Stigma Functionality and Fertility Are Reduced by Heat and Drought Co-stress in Wheat

Surviving a Double-Edged Sword: Response of Horticultural Crops to Multiple Abiotic Stressors

Climate change-induced weather events, such as extreme temperatures, prolonged drought spells, or flooding, pose an enormous risk to crop productivity. Studies on the implications of multiple stresses may vary from those on a single stress. Usually, these stresses coincide, amplifying the extent of collateral damage and contributing to significant financial losses. The breadth of investigations focusing on the response of horticultural crops to a single abiotic stress is immense. However, the tolerance mechanisms of horticultural crops to multiple abiotic stresses remain poorly understood. In this review, we described the most prevalent types of abiotic stresses that occur simultaneously and discussed them in in-depth detail regarding the physiological and molecular responses of horticultural crops. In particular, we discussed the transcriptional, posttranscriptional, and metabolic responses of horticultural crops to multiple abiotic stresses. Strategies to breed multi-stress-resilient lines have been presented. Our manuscript presents an interesting amount of proposed knowledge that could be valuable in generating resilient genotypes for multiple stressors.

Reactive oxygen species are signaling molecules that modulate plant reproduction

Reactive oxygen species (ROS) can serve as signaling molecules that are essential for plant growth and development but abiotic stress can lead to ROS increases to supraoptimal levels resulting in cellular damage. To ensure efficient ROS signaling, cells have machinery to locally synthesize ROS to initiate cellular responses and to scavenge ROS to prevent it from reaching damaging levels. This review summarizes experimental evidence revealing the role of ROS during multiple stages of plant reproduction. Localized ROS synthesis controls the formation of pollen grains, pollen-stigma interactions, pollen tube growth, ovule development, and fertilization. Plants utilize ROS-producing enzymes such as respiratory burst oxidase homologs and organelle metabolic pathways to generate ROS, while the presence of scavenging mechanisms, including synthesis of antioxidant proteins and small molecules, serves to prevent its escalation to harmful levels. In this review, we summarized the function of ROS and its synthesis and scavenging mechanisms in all reproductive stages from gametophyte development until completion of fertilization. Additionally, we further address the impact of elevated temperatures induced ROS on impairing these reproductive processes and of flavonol antioxidants in maintaining ROS homeostasis to minimize temperature stress to combat the impact of global climate change on agriculture.

Concurrent effect of drought and heat stress in rice (Oryza sativa L.): physio-biochemical and molecular approach

The present study was carried out to investigate the physio-biochemical and molecular responses of two rice genotypes (Noichi and N22) under drought, heat and combined drought/heat stress conditions. The antagonistic stomatal activity was found under the combined stress conditions; stomata were open under control and heat stress, conversely, stomata remained closed under drought and combined stress levels. Photosynthetic activity and chlorophyll content are decreased by the overproduction of reactive oxygen species and increased lipid peroxidation in both rice genotypes. To prevent oxidative damage, many antioxidant enzymes like catalase (CAT), ascorbate peroxidase (APX) and superoxide dismutase (SOD) are produced in both genotypes under these conditions. Under the single stress conditions, CAT activity were increased in N22, whereas combined stress levels, SOD and APX activity were higher for both genotypes. Proline accumulation was also increased under single as well as combined stress conditions for both genotypes to combat stress injuries. Pollen viability was lost under all stress levels but severe loss was found under combined stress levels, which causes spikelet sterility leading to yield losses for both genotypes. As evident from transcript levels, HSP71.18 and HSP71.10 expressions were higher under single and combined conditions, butHSP72.57 gene expression increased only by individual stress levels. WRKY11, WRKY 55, DREB 2A, LEA3 and DHN1 were positively expressed under all stress levels. Conversely, expression of DREB2B genes was higher only under single stress levels. In summary, these results suggest that the effect of combined stress is different from the single stress and it is more severe than the individual stress.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-024-03980-1.

Heat stress during male meiosis impairs cytoskeletal organization, spindle assembly and tapetum degeneration in wheat

The significance of heat stress in agriculture is ever-increasing with the progress of global climate changes. Due to a negative effect on the yield of staple crops, including wheat, the impairment of plant reproductive development triggered by high ambient temperature became a restraint in food production. Although the heat sensitivity of male meiosis and the following gamete development in wheat has long been recognized, a detailed structural characterization combined with a comprehensive gene expression analysis has not been done about this phenomenon. We demonstrate here that heat stress severely alters the cytoskeletal configuration, triggers the failure of meiotic division in wheat. Moreover, it changes the expression of genes related to gamete development in male meiocytes and the tapetum layer in a genotype-dependent manner. 'Ellvis', a heat-tolerant winter wheat cultivar, showed high spikelet fertility rate and only scarce structural aberrations upon exposure to high temperature. In addition, heat shock genes and genes involved in scavenging reactive oxygen species were significantly upregulated in 'Ellvis', and the expression of meiosis-specific and major developmental genes showed high stability in this cultivar. In the heat-sensitive 'Mv 17-09', however, genes participating in cytoskeletal fiber nucleation, the spindle assembly checkpoint genes, and tapetum-specific developmental regulators were downregulated. These alterations may be related to the decreased cytoskeleton content, frequent micronuclei formation, and the erroneous persistence of the tapetum layer observed in the sensitive genotype. Our results suggest that understanding the heat-sensitive regulation of these gene functions would be an essential contribution to the development of new, heat-tolerant cultivars.

Uncovering the wide protective responses in Coffea spp. leaves to single and superimposed exposure of warming and severe water deficit

Climate changes boosted the frequency and severity of drought and heat events, with aggravated when these stresses occur simultaneously, turning crucial to unveil the plant response mechanisms to such harsh conditions. Therefore, plant responses/resilience to single and combined exposure to severe water deficit (SWD) and heat were assessed in two cultivars of the main coffee-producing species: Coffea arabica cv. Icatu and C. canephora cv. Conilon Clone 153 (CL153). Well-watered plants (WW) were exposed to SWD under an adequate temperature of 25/20°C (day/night), and thereafter submitted to a gradual increase up to 42/30°C, and a 14-d recovery period (Rec14). Greater protective response was found to single SWD than to single 37/28°C and/or 42/30°C (except for HSP70) in both cultivars, but CL153-SWD plants showed the larger variations of leaf thermal imaging crop water stress index (CWSI, 85% rise at 37/28°C) and stomatal conductance index (IG, 66% decline at 25/20°C). Both cultivars revealed great resilience to SWD and/or 37/28°C, but a tolerance limit was surpassed at 42/30°C. Under stress combination, Icatu usually displayed lower impacts on membrane permeability, and PSII function, likely associated with various responses, usually mostly driven by drought (but often kept or even strengthened under SWD and 42/30°C). These included the photoprotective zeaxanthin and lutein, antioxidant enzymes (superoxide dismutase, Cu,Zn-SOD; ascorbate peroxidase, APX), HSP70, arabinose and mannitol (involving de novo sugar synthesis), contributing to constrain lipoperoxidation. Also, only Icatu showed a strong reinforcement of glutathione reductase activity under stress combination. In general, the activities of antioxidative enzymes declined at 42/30°C (except Cu,Zn-SOD in Icatu and CAT in CL153), but HSP70 and raffinose were maintained higher in Icatu, whereas mannitol and arabinose markedly increased in CL153. Overall, a great leaf plasticity was found, especially in Icatu that revealed greater responsiveness of coordinated protection under all experimental conditions, justifying low PIChr and absence of lipoperoxidation increase at 42/30°C. Despite a clear recovery by Rec14, some aftereffects persisted especially in SWD plants (e.g., membranes), relevant in terms of repeated stress exposure and full plant recovery to stresses.

