Ensuring Reproduction at High Temperatures: The Heat Stress Response during Anther and Pollen Development

Enhanced pollen tube performance at high temperature contributes to thermotolerant fruit production in tomato

Rising temperature extremes during critical reproductive periods threaten the yield of major grain and fruit crops. Flowering plant reproduction depends on development of sufficient numbers of pollen grains and on their ability to generate a cellular extension, the pollen tube, which elongates through the pistil to deliver sperm cells to female gametes for double fertilization. These critical phases of the life cycle are sensitive to temperature and limit productivity under high temperature (HT). Previous studies have investigated the effects of HT on pollen development, but little is known about how HT applied during the pollen tube growth phase affects fertility. Here, we used tomato as a model fruit crop to determine how HT affects the pollen tube growth phase, taking advantage of cultivars noted for fruit production in exceptionally hot growing seasons. We found that exposure to HT solely during the pollen tube growth phase limits fruit biomass and seed set more significantly in thermosensitive cultivars than in thermotolerant cultivars. Importantly, we found that pollen tubes from the thermotolerant Tamaulipas cultivar have enhanced growth in vivo and in vitro under HT. Analysis of the pollen tube transcriptome's response to HT allowed us to develop hypotheses for the molecular basis of cellular thermotolerance in the pollen tube and we define two response modes (enhanced induction of stress responses, and higher basal levels of growth pathways repressed by heat stress) associated with reproductive thermotolerance. Importantly, we define key components of the pollen tube stress response identifying enhanced ROS homeostasis and pollen tube callose synthesis and deposition as important components of reproductive thermotolerance in Tamaulipas. Our work identifies the pollen tube growth phase as a viable target to enhance reproductive thermotolerance and delineates key pathways that are altered in crop varieties capable of fruiting under HT conditions.

Heat-stress-induced ROS in maize silks cause late pollen tube growth arrest and sterility

The reproductive phase of plants is highly sensitive to ambient temperature stresses. To investigate sensitivity of female reproductive organs in grass crops during the pollination phase, we exposed the elongated stigma (silk) of maize to ambient environment at the silking stage. Moderate heat stress causes cell death of silk hair cells but did not affect early pollen tube growth inside the silk. Late pollen tube growth arrest was observed, leading to sterility. Heat stress causes elevated levels of reactive oxygen species (ROS) in silks, whose levels can be reduced by scavengers partly restoring pollen tube growth and fertility. A number of biological processes including hydrogen peroxide catabolic processes and bHLH transcription factor genes are downregulated by heat stress, while some NAC transcription factor genes are strongly upregulated. In conclusion, this study now provides a basis to select genes for engineering heat-stress-tolerant grass crops during the pollination phase.

Non-chemical weed management: Harnessing flame weeding for effective weed control

The goal of the current study was to create and assess the effectiveness of a hand-pulled ergonomically designed flame weeder. The developed weeder was tested in the field at three operating pressures (20, 30 and 40 Psi) and forward speeds (1.00, 1.25 and 1.50 km/h) to study their effects on plant damage, survival rates, weight preservation rates, weed management effectiveness, soil temperatures, and gas and energy consumption. Thereafter, at optimized values of forward speed and operating pressure, a comparative assessment of flame weeding with traditional methods (mechanical and manual weeding) was done in terms of weed control effectiveness, operational time, energy consumption, and cost of operation. Results showed that the optimal performance of the designed flame weeder was achieved when operated at a speed of 1 km/h and an operating pressure of 40 psi. The survival rate, weight preservation rate, weed control efficiency, change in soil temperature, recovery rate, plant damage, gas consumption, and energy consumption were observed to be 27.3 %, 32.5 %, 91.1 %, 40.74 °C, 8.5 %, 2.2 %, 4.05 kg/h, and 2500.24 MJ/ha, respectively, at optimized values of forward speed (1.00 km/h) and operating pressure (40 Psi). The actual field capacity, field efficiency and operating cost of the flame weeder were 0.0755 ha/h, 94.94 %, and 3620.81 ₹/ha, respectively. Hand weeding had the best level of weed control effectiveness, but it was a laborious, time-consuming process. When compared to manual weeding, flame weeding was 50.42 % cheaper and 94.82 % faster.

Deleterious Effects of Heat Stress on the Tomato, Its Innate Responses, and Potential Preventive Strategies in the Realm of Emerging Technologies

The tomato is a fruit vegetable rich in nutritional and medicinal value grown in greenhouses and fields worldwide. It is severely sensitive to heat stress, which frequently occurs with rising global warming. Predictions indicate a 0.2 °C increase in average surface temperatures per decade for the next three decades, which underlines the threat of austere heat stress in the future. Previous studies have reported that heat stress adversely affects tomato growth, limits nutrient availability, hammers photosynthesis, disrupts reproduction, denatures proteins, upsets signaling pathways, and damages cell membranes. The overproduction of reactive oxygen species in response to heat stress is toxic to tomato plants. The negative consequences of heat stress on the tomato have been the focus of much investigation, resulting in the emergence of several therapeutic interventions. However, a considerable distance remains to be covered to develop tomato varieties that are tolerant to current heat stress and durable in the perspective of increasing global warming. This current review provides a critical analysis of the heat stress consequences on the tomato in the context of global warming, its innate response to heat stress, and the elucidation of domains characterized by a scarcity of knowledge, along with potential avenues for enhancing sustainable tolerance against heat stress through the involvement of diverse advanced technologies. The particular mechanism underlying thermotolerance remains indeterminate and requires further elucidatory investigation. The precise roles and interplay of signaling pathways in response to heat stress remain unresolved. The etiology of tomato plants' physiological and molecular responses against heat stress remains unexplained. Utilizing modern functional genomics techniques, including transcriptomics, proteomics, and metabolomics, can assist in identifying potential candidate proteins, metabolites, genes, gene networks, and signaling pathways contributing to tomato stress tolerance. Improving tomato tolerance against heat stress urges a comprehensive and combined strategy including modern techniques, the latest apparatuses, speedy breeding, physiology, and molecular markers to regulate their physiological, molecular, and biochemical reactions.

Small but mighty: Peptides regulating abiotic stress responses in plants

Throughout evolution, plants have developed strategies to confront and alleviate the detrimental impacts of abiotic stresses on their growth and development. The combat strategies involve intricate molecular networks and a spectrum of early and late stress-responsive pathways. Plant peptides, consisting of fewer than 100 amino acid residues, are at the forefront of these responses, serving as pivotal signalling molecules. These peptides, with roles similar to phytohormones, intricately regulate plant growth, development and facilitate essential cell-to-cell communications. Numerous studies underscore the significant role of these small peptides in coordinating diverse signalling events triggered by environmental challenges. Originating from the proteolytic processing of larger protein precursors or directly translated from small open reading frames, including microRNA (miRNA) encoded peptides from primary miRNA, these peptides exert their biological functions through binding with membrane-embedded receptor-like kinases. This interaction initiates downstream cellular signalling cascades, often involving major phytohormones or reactive oxygen species-mediated mechanisms. Despite these advances, the precise modes of action for numerous other small peptides remain to be fully elucidated. In this review, we delve into the dynamics of stress physiology, mainly focusing on the roles of major small signalling peptides, shedding light on their significance in the face of changing environmental conditions.

