Thermal tolerance of a freshwater amphipod Gammarus lacustris can be enhanced by acclimation to higher mineralization

Temperature and mineralization are among the most important environmental factors affecting all processes of aquatic ecosystems, including geographical distribution of water animals. Previously we showed that a brackish water population of Gammarus lacustris, a widespread amphipod, demonstrates substantially higher thermotolerance than a freshwater population. A possible reason for this difference is the fact that brackish water conditions are closer to internal media mineralization. Here we aimed to test this hypothesis and relate the observed effects in animal survival under the heat shock to the status of cellular defence systems. We acclimated four groups of amphipods from the same freshwater population to 0.5 ‰ and 15 ‰ at the temperatures of 6°С or 15°С. Acclimation at 6°С, but not at 15°С, to 15 ‰ significantly increased resistance of the amphipods to heat shock at 30°C. At 6°С activities of antioxidant enzymes and levels of the lipid peroxidation products in G. lacustris did not react to the increase in mineralization and the heat shock, while the level of HSP70 elevated two-fold in amphipods acclimated to mineralization of 15 ‰ compared to animals acclimated to 0.5 ‰. Thus, the observed increase in thermotolerance could be explained by the higher initial level of HSP70 and potentially other heat shock proteins caused by a less energy-demanding, more isotonic level.

Genomic predictions and GWAS for heat tolerance in pigs based on reaction norm models with performance records and data from public weather stations considering alternative temperature thresholds

Genetic improvement of livestock productivity has resulted in greater production of metabolic heat and potentially greater susceptibility to heat stress. Various studies have demonstrated that there is genetic variability for heat tolerance and genetic selection for more heat tolerant individuals is possible. The rate of genetic progress tends to be greater when genomic information is incorporated into the analyses as more accurate breeding values can be obtained for young individuals. Therefore, this study aimed (1) to evaluate the predictive ability of genomic breeding values for heat tolerance based on routinely recorded traits, and (2) to investigate the genetic background of heat tolerance based on single-step genome-wide association studies for economically important traits related to body composition, growth and reproduction in Large White pigs. Pedigree information was available for 265,943 animals and genotypes for 8686 animals. The studied traits included ultrasound backfat thickness (BFT), ultrasound muscle depth (MDP), piglet weaning weight (WW), off-test weight (OTW), interval between farrowing (IBF), total number of piglets born (TNB), number of piglets born alive (NBA), number of piglets born dead (NBD), number of piglets weaned (WN) and weaning-to-estrus interval (IWE). The number of phenotypic records ranged from 6059 (WN) to 172,984 (TNB). Single-step genomic reaction norm predictions were used to calculate the genomic estimated breeding values for each individual. Predictions of breeding values for the validation population individuals were compared between datasets containing phenotypic records measured in the whole range of temperatures (WR) and datasets containing only phenotypic records measured when the weather station temperature was above 10°C (10C) or 15°C (15C), to evaluate the usefulness of these datasets that may better reflect the within-barn temperature. The use of homogeneous or heterogeneous residual variance was found to be trait-dependent, where homogeneous variance presented the best fit for MDP, BFT, OTW, TNB, NBA, WN and IBF, while the other traits (WW and IWE) had better fit with heterogeneous variance. The average prediction accuracy, dispersion and bias values considering all traits for WR were 0.36 ± 0.05, -0.07 ± 0.13 and 0.76 ± 0.10, respectively; for 10C were 0.39 ± 0.02, -0.05 ± 0.07 and 0.81 ± 0.05, respectively; and for 15C were 0.32 ± 0.05, -0.05 ± 0.11 and 0.84 ± 0.10, respectively. Based on the studied traits, using phenotypic records collected when the outside temperature (from public weather stations) was above 10°C provided better predictions for most of the traits. Forty-three and 62 candidate genomic regions were associated with the intercept (overall performance level) and slope term (specific biological mechanisms related to environmental sensitivity), respectively. Our results contribute to improve genomic predictions using existing datasets and better understand the genetic background of heat tolerance in pigs. Furthermore, the genomic regions and candidate genes identified will contribute to future genomic studies and breeding applications.

Are farmers motivated to select for heat tolerance? Linking attitudinal factors, perceived climate change impacts, and social trust to farmers' breeding desires

This study provides an understanding of dairy farmers' willingness to include heat tolerance in breeding goals and the modulating effect of sociopsychological factors and farm profile. A survey instrument including a choice experiment was designed to specifically address the trade-off between heat tolerance and milk production level. A total of 122 farmers across cattle, goat, and sheep farms were surveyed face-to-face. The results of the experiment show that most farmers perceive that heat stress and climate change are increasingly important problems, and that farming communities should invest more in generating knowledge and resources on mitigation strategies. However, we found limited initial support for selection for heat tolerance. This attitude changed when farmers were presented with objective information on the benefits and limitations of the different breeding choices, after which most farmers supported selection for heat tolerance, but only if doing so would compromise milk production gains to a small extent. Our results show that farmers' selection choices are driven by the interactions between heat stress risk perception, attitudes toward breeding tools, social trust, the species reared, and farm production level. In general, farmers willing to support selection of heat-tolerant animals are those with positive attitudes toward genetic values and genomic information and a strong perception of climate change and heat stress impacts on farms. On the contrary, negative support for selection for heat tolerance is found among farmers with high milk production levels; high trust in farming magazines, livestock farmers' associations, and veterinarians; and low trust in environmental and animalist groups.

Wolbachia infection negatively impacts Drosophila simulans heat tolerance in a strain- and trait-specific manner

The susceptibility of insects to rising temperatures has largely been measured by their ability to survive thermal extremes. However, the capacity for maternally inherited endosymbionts to influence insect heat tolerance has been overlooked. Further, while some studies have addressed the impact of heat on traits like fertility, which can decline at temperatures below lethal thermal limits, none have considered the impact of endosymbionts. Here, we assess the impact of three Wolbachia strains (wRi, wAu and wNo) on the survival and fertility of Drosophila simulans exposed to heat stress during development or as adults. The effect of Wolbachia infection on heat tolerance was generally small and trait/strain specific. Only the wNo infection significantly reduced the survival of adult males after a heat shock. When exposed to fluctuating heat stress during development, the wRi and wAu strains reduced egg-to-adult survival but only the wNo infection reduced male fertility. Wolbachia densities of all three strains decreased under developmental heat stress, but reductions occurred at temperatures above those that reduced host fertility. These findings emphasize the necessity to account for endosymbionts and their effect on both survival and fertility when investigating insect responses to heat stress.

Improvements in Orthostatic Tolerance with Exercise Are Augmented by Heat Acclimation: A Randomized Controlled Trial

INTRODUCTION

Heat adaptation is protective against heat illness; however, its role in heat syncope, due to reflex mechanisms, has not been conclusively established. The aim of this study was to evaluate if heat acclimation (HA) was protective against heat syncope and to ascertain underlying physiological mechanisms.

METHODS

Twenty (15 males, 5 females) endurance-trained athletes were randomized to either 8 d of mixed active and passive HA (HEAT) or climatically temperate exercise (CONTROL). Before, and after, the interventions participants underwent a head up tilt (HUT) with graded lower body negative pressure (LBNP), in a thermal chamber (32.0 ± 0.3°C), continued until presyncope with measurement of cardiovascular parameters. Heat stress tests (HST) were performed to determine physiological and perceptual measures of HA.

RESULTS

There was a significant increase in orthostatic tolerance (OT), as measured by HUT/LBNP, in the HEAT group (preintervention; 28 ± 9 min, postintervention; 40 ± 7 min) compared with CONTROL (preintervention; 30 ± 8 mins, postintervention; 33 ± 5 min) ( P = 0.01). Heat acclimation resulted in a significantly reduced peak and mean rectal and skin temperature ( P < 0.01), peak heat rate ( P < 0.003), thermal comfort ( P < 0.04), and rating of perceived exertion ( P < 0.02) during HST. There was a significantly increased plasma volume (PV) in the HEAT group in comparison to CONTROL ( P = 0.03).

CONCLUSIONS

Heat acclimation causes improvements in OT and is likely to be beneficial in patients with heat exacerbated reflex syncope. Heat acclimation-mediated PV expansion is a potential physiological mechanism underlying improved OT.

A Novel Conceptual Model for Human Heat Tolerance

Human "heat tolerance" has no accepted definition or physiological underpinnings; rather, it is almost always discussed in relative or comparative terms. We propose to use environmental limits to heat balance accounting for metabolic rate and clothing, that is, the environments for which heat stress becomes uncompensable for a specified metabolic rate and clothing, as a novel metric for quantifying heat tolerance.