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.

From the floret to the canopy: High temperature tolerance during flowering

Heat waves induced by climate warming have become common in food-producing regions worldwide, frequently coinciding with high temperature (HT)-sensitive stages of many crops and thus threatening global food security. Understanding the HT sensitivity of reproductive organs is currently of great interest for increasing seed set. The responses of seed set to HT involve multiple processes in both male and female reproductive organs, but we currently lack an integrated and systematic summary of these responses for the world's three leading food crops (rice, wheat, and maize). In the present work, we define the critical high temperature thresholds for seed set in rice (37.2°C ± 0.2°C), wheat (27.3°C ± 0.5°C), and maize (37.9°C ± 0.4°C) during flowering. We assess the HT sensitivity of these three cereals from the microspore stage to the lag period, including effects of HT on flowering dynamics, floret growth and development, pollination, and fertilization. Our review synthesizes existing knowledge about the effects of HT stress on spikelet opening, anther dehiscence, pollen shedding number, pollen viability, pistil and stigma function, pollen germination on the stigma, and pollen tube elongation. HT-induced spikelet closure and arrest of pollen tube elongation have a catastrophic effect on pollination and fertilization in maize. Rice benefits from pollination under HT stress owing to bottom anther dehiscence and cleistogamy. Cleistogamy and secondary spikelet opening increase the probability of pollination success in wheat under HT stress. However, cereal crops themselves also have protective measures under HT stress. Lower canopy/tissue temperatures compared with air temperatures indicate that cereal crops, especially rice, can partly protect themselves from heat damage. In maize, husk leaves reduce inner ear temperature by about 5°C compared with outer ear temperature, thereby protecting the later phases of pollen tube growth and fertilization processes. These findings have important implications for accurate modeling, optimized crop management, and breeding of new varieties to cope with HT stress in the most important staple crops.

Pollen viability, longevity, and function in angiosperms: key drivers and prospects for improvement

Pollen grains are central to sexual plant reproduction and their viability and longevity/storage are critical for plant physiology, ecology, plant breeding, and many plant product industries. Our goal is to present progress in assessing pollen viability/longevity along with recent advances in our understanding of the intrinsic and environmental factors that determine pollen performance: the capacity of the pollen grain to be stored, germinate, produce a pollen tube, and fertilize the ovule. We review current methods to measure pollen viability, with an eye toward advancing basic research and biotechnological applications. Importantly, we review recent advances in our understanding of how basic aspects of pollen/stigma development, pollen molecular composition, and intra- and intercellular signaling systems interact with the environment to determine pollen performance. Our goal is to point to key questions for future research, especially given that climate change will directly impact pollen viability/longevity. We find that the viability and longevity of pollen are highly sensitive to environmental conditions that affect complex interactions between maternal and paternal tissues and internal pollen physiological events. As pollen viability and longevity are critical factors for food security and adaptation to climate change, we highlight the need to develop further basic research for better understanding the complex molecular mechanisms that modulate pollen viability and applied research on developing new methods to maintain or improve pollen viability and longevity.

The transcriptome of soybean reproductive tissues subjected to water deficit, heat stress, and a combination of water deficit and heat stress

Global warming and climate change are driving an alarming increase in the frequency and intensity of extreme climate events, such as droughts, heat waves, and their combination, inflicting heavy losses to agricultural production. Recent studies revealed that the transcriptomic responses of different crops to water deficit (WD) or heat stress (HS) are very different from that to a combination of WD + HS. In addition, it was found that the effects of WD, HS, and WD + HS are significantly more devastating when these stresses occur during the reproductive growth phase of crops, compared to vegetative growth. As the molecular responses of different reproductive and vegetative tissues of plants to WD, HS, or WD + HS could be different from each other and these differences could impact many current and future attempts to enhance the resilience of crops to climate change through breeding and/or engineering, we conducted a transcriptomic analysis of different soybean (Glycine max) tissues to WD, HS, and WD + HS. Here we present a reference transcriptomic dataset that includes the response of soybean leaf, pod, anther, stigma, ovary, and sepal to WD, HS, and WD + HS conditions. Mining this dataset for the expression pattern of different stress response transcripts revealed that each tissue had a unique transcriptomic response to each of the different stress conditions. This finding is important as it suggests that enhancing the overall resilience of crops to climate change could require a coordinated approach that simultaneously alters the expression of different groups of transcripts in different tissues in a stress-specific manner.

High Temperature Tolerance in a Novel, High-Quality Phaseolus vulgaris Breeding Line Is Due to Maintenance of Pollen Viability and Successful Germination on the Stigma

The common bean (Phaseolus vulgaris L.) is an important nutritional source globally but is sensitive to high temperatures and thus particularly vulnerable to climate change. Derived from a breeding program at CIAT (Colombia), a heat-tolerant breeding line, named heat-tolerant Andean-type 4 (HTA4), was developed by a series of crosses of parents with a small-bean tepary genotype (Phaseolus acutifolius L.) in their pedigree, which might be the donor of heat stress (HS) tolerance. Importantly, in HTA4, the large, commercially desirable Andean-type beans was restored. To assess underlying tolerance mechanisms, HTA4, together with a heat-sensitive Colombian variety (Calima), was exposed to HS (31 °C/24 °C HS vs. 26 °C/19 °C day/night) under controlled environment conditions. Vegetative growth and photosynthetic performance were not negatively impacted by HS in either genotype, although senescence was delayed in Calima. HS during the reproductive stage caused an increase in pod number in Calima but with few fully developed seeds and many pods aborted and/or abscised. In contrast, HTA4 maintained a similar filled pod number under HS and a higher seed weight per plant. Pollen showed high sterility in Calima, with many non-viable pollen grains (24.9% viability compared to 98.4% in control) with a thicker exine and fewer starch granules under HS. Calima pollen failed to adhere to the stigma and germinate under HS. In HTA4, pollen viability was significantly higher than in Calima (71.1% viability compared to 95.4% under control), and pollen successfully germinated and formed pollen tubes in the style under HS. It is concluded that HTA4 is heat tolerant and maintains a high level of reproductive output due to its ability to produce healthy pollen that is able to adhere to the stigma.