Stomatal effects and ABA metabolism mediate differential regulation of leaf and flower cooling in tomato cultivars exposed to heat and drought stress

Co-occurring heat and drought stresses challenge crop performance. Stomata open to promote evaporative cooling during heat stress, but close to retain water during drought stress, which resulted in complex stomatal regulation under combined heat and drought. We aimed to investigate stomatal regulation in leaves and flowers of perennial, indeterminate cultivars of tomatoes subjected to individual and combined heat and drought stress followed by a recovery period, measuring morphological, physiological, and biochemical factors involved in stomatal regulation. Under stress, stomata of leaves were predominantly affected by drought, with lower stomatal density and stomatal closing, resulting in significantly decreased photosynthesis and higher leaf temperature. Conversely, stomata in sepals seemed affected mainly by heat during stress. The differential patterns in stomatal regulation in leaves and flowers persisted into the recovery phase as contrasting patterns in stomatal density. We show that flower transpiration is regulated by temperature, but leaf transpiration is regulated by soil water availability during stress. Organ-specific patterns of stomatal development and abscisic acid metabolism mediated this phenomenon. Our results throw light on the dual role of stomata in heat and drought tolerance of vegetative and generative organs, and demonstrate the importance of considering flower surfaces in the phenotyping of stomatal reactions to stress.

An E3 ubiquitin ligase CSIT2 controls critical sterility-inducing temperature of thermo-sensitive genic male sterile rice

Thermo-sensitive genic male sterile (TGMS) lines are the core of two-line hybrid rice (Oryza sativa). However, elevated or unstable critical sterility-inducing temperatures (CSITs) of TGMS lines are bottlenecks that restrict the development of two-line hybrid rice. However, the genes and molecular mechanisms controlling CSIT remain unknown. Here, we report the CRITICAL STERILITY-INDUCING TEMPERATURE 2 (CSIT2) that encodes a really interesting new gene (RING) type E3 ligase, controlling the CSIT of thermo-sensitive male sterility 5 (tms5)-based TGMS lines through ribosome-associated protein quality control (RQC). CSIT2 binds to the large and small ribosomal subunits and ubiquitinates 80S ribosomes for dissociation, and may also ubiquitinate misfolded proteins for degradation. Mutation of CSIT2 inhibits the possible damage to ubiquitin system and protein translation, which allows more proteins such as catalases to accumulate for anther development and inhibits abnormal accumulation of reactive oxygen species (ROS) and premature programmed cell death (PCD) in anthers, partly rescuing male sterility and raised the CSIT of tms5-based TGMS lines. These findings reveal a mechanism controlling CSIT and provide a strategy for solving the elevated or unstable CSITs of tms5-based TGMS lines in two-line hybrid rice.

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.

Combined effects of shade and drought on physiology, growth, and yield of mature cocoa trees

Climate models predict decreasing precipitation and increasing air temperature, causing concern for the future of cocoa in the major producing regions worldwide. It has been suggested that shade could alleviate stress by reducing radiation intensity and conserving soil moisture, but few on-farm cocoa studies are testing this hypothesis. Here, for 33 months, we subjected twelve-year cocoa plants in Ghana to three levels of rainwater suppression (full rainwater, 1/3 rainwater suppression and 2/3 rainwater suppression) under full sun or 40 % uniform shade in a split plot design, monitoring soil moisture, physiological parameters, growth, and yield. Volumetric soil moisture (ϴw) contents in the treatments ranged between 0.20 and 0.45 m3m-3 and increased under shade. Rainwater suppression decreased leaf water potentials (ѱw), reaching -1.5 MPa in full sun conditions indicating severe drought. Stomatal conductance (gs) was decreased under the full sun but was not affected by rainwater suppression, illustrating the limited control of water loss in cocoa plants. Although pre-dawn chlorophyll fluorescence (Fv/Fm) indicated photoinhibition, rates of photosynthesis (Pn) were highest in full sun. On the other hand, litter fall was highest in the full sun and under water stress, while diameter growth and carbon accumulation increased in the shade but was negatively affected by rainwater suppression. Abortion of fruits and damage to pods were high under shade, but dry bean yield was higher compared to under the full sun. The absence of interactions between shade treatments and rainwater suppression suggests that shade may improve the performance of cocoa, but not sufficiently to counteract the negative effects of water stress under field conditions.

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.

Various tomato cultivars display contrasting morphological and molecular responses to a chronic heat stress

Climate change is one of the biggest threats that human society currently needs to face. Heat waves associated with global warming negatively affect plant growth and development and will increase in intensity and frequency in the coming years. Tomato is one of the most produced and consumed fruit in the world but remarkable yield losses occur every year due to the sensitivity of many cultivars to heat stress (HS). New insights into how tomato plants are responding to HS will contribute to the development of cultivars with high yields under harsh temperature conditions. In this study, the analysis of microsporogenesis and pollen germination rate of eleven tomato cultivars after exposure to a chronic HS revealed differences between genotypes. Pollen development was either delayed and/or desynchronized by HS depending on the cultivar considered. In addition, except for two, pollen germination was abolished by HS in all cultivars. The transcriptome of floral buds at two developmental stages (tetrad and pollen floral buds) of five cultivars revealed common and specific molecular responses implemented by tomato cultivars to cope with chronic HS. These data provide valuable insights into the diversity of the genetic response of floral buds from different cultivars to HS and may contribute to the development of future climate resilient tomato varieties.

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.

Insights into morphological and physio-biochemical adaptive responses in mungbean (Vigna radiata L.) under heat stress

Mungbean (Vigna radiata L. Wilczek) is an important food legume crop which contributes significantly to nutritional and food security of South and Southeast Asia. The crop thrives in hot and humid weather conditions, with an optimal temperature range of 28°-35°C, and is mainly cultivated under rainfed environments. However, the rising global temperature has posed a serious threat to mungbean cultivation. Optimal temperature is a vital factor in cellular processes, and every crop species has evolved with its specific temperature tolerance ability. Moreover, variation within a crop species is inevitable, given the diverse environmental conditions under which it has evolved. For instance, various mungbean germplasm can grow and produce seeds in extreme ambient temperatures as low as 20°C or as high as 45°C. This range of variation in mungbean germplasm for heat tolerance plays a crucial role in developing heat tolerant and high yielding mungbean cultivars. However, heat tolerance is a complex mechanism which is extensively discussed in this manuscript; and at the same time individual genotypes have evolved with various ways of heat stress tolerance. Therefore, to enhance understanding towards such variability in mungbean germplasm, we studied morphological, anatomical, physiological, and biochemical traits which are responsive to heat stress in plants with more relevance to mungbean. Understanding heat stress tolerance attributing traits will help in identification of corresponding regulatory networks and associated genes, which will further help in devising suitable strategies to enhance heat tolerance in mungbean. The major pathways responsible for heat stress tolerance in plants are also discussed.

The hydroxyproline O-arabinosyltransferase FIN4 is required for tomato pollen intine development

The pollen grain cell wall is a highly specialized structure composed of distinct layers formed through complex developmental pathways. The production of the innermost intine layer, composed of cellulose, pectin and other polymers, is particularly poorly understood. Here we demonstrate an important and specific role for the hydroxyproline O-arabinosyltransferase (HPAT) FIN4 in tomato intine development. HPATs are plant-specific enzymes which initiate glycosylation of certain cell wall structural proteins and signaling peptides. FIN4 was expressed throughout pollen development in both the developing pollen and surrounding tapetal cells. A fin4 mutant with a partial deletion of the catalytic domain displayed significantly reduced male fertility in vivo and compromised pollen hydration and germination in vitro. However, fin4 pollen that successfully germinated formed morphologically normal pollen tubes with the same growth rate as the wild-type pollen. When we examined mature fin4 pollen, we found they were cytologically normal, and formed morphologically normal exine, but produced significantly thinner intine. During intine deposition at the late stages of pollen development we found fin4 pollen had altered polymer deposition, including reduced cellulose and increased detection of pectin, specifically homogalacturonan with both low and high degrees of methylesterification. Therefore, FIN4 plays an important role in intine formation and, in turn pollen hydration and germination and the process of intine formation involves dynamic changes in the developing pollen cell wall.