The development of abamectin resistance in Liriomyza trifolii and its contribution to thermotolerance

BACKGROUND

Liriomyza trifolii is an economically significant, invasive pest of horticultural and vegetable crops. The larvae form tunnels in foliage and hasten senescence and death. Outbreaks of L. trifolii often erupt in hot weather and are driven by thermotolerance; furthermore, the poor effectiveness of pesticides has made outbreaks more severe. But it is still unclear whether the development of insecticide tolerance will contribute to thermotolerance in L. trifolii.

RESULTS

To explore potential synergistic relationships between insecticide exposure and thermotolerance in L. trifolii, we first generated an abamectin-resistant (AB-R) strain. Knockdown behavior, eclosion and survival rates, and expression levels of genes encoding heat shock proteins (Hsps) in L. trifolii were then examined in AB-R and abamectin-susceptible (AB-S) strains. Our results demonstrated that long-term selection pressure for abamectin resistance made L. trifolii more prone to develop cross-resistance to other insecticides containing similar ingredients. Furthermore, the AB-R strain exhibited enhanced thermotolerance and possessed an elevated critical thermal maximum temperature, and upregulated expression levels of Hsps during heat stress.

CONCLUSION

Collectively, our results indicate that thermal adaptation in L. trifolii was accompanied by emerging abamectin resistance. This study provides a theoretical basis for investigating the synergistic or cross-adaptive mechanisms that insects use to cope with adversity and demonstrates the complexity of insect adaptation to environmental and chemical stress. © 2023 Society of Chemical Industry.

Embryonic thermal manipulation reduces hatch time, increases hatchability, thermotolerance, and liver metabolism in broiler embryos

The broilers' health and growth performance are affected by egg quality, incubation conditions, and posthatch management. Broilers are more susceptible to heat stress because they have poor thermoregulatory capacity. So, it is crucial to develop a strategy to make chicks thermotolerant and cope with heat stress in post-hatch life. This study investigated the effects of embryonic thermal manipulation (TM) on different hatching parameters (hatch time, hatchability, and hatch weight), brain thermotolerance, and liver metabolism. Six hundred fertile Cobb 500 eggs were incubated for 21 d. After candling on embryonic day (ED) 10, 238 eggs were thermally manipulated at 38.5°C with 55% relative humidity (RH) from ED 12 to 18, then transferred to the hatcher (ED 19-21, standard temperature, 37.5°C) and 236 eggs were incubated at a standard temperature (37.5°C) till hatch. The samples were collected from the Control and TM groups on ED 15 and 18 of the embryonic periods. Hatchability was significantly higher (P < 0.05) in the TM group (94.50%) than in the control group (91.0%). Hatch weight did not differ significantly between the TM group (50.54 g) and the Control group (50.39 g). Most importantly, hatch time was significantly lower (P < 0.05) in the TM group than in the Control. In the D15 embryo brain, the mRNA expression of TRPV1,TRPV2, TRPV3, and the epigenetic marker H3K27 were significantly lower (P < 0.05) in the TM group compared to the Control group. However, in the D18 brain, the expression of TRPV1, TRPV2, and CRHR1 was significantly higher (P < 0.05) in the TM group than in the Control group. In the liver, the mRNA expression of SLC6A14 was significantly lower (P < 0.05) in the D15 TM group than in the D15 Control group. Conversely, the DIO3 mRNA expression was significantly higher (P < 0.05) in the D15 TM group than in the D15 Control group. The expression of GPX3, FOXO1, IGF2, and GHR in the liver was significantly higher in the D18 TM group compared to the D18 Control group (P < 0.05). In conclusion, increased expression of the aforementioned markers during the later embryonic period has been linked to reduced hatch time by increasing liver metabolism and thermotolerance capacity in the brain.

Cassava phosphatase PP2C1 modulates thermotolerance via fine-tuning dephosphorylation of antioxidant enzymes

Global warming is an adverse environmental factor that threatens crop yields and food security. 2C-type protein phosphatases (PP2Cs), as core protein phosphatase components, play important roles in plant hormone signaling to cope with various environmental stresses. However, the function and underlying mechanism of PP2Cs in the heat stress response remain elusive in tropical crops. Here, we report that MePP2C1 negatively regulated thermotolerance in cassava (Manihot esculenta Crantz), accompanied by the modulation of reactive oxygen species (ROS) accumulation and the underlying antioxidant enzyme activities of catalase (CAT) and ascorbate peroxidase (APX). Further investigation found that MePP2C1 directly interacted with and dephosphorylated MeCAT1 and MeAPX2 at serine (S) 112 and S160 residues, respectively. Moreover, in vitro and in vivo assays showed that protein phosphorylation of MeCAT1S112 and MeAPX2S160 was essential for their enzyme activities, and MePP2C1 negatively regulated thermotolerance and redox homeostasis by dephosphorylating MeCAT1S112 and MeAPX2S160. Taken together, this study illustrates the direct relationship between MePP2C1-mediated protein dephosphorylation of MeCAT1 and MeAPX2 and ROS accumulation in thermotolerance to provide insights for adapting to global warming via fine-tuning thermotolerance of the tropical crop cassava.

Does heat tolerance actually predict animals' geographic thermal limits?

The "climate extremes hypothesis" is a major assumption of geographic studies of heat tolerance and climatic vulnerability. However, this assumption remains vastly untested across taxa, and multiple factors may contribute to uncoupling heat tolerance estimates and geographic limits. Our dataset includes 1000 entries of heat tolerance data and maximum temperatures for each species' known geographic limits (hereafter, Tmax). We gathered this information across major animal taxa, including marine fish, terrestrial arthropods, amphibians, non-avian reptiles, birds, and mammals. We first tested if heat tolerance constrains the Tmax of sites where species could be observed. Secondly, we tested if the strength of such restrictions depends on how high Tmax is relative to heat tolerance. Thirdly, we correlated the different estimates of Tmax among them and across species. Restrictions are strong for amphibians, arthropods, and birds but often weak or inconsistent for reptiles and mammals. Marine fish describe a non-linear relationship that contrasts with terrestrial groups. Traditional heat tolerance measures in thermal vulnerability studies, like panting temperatures and the upper set point of preferred temperatures, do not predict Tmax or are inversely correlated to it, respectively. Heat tolerance restricts the geographic warm edges more strongly for species that reach sites with higher Tmax for their heat tolerance. These emerging patterns underline the importance of reliable species' heat tolerance indexes to identify their thermal vulnerability at their warm range edges. Besides, the tight correlations of Tmax estimates across on-land microhabitats support a view of multiple types of thermal challenges simultaneously shaping ranges' warm edges for on-land species. The heterogeneous correlation of Tmax estimates in the ocean supports the view that fish thermoregulation is generally limited, too. We propose new hypotheses to understand thermal restrictions on animal distribution.

Heat shock and thermotolerance in Caenorhabditis elegans: An overview of laboratory techniques

The soil nematode worm Caenorhabditis elegans is a simple and well-established model for the study of many biological processes. Heat shock and thermotolerance assays have been developed for this nematode, and have been used to decipher the molecular relationships between thermal stress and aging, among others. Nevertheless, a systematic and methodological comparison of the different approaches and tools utilized is lacking in the literature. Here, we aim to provide a comprehensive summary of the most commonly used strategies for carrying out heat shock and thermotolerance assays that have been reported, highlighting specific readouts and scientific questions that can be addressed. Furthermore, we offer examples of thermotolerance assays performed with wild type nematodes, that can serve as a gauge of the animal survival under diverse conditions of stress.

Are Thermotolerant and Osmotolerant Characteristics of Acanthamoeba Species an Indicator of Pathogenicity?

Objective

The aim of this study was to evaluate the pathogenicity of Acanthamoeba strains with T4, T5, T11, and T12 genotypes by comparing the osmotolerance and thermotolerance characteristics of Acanthamoeba strains isolated from genotype groups, within species with the same genotype, and from environmental and keratitis cases.

Methods

In this study, after axenic cultures of 22 Acanthamoeba strains with T4 (Neff, A, B, D, E), T5, T11, and T12 genotypes isolated from clinical and environmental samples, thermotolerance (37 °C, 39 °C and 41 °C) and osmotolerance (0.5 M, 1 M) tests were performed.

Results

All strains showed growth ability at 37 °C and 0.5 M osmolarity. While all five strains isolated from patients with Acanthamoeba keratitis showed growth ability at 37 °C and 0.5 M osmolarity, no growth was detected at 41 °C and 1 M osmolarity. When the tolerance characteristics of the strains with the same genotype were evaluated, the strains with the T5 and T4E genotypes showed the same characteristics. When Acanthamoeba strains with the T4 genotype were evaluated in general, 31.25% of the strains were found to grow at 39 °C and 6.25% at 41 °C. Of the T4Neff strains, only one strain did not show the ability to reproduce at 39 °C and showed a different feature from the other strains. While the strain with the T11 genotype grew at all temperatures, the strain with the T12 genotype did not grow at 41 °C.

