions, these 2 stresses rarely occur separately. Under well-watered conditions, plants avoid heat stress by transpirational cooling, while this is not possible under drought conditions. Although investigating combined drought and heat stress is clearly more agronomically relevant than analyzing the effects of the single stresses, only a few studies of this stress combination, in particular under field conditions, have been published.

RESULTS

Three rice cultivars differing in drought and heat tolerance were grown in the field under control and drought conditions in 3 consecutive years. Drought was applied either during flowering or during early grain filling and resulted in simultaneous heat stress, leading to reduced grain yield and quality. Analysis by gas chromatography-mass spectrometry showed distinct metabolic profiles for the 3 investigated organs (flag leaves, flowering spikelets, developing seeds). The metabolic stress responses of the plants also strongly differed between cultivars and organs. Correlation analysis identified potential metabolic markers for grain yield and quality under combined drought and heat stress from both stress-regulated metabolites and from metabolites with constitutive differences between the cultivars.

CONCLUSIONS

Gas chromatography-mass spectrometry resolved metabolic responses to combined drought and heat stress in different organs of field-grown rice. The metabolite profiles can be used to identify potential marker metabolites for yield stability and grain quality that are expected to improve breeding efforts towards developing rice cultivars that are resilient to climate change.

Methylglyoxal triggers the heat tolerance in maize seedlings by driving AsA-GSH cycle and reactive oxygen species-/methylglyoxal-scavenging system

Traditionally, methylglyoxal (MG) was looked upon as a toxic byproduct of cellular metabolism. Nowadays, MG has been found to be a novel signaling molecule. However, whether MG can trigger the heat tolerance in maize seedlings and the underlying mechanisms is still elusive. In this study, the maize seedlings irrigated with MG increased the survival percentage of seedlings under heat stress (HS), remitted a decrease in tissue vitality and an increase in electrolyte leakage, and reduced membrane lipid peroxidation, implying MG could trigger the heat tolerance of maize seedlings. The further experiments showed that MG drove the ascorbic acid (AsA)-glutathione (GSH) cycle by activating enzymes (glutathione reductase, monodehydroascorbate reductase, dehydroascorbate reductase, and ascorbate peroxidase) and increasing the contents of antioxidants (AsA and GSH) and the ratio of GSH/(GSH + oxidized glutathione) and AsA/(AsA + dehydroascorbate) under both non-HS and HS. Also, the reactive oxygen species (ROS)-scavenger system (catalase, guaiacol peroxidase, carotenoid, total phenols, and flavonoids) and MG-scavenger system (glyoxalase I and glyoxalas II) also were up-regulated in maize seedlings pretreated with MG under non-HS and HS. This work for the first time reported that MG could trigger the heat tolerance of maize seedlings by driving the AsA-GSH cycle and ROS-/MG-scavenging system.

Climate change and abiotic stress mechanisms in plants

Predicted global climatic change will perturb the productivity of our most valuable crops as well as detrimentally impact ecological fitness. The most important aspects of climate change with respect to these effects relate to water availability and heat stress. Over multiple decades, the plant research community has amassed a highly comprehensive understanding of the physiological mechanisms that facilitate the maintenance of productivity in response to drought, flooding, and heat stress. Consequently, the foundations necessary to begin the development of elite crop varieties that are primed for climate change are in place. To meet the food and fuel security concerns of a growing population, it is vital that biotechnological and breeding efforts to harness these mechanisms are accelerated in the coming decade. Despite this, those concerned with crop improvement must approach such efforts with caution and ensure that potentially harnessed mechanisms are viable under the context of a dynamically changing environment.

Interactive effects of drought and heat stresses on morpho-physiological attributes, yield, nutrient uptake and oxidative status in maize hybrids

Maize is a sensitive crop to drought and heat stresses, particularly at the reproductive stages of development. The present study investigated the individual and interactive effects of drought (50% field capacity) and heat (38 °C/30 °C) stresses on morpho-physiological growth, yield, nutrient uptake and oxidative metabolism in two maize hybrids i.e., 'Xida 889' and 'Xida 319'. The stress treatments were applied at tasseling stage for 15 days. Drought, heat and drought + heat stress caused oxidative stress by the over-production of ROS (O2-, H2O2, OH-) and enhanced malondialdehyde contents, which led to reduced photosynthetic components, nutrients uptake and yield attributes. The concurrent occurrence of drought and heat was more severe for maize growth than the single stress. However, both stresses induced the metabolites accumulation and enzymatic and non-enzymatic antioxidants to prevent the oxidative damage. The performance of Xida 899 was more prominent than the Xida 319. The greater tolerance of Xida 889 to heat and drought stresses was attributed to strong antioxidant defense system, higher osmolyte accumulation, and maintenance of photosynthetic pigments and nutrient balance compared with Xida 319.

Induced, Imprinted, and Primed Responses to Changing Environments: Does Metabolism Store and Process Information?

Metabolism is the system layer that determines growth by the rate of matter uptake and conversion into biomass. The scaffold of enzymatic reaction rates drives the metabolic network in a given physico-chemical environment. In response to the diverse environmental stresses, plants have evolved the capability of integrating macro- and micro-environmental events to be prepared, i.e., to be primed for upcoming environmental challenges. The hierarchical view on stress signaling, where metabolites are seen as final downstream products, has recently been complemented by findings that metabolites themselves function as stress signals. We present a systematic concept of metabolic responses that are induced by environmental stresses and persist in the plant system. Such metabolic imprints may prime metabolic responses of plants for subsequent environmental stresses. We describe response types with examples of biotic and abiotic environmental stresses and suggest that plants use metabolic imprints, the metabolic changes that last beyond recovery from stress events, and priming, the imprints that function to prepare for upcoming stresses, to integrate diverse environmental stress histories. As a consequence, even genetically identical plants should be studied and understood as phenotypically plastic organisms that continuously adjust their metabolic state in response to their individually experienced local environment. To explore the occurrence and to unravel functions of metabolic imprints, we encourage researchers to extend stress studies by including detailed metabolic and stress response monitoring into extended recovery phases.

Impact on physiology and malting quality of barley exposed to heat, drought and their combination during different growth stages under controlled environment

Drought and heat stress are two major abiotic stresses that tend to co-occur in nature. Recent climate change models predict that the frequency and duration of periods of high temperatures and moisture-deficits are on the rise and can be detrimental to crop production and hence a serious threat for global food security. In this study we examined the impact of short-term heat, drought and combined heat and drought stress on four barley varieties. These stresses were applied during vegetative stage or during heading stages. The impact on root and shoot biomass as well as seed yields were analyzed. This study demonstrated that sensitivity to combined stress was generally greater than heat or drought individually, and greater when imposed at heading than at the vegetative stages. Micromalted seeds collected from plants stressed during heading showed differences in malt extract, beta-glucan content and percent soluble protein. Screening barley germplasm during heading stage is recommended to identify novel sources of tolerance to combined stress. Apart from seed yield, assessing the seed quality traits of concern for the stakeholders and/or consumers should be an integral part of breeding programs for developing new barley varieties with improved heat and drought stress tolerance.