Brassinosteroids mediate moderate soil-drying to alleviate spikelet degeneration under high temperature during meiosis of rice

This study tested the hypothesis that brassinosteroids (BRs) mediate moderate soil-drying (MD) to alleviate spikelet degeneration under high temperature (HT) stress during meiosis of rice (Oryza sativa L.). A rice cultivar was pot-grown and subjected to normal temperature (NT) and HT treatments during meiosis, and two irrigation regimes including well-watered (WW) and MD were imposed to the plants simultaneously. The MD effectively alleviated the spikelet degeneration and yield loss under HT stress mainly via improving root activity and canopy and panicle traits including higher photosynthetic capacity, tricarboxylic acid cycle activity, and antioxidant capacity than WW. These parameters were regulated by BRs levels in plants. The decrease in BRs levels at HT was due mainly to the enhanced BRs decomposition, and the MD could rescue the BRs deficiency at HT via enhancing BRs biosynthesis and impeding decomposition. The connection between BRs and HT was verified by using rice BRs-deficient mutants, transgenic rice lines, and chemical regulators. Similar results were obtained in the open-air field experiment. The results suggest that BRs can mediate the MD to alleviate spikelet degeneration under HT stress during meiosis mainly via enhancing root activity, canopy traits, and young panicle traits of rice.

M, Paudel K. Influence of high-temperature stress on rice growth and development. A review

High-temperature stress (HS) has become an alarming threat to the global food system. Rice, an important crop that supports almost half of the global population, is vulnerable to heat stress. Under the influence of HS, it shows various physiological and morphological symptoms that increase spikelet sterility, reduce grain yield, and even cause total crop failure. HS affects growth and yield in two ways: hindrance in the process of pollination and fertilization and reduction of the grain weight. The former is caused by (i) distortion of floral organs, (ii) tapetum degeneration, (iii) low pollen protein concentration, (iv) decline in pollen viability, (v) reduction in dehiscence of anther, (vi) low pollen dispersal, (vii) decrease in number of pollens on stigma, (viii) reduction in pollen grain germination, (ix) hindrance in extension of pollen tubes, and (x) shrinkage of stigma which ultimately cause spikelet infertility. The latter is caused by (i)reduced photosynthetic rate, (ii) a boost in senescence of functional leaves, (iii) reduction of biological synthesis of starch, (iv)reduced starch augmentation, (v) shrunk duration of grain filling, and (vi) declined grain weight which ultimately reduce the grain yield. However, some agronomic and breeding approaches have been adopted for developing thermo-resistant cultivars but the success is limited. In this paper, we have summarized the the morpho-physiological and molecular response of plant to HS, and a few possible management strategies.

Enhancement of Heat and Drought Stress Tolerance in Rice by Genetic Manipulation: A Systematic Review

As a result of global warming, plants are subjected to ever-increasing abiotic stresses including heat and drought. Drought stress frequently co-occurs with heat stress as a result of water evaporation. These stressors have adverse effects on crop production, which in turn affects human food security. Rice is a major food resource grown widely in crop-producing regions throughout the world. However, increasingly common heat and drought stresses in growth regions can have negative impacts on seedling morphogenesis, reproductive organ establishment, overall yield, and quality. This review centers on responses to heat and drought stress in rice. Current knowledge of molecular regulation mechanisms is summarized. We focus on approaches to cope with heat and drought stress, both at the genetic level and from an agricultural practice perspective. This review establishes a basis for improving rice stress tolerance, grain quality, and yield for human benefit.

Metabolomic and transcriptomic analyses reveal that sucrose synthase regulates maize pollen viability under heat and drought stress

Maize pollen is highly sensitive to heat and drought, but few studies have investigated the combined effects of heat and drought on pollen viability. In this study, pollen's structural and physiological characteristics were determined after heat, drought, and combined stressors. Furthermore, integrated metabolomic and transcriptomic analyses of maize pollen were conducted to identify potential mechanisms of stress responses. Tassel growth and spikelet development were considerably suppressed, pollen viability was negatively impacted, and pollen starch granules were depleted during anthesis under stress. The inhibitory effects were more significant due to combined stresses than to heat or drought individually. The metabolic analysis identified 71 important metabolites in the combined stress compared to the other treatments, including sugars and their derivatives related to pollen viability. Transcriptomics also revealed that carbohydrate metabolism was significantly altered under stress. Moreover, a comprehensive metabolome-transcriptome analysis identified a central mechanism in the biosynthesis of UDP-glucose involved in reducing the activity of sucrose synthase SH-1 (shrunken 1) and sus1 (sucrose synthase 1) that suppressed sucrose transfer to UDP-glucose, leading to pollen viability exhaustion under stress. In conclusion, the lower pollen viability after heat and drought stress was associated with poor sucrose synthase activity due to the stress treatments.

Reduction of heat stress pressure and activation of photosystem II repairing system are crucial for citrus tolerance to multiple abiotic stress combination

Drought, heat and high irradiance are abiotic stresses that negatively affect plant development and reduce crop productivity. The confluence of these three factors is common in nature, causing extreme situations for plants that compromise their viability. Drought and heat stresses increase the saturation of the photosystem reaction centers, increasing sensitivity to high irradiance. In addition, these stress conditions affect photosystem II (PSII) integrity, alter redox balance of the electron transport chain and decrease the photosynthetic rate. Here, we studied the effect of the stress combinations on the photosynthetic apparatus of two citrus genotypes, Carrizo citrange (Citrus sinensis × Poncirus trifoliata) and Cleopatra mandarin (Citrus reshni). Results obtained showed that physiological responses, such as modulation of stomatal aperture and transpiration rate, aimed to reduce leaf temperature, are key to diminishing heat impact on photosynthetic apparatus and increasing tolerance to double and triple combinations of drought, high irradiance and high temperatures. By using transcriptomic and proteomic analyses, we have demonstrated that under these abiotic stress combinations, Carrizo plants were able to increase expression of genes and proteins related to the photosystem repairing machinery (which better maintained the integrity of PSII) and other components of the photosynthetic apparatus. Our findings reveal crucial physiological and genetic responses in citrus to increase tolerance to the combination of multiple abiotic stresses that could be the basis for breeding programs that ensure a sustainable citrus production.