Conclusion

According to our research results, we believe that tolerance to 39 °C and 1 M mannitol is not an indicator of pathogenicity. More studies with Acanthamoeba strains are required to clarify this issue.

Beyond a single temperature threshold: Applying a cumulative thermal stress framework to plant heat tolerance

Most plant thermal tolerance studies focus on single critical thresholds, which limit the capacity to generalise across studies and predict heat stress under natural conditions. In animals and microbes, thermal tolerance landscapes describe the more realistic, cumulative effects of temperature. We tested this in plants by measuring the decline in leaf photosynthetic efficiency (FV/FM) following a combination of temperatures and exposure times and then modelled these physiological indices alongside recorded environmental temperatures. We demonstrate that a general relationship between stressful temperatures and exposure durations can be effectively employed to quantify and compare heat tolerance within and across plant species and over time. Importantly, we show how FV/FM curves translate to plants under natural conditions, suggesting that environmental temperatures often impair photosynthetic function. Our findings provide more robust descriptors of heat tolerance in plants and suggest that heat tolerance in disparate groups of organisms can be studied with a single predictive framework.

Auditing the Representation of Females Versus Males in Heat Adaptation Research

The aim of this audit was to quantify female representation in research on heat adaptation. Using a standardized audit tool, the PubMed database was searched for heat adaptation literature from inception to February 2023. Studies were included if they investigated heat adaptation among female and male adults (≥18-50 years) who were free from noncommunicable diseases, with heat adaptation the primary or secondary outcome of interest. The number and sex of participants, athletic caliber, menstrual status, research theme, journal impact factor, Altmetric score, Field-Weighted Citation Impact, and type of heat exposure were extracted. A total of 477 studies were identified in this audit, including 7,707 participants with ∼13% of these being female. Most studies investigated male-only cohorts (∼74%, n = 5,672 males), with ∼5% (n = 360 females) including female-only cohorts. Of the 126 studies that included females, only 10% provided some evidence of appropriate methodological control to account for ovarian hormone status, with no study meeting best-practice recommendations. Of the included female participants, 40% were able to be classified to an athletic caliber, with 67% of these being allocated to Tier 2 (i.e., trained/developmental) or below. Exercise heat acclimation was the dominant method of heat exposure (437 interventions), with 21 studies investigating sex differences in exercise heat acclimation interventions. We recommend that future research on heat adaptation in female participants use methodological approaches that consider the potential impact of sexual dimorphism on study outcomes to provide evidence-based guidelines for female athletes preparing for exercise or competition in hot conditions.

Bees display limited acclimation capacity for heat tolerance

Bees are essential pollinators and understanding their ability to cope with extreme temperature changes is crucial for predicting their resilience to climate change, but studies are limited. We measured the response of the critical thermal maximum (CTMax) to short-term acclimation in foragers of six bee species from the Greek island of Lesvos, which differ in body size, nesting habit, and level of sociality. We calculated the acclimation response ratio as a metric to assess acclimation capacity and tested whether bees' acclimation capacity was influenced by body size and/or CTMax. We also assessed whether CTMax increases following acute heat exposure simulating a heat wave. Average estimate of CTMax varied among species and increased with body size but did not significantly shift in response to acclimation treatment except in the sweat bee Lasioglossum malachurum. Acclimation capacity averaged 9% among species and it was not significantly associated with body size or CTMax. Similarly, the average CTMax did not increase following acute heat exposure. These results indicate that bees might have limited capacity to enhance heat tolerance via acclimation or in response to prior heat exposure, rendering them physiologically sensitive to rapid temperature changes during extreme weather events. These findings reinforce the idea that insects, like other ectotherms, generally express weak plasticity in CTMax, underscoring the critical role of behavioral thermoregulation for avoidance of extreme temperatures. Conserving and restoring native vegetation can provide bees temporary thermal refuges during extreme weather events.

Lily LlHSFC2 coordinates with HSFAs to balance heat stress response and improve thermotolerance

Heat stress transcription factors (HSFs) are core regulators of plant heat stress response. Much research has focused on class A and B HSFs, leaving those of class C relatively understudied. Here, we reported a lily (Lilium longiflorum) heat-inducible HSFC2 homology involved in thermotolerance. LlHSFC2 was located in the nucleus and cytoplasm and exhibited a repression ability by binding heat stress element. Overexpression of LlHSFC2 in Arabidopsis, tobacco (Nicotiana benthamiana), and lily, all increased the thermotolerance. Conversely, silencing of LlHSFC2 in lily reduced its thermotolerance. LlHSFC2 could interact with itself, or interact with LlHSFA1, LlHSFA2, LlHSFA3A, and LlHSFA3B of lily, AtHSFA1e and AtHSFA2 of Arabidopsis, and NbHSFA2 of tobacco. LlHSFC2 interacted with HSFAs to accelerate their transactivation ability and act as a transcriptional coactivator. Notably, compared with the separate LlHSFA3A overexpression, co-overexpression of LlHSFC2/LlHSFA3A further enhanced thermotolerance of transgenic plants. In addition, after suffering HS, the homologous interaction of LlHSFC2 was repressed, but its heterologous interaction with the heat-inducible HSFAs was promoted, enabling it to exert its co-activation effect for thermotolerance establishment and maintenance. Taken together, we identified that LlHSFC2 plays an active role in the general balance and maintenance of heat stress response by cooperating with HSFAs, and provided an important candidate for the enhanced thermotolerance breeding of crops and horticulture plants.

Plasticity of cold and heat stress tolerance induced by hardening and acclimation in the melon thrips

Extreme temperatures threaten species under climate change and can limit range expansions. Many species cope with changing environments through plastic changes. This study tested phenotypic changes in heat and cold tolerance under hardening and acclimation in the melon thrips, Thrips palmi Karny (Thysanoptera: Thripidae), an agricultural pest of many vegetables. We first measured the critical thermal maximum (CTmax) of the species by the knockdown time under static temperatures and found support for an injury accumulation model of heat stress. The inferred knockdown time at 39 °C was 82.22 min. Rapid heat hardening for 1 h at 35 °C slightly increased CTmax by 1.04 min but decreased it following exposure to 31 °C by 3.46 min and 39 °C by 6.78 min. Heat acclimation for 2 and 4 days significantly increased CTmax at 35 °C by 1.83, and 6.83 min, respectively. Rapid cold hardening at 0 °C and 4 °C for 2 h, and cold acclimation at 10 °C for 3 days also significantly increased cold tolerance by 6.09, 5.82, and 2.00 min, respectively, while cold hardening at 8 °C for 2 h and acclimation at 4 °C and 10 °C for 5 days did not change cold stress tolerance. Mortality at 4 °C for 3 and 5 days reached 24.07 % and 43.22 % respectively. Our study showed plasticity for heat and cold stress tolerance in T. palmi, but the thermal and temporal space for heat stress induction is narrower than for cold stress induction.

Effects of medical plants on alleviating the effects of heat stress on chickens

Over the past decades, global climate change has led to a significant increase in the average ambient temperature causing heat stress (HS) waves. This increase has resulted in more frequent heat waves during the summer periods. HS can have detrimental effects on poultry, including growth retardation, imbalance in immune/antioxidant pathways, inflammation, intestinal dysfunction, and economic losses in the poultry industry. Therefore, it is crucial to find an effective, safe, applicable, and economically efficient method for reducing these negative influences. Medicinal plants (MPs) contain various bioactive compounds with antioxidant, antimicrobial, anti-inflammatory, and immunomodulatory effects. Due to the biological activities of MPs, it could be used as promising thermotolerance agents in poultry diets during HS conditions. Nutritional supplementation with MPs has been shown to improve growth performance, antioxidant status, immunity, and intestinal health in heat-exposed chickens. As a result, several types of herbs have been supplemented to mitigate the harmful effects of heat stress in chickens. Therefore, several types of herbs have been supplemented to mitigate the harmful effects of heat stress in chickens. This review aims to discuss the negative consequences of HS in poultry and explore the use of different traditional MPs to enhance the health status of chickens.