Evaluation of methods to assess the quality of cryopreserved Solanaceae pollen

Solanaceae pollen cryopreservation is a common practice in the hybrid seed production industry worldwide, enabling effective hybridization across geographical and seasonal limitations. As pollination with low quality pollen can result in significant seed yield loss, monitoring the pollen quality has become an important risk management tool. In this study, pollen quality analysis methods were evaluated for their suitability for routine quality control of cryopreserved pollen batches. The assessments, including pollen viability, pollen germinability and pollen vigor analysis, were conducted in two locations on a diverse set of cryopreserved tomato and pepper pollen batches. While the viability obtained by Impedance Flow Cytometry (IFC) can be interpreted as the pollen's potential to germinate, the in vitro germination assay directly quantifies this functionality under given assay conditions. A linear correlation was found between pollen viability obtained by IFC and in vitro germinability. In conclusion, IFC is the most suitable tool for applications and industries requiring a high degree of automation, throughput, repeatability, and reproducibility. In vitro germination assays are suitable for studies within certain temporal and geographic limitations, due to difficulties in standardization. On the other hand, vigor assessments are not sufficiently addressing the needs of the industry due to poor reproducibility and low throughput.

Growth and Molecular Responses of Tomato to Prolonged and Short-Term Heat Exposure

Tomatoes are one of the most important vegetables for human consumption. In the Mediterranean's semi-arid and arid regions, where tomatoes are grown in the field, global average surface temperatures are predicted to increase. We investigated tomato seed germination at elevated temperatures and the impact of two different heat regimes on seedlings and adult plants. Selected exposures to 37 °C and heat waves at 45 °C mirrored frequent summer conditions in areas with a continental climate. Exposure to 37 °C or 45 °C differently affected seedlings' root development. Both heat stresses inhibited primary root length, while lateral root number was significantly suppressed only after exposure to 37 °C. Heat stress treatments induced significant accumulation of indole-3-acetic acid (IAA) and reduced abscisic acid (ABA) levels in seedlings. As opposed to the heat wave treatment, exposure to 37 °C increased the accumulation of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), which may have been involved in the root architecture modification of seedlings. Generally, more drastic phenotypic changes (chlorosis and wilting of leaves and bending of stems) were found in both seedlings and adult plants after the heat wave-like treatment. This was also reflected by proline, malondialdehyde and heat shock protein HSP90 accumulation. The gene expression of heat stress-related transcription factors was perturbed and DREB1 was shown to be the most consistent heat stress marker.

Hydrogen Peroxide and GA3 Levels Regulate the High Night Temperature Response in Pistils of Wheat (Triticum aestivum L.)

High night temperature (HNT) impairs crop productivity through the reproductive failure of gametes (pollen and pistil). Though female gametophyte (pistil) is an equal partner in the seed-set, the knowledge of the antioxidant system(s) and hormonal control of HNT tolerance or susceptibility of pistils is limited and lacking. The objectives of this study were to determine the antioxidant mechanism for homeostatic control of free radicals, and the involvement of abscisic acid (ABA) and gibberellic acid (GA₃) in HNT stress protection in the wheat pistils of contrasting wheat genotypes. We hypothesized that HNT tolerance is attributed to the homeostatic control of reactive oxygen species (ROS) and hormonal readjustment in pistils of the tolerant genotype. The ears of two contrasting wheat genotypes-HD 2329 (susceptible) and Raj 3765 (tolerant) were subjected to two HNTs (+5 °C and +8 °C) over ambient, in the absence and presence of dimethylthiourea (DMTU), a chemical trap of hydrogen peroxide (H₂O₂). Results showed that HNTs significantly increased ROS in pistils of susceptible genotype HD 2329 to a relatively greater extent compared to tolerant genotype Raj 3765. The response was similar in the presence or absence of DMTU, but the H₂O₂ values were lower in the presence of DMTU. The ROS levels were balanced by increased activity of peroxidase under HNT to a greater extent in the tolerant genotype. Cytosolic glyceraldehyde-3-phosphate dehydrogenase (GAPC) activity was inversely related to H₂O₂ production within a critical range in Raj 3765, indicating its modulation by H₂O₂ levels as no change was observed at the transcriptional level. The hormonal status showed increased ABA and decreased GA₃ contents with increasing temperature. Our study elucidates the role of H₂O₂ and GA₃ in stress tolerance of pistils of tolerant genotype where GAPC acts as a ROS sensor due to H₂O₂-mediated decrease in its activity.

Divergent responses of cropland soil organic carbon to warming across the Sichuan Basin of China

Cropland soils are considered to have the potential to sequester carbon (C). Warming can increase soil organic C (SOC) by enhancing primary production, but it can also cause carbon release from soils. However, the role of warming in governing cropland SOC dynamics over broad geographic scales remains poorly understood. Using over 4000 soil samples collected in the 1980s and 2010s across the Sichuan Basin of China, this study assessed the warming-induced cropland SOC change and the correlations with precipitation, cropland type and soil type. Results showed mean SOC content increased from 11.10 to 13.85 g C kg^-1. Larger SOC increments were observed under drier conditions (precipitation < 1050 mm, dryland and paddy-dryland rotation cropland), which were 1.67-2.23 times higher than under wetter conditions (precipitation > 1050 mm and paddy fields). Despite the significant associations of SOC increment with crop productivity, precipitation, fertilization, cropland type and soil type, warming also acted as one of major contributors to cropland SOC change. The SOC increment changed parabolically with the rise in temperature increase rate under relatively drier conditions, while temperature increase had no impact on cropland SOC increment under wetter conditions. Meanwhile, the patterns of the parabolical relationship varied with soil types in drylands, where the threshold of temperature increase rate, the point at which the SOC increment switched from increasing to decreasing with warming, was lower for clayey soils (Ali-Perudic Argosols) than for sandy soils (Purpli-Udic Cambosols). These results illustrate divergent responses of cropland SOC to warming under different environments, which were contingent on water conditions and soil types. Our findings emphasize the importance of formulating appropriate field water management for sustainable C sequestration and the necessity of incorporating environment-specific mechanisms in Earth system models for better understanding of the soil C-climate feedback in complex environments.

Overcoming Reproductive Compromise Under Heat Stress in Wheat: Physiological and Genetic Regulation, and Breeding Strategy

The reproductive compromise under heat stress is a major obstacle to achieve high grain yield and quality in wheat worldwide. Securing reproductive success is the key solution to sustain wheat productivity by understanding the physiological mechanism and molecular basis in conferring heat tolerance and utilizing the candidate gene resources for breeding. In this study, we examined the performance on both carbon supply source (as leaf photosynthetic rate) and carbon sink intake (as grain yields and quality) in wheat under heat stress varying with timing, duration, and intensity, and we further surveyed physiological processes from source to sink and the associated genetic basis in regulating reproductive thermotolerance; in addition, we summarized the quantitative trait loci (QTLs) and genes identified for heat stress tolerance associated with reproductive stages. Discovery of novel genes for thermotolerance is made more efficient via the combination of transcriptomics, proteomics, metabolomics, and phenomics. Gene editing of specific genes for novel varieties governing heat tolerance is also discussed.

Morphological analysis and stage determination of anther development in Sorghum [Sorghum bicolor (L.) Moench]

The characteristics of sorghum anthers at 18 classified developmental stages provide an important reference for future studies on sorghum reproductive biology and abiotic stress tolerance of sorghum pollen. Sorghum (Sorghum bicolor L. Moench) is the fifth-most important cereal crop in the world. It has relatively high resilience to drought and high temperature stresses during vegetative growing stages comparing to other major cereal crops. However, like other cereal crops, the sensitivity of male organ to heat and drought can severely depress sorghum yield due to reduced fertility and pollination efficiency if the stress occurs at the reproductive stage. Identification of the most vulnerable stages and the genes and genetic networks that differentially regulate the abiotic stress responses during anther development are two critical prerequisites for targeted molecular trait selection and for enhanced environmentally resilient sorghum in breeding using a variety of genetic modification strategies. However, in sorghum, anther developmental stages have not been determined. The distinctive cellular characteristics associated with anther development have not been well examined. Lack of such critical information is a major obstacle in the studies of anther and pollen development in sorghum. In this study, we examined the morphological changes of sorghum anthers at cellular level during entire male organ development processes using a modified high-throughput imaging variable pressure scanning electron microscopy and traditional light microscopy methods. We divided sorghum anther development into 18 distinctive stages and provided detailed description of the morphological changes in sorghum anthers for each stage. The findings of this study will serve as an important reference for future studies focusing on sorghum physiology, reproductive biology, genetics, and genomics.