Inquiring the inter-relationships amongst grain-filling, grain-yield, and grain-quality of Japonica rice at high latitudes of China

The widespread impacts of projected global and regional climate change on rice yield have been investigated by different indirect approaches utilizing various simulation models. However, direct approaches to assess the impacts of climatic variabilities on rice growth and development may provide more reliable evidence to evaluate the effects of climate change on rice productivity. Climate change has substantially impacted rice production in the mid-high latitudes of China, especially in Northeast China (NEC). Climatic variabilities occurring in NEC since the 1970s have resulted in an obvious warming trend, which made this region one of the three major rice-growing regions in China. However, the projections of future climate change have indicated the likelihood of more abrupt and irregular climatic changes, posing threats to rice sustainability in this region. Hence, understanding the self-adaptability and identifying adjustive measures to climate variability in high latitudes has practical significance for establishing a sustainable rice system to sustain future food security in China. A well-managed field study under randomized complete block design (RCBD) was conducted in 2017 and 2018 at two study sites in Harbin and Qiqihar, located in Heilongjiang province in NEC. Four different cultivars were evaluated: Longdao-18, Longdao-21 (longer growth duration), Longjing-21, and Suijing-18 (shorter growth duration) to assess the inter-relationships among grain-filling parameters, grain yield and yield components, and grain quality attributes. To better compare the adaptability mechanisms between grain-filling and yield components, the filling phase was divided into three sub-phases (start, middle, and late). The current study evaluated the formation and accumulation of the assimilates in superior and inferior grains during grain-filling, mainly in the middle sub-phase, which accounted for 59.60% of the yield. The grain yields for Suijing-18, Longjing-21, Longdao-21, and Longdao-18 were 8.02%, 12.78%, 17.19%, and 20.53% higher in Harbin than those in Qiqihar, respectively in 2017, with a similar trend observed in 2018. At Harbin, a higher number of productive tillers was noticed in Suijing-18, with averages of 17 and 15 in 2017 and 2018, respectively. The grain-filling parameters of yield analysis showed that the filling duration in Harbin was conducive to increased yield but the low dry weight of inferior grains was a main factor limiting the yield in Qiqihar. The average protein content values in Harbin were significantly higher (8.54% and 9.13%) than those in Qiqihar (8.34% and 9.14%) in 2017 and 2018, respectively. The amylose content was significantly higher in Harbin (20.03% and 22.27%) than those in Qiqihar (14.44% and 14.67%) in 2017 and 2018, respectively. The chalkiness percentage was higher in Qiqihar, indicating that Harbin produced good quality rice. This study provides more direct evidence of the relative changes in rice grain yield due to changes in grain-filling associated with relative changes in environmental components. These self-adaptability mechanisms to climatic variability and the inter-relationships between grain-filling and grain yield underscore the urgent to investigate and explore measures to improve Japonica rice sustainability, with better adaptation to increasing climatic variabilities. These findings may also be a reference for other global rice regions at high latitudes in addressing the impacts of climate change on future rice sustainability.

Precision phenotyping across the life cycle to validate and decipher drought-adaptive QTLs of wild emmer wheat (Triticum turgidum ssp. dicoccoides) introduced into elite wheat varieties

Drought events or the combination of drought and heat conditions are expected to become more frequent due to global warming, and wheat yields may fall below their long-term average. One way to increase climate-resilience of modern high-yielding varieties is by their genetic improvement with beneficial alleles from crop wild relatives. In the present study, the effect of two beneficial QTLs introgressed from wild emmer wheat and incorporated in the three wheat varieties BarNir, Zahir and Uzan was studied under well-watered conditions and under drought stress using non-destructive High-throughput Phenotyping (HTP) throughout the life cycle in a single pot-experiment. Plants were daily imaged with RGB top and side view cameras and watered automatically. Further, at two time points, the quantum yield of photosystem II was measured with a top view FluorCam. The QTL carrying near isogenic lines (NILs) were compared with their corresponding parents by t-test for all non-invasively obtained traits and for the manually determined agronomic and yield parameters. Data quality of phenotypic traits (repeatability) in the controlled HTP experiment was above 85% throughout the life cycle and at maturity. Drought stress had a strong effect on growth in all wheat genotypes causing biomass reduction from 2% up to 70% at early and late points in the drought period, respectively. At maturity, the drought caused 47-55% decreases in yield-related traits grain weight, straw weight and total biomass and reduced TKW by 10%, while water use efficiency (WUE) increased under drought by 29%. The yield-enhancing effect of the introgressed QTLs under drought conditions that were previously demonstrated under field/screenhouse conditions in Israel, could be mostly confirmed in a greenhouse pot experiment using HTP. Daily precision phenotyping enabled to decipher the mode of action of the QTLs in the different genetic backgrounds throughout the entire wheat life cycle. Daily phenotyping allowed a precise determination of the timing and size of the QTLs effect (s) and further yielded information about which image-derived traits are informative at which developmental stage of wheat during the entire life cycle. Maximum height and estimated biovolume were reached about a week after heading, so experiments that only aim at exploring these traits would not need a longer observation period. To obtain information on different onset and progress of senescence, the CVa curves represented best the ongoing senescence of plants. The QTL on 7A in the BarNir background was found to improve yield under drought by increased biomass growth, a higher photosynthetic performance, a higher WUE and a "stay green effect."

A scalable phenotyping approach for female floral organ development and senescence in the absence of pollination in wheat

In the absence of pollination, female reproductive organs senesce, leading to an irrevocable loss in the reproductive potential of the flower, which directly affects seed set. In self-pollinating crops like wheat (Triticum aestivum), the post-anthesis viability of unpollinated carpels has been overlooked, despite its importance for hybrid seed production systems. To advance our knowledge of carpel development in the absence of pollination, we created a high-throughput phenotyping approach to quantify stigma and ovary morphology. We demonstrate the suitability of the approach, which uses light-microscopy imaging and machine learning, for the analysis of floral organ traits in field-grown plants using fresh and fixed samples. We show that the unpollinated carpel undergoes a well-defined initial growth phase, followed by a peak phase in which stigma area reaches its maximum and the radial expansion of the ovary slows, and a final deterioration phase. These developmental dynamics were consistent across years and could be used to classify male-sterile cultivars. This phenotyping approach provides a new tool for examining carpel development, which we hope will advance research into female fertility of wheat.