Heat tolerance in Drosophila melanogaster is influenced by oxygen conditions and mutations in cell size control pathways

Understanding metabolic performance limitations is key to explaining the past, present and future of life. We investigated whether heat tolerance in actively flying Drosophila melanogaster is modified by individual differences in cell size and the amount of oxygen in the environment. We used two mutants with loss-of-function mutations in cell size control associated with the target of rapamycin (TOR)/insulin pathways, showing reduced (mutant rictorΔ2) or increased (mutant Mnt1) cell size in different body tissues compared to controls. Flies were exposed to a steady increase in temperature under normoxia and hypoxia until they collapsed. The upper critical temperature decreased in response to each mutation type as well as under hypoxia. Females, which have larger cells than males, had lower heat tolerance than males. Altogether, mutations in cell cycle control pathways, differences in cell size and differences in oxygen availability affected heat tolerance, but existing theories on the roles of cell size and tissue oxygenation in metabolic performance can only partially explain our results. A better understanding of how the cellular composition of the body affects metabolism may depend on the development of research models that help separate various interfering physiological parameters from the exclusive influence of cell size. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Threshold shifts and developmental temperature impact trade-offs between tolerance and plasticity

Mounting evidence suggests that ectotherms are already living close to their upper physiological thermal limits. Phenotypic plasticity has been proposed to reduce the impact of climate change in the short-term providing time for adaptation, but the tolerance-plasticity trade-off hypothesis predicts organisms with higher tolerance have lower plasticity. Empirical evidence is mixed, which may be driven by methodological issues such as statistical artefacts, nonlinear reaction norms, threshold shifts or selection. Here, we examine whether threshold shifts (organisms with higher tolerance require stronger treatments to induce maximum plastic responses) influence tolerance-plasticity trade-offs in hardening capacity for desiccation tolerance and critical thermal maximum (CTMAX) across Drosophila species with varying distributions/sensitivity to desiccation/heat stress. We found evidence for threshold shifts in both traits; species with higher heat/desiccation tolerance required longer hardening treatments to induce maximum hardening responses. Species with higher heat tolerance also showed reductions in hardening capacity at higher developmental acclimation temperatures. Trade-off patterns differed depending on the hardening treatment used and the developmental temperature flies were exposed to. Based on these findings, studies that do not consider threshold shifts, or that estimate plasticity under a narrow set of environments, will have a limited ability to assess trade-off patterns and differences in plasticity across species/populations more broadly.

Leaf thermotolerance of Hevea brasiliensis clones: intra- versus interclonal variation and relationships with other functional traits

Land surface temperature is predicted to increase by 0.2 °C per decade due to climate change, although with considerable regional variability, and heatwaves are predicted to increase markedly in the future. These changes will affect where crops can be grown in the future. Understanding the thermal limits of plant physiological functioning and how flexible such limits are is thus important. Here, we report on the measurements of a core foliar thermotolerance trait, T50, defined as the temperature at which the maximum quantum yield (Fv/Fm) of photosystem II declines by 50%, across nine different Malaysian Hevea brasiliensis clones. We explore the relative importance of interclonal versus intraclonal variation in T50 as well as its association with leaf and hydraulic traits. We find very low variation in T50 within individual clones (mean intraclonal coefficient of variation (CoV) of 1.26%) and little variation across clones (interclonal CoV of 2.1%). The interclonal variation in T50 was lower than for all other functional traits considered. The T50 was negatively related to leaf mass per area and leaf dry matter content, but it was not related to hydraulic traits such as embolism resistance (P50) or hydraulic safety margins (HSM50). The range of T50 observed (42.9-46.2 °C) is well above the current maximum air temperatures Tmax,obs (T50 - Tmax,obs >5.8 °C), suggesting that H. brasiliensis is likely thermally safe in this south-east Asian region of Malaysia.

Comparative transcriptome profiling of contrasting finger millet (Eleusine coracana (L.) Gaertn) genotypes under heat stress

BACKGROUND

Eleusine coracana (L.) Gaertn is a crucial C4 species renowned for its stress robustness and nutritional significance. Because of its adaptability traits, finger millet (ragi) is a storehouse of critical genomic resources for crop improvement. However, more knowledge about this crop's molecular responses to heat stress needs to be gained.

METHODS AND RESULTS

In the present study, a comparative RNA sequencing analysis was done in the leaf tissue of the finger millet, between the heat-sensitive (KJNS-46) and heat-tolerant (PES-110) cultivars of Ragi, in response to high temperatures. On average, each sample generated about 24 million reads. Interestingly, a comparison of transcriptomic profiling identified 684 transcripts which were significantly differentially expressed genes (DEGs) examined between the heat-stressed samples of both genotypes. The heat-induced change in the transcriptome was confirmed by qRT-PCR using a set of randomly selected genes. Pathway analysis and functional annotation analysis revealed the activation of various genes involved in response to stress specifically heat, oxidation-reduction process, water deprivation, and changes in heat shock protein (HSP) and transcription factors, calcium signaling, and kinase signaling. The basal regulatory genes, such as bZIP, were involved in response to heat stress, indicating that heat stress activates genes involved in housekeeping or related to basal regulatory processes. A substantial percentage of the DEGs belonged to proteins of unknown functions (PUFs), i.e., not yet characterized.

CONCLUSION

These findings highlight the importance of candidate genes, such as HSPs and pathways that can confer tolerance towards heat stress in ragi. These results will provide valuable information to improve the heat tolerance in heat-susceptible agronomically important varieties of ragi and other crops.

Elongation factor MdEF-Tu coordinates with heat shock protein MdHsp70 to enhance apple thermotolerance

High-temperature stress results in protein misfolding/unfolding and subsequently promotes the accumulation of cytotoxic protein aggregates that can compromise cell survival. Heat shock proteins (HSPs) function as molecular chaperones that coordinate the refolding and degradation of aggregated proteins to mitigate the detrimental effects of high temperatures. However, the relationship between HSPs and protein aggregates in apples under high temperatures remains unclear. Here, we show that an apple (Malus domestica) chloroplast-localized, heat-sensitive elongation factor Tu (MdEF-Tu), positively regulates apple thermotolerance when it is overexpressed. Transgenic apple plants exhibited higher photosynthetic capacity and better integrity of chloroplasts during heat stress. Under high temperatures, MdEF-Tu formed insoluble aggregates accompanied by ubiquitination modifications. Furthermore, we identified a chaperone heat shock protein (MdHsp70), as an interacting protein of MdEF-Tu. Moreover, we observed obviously elevated MdHsp70 levels in 35S: MdEF-Tu apple plants that prevented the accumulation of ubiquitinated MdEF-Tu aggregates, which positively contributes to the thermotolerance of the transgenic plants. Overall, our results provide new insights into the molecular chaperone function of MdHsp70, which mediates the homeostasis of thermosensitive proteins under high temperatures.

Fungal thermotolerance revisited and why climate change is unlikely to be supercharging pathogenic fungi (yet)

Thermotolerance has been viewed as an uncommon characteristic among the fungi and one of the reasons that less than 1% of the described species operate as opportunistic pathogens of humans. Growth at 37°C is certainly a requirement for a fungus that invades the body core, but tens of thousands of nonpathogenic species are also able to grow at this temperature. Ergo, body temperature does not serve as a thermal barrier to the development of infections by many harmless fungi. The absence of other virulence factors must be more demanding. This observation raises questions about the hypothetical links between climate change and the increasing number of life-threatening human mycoses. Given the widespread distribution of fungal thermotolerance and the 1°C (2°F) increase in global temperature over the last 140 years it seems unlikely that the warming climate has driven the evolution of more virulent strains of fungi. More compelling explanations for the changes in the behavior of fungi as disease agents include their adaptation to the widening use of azole antifungals in hospitals and the wholesale application of millions of tons of the same class of heterocyclic chemicals in agriculture. On the other hand, climate change is having a significant effect on the spread of human mycoses by extending the geographical range of pathogenic fungi. A related increase in fungal asthma caused by spore inhalation is another likely consequence of planetary change.

ZmMKK9-ZmMPK20-ZmRIN2 cascade boosts plant heat tolerance by regulating stomatal response

KEY MESSAGE

Recent research has unveiled that the ZmMKK9-ZmMPK20-ZmRIN2 cascade plays a role in suppressing stomatal opening induced by high temperatures and is a significant contributor to enhancing thermotolerance in plants.

Multi-tissue metabolic and transcriptomic responses to a short-term heat stress in swine

BACKGROUND

Heat stress (HS) is an increasing threat for pig production with a wide range of impacts. When submitted to high temperatures, pigs will use a variety of strategies to alleviate the effect of HS. While systemic adaptations are well known, tissue-specific changes remain poorly understood. In this study, thirty-two pigs were submitted to a 5-day HS at 32 °C.

RESULTS

Transcriptomic and metabolomic analyses were performed on several tissues. The results revealed differentially expressed genes and metabolites in different tissues. Specifically, 481, 1774, 71, 1572, 17, 164, and 169 genes were differentially expressed in muscle, adipose tissue, liver, blood, thyroid, pituitary, and adrenal glands, respectively. Regulatory glands (pituitary, thyroid, and adrenal) had a lower number of regulated genes, perhaps indicating an earlier sensitivity to HS. In addition, 7, 8, 2, and 8 metabolites were differentially produced in muscle, liver, plasma, and urine, respectively. The study also focused on the oxidative stress pathway in muscle and liver by performing a correlation analysis between genes and metabolites.