High temperatures during microsporogenesis fatally shorten pollen lifespan

Many crop species are cultivated to produce seeds and/or fruits and therefore need reproductive success to occur. Previous studies proved that high temperature on mature pollen at anther dehiscence reduce viability and germinability therefore decreasing crop productivity. We hypothesized that high temperature might affect pollen functionality even if the heat treatment is exerted only during the microsporogenesis. Experimental data on Solanum lycopersicum 'Micro-Tom' confirmed our hypothesis. Microsporogenesis successfully occurred at both high (30 °C) and optimal (22 °C) temperature. After the anthesis, viability and germinability of the pollen developed at optimal temperature gradually decreased and the reduction was slightly higher when pollen was incubated at 30 °C. Conversely, temperature effect was eagerly enhanced in pollen developed at high temperature. In this case, a drastic reduction of viability and a drop-off to zero of germinability occurred not only when pollen was incubated at 30 °C but also at 22 °C. Further ontogenetic analyses disclosed that high temperature significantly speeded-up the microsporogenesis and the early microgametogenesis (from vacuolated stage to bi-cellular pollen); therefore, gametophytes result already senescent at flower anthesis. Our work contributes to unravel the effects of heat stress on pollen revealing that high temperature conditions during microsporogenesis prime a fatal shortening of the male gametophyte lifespan.

Sugar metabolism during pre- and post-fertilization events in plants under high temperature stress

High temperature challenges global crop production by limiting the growth and development of the reproductive structures and seed. It impairs the developmental stages of male and female gametogenesis, pollination, fertilization, endosperm formation and embryo development. Among these, the male reproductive processes are highly prone to abnormalities under high temperature at various stages of development. The disruption of source-sink balance is the main constraint for satisfactory growth of the reproductive structures which is disturbed at the level of sucrose import and utilization within the tissue. Seed development after fertilization is affected by modulation in the activity of enzymes involved in starch metabolism. In addition, the alteration in the seed-filling rate and its duration affects the seed weight and quality. The present review critically discusses the role of sugar metabolism in influencing the various stages of gamete and seed development under high temperature stress. It also highlights the interaction of the sugars with hormones that mediate the transport of sugars to sink tissues. The role of transcription factors for the regulation of sugar availability under high temperature has also been discussed. Further, the omics-based systematic investigation has been suggested to understand the synergistic or antagonistic interactions between sugars, hormones and reactive oxygen species at various points of sucrose flow from source to sink under high temperature stress.

The Halophyte Species Solanum chilense Dun. Maintains Its Reproduction despite Sodium Accumulation in Its Floral Organs

Salinity is a growing global concern that affects the yield of crop species, including tomato (Solanum lycopersicum). Its wild relative Solanum chilense was reported to have halophyte properties. We compared salt resistance of both species during the reproductive phase, with a special focus on sodium localization in the flowers. Plants were exposed to NaCl from the seedling stage. Salinity decreased the number of inflorescences in both species but the number of flowers per inflorescence and sepal length only in S. lycopersicum. External salt supply decreased the stamen length in S. chilense, and it was associated with a decrease in pollen production and an increase in pollen viability. Although the fruit set was not affected by salinity, fruit weight and size decreased in S. lycopersicum. Concentrations and localization of Na, K, Mg, and Ca differed in reproductive structures of both species. Inflorescences and fruits of S. chilense accumulated more Na than S. lycopersicum. Sodium was mainly located in male floral organs of S. chilense but in non-reproductive floral organs in S. lycopersicum. The expression of Na transporter genes differed in flowers of both species. Overall, our results indicated that S. chilense was more salt-resistant than S. lycopersicum during the reproductive phase and that differences could be partly related to dissimilarities in element distribution and transport in flowers.

Epigenetic Regulation of Heat Stress in Plant Male Reproduction

In flowering plants, male reproductive development is highly susceptible to heat stress. In this mini-review, we summarized different anomalies in tapetum, microspores, and pollen grains during anther development under heat stress. We then discussed how epigenetic control, particularly DNA methylation, is employed to cope with heat stress in male reproduction. Further understanding of epigenetic mechanisms by which plants manage heat stress during male reproduction will provide new genetic engineering and molecular breeding tools for generating heat-resistant crops.

Genetic and Physiological Responses to Heat Stress in Brassica napus

Given the current rise in global temperatures, heat stress has become a major abiotic challenge affecting the growth and development of various crops and reducing their productivity. Brassica napus, the second largest source of vegetable oil worldwide, experiences a drastic reduction in seed yield and quality in response to heat. This review outlines the latest research that explores the genetic and physiological impact of heat stress on different developmental stages of B. napus with a special attention to the reproductive stages of floral progression, organogenesis, and post flowering. Several studies have shown that extreme temperature fluctuations during these crucial periods have detrimental effects on the plant and often leading to impaired growth and reduced seed production. The underlying mechanisms of heat stress adaptations and associated key regulatory genes are discussed. Furthermore, an overview and the implications of the polyploidy nature of B. napus and the regulatory role of alternative splicing in forming a priming-induced heat-stress memory are presented. New insights into the dynamics of epigenetic modifications during heat stress are discussed. Interestingly, while such studies are scarce in B. napus, opposite trends in expression of key genetic and epigenetic components have been identified in different species and in cultivars within the same species under various abiotic stresses, suggesting a complex role of these genes and their regulation in heat stress tolerance mechanisms. Additionally, omics-based studies are discussed with emphasis on the transcriptome, proteome and metabolome of B. napus, to gain a systems level understanding of how heat stress alters its yield and quality traits. The combination of omics approaches has revealed crucial interactions and regulatory networks taking part in the complex machinery of heat stress tolerance. We identify key knowledge gaps regarding the impact of heat stress on B. napus during its yield determining reproductive stages, where in-depth analysis of this subject is still needed. A deeper knowledge of heat stress response components and mechanisms in tissue specific models would serve as a stepping-stone to gaining insights into the regulation of thermotolerance that takes place in this important crop species and support future breeding of heat tolerant crops.

Genetic and Molecular Mechanisms Conferring Heat Stress Tolerance in Tomato Plants

Climate change is a major threat to global food security. Changes in climate can directly impact food systems by reducing the production and genetic diversity of crops and their wild relatives, thereby restricting future options for breeding improved varieties and reducing the ability to adapt crops to future challenges. The global surface temperature is predicted to rise by an average of 0.3°C during the next decade, and the Paris Agreement (Paris Climate Accords) aims to limit global warming to below an average of 2°C, preferably to 1.5°C compared to pre-industrial levels. Even if the goal of the Paris Agreement can be met, the predicted rise in temperatures will increase the likelihood of extreme weather events, including heatwaves, making heat stress (HS) a major global abiotic stress factor for many crops. HS can have adverse effects on plant morphology, physiology, and biochemistry during all stages of vegetative and reproductive development. In fruiting vegetables, even moderate HS reduces fruit set and yields, and high temperatures may result in poor fruit quality. In this review, we emphasize the effects of abiotic stress, especially at high temperatures, on crop plants, such as tomatoes, touching upon key processes determining plant growth and yield. Specifically, we investigated the molecular mechanisms involved in HS tolerance and the challenges of developing heat-tolerant tomato varieties. Finally, we discuss a strategy for effectively improving the heat tolerance of vegetable crops.