Transpirational Leaf Cooling Effect Did Not Contribute Equally to Biomass Retention in Wheat Genotypes under High Temperature

High temperature and water deficit are the most critical yield-limiting environmental factors for wheat in rainfed environments. It is important to understand the heat avoidance mechanisms and their associations with leaf morpho-physiological traits that allow crops to stay cool and retain high biomass under warm and dry conditions. We examined 20 morpho-physiologically diverse wheat genotypes under ambient and elevated temperatures (T_air) to investigate whether increased water use leads to high biomass retention due to increased leaf cooling. An experiment was conducted under well-watered conditions in two partially controlled glasshouses. We measured plant transpiration (T_r), leaf temperature (T_leaf), vapor pressure deficit (VPD), and associated leaf morpho-physiological characteristics. High water use and leaf cooling increased biomass retention under high temperatures, but increased use did not always increase biomass retention. Some genotypes maintained biomass, irrespective of water use, possibly through mechanisms other than leaf cooling, indicating their adaptation under water shortage. Genotypic differences in leaf cooling capacity did not always correlate with T_r (VPD) response. In summary, the contribution of high water use or the leaf cooling effect on biomass retention under high temperature is genotype-dependent and possibly due to variations in leaf morpho-physiological traits. These findings are useful for breeding programs to develop climate resilient wheat cultivars.

The Balance between Different ROS on Tobacco Stigma during Flowering and Its Role in Pollen Germination

The concept of ROS as an important factor controlling pollen germination and tube growth has become generally accepted in the last decade. However, the relationship between various ROS and their significance for the success of in vivo germination and fertilization remained unexplored. For the present study, we collected Nicotiana tabacum stigma exudate on different stages of stigma maturity before and after pollination. Electron paramagnetic resonance (EPR) and colorimetric analysis were used to assess levels of O^•₂⁻ and H₂O₂ on stigma. Superoxide dismutase activity in the stigma tissues at each stage was evaluated zymographically. As the pistil matured, the level of both ROS decreased markedly, while the activity of SOD increased, and, starting from the second stage, the enzyme was represented by two isozymes: Fe SOD and Cu/Zn SOD, which was demonstrated by the in-gel inhibitory analysis. Selective suppression of Cu/Zn SOD activity shifted the ROS balance, which was confirmed by EPR. This shift markedly reduced the rate of pollen germination in vivo and the fertilization efficiency, which was estimated by the seed set. This result showed that hydrogen peroxide is a necessary component of stigma exudate, accelerates germination and ensures successful reproduction. A decrease in O^•₂⁻ production due to NADPH oxidase inhibition, although it slowed down germination, did not lead to a noticeable decrease in the seed set. Thus, the role of the superoxide radical can be characterized as less important.

Citrus rootstocks modify scion antioxidant system under drought and heat stress combination

The activation of the antioxidant system under stress combination is a transmissible trait from the rootstock to the scion. Therefore, rootstock selection is key to improve crop performance and a sustainable production under changing climate conditions. Climate change is altering weather conditions such as mean temperatures and precipitation patterns. Rising temperatures, especially in certain regions, accelerates soil water depletion and increases drought risk, which affects agriculture yield. Previously, our research demonstrated that the citrus rootstock Carrizo citrange (Citrus sinensis × Poncirus trifoliata) is more tolerant than Cleopatra mandarin (C. reshni) to drought and heat stress combination, in part, due to a higher activation of the antioxidant system that alleviated damage produced by oxidative stress. Here, by using reciprocal grafts of both genotypes, we studied the importance of the rootstock on scion performance and antioxidant response under this stress combination. Carrizo rootstock, under stress combination, positively influenced Cleopatra scion by reducing H₂O₂ accumulation, increasing superoxide dismutase (SOD) and ascorbate peroxidase (APX) enzymatic activities and inducing SOD1, APX2 and catalase (CAT) protein accumulations. On the contrary, Cleopatra rootstock induced decreases in APX2 expression, CAT activity and SOD1, APX2 and CAT contents on Carrizo scion. Taken together, our findings indicate that the activation of the antioxidant system under stress combination is a transmissible trait from the rootstock to the scion and highlight the importance of the rootstock selection to improve crop performance and maintain citrus yield under the current scenario of climate change.

'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.

Integrative analysis of transcriptome and proteome provides insights into adaptation to cadmium stress in Sedum plumbizincicola

Sedum plumbizincicola, a cadmium (Cd) hyperaccumulating herbaceous plant, can accumulate large amounts of Cd in the above-ground tissues without being poisoned. However, the molecular mechanisms regulating the processes are not fully understood. In this study, Transcriptional and proteomic analyses were integrated to investigate the response of S. plumbizincicola plants to Cd stress and to identify key pathways that are potentially responsible for Cd tolerance and accumulation. A total of 630 DAPs (differentially abundant proteins, using fold change >1.5 and adjusted p-value <0.05) were identified from Tandem Mass Tag (TMT)- based quantitative proteomic profiling, which were enriched in processes including phenylpropanoid biosynthesis, protein processing in endoplasmic reticulum, and biosynthesis of secondary metabolites. Combined with the previous transcriptomic study, 209 genes and their corresponding proteins showed the identical expression pattern. The identified genes/proteins revealed the potential roles of several metabolism pathways, including phenylpropanoid biosynthesis, oxidative phosphorylation, phagosome, and glutathione metabolism, in mediating Cd tolerance and accumulation. Lignin staining and Cd accumulation assay of the transgenic lines over-expressing a selected Cd up-regulated gene SpFAOMT (Flavonoid 3',5'-methyltransferase) showed its functions in adapting to Cd stress, and provided insight into its role in lignin biosynthesis and Cd accumulation in S. plumbizincicola during Cd stress.

Differential Physio-Biochemical and Metabolic Responses of Peanut (Arachis hypogaea L.) under Multiple Abiotic Stress Conditions

The frequency and severity of extreme climatic conditions such as drought, salinity, cold, and heat are increasing due to climate change. Moreover, in the field, plants are affected by multiple abiotic stresses simultaneously or sequentially. Thus, it is imperative to compare the effects of stress combinations on crop plants relative to individual stresses. This study investigated the differential regulation of physio-biochemical and metabolomics parameters in peanut (Arachis hypogaea L.) under individual (salt, drought, cold, and heat) and combined stress treatments using multivariate correlation analysis. The results showed that combined heat, salt, and drought stress compounds the stress effect of individual stresses. Combined stresses that included heat had the highest electrolyte leakage and lowest relative water content. Lipid peroxidation and chlorophyll contents did not significantly change under combined stresses. Biochemical parameters, such as free amino acids, polyphenol, starch, and sugars, significantly changed under combined stresses compared to individual stresses. Free amino acids increased under combined stresses that included heat; starch, sugars, and polyphenols increased under combined stresses that included drought; proline concentration increased under combined stresses that included salt. Metabolomics data that were obtained under different individual and combined stresses can be used to identify molecular phenotypes that are involved in the acclimation response of plants under changing abiotic stress conditions. Peanut metabolomics identified 160 metabolites, including amino acids, sugars, sugar alcohols, organic acids, fatty acids, sugar acids, and other organic compounds. Pathway enrichment analysis revealed that abiotic stresses significantly affected amino acid, amino sugar, and sugar metabolism. The stress treatments affected the metabolites that were associated with the tricarboxylic acid (TCA) and urea cycles and associated amino acid biosynthesis pathway intermediates. Principal component analysis (PCA), partial least squares-discriminant analysis (PLS-DA), and heatmap analysis identified potential marker metabolites (pinitol, malic acid, and xylopyranose) that were associated with abiotic stress combinations, which could be used in breeding efforts to develop peanut cultivars that are resilient to climate change. The study will also facilitate researchers to explore different stress indicators to identify resistant cultivars for future crop improvement programs.