CONCLUSIONS

This study has identified various adaptation mechanisms in swine that enable them to cope with heat stress (HS). These mechanisms include a global decrease in energetic metabolism, as well as changes in metabolic precursors that are linked with protein and lipid catabolism and anabolism. Notably, the adaptation mechanisms differ significantly between regulatory (pituitary, thyroid and adrenal glands) and effector tissues (muscle, adipose tissue, liver and blood). Our findings provide new insights into the comprehension of HS adaptation mechanisms in swine.

Flying, nectar-loaded honey bees conserve water and improve heat tolerance by reducing wingbeat frequency and metabolic heat production

Heat waves are becoming increasingly common due to climate change, making it crucial to identify and understand the capacities for insect pollinators, such as honey bees, to avoid overheating. We examined the effects of hot, dry air temperatures on the physiological and behavioral mechanisms that honey bees use to fly when carrying nectar loads, to assess how foraging is limited by overheating or desiccation. We found that flight muscle temperatures increased linearly with load mass at air temperatures of 20 or 30 °C, but, remarkably, there was no change with increasing nectar loads at an air temperature of 40 °C. Flying, nectar-loaded bees were able to avoid overheating at 40 °C by reducing their flight metabolic rates and increasing evaporative cooling. At high body temperatures, bees apparently increase flight efficiency by lowering their wingbeat frequency and increasing stroke amplitude to compensate, reducing the need for evaporative cooling. However, even with reductions in metabolic heat production, desiccation likely limits foraging at temperatures well below bees' critical thermal maxima in hot, dry conditions.

Growth performance, histological and physiological responses of heat-stressed broilers in response to short periods of incubation during egg storage and thermal conditioning

The short periods of incubation during egg storage (SPIDES) method enhances the quality of chicks and improves hatching rates. Additionally, embryonic thermal conditioning (TC) is a technique used to enhance thermotolerance in birds. Previous studies have evaluated the effects of SPIDES and embryonic TC separately. Yet, our hypothesis postulated that a synergistic effect could be achieved by integrating TC and SPIDES, thereby enhancing the broilers' resilience to thermal stress. We conducted an experiment involving 800 Ross broiler eggs, divided into two groups. The first group, referred to as S0, was maintained under standard storage room conditions and acted as our control group. The second group, known as S1, underwent a process called SPIDES for 5 h at a temperature of 37.8 ± 0.1 °C, on three occasions: days 5, 10, and 15 following egg collection. Upon reaching the 14th day of incubation (DOI), each of these primary groups was randomly subdivided into two equal subgroups. The control subgroup, designated as TC0, remained in the usual incubation conditions. Meanwhile, the other subgroup, TC1, was subjected to prenatal heat conditioning at a temperature of 39.5 ± 0.1 °C for 6 h per day, commencing on the 14th embryonic day (E) and extending until the 18th embryonic day (E). This experimental setup resulted in four distinct experimental subgroups: S0TC0, S1TC0, S0TC1, and S1TC1. The findings indicated that the combined application of SPIDES and TC had a significant positive effect on chick performance after hatching. Specifically, the (S1TC1) group exhibited the heaviest live body weight (LBW) and body weight gain (BWG) at the marketing age in comparison to the other groups. Furthermore, both SPIDES and TC had a positive influence on the relative weights of breast muscles and their histological measurements. The (S1TC1) group displayed significantly higher values in terms of the relative weight of breast muscles and the number of myocytes. In conclusion, SPIDES and TC have beneficial effects on pre- and post-hatch characteristics of broiler chicks up until the marketing age. Additionally, TC techniques improve chick performance, particularly under conditions of heat stress, and enhance the yield of breast muscle in later stages of life.

Heat tolerance of a tropical-subtropical rainforest tree species Polyscias elegans: time-dependent dynamic responses of physiological thermostability and biochemistry

Heat stress interrupts physiological thermostability and triggers biochemical responses that are essential for plant survival. However, there is limited knowledge on the speed plants adjust to heat in hours and days, and which adjustments are crucial. Tropical-subtropical rainforest tree species (Polyscias elegans) were heated at 40°C for 5 d, before returning to 25°C for 13 d of recovery. Leaf heat tolerance was quantified using the temperature at which minimal chl a fluorescence sharply rose (Tcrit ). Tcrit , metabolites, heat shock protein (HSP) abundance and membrane lipid fatty acid (FA) composition were quantified. Tcrit increased by 4°C (48-52°C) within 2 h of 40°C exposure, along with rapid accumulation of metabolites and HSPs. By contrast, it took > 2 d for FA composition to change. At least 2 d were required for Tcrit , HSP90, HSP70 and FAs to return to prestress levels. The results highlight the multi-faceted response of P. elegans to heat stress, and how this response varies over the scale of hours to days, culminating in an increased level of photosynthetic heat tolerance. These responses are important for survival of plants when confronted with heat waves amidst ongoing global climate change.

Cross-species gut microbiota transplantation predictably affects host heat tolerance

The gut microbiome is known to influence and have regulatory effects in diverse physiological functions of host animals, but only recently has the relationship between host thermal biology and gut microbiota been explored. Here, we examined how early-life manipulations of the gut microbiota in larval amphibians influenced their critical thermal maximum (CTmax) at different acclimation temperatures. We stripped the resident microbiome from egg masses of wild-caught wood frogs (Lithobates sylvaticus) via an antibiotic wash, and then inoculated the eggs with pond water (control), no inoculation, or the intestinal microbiota of another species that has a wider thermal tolerance - green frogs (Lithobates clamitans). We predicted that this cross-species transplant would increase the CTmax of the recipient wood frog larvae relative to the other treatments. In line with this prediction, green frog microbiome-recipient larvae had the highest CTmax while those with no inoculum had the lowest CTmax. Both the microbiome treatment and acclimation temperature significantly influenced the larval gut microbiota communities and α-diversity indices. Green frog microbiome-inoculated larvae were enriched in Rikenellaceae relative to the other treatments, which produce short-chain fatty acids and could contribute to greater energy availability and enhanced heat tolerance. Larvae that received no inoculation had a higher relative abundance of potentially pathogenic Aeromonas spp., which negatively affects host health and performance. Our results are the first to show that cross-species gut microbiota transplants alter heat tolerance in a predictable manner. This finding has repercussions for the conservation of species that are threatened by climate change and demonstrates a need to further explore the mechanisms by which the gut microbiota modulate host thermal tolerance.

Behavioural and physiological responses of juvenile geoduck (Panopea zelandica) following acute thermal stress

Climate extremes, such as heatwaves, are expected to become more intense and of longer duration in the near future. These climatic conditions may have a significant impact on the prospects of establishing a new aquaculture industry for the endemic New Zealand geoduck, Panopea zelandica. This study focused on characterising animal behaviour, haemocytes , and heat shock protein (HSP70 & HSP90) mRNA expression following exposure to elevated temperatures, such as those encountered during marine heatwaves around 20 °C and an extreme scenario of 25 °C, contrasted to an ambient temperature of 17 °C. After 24 h of heat challenge, P. zelandica were found to be significantly influenced by the thermal changes, as there were differences recorded in all the responses examined. With increasing temperatures, juvenile geoduck were observed to fully emerge from the sediment a behaviour that has not previously been quantified nor associated with stress in this species. The ability of P. zelandica juveniles to re-bury still warrants further investigation, as adults are unable to do so. Haemocyte analyses revealed an increase in the abundance of granulocytes, cellular aggregations, and size of these aggregations at the highest temperature exposure. Increased expression of the hsp70 gene in the haemolymph after exposure at 25 °C for 24 h was detected and attributed to attempts to mitigate protein denaturation caused by thermal stress. The inter-individual variability in the response of heat shock proteins recorded could aid in future selective breeding programs if it is reflected in net thermotolerance. P. zelandica shows great potential for growing in subtidal habitats around New Zealand, and this study highlights the importance of temperature considerations when selecting potential farm and reseeding locations.

Gasotransmitter ammonia accelerates seed germination, seedling growth, and thermotolerance acquirement in maize

Ammonia (NH₃), as an intermediate product of nitrogen metabolism, is recognized as a novel gasotransmitter (namely gaseous signaling molecule), its signaling role being revealed in plants. NH₃ exists in two different chemical forms, namely the weak base (free molecule: NH₃) and the weak acid (ammonium: NH₄⁺), which are generally in equilibrium with each other in plants. However, the effect of NH₃ on seed germination, seedling growth, and thermotolerance acquirement in maize remains unclear. Here, maize seeds were imbibed in the different concentrations of NH₃·H₂O (NH₃ donor), and then germinated and calculated seed germination rate at the various time points. Also, the 60-h-old seedlings were irrigated in the different concentrations of NH₃·H₂O, and then subjected to heat stress and counted survival rate. The data implied that the appropriate concentrations (6, 9, and 12 mM) of NH₃·H₂O accelerated seed germination as well as increased seedling height and root length compared with the control without NH₃ treatment. Also, the suitable concentrations (2 and 4 mM) of NH₃·H₂O improved tissue vitality, relieved an increase in malondialdehyde content, and enhanced survival rate of maize seedlings under heat stress compared with the control. These results firstly suggest that NH₃ could accelerate seed germination, seedling growth, and thermotolerance acquirement in maize.