Chickpea tolerance to temperature stress: Status and opportunity for improvement

Chickpea is a globally important commercial crop and a key source of protein for vegetarian populations. Though chickpea was domesticated at least 3000 years ago, research into abiotic stress tolerance has been limited compared to cereal crops such as wheat. This review investigates the impacts of heat stress on chickpea, focusing on reproductive development. The fertilisation process is particularly sensitive to environmental stress, such as drought and heat that can reduce yields by up to 70%. Current research has largely focused on breeding cultivars that reach maturity faster to avoid stress rather than true thermotolerance and little is known of the impact of heat on cellular processes. This review suggests that there is ample variation within the chickpea gene pool for selective breeding to achieve improved abiotic stress tolerance. Rates of genetic progress will improve once key QTL are identified and the link between thermotolerance and pollen viability confirmed. Other benefits may arise from better understanding of heat shock proteins and molecular chaperones and their role in the protection of reproductive processes.

Nitric oxide secures reproductive efficiency in heat-stressed lentil (Lens culinaris Medik.) plants by enhancing the photosynthetic ability to improve yield traits

Rising temperatures, globally and locally, would be detrimental for cool- and summer-season food legumes, such as lentil (Lens culinaris Medik.). Lentil is highly sensitive to supra-optimal temperatures (> 30 °C), particularly during reproductive growth, resulting in flower and pod losses. Thus, suitable strategies are needed to introduce heat tolerance in this legume. Here, we evaluated the efficacy of nitric oxide (NO)-applied as foliar treatment of 1 mM sodium nitroprusside (SNP), twice (one day before final exposure to high temperature, and again five days later)-on heat-stressed (32/20 °C) lentil genotypes, differing in heat sensitivity. As a result of heat stress, endogenous NO increased significantly in heat-tolerant genotypes (46-62% in leaves and 66-68% in anthers, relative to the respective controls), while it decreased in heat-sensitive (HS) genotypes (27-30% in leaves and 28-33% in anthers, relative to the respective controls). Foliar supplementation with SNP markedly increased endogenous NO in leaves and anthers of both the control and heat-treated plants. Heat stress significantly accelerated phenology, damaged membranes, chlorophyll, chlorophyll fluorescence, cellular viability, and decreased leaf water status, carbon fixing and assimilating ability, less so in plants treated with SNP. Heat stress plus SNP significantly improved carbon fixation (as RuBisCo activity) and assimilation ability, (as sucrose concentration (in leaves and anthers), sucrose synthase and vacuolar acid invertase activity, reducing sugars), as well as osmolyte accumulation (proline and glycine betaine) in leaves and anthers. Moreover, SNP-treated plants had significantly less oxidative damage-measured as malondialdehyde and hydrogen peroxide concentrations-in leaves and anthers, relative to the respective control. Reproductive function-assessed as pollen grain germination and viability, stigma receptivity, and ovular viability-decreased markedly in plants exposed to heat stress alone, more so in HS genotypes, but increased significantly with SNP treatment as a consequence of improved leaf and anther function, to significantly increase the pod and seed numbers in heat-stressed lentil plants, relative to heat-stress alone.

Two Carbohydrate-Based Natural Extracts Stimulate in vitro Pollen Germination and Pollen Tube Growth of Tomato Under Cold Temperatures

To date, it is widely accepted by the scientific community that many agricultural regions will experience more extreme temperature fluctuations. These stresses will undoubtedly impact crop production, particularly fruit and seed yields. In fact, pollination is considered as one of the most temperature-sensitive phases of plant development and until now, except for the time-consuming and costly processes of genetic breeding, there is no immediate alternative to address this issue. In this work, we used a multidisciplinary approach using physiological, biochemical, and molecular techniques for studying the effects of two carbohydrate-based natural activators on in vitro tomato pollen germination and pollen tube growth cultured in vitro under cold conditions. Under mild and strong cold temperatures, these two carbohydrate-based compounds significantly enhanced pollen germination and pollen tube growth. The two biostimulants did not induce significant changes in the classical molecular markers implicated in pollen tube growth. Neither the number of callose plugs nor the CALLOSE SYNTHASE genes expression were significantly different between the control and the biostimulated pollen tubes when pollens were cultivated under cold conditions. PECTIN METHYLESTERASE (PME) activities were also similar but a basic PME isoform was not produced or inactive in pollen grown at 8°C. Nevertheless, NADPH oxidase (RBOH) gene expression was correlated with a higher number of viable pollen tubes in biostimulated pollen tubes compared to the control. Our results showed that the two carbohydrate-based products were able to reduce in vitro the effect of cold temperatures on tomato pollen tube growth and at least for one of them to modulate reactive oxygen species production.

Flower orientation influences floral temperature, pollinator visits and plant fitness

Effective insect pollination requires appropriate responses to internal and external environmental cues in both the plant and the pollinator. Helianthus annuus, a highly outcrossing species, is marked for its uniform eastward orientation of mature pseudanthia, or capitula. Here we investigate how this orientation affects floral microclimate and the consequent effects on plant and pollinator interactions and reproductive fitness. We artificially manipulated sunflower capitulum orientation and temperature in both field and controlled conditions and assessed flower physiology, pollinator visits, seed traits and siring success. East-facing capitula were found to have earlier style elongation, pollen presentation and pollinator visits compared with capitula manipulated to face west. East-facing capitula also sired more offspring than west-facing capitula and under some conditions produced heavier and better-filled seeds. Local ambient temperature change on the capitulum was found to be a key factor regulating the timing of style elongation, pollen emergence and pollinator visits. These results indicate that eastward capitulum orientation helps to control daily rhythms in floral temperature, with direct consequences on the timing of style elongation and pollen emergence, pollinator visitation, and plant fitness.

Abnormal anther development leads to lower spikelet fertility in rice (Oryza sativa L.) under high temperature during the panicle initiation stage

BACKGROUND

Decreased spikelet fertility is often responsible for reduction in grain yield in rice (Oryza sativa L.). In this study, two varieties with different levels of heat tolerance, Liangyoupeijiu (LYPJ, heat susceptible) and Shanyou63 (SY63, heat tolerant) were subjected to two temperature treatments for 28 days during the panicle initiation stage in temperature/relative humidity-controlled greenhouses: high temperature (HT; 37/27 °C; day/night) and control temperature (CK; 31/27 °C; day/night) to investigate changes in anther development under HT during panicle initiation and their relationship with spikelet fertility.

RESULTS

HT significantly decreased the grain yield of LYPJ by decreasing the number of spikelets per panicle and seed setting percentage. In addition, HT produced minor adverse effects in SY63. The decreased spikelet fertility was primarily attributed to decreased pollen viability and anther dehiscence, as well as poor pollen shedding of the anthers of LYPJ under HT. HT resulted in abnormal anther development (fewer vacuolated microspores, un-degraded tapetum, unevenly distributed Ubisch bodies) and malformation of pollen (obscure outline of the pollen exine with a collapsed bacula, disordered tectum, and no nexine of the pollen walls, uneven sporopollenin deposition on the surface of pollen grains) in LYPJ, which may have lowered pollen viability. Additionally, HT produced a compact knitted anther cuticle structure of the epidermis, an un-degraded septum, a thickened anther wall, unevenly distributed Ubisch bodies, and inhibition of the confluent locule, and these malformed structures may be partially responsible for the decreased anther dehiscence rate and reduced pollen shedding of the anthers in LYPJ. In contrast, the anther wall and pollen development of SY63 were not substantially changed under HT.

CONCLUSIONS

Our results suggest that disturbed anther walls and pollen development are responsible for the reduced spikelet fertility and grain yield of the tested heat susceptible variety, and noninvasive anthers and pollen formation in response to HT were associated with improved heat tolerance.