Opportunities and limits of controlled-environment plant phenotyping for climate response traits

Rising temperatures and changing precipitation patterns will affect agricultural production substantially, exposing crops to extended and more intense periods of stress. Therefore, breeding of varieties adapted to the constantly changing conditions is pivotal to enable a quantitatively and qualitatively adequate crop production despite the negative effects of climate change. As it is not yet possible to select for adaptation to future climate scenarios in the field, simulations of future conditions in controlled-environment (CE) phenotyping facilities contribute to the understanding of the plant response to special stress conditions and help breeders to select ideal genotypes which cope with future conditions. CE phenotyping facilities enable the collection of traits that are not easy to measure under field conditions and the assessment of a plant's phenotype under repeatable, clearly defined environmental conditions using automated, non-invasive, high-throughput methods. However, extrapolation and translation of results obtained under controlled environments to field environments is ambiguous. This review outlines the opportunities and challenges of phenotyping approaches under controlled environments complementary to conventional field trials. It gives an overview on general principles and introduces existing phenotyping facilities that take up the challenge of obtaining reliable and robust phenotypic data on climate response traits to support breeding of climate-adapted crops.

Developing climate-resilient crops: improving plant tolerance to stress combination

Global warming and climate change are driving an alarming increase in the frequency and intensity of different abiotic stresses, such as droughts, heat waves, cold snaps, and flooding, negatively affecting crop yields and causing food shortages. Climate change is also altering the composition and behavior of different insect and pathogen populations adding to yield losses worldwide. Additional constraints to agriculture are caused by the increasing amounts of human-generated pollutants, as well as the negative impact of climate change on soil microbiomes. Although in the laboratory, we are trained to study the impact of individual stress conditions on plants, in the field many stresses, pollutants, and pests could simultaneously or sequentially affect plants, causing conditions of stress combination. Because climate change is expected to increase the frequency and intensity of such stress combination events (e.g., heat waves combined with drought, flooding, or other abiotic stresses, pollutants, and/or pathogens), a concentrated effort is needed to study how stress combination is affecting crops. This need is particularly critical, as many studies have shown that the response of plants to stress combination is unique and cannot be predicted from simply studying each of the different stresses that are part of the stress combination. Strategies to enhance crop tolerance to a particular stress may therefore fail to enhance tolerance to this specific stress, when combined with other factors. Here we review recent studies of stress combinations in different plants and propose new approaches and avenues for the development of stress combination- and climate change-resilient crops.

High-Temperature and Drought Stress Effects on Growth, Yield and Nutritional Quality with Transpiration Response to Vapor Pressure Deficit in Lentil

High temperature and water deficit are among the major limitations reducing lentil (Lens culinaris Medik.) yield in many growing regions. In addition, increasing atmospheric vapor pressure deficit (VPD) due to global warming causes a severe challenge by influencing the water balance of the plants, thus also affecting growth and yield. In the present study, we evaluated 20 lentil genotypes under field conditions and controlled environments with the following objectives: (i) to investigate the impact of temperature stress and combined temperature-drought stress on traits related to phenology, grain yield, nutritional quality, and canopy temperature under field conditions, and (ii) to examine the genotypic variability for limited transpiration (TR_lim) trait in response to increased VPD under controlled conditions. The field experiment results revealed that high-temperature stress significantly affected all parameters compared to normal conditions. The protein content ranged from 23.4 to 31.9%, while the range of grain zinc and iron content varied from 33.1 to 64.4 and 62.3 to 99.3 mg kg^-1, respectively, under normal conditions. The grain protein content, zinc and iron decreased significantly by 15, 14 and 15% under high-temperature stress, respectively. However, the impact was more severe under combined temperature-drought stress with a reduction of 53% in protein content, 18% in zinc and 20% in iron. Grain yield declined significantly by 43% in temperature stress and by 49% in the combined temperature-drought stress. The results from the controlled conditions showed a wide variation in TR among studied lentil genotypes. Nine genotypes displayed TR_lim at 2.76 to 3.51 kPa, with the genotypes ILL 7833 and ILL 7835 exhibiting the lowest breakpoint. Genotypes with low breakpoints had the ability to conserve water, allowing it to be used at later stages for increased yield. Our results identified promising genotypes including ILL 7835, ILL 7814 and ILL 4605 (Bakria) that could be of great interest in breeding for high yields, protein and micronutrient contents under high-temperature and drought stress. In addition, it was found that the TR_lim trait has the potential to select for increased lentil yields under field water-deficit environments.

The impact of stress combination on reproductive processes in crops

Historically, extended droughts combined with heat waves caused severe reductions in crop yields estimated at billions of dollars annually. Because global warming and climate change are driving an increase in the frequency and intensity of combined water-deficit and heat stress episodes, understanding how these episodes impact yield is critical for our efforts to develop climate change-resilient crops. Recent studies demonstrated that a combination of water-deficit and heat stress exacerbates the impacts of water-deficit or heat stress on reproductive processes of different cereals and legumes, directly impacting grain production. These studies identified several different mechanisms potentially underlying the effects of stress combination on anthers, pollen, and stigma development and function, as well as fertilization. Here we review some of these findings focusing on unbalanced reactive oxygen accumulation, altered sugar concentrations, and conflicting functions of different hormones, as contributing to the reduction in yield during a combination of water-deficit and heat stress. Future studies focused on the effects of water-deficit and heat stress combination on reproduction of different crops are likely to unravel additional mechanisms, as well as reveal novel ways to develop stress combination-resilient crops. These could mitigate some of the potentially devastating impacts of this stress combination on agriculture.