Calcium Signaling and the Response to Heat Shock in Crop Plants

Climate change and the increasing frequency of high temperature (HT) events are significant threats to global crop yields. To address this, a comprehensive understanding of how plants respond to heat shock (HS) is essential. Signaling pathways involving calcium (Ca2+), a versatile second messenger in plants, encode information through temporal and spatial variations in ion concentration. Ca2+ is detected by Ca2+-sensing effectors, including channels and binding proteins, which trigger specific cellular responses. At elevated temperatures, the cytosolic concentration of Ca2+ in plant cells increases rapidly, making Ca2+ signals the earliest response to HS. In this review, we discuss the crucial role of Ca2+ signaling in raising plant thermotolerance, and we explore its multifaceted contributions to various aspects of the plant HS response (HSR).

Longitudinal genomic analyses of automatically-recorded vaginal temperature in lactating sows under heat stress conditions based on random regression models

BACKGROUND

Automatic and continuous recording of vaginal temperature (TV) using wearable sensors causes minimal disruptions to animal behavior and can generate data that enable the evaluation of temporal body temperature variation under heat stress (HS) conditions. However, the genetic basis of TV in lactating sows from a longitudinal perspective is still unknown. The objectives of this study were to define statistical models and estimate genetic parameters for TV in lactating sows using random regression models, and identify genomic regions and candidate genes associated with HS indicators derived from automatically-recorded TV.

RESULTS

Heritability estimates for TV ranged from 0.14 to 0.20 over time (throughout the day and measurement period) and from 0.09 to 0.18 along environmental gradients (EG, - 3.5 to 2.2, which correspond to dew point values from 14.87 to 28.19 ˚C). Repeatability estimates of TV over time and along EG ranged from 0.57 to 0.66 and from 0.54 to 0.77, respectively. TV measured from 12h00 to 16h00 had moderately high estimates of heritability (0.20) and repeatability (0.64), indicating that this period might be the most suitable for recording TV for genetic selection purposes. Significant genotype-by-environment interactions (GxE) were observed and the moderately high estimates of genetic correlations between pairs of extreme EG indicate potential re-ranking of selection candidates across EG. Two important genomic regions on chromosomes 10 (59.370-59.998 Mb) and16 (21.548-21.966 Mb) were identified. These regions harbor the genes CDC123, CAMK1d, SEC61A2, and NUDT5 that are associated with immunity, protein transport, and energy metabolism. Across the four time-periods, respectively 12, 13, 16, and 10 associated genomic regions across 14 chromosomes were identified for TV. For the three EG classes, respectively 18, 15, and 14 associated genomic windows were identified for TV, respectively. Each time-period and EG class had uniquely enriched genes with identified specific biological functions, including regulation of the nervous system, metabolism and hormone production.

CONCLUSIONS

TV is a heritable trait with substantial additive genetic variation and represents a promising indicator trait to select pigs for improved heat tolerance. Moderate GxE for TV exist, indicating potential re-ranking of selection candidates across EG. TV is a highly polygenic trait regulated by a complex interplay of physiological, cellular and behavioral mechanisms.

Low heat tolerance and high desiccation resistance in nocturnal bees and the implications for nocturnal pollination under climate change

Predicting insect responses to climate change is essential for preserving ecosystem services and biodiversity. Due to high daytime temperatures and low humidity levels, nocturnal insects are expected to have lower heat and desiccation tolerance compared to diurnal species. We estimated the lower (CTMin) and upper (CTMax) thermal limits of Megalopta, a group of neotropical, forest-dwelling bees. We calculated warming tolerance (WT) as a metric to assess vulnerability to global warming and measured survival rates during simulated heatwaves and desiccation stress events. We also assessed the impact of body size and reproductive status (ovary area) on bees' thermal limits. Megalopta displayed lower CTMin, CTMax, and WTs than diurnal bees (stingless bees, orchid bees, and carpenter bees), but exhibited similar mortality during simulated heatwave and higher desiccation tolerance. CTMin increased with increasing body size across all bees but decreased with increasing body size and ovary area in Megalopta, suggesting a reproductive cost or differences in thermal environments. CTMax did not increase with increasing body size or ovary area. These results indicate a greater sensitivity of Megalopta to temperature than humidity and reinforce the idea that nocturnal insects are thermally constrained, which might threaten pollination services in nocturnal contexts during global warming.

Investigating the impact of non-additive genetic effects in the estimation of variance components and genomic predictions for heat tolerance and performance traits in crossbred and purebred pig populations

BACKGROUND

Non-additive genetic effects are often ignored in livestock genetic evaluations. However, fitting them in the models could improve the accuracy of genomic breeding values. Furthermore, non-additive genetic effects contribute to heterosis, which could be optimized through mating designs. Traits related to fitness and adaptation, such as heat tolerance, tend to be more influenced by non-additive genetic effects. In this context, the primary objectives of this study were to estimate variance components and assess the predictive performance of genomic prediction of breeding values based on alternative models and two independent datasets, including performance records from a purebred pig population and heat tolerance indicators recorded in crossbred lactating sows.

RESULTS

Including non-additive genetic effects when modelling performance traits in purebred pigs had no effect on the residual variance estimates for most of the traits, but lower additive genetic variances were observed, especially when additive-by-additive epistasis was included in the models. Furthermore, including non-additive genetic effects did not improve the prediction accuracy of genomic breeding values, but there was animal re-ranking across the models. For the heat tolerance indicators recorded in a crossbred population, most traits had small non-additive genetic variance with large standard error estimates. Nevertheless, panting score and hair density presented substantial additive-by-additive epistatic variance. Panting score had an epistatic variance estimate of 0.1379, which accounted for 82.22% of the total genetic variance. For hair density, the epistatic variance estimates ranged from 0.1745 to 0.1845, which represent 64.95-69.59% of the total genetic variance.

CONCLUSIONS

Including non-additive genetic effects in the models did not improve the accuracy of genomic breeding values for performance traits in purebred pigs, but there was substantial re-ranking of selection candidates depending on the model fitted. Except for panting score and hair density, low non-additive genetic variance estimates were observed for heat tolerance indicators in crossbred pigs.

Does acclimation in distinct light conditions determine differences in the photosynthetic heat tolerance of coffee plants?

Worldwide coffee production is threatened by climate change, which highlights the importance of heat tolerance studies. Here we tested the hypothesis that photosynthetic heat tolerance in coffee varieties changes according to acclimation to distinct light conditions. Furthermore, we tested if heat tolerance is associated with the habitat of origin of the coffee species. We evaluated heat tolerance using chlorophyll fluorescence in varieties of Coffea arabica (Mundo Novo and Catuai Amarelo) and C. canephora (Conilon) grown in a common garden under two conditions: high (HS) and low (LS) sunlight. Leaf traits associated with leaf cooling were evaluated in plants grown in LS and HS and associations of heat tolerance with these traits were determined. The varieties tested had high photosynthetic heat tolerance, with temperatures above 54 °C leading to a 50% reduction in Fv /Fm (T50 ). The heat tolerance of each Coffea variety was unaffected by growth in distinct light conditions. Leaves of plants grown in LS were larger and had a lower fraction of the leaf area occupied by stomata (nast ). Heat tolerance was positively associated with leaf size and negatively with nast . C. canephora exhibited higher heat tolerance than C. arabica. The limited plasticity of heat tolerance in response to acclimation under distinct light conditions contradicts the prediction that plants acclimated to HS would have higher photosynthetic heat tolerance than those acclimated to LS. Our results on heat tolerance among Coffea species/varieties in HS and LS indicate the possibility of selection of varieties for better acclimation to ongoing climate changes.