PIF4 negatively modulates cold tolerance in tomato anthers via temperature-dependent regulation of tapetal cell death

Extreme temperature conditions seriously impair male reproductive development in plants; however, the molecular mechanisms underlying the response of anthers to extreme temperatures remain poorly described. The transcription factor phytochrome-interacting factor4 (PIF4) acts as a hub that integrates multiple signaling pathways to regulate thermosensory growth and architectural adaptation in plants. Here, we report that SlPIF4 in tomato (Solanum lycopersicum) plays a pivotal role in regulating cold tolerance in anthers. CRISPR (clustered regularly interspaced short palindromic repeats)-associated nuclease Cas9-generated SlPIF4 knockout mutants showed enhanced cold tolerance in pollen due to reduced temperature sensitivity of the tapetum, while overexpressing SlPIF4 conferred pollen abortion by delaying tapetal programmed cell death (PCD). SlPIF4 directly interacts with SlDYT1, a direct upstream regulator of SlTDF1, both of which (SlDYT1 and SlTDF1) play important roles in regulating tapetum development and tapetal PCD. Moderately low temperature (MLT) promotes the transcriptional activation of SlTDF1 by the SlPIF4-SlDYT1 complex, resulting in pollen abortion, while knocking out SlPIF4 blocked the MLT-induced activation of SlTDF1. Furthermore, SlPIF4 directly binds to the canonical E-box sequence in the SlDYT1 promoter. Collectively, these findings suggest that SlPIF4 negatively regulates cold tolerance in anthers by directly interacting with the tapetal regulatory module in a temperature-dependent manner. Our results shed light on the molecular mechanisms underlying the adaptation of anthers to low temperatures.

Contrasting processing tomato cultivars unlink yield and pollen viability under heat stress

Climate change is causing temperature increment in crop production areas worldwide, generating conditions of heat stress that negatively affect crop productivity. Tomato (Solanum lycopersicum), a major vegetable crop, is highly susceptible to conditions of heat stress. When tomato plants are exposed to ambient day/night temperatures that exceed 32 °C/20 °C, respectively, during the reproductive phase, fruit set and fruit weight are reduced, leading to a significant decrease in yield. Processing tomato cultivars are cultivated in open fields, where environmental conditions are not controlled; therefore, plants are exposed to multiple abiotic stresses, including heat stress. Nonetheless, information on stress response in processing tomatoes is very limited. Understanding the physiological response of modern processing tomato cultivars to heat stress may facilitate the development of thermotolerant cultivars. Here, we compared two tomato processing cultivars, H4107 and H9780, that we found to be constantly differing in yield performance. Using field and temperature-controlled greenhouse experiments, we show that the observed difference in yield is attributed to the occurrence of heat stress conditions. In addition, fruit set and seed production were significantly higher in the thermotolerant cultivar H4107, compared with H9780. Despite the general acceptance of pollen viability as a measure of thermotolerance, there was no difference in the percentage of viable pollen between H4107 and H9780 under either of the conditions tested. In addition to observations of similar pollen germination and bud abscission rates, our results suggest that processing tomato cultivars may present a particular case, in which pollen performance is not determining reproductive thermotolerance. Our results also demonstrate the value of combining controlled and uncontrolled experimental settings, in order to validate and identify heat stress-related responses, thus facilitating the development of thermotolerant processing tomato cultivars.

Sequential Deposition and Remodeling of Cell Wall Polymers During Tomato Pollen Development

The cell wall of a mature pollen grain is a highly specialized, multilayered structure. The outer, sporopollenin-based exine provides protection and support to the pollen grain, while the inner intine, composed primarily of cellulose, is important for pollen germination. The formation of the mature pollen grain wall takes place within the anther with contributions of cell wall material from both the developing pollen grain as well as the surrounding cells of the tapetum. The process of wall development is complex; multiple cell wall polymers are deposited, some transiently, in a controlled sequence of events. Tomato (Solanum lycopersicum) is an important agricultural crop, which requires successful fertilization for fruit production as do many other members of the Solanaceae family. Despite the importance of pollen development for tomato, little is known about the detailed pollen gain wall developmental process. Here, we describe the structure of the tomato pollen wall and establish a developmental timeline of its formation. Mature tomato pollen is released from the anther in a dehydrated state and is tricolpate, with three long apertures without overlaying exine from which the pollen tube may emerge. Using histology and immunostaining, we determined the order in which key cell wall polymers were deposited with respect to overall pollen and anther development. Pollen development began in young flower buds when the premeiotic microspore mother cells (MMCs) began losing their cellulose primary cell wall. Following meiosis, the still conjoined microspores progressed to the tetrad stage characterized by a temporary, thick callose wall. Breakdown of the callose wall released the individual early microspores. Exine deposition began with the secretion of the sporopollenin foot layer. At the late microspore stage, exine deposition was completed and the tapetum degenerated. The pollen underwent mitosis to produce bicellular pollen; at which point, intine formation began, continuing through to pollen maturation. The entire cell wall development process was also punctuated by dynamic changes in pectin composition, particularly changes in methyl-esterified and de-methyl-esterified homogalacturonan.

Signaling Peptides Regulating Abiotic Stress Responses in Plants

As sessile organisms, plants are exposed to constantly changing environments that are often stressful for their growth and development. To cope with these stresses, plants have evolved complex and sophisticated stress-responsive signaling pathways regulating the expression of transcription factors and biosynthesis of osmolytes that confer tolerance to plants. Signaling peptides acting like phytohormones control various aspects of plant growth and development via cell-cell communication networks. These peptides are typically recognized by membrane-embedded receptor-like kinases, inducing activation of cellular signaling to control plant growth and development. Recent studies have revealed that several signaling peptides play important roles in plant responses to abiotic stress. In this mini review, we provide recent findings on the roles and signaling pathways of peptides that are involved in coordinating plant responses to abiotic stresses, such as dehydration, high salinity, reactive oxygen species, and heat. We also discuss recent developments in signaling peptides that play a role in plant adaptation responses to nutrient deficiency stress, focusing on nitrogen and phosphate deficiency responses.

The miR166-SlHB15A regulatory module controls ovule development and parthenocarpic fruit set under adverse temperatures in tomato

Fruit set is inhibited by adverse temperatures, with consequences on yield. We isolated a tomato mutant producing fruits under non-permissive hot temperatures and identified the causal gene as SlHB15A, belonging to class III homeodomain leucine-zipper transcription factors. SlHB15A loss-of-function mutants display aberrant ovule development that mimics transcriptional changes occurring in fertilized ovules and leads to parthenocarpic fruit set under optimal and non-permissive temperatures, in field and greenhouse conditions. Under cold growing conditions, SlHB15A is subjected to conditional haploinsufficiency and recessive dosage sensitivity controlled by microRNA 166 (miR166). Knockdown of SlHB15A alleles by miR166 leads to a continuum of aberrant ovules correlating with parthenocarpic fruit set. Consistent with this, plants harboring an Slhb15a-miRNA166-resistant allele developed normal ovules and were unable to set parthenocarpic fruit under cold conditions. DNA affinity purification sequencing and RNA-sequencing analyses revealed that SlHB15A is a bifunctional transcription factor expressed in the ovule integument. SlHB15A binds to the promoters of auxin-related genes to repress auxin signaling and to the promoters of ethylene-related genes to activate their expression. A survey of tomato genetic biodiversity identified pat and pat-1, two historical parthenocarpic mutants, as alleles of SlHB15A. Taken together, our findings demonstrate the role of SlHB15A as a sentinel to prevent fruit set in the absence of fertilization and provide a mean to enhance fruiting under extreme temperatures.