Discovery of miRNAs and Development of Heat-Responsive miRNA-SSR Markers for Characterization of Wheat Germplasm for Terminal Heat Tolerance Breeding

A large proportion of the Asian population fulfills their energy requirements from wheat (Triticum aestivum L.). Wheat quality and yield are critically affected by the terminal heat stress across the globe. It affects approximately 40% of the wheat-cultivating regions of the world. Therefore, there is a critical need to develop improved terminal heat-tolerant wheat varieties. Marker-assisted breeding with genic simple sequence repeats (SSR) markers have been used for developing terminal heat-tolerant wheat varieties; however, only few studies involved the use of microRNA (miRNA)-based SSR markers (miRNA-SSRs) in wheat, which were found as key players in various abiotic stresses. In the present study, we identified 104 heat-stress-responsive miRNAs reported in various crops. Out of these, 70 miRNA-SSR markers have been validated on a set of 20 terminal heat-tolerant and heat-susceptible wheat genotypes. Among these, only 19 miRNA-SSR markers were found to be polymorphic, which were further used to study the genetic diversity and population structure. The polymorphic miRNA-SSRs amplified 61 SSR loci with an average of 2.9 alleles per locus. The polymorphic information content (PIC) value of polymorphic miRNA-SSRs ranged from 0.10 to 0.87 with a mean value of 0.48. The dendrogram constructed using unweighted neighbor-joining method and population structure analysis clustered these 20 wheat genotypes into 3 clusters. The target genes of these miRNAs are involved either directly or indirectly in providing tolerance to heat stress. Furthermore, two polymorphic markers miR159c and miR165b were declared as very promising diagnostic markers, since these markers showed specific alleles and discriminated terminal heat-tolerant genotypes from the susceptible genotypes. Thus, these identified miRNA-SSR markers will prove useful in the characterization of wheat germplasm through the study of genetic diversity and population structural analysis and in wheat molecular breeding programs aimed at terminal heat tolerance of wheat varieties.

Heat Priming of Lentil (Lens culinaris Medik.) Seeds and Foliar Treatment with γ-Aminobutyric Acid (GABA), Confers Protection to Reproductive Function and Yield Traits under High-Temperature Stress Environments

Gradually increasing temperatures at global and local scales are causing heat stress for cool and summer-season food legumes, such as lentil (Lens culinaris Medik.), which is highly susceptible to heat stress, especially during its reproductive stages of development. Hence, suitable strategies are needed to develop heat tolerance in this legume. In the present study, we tested the effectiveness of heat priming (HPr; 6 h at 35 °C) the lentil seeds and a foliar treatment of γ-aminobutyric acid (GABA; 1 mM; applied twice at different times), singly or in combination (HPr+GABA), under heat stress (32/20 °C) in two heat-tolerant (HT; IG2507, IG3263) and two heat-sensitive (HS; IG2821, IG2849) genotypes to mitigate heat stress. The three treatments significantly reduced heat injury to leaves and flowers, particularly when applied in combination, including leaf damage assessed as membrane injury, cellular oxidizing ability, leaf water status, and stomatal conductance. The combined HPr+GABA treatment significantly improved the photosynthetic function, measured as photosynthetic efficiency, chlorophyll concentration, and sucrose synthesis; and significantly reduced the oxidative damage, which was associated with a marked up-regulation in the activities of enzymatic antioxidants. The combined treatment also facilitated the synthesis of osmolytes, such as proline and glycine betaine, by upregulating the expression of their biosynthesizing enzymes (pyrroline-5-carboxylate synthase; betaine aldehyde dehydrogenase) under heat stress. The HPr+GABA treatment caused a considerable enhancement in endogenous levels of GABA in leaves, more so in the two heat-sensitive genotypes. The reproductive function, measured as germination and viability of pollen grains, receptivity of stigma, and viability of ovules, was significantly improved with combined treatment, resulting in enhanced pod number (21-23% in HT and 35-38% in HS genotypes, compared to heat stress alone) and seed yield per plant (22-24% in HT and 37-40% in HS genotypes, in comparison to heat stress alone). The combined treatment (HPr+GABA) was more effective and pronounced in heat-sensitive than heat-tolerant genotypes for all the traits tested. This study offers a potential solution for tackling and protecting heat stress injury in lentil plants.

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.

Transcripts of wheat at a target locus on chromosome 6B associated with increased yield, leaf mass and chlorophyll index under combined drought and heat stress

Drought and heat stress constrain wheat (Triticum aestivum L.) yields globally. To identify putative mechanisms and candidate genes associated with combined drought and heat stress tolerance, we developed bread wheat near-isogenic lines (NILs) targeting a quantitative trait locus (QTL) on chromosome 6B which was previously associated with combined drought and heat stress tolerance in a diverse panel of wheats. Genotyping-by-sequencing was used to identify additional regions that segregated in allelic pairs between the recurrent and the introduced exotic parent, genome-wide. NILs were phenotyped in a gravimetric platform with precision irrigation and exposed to either drought or to combined drought and heat stress from three days after anthesis. An increase in grain weight in NILs carrying the exotic allele at 6B locus was associated with thicker, greener leaves, higher photosynthetic capacity and increased water use index after re-watering. RNA sequencing of developing grains at early and later stages of treatment revealed 75 genes that were differentially expressed between NILs across both treatments and timepoints. Differentially expressed genes coincided with the targeted QTL on chromosome 6B and regions of genetic segregation on chromosomes 1B and 7A. Pathway enrichment analysis showed the involvement of these genes in cell and gene regulation, metabolism of amino acids and transport of carbohydrates. The majority of these genes have not been characterized previously under drought or heat stress and they might serve as candidate genes for improved abiotic stress tolerance.

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.

The importance of accurate developmental staging

This article comments on: Fernández-Gómez J, Talle B, Tidy A, Wilson ZA. 2020. Accurate staging of reproduction development in Cadenza wheat by non-destructive spike analysis. Journal of Experimental Botany71, 3475–3484.

Effects of high temperature stress during anthesis and grain filling periods on photosynthesis, lipids and grain yield in wheat

BACKGROUND

Short episodes of high temperature (HT) stress during reproductive stages of development cause significant yield losses in wheat (Triticum aestivum L.). Two independent experiments were conducted to quantify the effects of HT during anthesis and grain filling periods on photosynthesis, leaf lipidome, and yield traits in wheat. In experiment I, wheat genotype Seri82 was exposed to optimum temperature (OT; 22/14 °C; day/night) or HT (32/22 °C) for 14 d during anthesis stage. In experiment II, the plants were exposed to OT or HT for 14 d during the grain filling stage. During the HT stress, chlorophyll index, thylakoid membrane damage, stomatal conductance, photosynthetic rate and leaf lipid composition were measured. At maturity, grain yield and its components were quantified.

RESULTS

HT stress during anthesis or grain filling stage decreased photosynthetic rate (17 and 25%, respectively) and grain yield plant^- 1 (29 and 44%, respectively), and increased thylakoid membrane damage (61 and 68%, respectively) compared to their respective control (OT). HT stress during anthesis or grain filling stage increased the molar percentage of less unsaturated lipid species [36:5- monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG)]. However, at grain filling stage, HT stress decreased the molar percentage of more unsaturated lipid species (36:6- MGDG and DGDG). There was a significant positive relationship between photosynthetic rate and grain yield plant^- 1, and a negative relationship between thylakoid membrane damage and photosynthetic rate.