HoTDAM! An easy-to-use automated assay expands the inducible thermotolerance phenotype in Drosophila melanogaster: Heat hardening reduces motility

To quantify heat tolerance in insects, two manual observation measures are typically implemented: the time to physiological collapse at a static noxious temperature (time to knockdown; TKD) or the temperature at which collapse occurs as temperature increases (critical thermal maximum; CTmax). Both assay modalities focus on physiological collapse, neglecting the prior behavioral processes. In this study, the locomotion response of Drosophila melanogaster to relatively high temperature (39 and 40.5 °C) was quantified using the TriKinetics Drosophila Activity Monitor (DAM2 system). The absence of locomotion was defined as the state of physiological collapse resulting from extended exposure to high temperature. An easy-to-use executable application that allows the user to automatically extract individual TKD from the activity data was developed. For validation, manual TKD assays were performed in parallel to automated assays across multiple factors, including sex, hardening, recovery time after hardening, and assay temperature, which gave similar results. In terms of behavioral aspects, heat hardening consistently led to reduced activity during a subsequent heat stress, irrespective of assay temperature, sex, or recovery time after hardening. Our automated heat tolerance assay utilizing the DAM2 system is one way to expand the scope of the heat tolerance phenotype to include a behavioral component in conjunction with the traditional TKD measure.

Heat-evolved algal symbionts enhance bleaching tolerance of adult corals without trade-off against growth

Ocean warming has caused coral mass bleaching and mortality worldwide and the persistence of symbiotic reef-building corals requires rapid acclimation or adaptation. Experimental evolution of the coral's microalgal symbionts followed by their introduction into coral is one potential method to enhance coral thermotolerance. Heat-evolved microalgal symbionts of the generalist species, Cladocopium proliferum (strain SS8), were exposed to elevated temperature (31°C) for ~10 years, and were introduced into four genotypes of chemically bleached adult fragments of the scleractinian coral, Galaxea fascicularis. Two of the four coral genotypes acquired SS8. The new symbionts persisted for the 5 months of the experiment and enhanced adult coral thermotolerance, compared with corals that were inoculated with the wild-type C. proliferum strain. Thermotolerance of SS8-corals was similar to that of coral fragments from the same colony hosting the homologous symbiont, Durusdinium sp., which is naturally heat tolerant. However, SS8-coral fragments exhibited faster growth and recovered cell density and photochemical efficiency more quickly following chemical bleaching and inoculation under ambient temperature relative to Durusdinium-corals. Mass spectrometry imaging suggests that algal pigments involved in photobiology and oxidative stress were the greatest contributors to the thermotolerance differences between coral hosting heat-evolved versus wild-type C. proliferum. These pigments may have increased photoprotection in the heat-evolved symbionts. This is the first laboratory study to show that thermotolerance of adult corals (G. fascicularis) can be enhanced via the uptake of exogenously supplied, heat-evolved symbionts, without a trade-off against growth under ambient temperature. Importantly, heat-evolved C. proliferum remained in the corals in moderate abundance 2 years after first inoculation, suggesting long-term stability of this novel symbiosis and potential long-term benefits to coral thermotolerance.

Assessing the contribution of bacteria to the heat tolerance of experimentally evolved coral photosymbionts

Coral reefs are extremely vulnerable to ocean warming, which triggers coral bleaching-the loss of endosymbiotic microalgae (Symbiodiniaceae) from coral tissues, often leading to death. To enhance coral climate resilience, the symbiont, Cladocopium proliferum was experimentally evolved for >10 years under elevated temperatures resulting in increased heat tolerance. Bacterial 16S rRNA gene metabarcoding showed the composition of intra- and extracellular bacterial communities of heat-evolved strains was significantly different from that of wild-type strains, suggesting bacteria responded to elevated temperatures, and may even play a role in C. proliferum thermal tolerance. To assess whether microbiome transplantation could enhance heat tolerance of the sensitive wild-type C. proliferum, we transplanted bacterial communities from heat-evolved to the wild-type strain and subjected it to acute heat stress. Microbiome transplantation resulted in the incorporation of only 30 low-abundance strains into the microbiome of wild-type cultures, while the relative abundance of 14 pre-existing strains doubled in inoculated versus uninoculated samples. Inoculation with either wild-type or heat-evolved bacterial communities boosted C. proliferum growth, although no difference in heat tolerance was observed between the two inoculation treatments. This study provides evidence that Symbiodiniaceae-associated bacterial communities respond to heat selection and may contribute to coral adaptation to climate change.

Heat stress adaptation in cows - Physiological responses and underlying molecular mechanisms

Heat stress is a key abiotic stressor for dairy production in the tropics which is further compounded by the ongoing climate change. Heat stress not only adversely impacts the production and welfare of dairy cows but severely impacts the economics of dairying due to production losses and increased cost of rearing. Over the years, selection has ensured development of high producing breeds, however, the thermotolerance ability of animals has been largely overlooked. In the past decade, the ill effects of climate change have made it pertinent to rethink the selection strategies to opt for climate resilient breeds, to ensure optimum production and reproduction. This has led to renewed interest in evaluation of the impacts of heat stress on cows and the underlying mechanisms that results in their acclimatization and adaptation to varied thermal ambience. The understanding of heat stress and associated responses at various level of animal is crucial to device amelioration strategies to secure optimum production and welfare of cows. With this review, an effort has been made to provide an overview on temperature humidity index as an important indicator of heat stress, general effect of heat stress in dairy cows, and impact of heat stress and subsequent response at physiological, haematological, molecular and genetic level of dairy cows.

PWL1, a G-type lectin receptor-like kinase, positively regulates leaf senescence and heat tolerance but negatively regulates resistance to Xanthomonas oryzae in rice

Plant leaf senescence, caused by multiple internal and environmental factors, has an important impact on agricultural production. The lectin receptor-like kinase (LecRLK) family members participate in plant development and responses to biotic and abiotic stresses, but their roles in regulating leaf senescence remain elusive. Here, we identify and characterize a rice premature withered leaf 1 (pwl1) mutant, which exhibits premature leaf senescence throughout the plant life cycle. The pwl1 mutant displayed withered and whitish leaf tips, decreased chlorophyll content, and accelerated chloroplast degradation. Map-based cloning revealed an amino acid substitution (Gly412Arg) in LOC_Os03g62180 (PWL1) was responsible for the phenotypes of pwl1. The expression of PWL1 was detected in all tissues, but predominantly in tillering and mature leaves. PWL1 encodes a G-type LecRLK with active kinase and autophosphorylation activities. PWL1 is localized to the plasma membrane and can self-associate, mainly mediated by the plasminogen-apple-nematode (PAN) domain. Substitution of the PAN domain significantly diminished the self-interaction of PWL1. Moreover, the pwl1 mutant showed enhanced reactive oxygen species (ROS) accumulation, cell death, and severe DNA fragmentation. RNA sequencing analysis revealed that PWL1 was involved in the regulation of multiple biological processes, like carbon metabolism, ribosome, and peroxisome pathways. Meanwhile, interfering of biological processes induced by the PWL1 mutation also enhanced heat sensitivity and resistance to bacterial blight and bacterial leaf streak with excessive accumulation of ROS and impaired chloroplast development in rice. Natural variation analysis indicated more variations in indica varieties, and the vast majority of japonica varieties harbour the PWL1Hap1 allele. Together, our results suggest that PWL1, a member of LecRLKs, exerts multiple roles in regulating plant growth and development, heat-tolerance, and resistance to bacterial pathogens.

Exploring the Role of TaPLC1-2B in Heat Tolerance at Seedling and Adult Stages of Wheat through Transcriptome Analysis

Heat stress is a major abiotic stress that can cause serious losses of a crop. Our previous work identified a gene involved in heat stress tolerance in wheat, TaPLC1-2B. To further investigate its mechanisms, in the present study, TaPLC1-2B RNAi-silenced transgenic wheat and the wild type were comparatively analyzed at both the seedling and adult stages, with or without heat stress, using transcriptome sequencing. A total of 15,549 differentially expressed genes (DEGs) were identified at the adult stage and 20,535 DEGs were detected at the seedling stage. After heat stress, an enrichment of pathways such as phytohormones and mitogen-activated protein kinase signaling was mainly found in the seedling stage, and pathways related to metabolism, glycerophospholipid metabolism, circadian rhythms, and ABC transporter were enriched in the adult stage. Auxin and abscisic acid were downregulated in the seedling stage and vice versa in the adult stage; and the MYB, WRKY, and no apical meristem gene families were downregulated in the seedling stage in response to heat stress and upregulated in the adult stage in response to heat stress. This study deepens our understanding of the mechanisms of TaPLC1-2B in regard to heat stress in wheat at the seedling and adult stages.

Implication of amino acid metabolism and cell surface integrity for the thermotolerance mechanism in the thermally adapted acetic acid bacterium Acetobacter pasteurianus TH-3

IMPORTANCE

Acetobacter pasteurianus, an industrial vinegar-producing strain, is suffered by fermentation stress such as fermentation heat and/or high concentrations of acetic acid. By an experimental evolution approach, we have obtained a stress-tolerant strain, exhibiting significantly increased growth and acetic acid fermentation ability at higher temperatures. In this study, we report that only the three gene mutations of ones accumulated during the adaptation process, ansP, dctD, and glnD, were sufficient to reproduce the increased thermotolerance of A. pasteurianus. These mutations resulted in cell envelope modification, including increased phospholipid and lipopolysaccharide synthesis, increased respiratory activity, and cell size reduction. The phenotypic changes may cooperatively work to make the adapted cell thermotolerant by enhancing cell surface integrity, nutrient or oxygen availability, and energy generation.