Pollen development in cotton (Gossypium hirsutum) is highly sensitive to heat exposure during the tetrad stage

The development of gametes in plants is acutely susceptible to heatwaves as brief as a few days, adversely affecting pollen maturation and reproductive success. Pollen in cotton (Gossypium hirsutum) was differentially affected when tetrad and binucleate stages were exposed to heat, revealing new insights into the interaction between heat and pollen development. Squares were tagged and exposed to 36/25°C (day/night, moderate heat) or 40/30°C (day/night, extreme heat) for 5 days. Mature pollen grains and leaves were collected for physiological and proteomic responses. While photosynthetic competence was not compromised even at 40°C, leaf tissues became leakier. In contrast, pollen grains were markedly smaller after the tetrad stage was exposed to 40°C and boll production was reduced by 65%. Sugar levels in pollen grains were elevated after exposure to heat, eliminating carbohydrate deficits as a likely cause of poor reproductive capacity. Proteomic analysis of pure pollen samples revealed a particularly high abundance of 70-kDa heat shock (Hsp70s) and cytoskeletal proteins. While short-term bursts of heat had a minor impact on leaves, male gametophyte development was profoundly damaged. Cotton acclimates to maxima of 36°C at both the vegetative and reproductive stages but 5-days exposure to 40°C significantly impairs reproductive development.

Heat stress response mechanisms in pollen development

Being rooted in place, plants are faced with the challenge of responding to unfavourable local conditions. One such condition, heat stress, contributes massively to crop losses globally. Heatwaves are predicted to increase, and it is of vital importance to generate crops that are tolerant to not only heat stress but also to several other abiotic stresses (e.g. drought stress, salinity stress) to ensure that global food security is protected. A better understanding of the molecular mechanisms that underlie the temperature stress response in pollen will be a significant step towards developing effective breeding strategies for high and stable production in crop plants. While most studies have focused on the vegetative phase of plant growth to understand heat stress tolerance, it is the reproductive phase that requires more attention as it is more sensitive to elevated temperatures. Every phase of reproductive development is affected by environmental challenges, including pollen and ovule development, pollen tube growth, male-female cross-talk, fertilization, and embryo development. In this review we summarize how pollen is affected by heat stress and the molecular mechanisms employed during the stress period, as revealed by classical and -omics experiments.

Bulk RNA-Seq analysis to dissect the regulation of stigma position in tomato

Transcriptomic analysis of tomato genotypes contrasting for stigma position suggests that stigma insertion occurred by the disruption of a process that finds a parallel in Arabidopsis gynoecium development. Domestication of cultivated tomato (Solanum lycopersicum L.) included the transition from allogamy to autogamy that occurred through the loss of self-incompatibilty and the retraction of the stigma within the antheridial cone. Although the inserted stigma is an established phenotype in modern tomatoes, an exserted stigma is still present in several landraces or vintage varieties. Moreover, exsertion of the stigma is a frequent response to high temperature stress and, being a cause of reduced fertility, a trait of increasing importance. Few QTLs for stigma position have been described and only one of the underlying genes identified. To gain insights on genes involved in stigma position in tomato, a bulk RNA sequencing (RNA-Seq) approach was adopted, using two groups of contrasting genotypes. Phenotypic analysis confirmed the extent and the stability of stigma position in the selected genotypes, whereas they were highly heterogeneous for other reproductive and productive traits. The RNA-Seq analysis yielded 801 differentially expressed genes (DEGs), 566 up-regulated and 235 down-regulated in the genotypes with exserted stigma. Validation by quantitative PCR indicated a high reliability of the RNA-Seq data. Up-regulated DEGs were enriched for genes involved in the cell wall metabolism, lipid transport, auxin response and flavonoid biosynthesis. Down-regulated DEGs were enriched for genes involved in translation. Validation of selected genes on pistil tissue of the 26 single genotypes revealed that differences between bulks could both be due to a general trend of the bulk or to the behaviour of single genotypes. Novel candidate genes potentially involved in the control of stigma position in tomato are discussed.

Heat-Induced Oxidation of the Nuclei and Cytosol

The concept that heat stress (HS) causes a large accumulation of reactive oxygen species (ROS) is widely accepted. However, the intracellular compartmentation of ROS accumulation has been poorly characterized. We therefore used redox-sensitive green fluorescent protein (roGFP2) to provide compartment-specific information on heat-induced redox changes of the nuclei and cytosol of Arabidopsis leaf epidermal and stomatal guard cells. We show that HS causes a large increase in the degree of oxidation of both compartments, causing large shifts in the glutathione redox potentials of the cells. Heat-induced increases in the levels of the marker transcripts, heat shock protein (HSP)101, and ascorbate peroxidase (APX)2 were maximal after 15 min of the onset of the heat treatment. RNAseq analysis of the transcript profiles of the control and heat-treated seedlings revealed large changes in transcripts encoding HSPs, mitochondrial proteins, transcription factors, and other nuclear localized components. We conclude that HS causes extensive oxidation of the nucleus as well as the cytosol. We propose that the heat-induced changes in the nuclear redox state are central to both genetic and epigenetic control of plant responses to HS.

Enhanced Reproductive Thermotolerance of the Tomato high pigment 2 Mutant Is Associated With Increased Accumulation of Flavonols in Pollen

Climate change has created an environment where heat stress conditions are becoming more frequent as temperatures continue to raise in crop production areas around the world. This situation leads to decreased crop production due to plant sensitivity to heat stress. Reproductive success is critically dependent on plants' ability to produce functional pollen grains, which are the most thermo-sensitive tissue. Flavonols are plant secondary metabolites known for their potent antioxidative activity, essential for male fertility in several species including tomato, and implicated in heat stress tolerance. Since flavonols are highly abundant in fruits of the tomato high pigment 2 (hp2) mutant, we tested the level of flavonols in pollen of this mutant, under the hypothesis that increased accumulation of flavonols would render pollen more tolerant to heat stress. Indeed, pollen from two alleles of the hp2 mutant was found to have flavonols levels increased by 18 and 280% compared with wild-type (WT) under moderate chronic heat stress (MCHS) conditions. This mutant produced on average 7.8-fold higher levels of viable pollen and displayed better germination competence under heat stress conditions. The percentage of fully seeded fruits and the number of seeds per fruit were maintained in the mutant under heat stress conditions while decreased in wild-type plants. Our results strongly suggest that increased concentrations of pollen flavonols enhance pollen thermotolerance and reproductive success under heat stress conditions. Thus, the high flavonols trait may help frame the model for improving crop resilience to heat stress.

Understanding the molecular mechanism of anther development under abiotic stresses

The developmental stage of anther development is generally more sensitive to abiotic stress than other stages of growth. Specific ROS levels, plant hormones and carbohydrate metabolism are disturbed in anthers subjected to abiotic stresses. As sessile organisms, plants are often challenged to multiple extreme abiotic stresses, such as drought, heat, cold, salinity and metal stresses in the field, which reduce plant growth, productivity and yield. The development of reproductive stage is more susceptible to abiotic stresses than the vegetative stage. Anther, the male reproductive organ that generate pollen grains, is more sensitive to abiotic stresses than female organs. Abiotic stresses affect all the processes of anther development, including tapetum development and degradation, microsporogenesis and pollen development, anther dehiscence, and filament elongation. In addition, abiotic stresses significantly interrupt phytohormone, lipid and carbohydrate metabolism, alter reactive oxygen species (ROS) homeostasis in anthers, which are strongly responsible for the loss of pollen fertility. At present, the precise molecular mechanisms of anther development under adverse abiotic stresses are still not fully understood. Therefore, more emphasis should be given to understand molecular control of anther development during abiotic stresses to engineer crops with better crop yield.