CONCLUSIONS

The study suggests that maintaining thylakoid membrane stability, and seed-set per cent and individual grain weight under HT stress can improve the photosynthetic rate and grain yield, respectively.

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.

Roles of phytohormone changes in the grain yield of rice plants exposed to heat: a review

During its reproductive phase, rice is susceptible to heat stress. Heat events will occur at all stages during the reproductive phase of rice as a result of global warming. Moreover, rice yield traits respond differently to heat stress during panicle initiation, flowering and grain filling. The reduction in the number of spikelets per panicle of heat-stressed plants is due to the attenuated differentiation of secondary branches and their attached florets as well as the promotion of their degradation during the panicle-initiation stage but is not affected by heat stress thereafter. Spikelet sterility as a result of heat stress is attributed not only to physiological abnormalities in the reproductive organs during the flowering stage but also to structural and morphological abnormalities in reproductive organs during the panicle-initiation stage. The reduced grain weight of heat-stressed plants is due to a reduction in nonstructural carbohydrates, undeveloped vascular bundles, and a reduction in glume size during the panicle-initiation stage, while a shortened grain-filling duration, reduced grain-filling rate, and decreased grain width contribute to reduced grain weight during the grain-filling stage. Thus, screening and breeding rice varieties that have comprehensive tolerance to heat stress at all time points during their reproductive stage may be possible to withstand unpredictable heat events in the future. The responses of yield traits to heat stress are regulated by phytohormone levels, which are determined by phytohormone homeostasis. Currently, the biosynthesis and transport of phytohormones are the key processes that determine phytohormone levels in and grain yield of rice under heat stress. Studies on phytohormone homeostatic responses are needed to further reveal the key processes that determine phytohormone levels under heat conditions.

Unique features of the grapevine VvK5.1 channel support novel functions for outward K+ channels in plants

Grapevine (Vitis vinifera L.), one of the most important fruit crops, is a model plant for studying the physiology of fleshy fruits. Here, we report on the characterization of a new grapevine Shaker-type K+ channel, VvK5.1. Phylogenetic analysis revealed that VvK5.1 belongs to the SKOR-like subfamily. Our functional characterization of VvK5.1 in Xenopus oocytes confirms that it is an outwardly rectifying K+ channel that displays strict K+ selectivity. Gene expression level analyses by real-time quantitative PCR showed that VvK5.1 expression was detected in berries, roots, and flowers. In contrast to its Arabidopsis thaliana counterpart that is involved in K+ secretion in the root pericycle, allowing root to shoot K+ translocation, VvK5.1 expression territory is greatly enlarged. Using in situ hybridization we showed that VvK5.1 is expressed in the phloem and perivascular cells of berries and in flower pistil. In the root, in addition to being expressed in the root pericycle like AtSKOR, a strong expression of VvK5.1 is detected in small cells facing the xylem that are involved in lateral root formation. This fine and selective expression pattern of VvK5.1 at the early stage of lateral root primordia supports a role for outward channels to switch on cell division initiation.

Enhanced antioxidant enzyme activities in developing anther contributes to heat stress alleviation and sustains grain yield in wheat

Climatic variations along with a rise in temperature during the winter season impose severe heat stress during the anthesis stage of spring wheat, resulting in severe yield losses. The present study was conducted to evaluate the influence of heat stress on redox homeostasis in developing anthers and flag leaves of wheat. Five Indian bread wheat genotypes were studied under field conditions during the dry season, with two extreme sowing dates (timely and very late sown) to explore the effect of heat stress on anthesis stage. Results showed that elevated temperature during anthesis caused significant increase in reactive oxygen species (ROS) content and malondialdehyde (MDA) accumulation in developing anthers, triggering pollen mortality. Moreover, defective source (leaf) to the sink (anthers) mobilisation of starch also contributes in reducing pollen viability. However, ROS-induced oxidative damage of developing anthers under heat stress varied among the wheat genotypes depending upon differential antioxidant enzyme activities. Wheat genotype with enhanced antioxidant activities and reduced ROS built up in developing anthers sustained their grain yield, suggesting thermo-tolerance in wheat to be associated with antioxidant enzyme-mediated improved ROS-scavenging mechanism not only in leaves even in developing anther also. In the present study, heat stressed wheat genotype WH 730 exhibited effective source to sink mobilisation and sustainable grain yield with improved ROS scavenging, conferring greater potential for heat tolerance. We conclude that redox homeostasis and balanced source sink activity played a significant role for sustainable yield and heat tolerance in wheat.

Pollen development at high temperature and role of carbon and nitrogen metabolites

Fruit and seed crop production heavily relies on successful stigma pollination, pollen tube growth, and fertilization of female gametes. These processes depend on production of viable pollen grains, a process sensitive to high-temperature stress. Therefore, rising global temperatures threaten worldwide crop production. Close observation of plant development shows that high-temperature stress causes morpho-anatomical changes in male reproductive tissues that contribute to reproductive failure. These changes include early tapetum degradation, anther indehiscence, and deformity of pollen grains, all of which are contributing factors to pollen fertility. At the molecular level, reactive oxygen species (ROS) accumulate when plants are subjected to high temperatures. ROS is a signalling molecule that can be beneficial or detrimental for plant cells depending on its balance with the endogenous cellular antioxidant system. Many metabolites have been linked with ROS over the years acting as direct scavengers or molecular stabilizers that promote antioxidant enzyme activity. This review highlights recent advances in research on anther and pollen development and how these might explain the aberrations seen during high-temperature stress; recent work on the role of nitrogen and carbon metabolites in anther and pollen development is discussed including their potential role at high temperature.

Novel Alleles for Combined Drought and Heat Stress Tolerance in Wheat

Drought and heat waves commonly co-occur in many wheat-growing regions causing significant crop losses. The identification of stress associated quantitative trait loci, particularly those for yield, is problematic due to their association with plant phenology and the high genetic × environment interaction. Here we studied a panel of 315 diverse, spring type accessions of bread wheat (Triticum aestivum) in pots in a semi-controlled environment under combined drought and heat stress over 2 years. Importantly, we treated individual plants according to their flowering time. We found 134 out of the 145 identified loci for grain weight that were not associated with either plant phenology or plant height. The majority of loci uncovered here were novel, with favorable alleles widespread in Asian and African landraces providing opportunities for their incorporation into modern varieties through breeding. Using residual heterozygosity in lines from a nested association mapping population, we were able to rapidly develop near-isogenic lines for important target loci. One target locus on chromosome 6A contributed to higher grain weight, harvest index, thousand kernel weight, and grain number under drought and heat stress in field conditions consistent with allelic effects demonstrated in the genome-wide association study.