Investigating desiccation resistance, post-rehydration growth, and heat tolerance in desiccation-injured cells of Salmonella enterica isolated from the soybean production chain

This study compared the resistance to different desiccation conditions of 190 Salmonella enterica strains previously isolated from the soybean meal production chain and belonging to 23 serovars. Additionally, the post-rehydration growth and heat tolerance of the strains previously exposed to desiccation were determined. Variability in desiccation resistance was observed both within and between serovars. Strains belonging to S. Havana and S. Schwarzengrund serovars were the most resistant, regardless of storage condition. The drying temperature (20 °C and 30 °C) did not influence the desiccation resistance of the Salmonella strains. On the other hand, increasing drying time from 1 to 7 days reduced Salmonella counts. The origin (isolation sources) also influenced the desiccation resistance of the Salmonella strains. The growth of the Salmonella strains after rehydration varied considerably depending on the drying conditions and incubation temperature during cultivation. An increase in the time and temperature of drying led to a reduction in population of most Salmonella strains after rehydration. Salmonella strains previously desiccated also showed differences in the heat tolerance in all temperature-time binomials tested. Some strains were highly resistant to heat tolerance conditions, presenting <1 log CFU/mL reduction from the initial population. The results obtained in this study suggest that the strategies to mitigate Salmonella in low-aw foods must consider the existence of high-stress resistant strains and their multiple-stress adaptability profiles, including effects of processing, food composition, and storage conditions.

Acclimation response and management strategies to combat heat stress in wheat for sustainable agriculture: A state-of-the-art review

Unpredicted variability in climate change on the planet is associated with frequent extreme high-temperature events impacting crop yield globally. Wheat is an economically and nutritionally important crop that fulfils global food requirements and each degree rise in temperature results in ∼6% of its yield reduction. Thus, understanding the impact of climate change, especially the terminal heat stress on global wheat production, becomes critically important for policymakers, crop breeders, researchers and scientists to ensure global food security. This review describes how wheat perceives heat stress and induces stress adaptation events by its morpho-physiological, phenological, molecular, and biochemical makeup. Temperature above a threshold level in crop vicinity leads to irreversible injuries, viz. destruction of cellular membranes and enzymes, generation of active oxygen species, redox imbalance, etc. To cope with these changes, wheat activates its heat tolerance mechanisms characterized by hoarding up soluble carbohydrates, signalling molecules, and heat tolerance gene expressions. Being vulnerable to heat stress, increasing wheat production without delay seeks strategies to mitigate the detrimental effects and provoke the methods for its sustainable development. Thus, to ensure the crop's resilience to stress and increasing food demand, this article circumscribes the integrated management approaches to enhance wheat's performance and adaptive capacity besides its alleviating risks of increasing temperature anticipated with climate change. Implementing these integrated strategies in the face of risks from rising temperatures will assist us in producing sustainable wheat with improved yield.

Enhanced heat tolerance of freeze-dried Enterococcus faecium NRRL B-2354 as valid Salmonella surrogate in low-moisture foods

In microbial studies of low-moisture foods (LMFs, water activity less than 0.85), freeze-dried bacteria benefit us to inoculate LMFs without introducing extra water or altering food physiochemical properties. However, the freeze-drying process would bring unavoidable damage to bacterial cells and results in less-resistant inoculum that are unlikely to be qualified in microbial studies. Herein, we enhanced bacterial heat tolerance by subjecting the cells to mild heat (42-50 °C) to counteract the reduced heat tolerance and survivability of freeze-dried bacteria. Enterococcus faecium NRRL B-2354 (E. faecium), a Salmonella surrogate in LMFs, was used as the target microorganism because it was widely accepted in microbial validation of thermal pasteurizing LMFs. Three types of LMFs (peanut powder, protein powder, and onion powder) were used as LMFs models to validate the freeze-dried E. faecium in comparison with Salmonella enterica Enteritidis PT 30 (S. Enteritidis) prepared by the traditional aqueous method. The heat tolerance (D65℃ value) of E. faecium increased at all treatments and peaked (+31.48 ± 0.13%) at temperature-time combinations of 45 °C-60 min and 50 °C-5 min. Survivability of freeze-dried inoculum and its heat tolerance retained well within 50 d storage. The freeze-dried E. faecium was prepared in this study brought equal or higher heat tolerance (D85℃ or D75℃) than S. Enteritidis in tested LMFs models. For instance, the D85℃ of freeze-dried E. faecium (heat-treated at 50 °C for 5 min) and S. Enteritidis in whole egg powder are 35.56 ± 1.52 min and 28.41 ± 0.41 min, respectively. The freeze-dried E. faecium with enhanced heat tolerance appears to be a suitable Salmonella surrogate for dry-inoculating LMFs. Our protocol also enables industry-scale production of freeze-dried inoculum by broth-cultivation method combined with mild-heat treatment.

Evaluation of species thermal sensitivity with individual-based physiological performance

Ignoring intraspecific variations can prevent us from accurately assessing species' thermal sensitivity to global warming. Individual-based physiological performance provides a feasible solution to depict species' thermal sensitivity using a bottom-up approach. We measured the cardiac performance of intertidal bivalves (1159 individuals from multiple populations of six bivalves), determined the upper thermal limit of each individual, calculated the proportions of individuals suffering sublethal/lethal heat stress, and mapped sensitive regions to high temperatures. Results showed that high inter-individual variations of physiological performance existed in levels of populations and species, and species' thermal sensitivity was positively related to the intraspecific variations of heat tolerance. This bottom-up approach scaled up from individual, population to species emphasizes the importance of individual-based physiology performance in assessing thermal sensitivity across different hierarchical levels and enables better evaluating and forecasting of species responses to global warming.

Transgenic strategies to improve the thermotolerance of photosynthesis

Warming driven by the accumulation of greenhouse gases in the atmosphere is irreversible over at least the next century, unless practical technologies are rapidly developed and deployed at scale to remove and sequester carbon dioxide from the atmosphere. Accepting this reality highlights the central importance for crop agriculture to develop adaptation strategies for a warmer future. While nearly all processes in plants are impacted by above optimum temperatures, the impact of heat stress on photosynthetic processes stand out for their centrality. Here, we review transgenic strategies that show promise in improving the high-temperature tolerance of specific subprocesses of photosynthesis and in some cases have already been shown in proof of concept in field experiments to protect yield from high temperature-induced losses. We also highlight other manipulations to photosynthetic processes for which full proof of concept is still lacking but we contend warrant further attention. Warming that has already occurred over the past several decades has had detrimental impacts on crop production in many parts of the world. Declining productivity presages a rapidly developing global crisis in food security particularly in low income countries. Transgenic manipulation of photosynthesis to engineer greater high-temperature resilience holds encouraging promise to help meet this challenge.

Temporal variation of thermal sensitivity to global warming: Acclimatization in the guitarist beetle, Megelenophorus americanus (Coleoptera: Tenebrionidae) from the Monte Desert

Global warming is a major threat to biodiversity, the increase in mean temperature plus the higher rate and intensity of heat waves can severely affect organisms by exposing them to temperatures beyond their tolerance limits. Desert ectotherms are particularly vulnerable due to their dependence on environmental temperatures in an extreme habitat. Thermal tolerance changes depending on environmental conditions, studying these fluctuations provides a better understanding of species susceptibility to global warming. Tenebrionids are successful desert-inhabiting ectotherm taxa because of a series of adaptations for heat tolerance and water loss. We studied the seasonal variation (acclimatization) of thermal tolerance in Megelenophorus americanus, a widely distributed species in the Monte Desert (Argentina). To do this, we measured environmental and operative temperatures: body temperature (Tb), soil temperature (Ts), air temperature (Ta), environmental temperature (Te) and maximum temperature (Tmax), and tolerance proxies volunteer thermal maximum (VTmax), Fluid release (FR) and critical thermal maximum (CTmax) in a population of M. americanus from San Juan province, Argentina from October to March (full activity season). We found that Ts and Ta are accurate predictors of Tb, suggesting thermoconformism. All tolerance proxies showed differences among months, suggesting a natural acclimatization process in situ. Insects were found operating beyond VTmax (thermal stress) but they were far from reaching CTmax under natural conditions. Organisms present different degrees of tolerance plasticity that should be considered when predicting potential impacts of climate change.