Effect of Photo-Selective Shade Nets on Pollination Process and Nut Development of Corylus avellana L

Hazelnut (Corylus avellana L.) is one of the most appreciated nut crops, which is motivating the cultivation outside its historical production areas. Despite that, there is still limited knowledge about the floral biology of the species and its developmental fruiting stages under different environments. Adverse climatic conditions can threaten the pollination process and fruit development. In South Africa, the deciduous fruit industry identified the net shading as a tool to mitigate the effects of unfavorable abiotic events. The objective of this work was to investigate the effects of photo-selective nets on the pollination process and nut development of C. avellana. Mature hazelnut trees were maintained under netting and compared with the ones in open field. Microscopic examination of female flower and developing nuts were conducted in order to observe the pollen tube growth and the pattern of disodium fluorescein transport into the funiculus and ovule. The results showed differences in pollen tubes growth and timing between the treatments. Generally, trees under nets showed higher rate in pollen tubes developing and reaching the base of the style. On the contrary, the tests carried out in open field showed a higher ratio of pollen tubes arrested in the style. The results also indicated differences in ovules abortion. Developing fruits that showed an interruption point at the funicle level or at junction point of the ovule were classified as aborting fruits (blank nuts at harvest time). A higher rate of abortion was detected in open field compared to the plants under netting. In conclusion, the shade nets influenced the pollen tube growth and the nut development, principally due to micro-climate modification. Therefore, further investigations are needed to analyze the influence of light spectra and to determine the sustainability of photo-selective nets over several years.

miRNAs involved in transcriptome remodeling during pollen development and heat stress response in Solanum lycopersicum

Cellular transitions during development and stress response depend on coordinated transcriptomic and proteomic alterations. Pollen is particular because its development is a complex process that includes meiotic and mitotic divisions which causes a high heat sensitivity of these cells. Development and stress response are accompanied by a reprogramming of the transcriptome, e.g. by post-transcriptional regulation via miRNAs. We identified known and potentially novel miRNAs in the transcriptome of developing and heat-stressed pollen of Solanum lycopersicum (tomato). The prediction of target mRNAs yielded an equal number of predicted target-sites in CDS and 3'UTR regions of target mRNAs. The result enabled the postulation of a possible link between miRNAs and a fine-tuning of transcription factor abundance during pollen development. miRNAs seem to play a role in the pollen heat stress response as well. We identified several heat stress transcription factors and heat shock proteins as putative targets of miRNAs in response to heat stress, thereby placing these miRNAs as important elements of thermotolerance. Moreover, for members of the AP2, SBP and ARF family members we could predict a miRNA-mediated regulation during development via the miR172, mir156 and mir160-family strengthening the current concept of a cross-connection between development and stress response in plants.

Differential cell persistence is observed in the Arabidopsis female gametophyte during heat stress

The central cell withstands heat stress better than the egg and antipodal cells. Insilco analysis of transcriptomic data identified several heat responsive genes which are central cell specific. Crop damage due to heat stress (HS) is a major cause of yield lost. Plants are particularly susceptible to negative effects of HS during gametophyte development and fertilization. Extensive studies have been performed on the male gametophyte under HS, but how the female gametophyte copes with HS is largely unknown. To learn how the different cell types of the female gametophyte reacts to HS, we studied unfertilized CDC123::H2B:YFP ovules. We found that the YFP-specific florescent signal persisted in the central cell during HS significantly more than the egg cell. We also found that the fluorescent signal persistence was the lowest in the antipodal cells. This finding suggests that the reaction of the female gametophyte to HS is rather unique and differentially mediated according to the cell's identity. In addition, mining through published transcriptomic datasets we found that several important heat stress responsive genes which are extremely upregulated during HS (more than 64-fold) are specifically expressed in the CC but not in the EC. Further research such as comparative transcriptomics and cell biology will likely shed more light on the phenomena reported here and increase our basic understandings about the ways sexual reproduction processes are affected by heat stress.

High temperature susceptibility of sexual reproduction in crop plants

Climate change-induced increases in the frequency of extreme weather events, particularly heatwaves, are a serious threat to crop productivity. The productivity of grain crops is dependent on the success of sexual reproduction, which is very sensitive to heat stress. Male gametophyte development has been identified as the most heat-vulnerable stage. This review outlines the susceptibility of the various stages of sexual reproduction in flowering plants from the time of floral transition to double fertilization. We summarize current knowledge concerning the molecular mechanisms underpinning the heat stress-induced aberrations and abnormalities at flowering, male reproductive development, female reproductive development, and fertilization. We highlight the stage-specific bottlenecks in sexual reproduction, which regulate seed set and final yields under high-temperature conditions, together with the outstanding research questions concerning genotypic and species-specific differences in thermotolerance observed in crops. This knowledge is essential for trait selection and genetic modification strategies for the development of heat-tolerant genotypes and high-temperature-resilient crops.

Accelerating Breeding for Heat Tolerance in Tomato (Solanum lycopersicum L.): An Integrated Approach

Heat stress is a major limiting factor for crop productivity. Tomato is highly sensitive to heat stress, which can result in a total yield loss. To adapt to current and future heat stress, there is a dire need to develop heat tolerant cultivars. Here, we review recent attempts to improve screening for heat tolerance and to exploit genetic and genomic resources in tomatoes. We provide key factors related to phenotyping environments and traits (morphological, physiological, and metabolic) to be considered to identify and breed thermo-tolerant genotypes. There is significant variability in tomato germplasm that can be harnessed to breed for thermo-tolerance. Based on our review, we propose that the use of advanced backcross populations and chromosome segments substitution lines is the best means to exploit variability for heat tolerance in non-cultivated tomato species. We applied a meta quantitative trait loci (MQTL) analysis on data from four mapping experiments to co-localize QTL associated with heat tolerance traits (e.g., pollen viability, number of pollen, number of flowers, style protrusion, style length). The analysis revealed 13 MQTL of which 11 were composed of a cluster of QTL. Overall, there was a reduction of about 1.5-fold in the confidence interval (CI) of the MQTL (31.82 cM) compared to the average CI of individual QTL (47.4 cM). This confidence interval is still large and additional mapping resolution approaches such as association mapping and multi-parent linkage mapping are needed. Further investigations are required to decipher the genetic architecture of heat tolerance surrogate traits in tomatoes. Genomic selection and new breeding techniques including genome editing and speed breeding hold promise to fast-track development of improved heat tolerance and other farmer- and consumer-preferred traits in tomatoes.

Male Sterility in Maize after Transient Heat Stress during the Tetrad Stage of Pollen Development

Shifts in the duration and intensity of ambient temperature impair plant development and reproduction, particularly male gametogenesis. Stress exposure causes meiotic defects or premature spore abortion in male reproductive organs, leading to male sterility. However, little is known about the mechanisms underlying stress and male sterility. To elucidate these mechanisms, we imposed a moderate transient heat stress on maize (Zea mays) plants at the tetrad stage of pollen development. After completion of pollen development at optimal conditions, stress responses were assessed in mature pollen. Transient heat stress resulted in reduced starch content, decreased enzymatic activity, and reduced pollen germination, resulting in sterility. A transcriptomic comparison pointed toward misregulation of starch, lipid, and energy biosynthesis-related genes. Metabolomic studies showed an increase of Suc and its monosaccharide components, as well as a reduction in pyruvate. Lipidomic analysis showed increased levels of unsaturated fatty acids and decreased levels of saturated fatty acids. In contrast, the majority of genes involved in developmental processes such as those required for auxin and unfolded protein responses, signaling, and cell wall biosynthesis remained unaltered. It is noteworthy that changes in the regulation of transcriptional and metabolic pathway genes, as well as heat stress proteins, remained altered even though pollen could recover during further development at optimal conditions. In conclusion, our findings demonstrate that a short moderate heat stress during the highly susceptible tetrad stage strongly affects basic metabolic pathways and thus generates germination-defective pollen, ultimately leading to severe yield losses in maize.