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Haloxylon ammodendron, an important shrub utilized for afforestation in desert areas, can withstand harsh ecological conditions such as drought, high salt and extreme heat. A better understanding of the stress adaptation mechanisms of H. ammodendron is vital for ecological improvement in desert areas. In this study, the role of the H. ammodendron 14-3-3 protein HaFT-1 in thermotolerance was investigated. qRT-PCR analysis showed that heat stress (HS) priming (the first HS) enhanced the expression of HaFT-1 during the second HS and subsequent recovery phase. The subcellular localization of YFP-HaFT-1 fusion protein was mainly detected in cytoplasm. HaFT-1 overexpression increased the germination rate of transgenic Arabidopsis seeds, and the survival rate of HaFT-1 overexpression seedlings was higher than that of wild-type (WT) Arabidopsis after priming-and-triggering and non-primed control treatments. Cell death staining showed that HaFT-1 overexpression lines exhibited significantly reduced cell death during HS compared to WT. Transcriptome analysis showed that genes associated with energy generation, protein metabolism, proline metabolism, autophagy, chlorophyll metabolism and reactive oxygen species (ROS) scavenging were important to the thermotolerance of HS-primed HaFT-1 transgenic plants. Growth physiology analysis indicated that priming-and-triggering treatment of Arabidopsis seedlings overexpressing HaFT-1 increased proline content and strengthened ROS scavenging activity. These results demonstrated that overexpression of HaFT-1 increased not only HS priming but also tolerance to the second HS of transgenic Arabidopsis, suggesting that HaFT-1 is a positive regulator in acquired thermotolerance. Key message: Over-expression of HaFT-1 enhances heat stress priming and tolerance to the second HS in transgenic Arabidopsis, suggesting HaFT-1 as a positive regulator in acquired thermotolerance. [ABSTRACT FROM AUTHOR]

Acquired tolerance to heat stress is an increased resistance to elevated temperature following a prior exposure to heat. Acquired thermotolerance can significantly increase bivalve's heat adaptation capacity while leaving its basal thermotolerance unchanged. However, its characteristics are not yet fully understood, which limit its potential for utilization in enhancing heat tolerance. This study systematically characterizes the acquired thermotolerance of adult Fujian oyster by observing the effects of different priming temperatures (34 °C, 36 °C, 38 °C, 40 °C, and 42 °C with 1 h), recovery temperatures (16 °C, 20 °C, 24 °C, 28 °C, and 32 °C with 72 h) and durations (0 h, 4 h, 24 h, 3 days, 7 days, 14 days, 21 days, 28 days, 35 days, and 40 days under 20 °C) on the survival rates and the expression of heat shock protein genes (HSP10, HSP27, HSP68 and HSP90) in gill. Results indicated that a priming temperature of 34 °C did not induce tolerance to lethal heat stress. However, thermotolerance increased with increasing priming temperature in the range of 36 °C to 40 °C. In addition, it was found that acquired thermotolerance can be achieved with a lower recovery temperature, but can be reduced by a high recovery temperature (> 24 °C). It was also observed that a recovery durations of 0 h and 4 h are insufficient to induce thermotolerance. The maximum duration of acquired thermotolerance is 40 days in the Fujian oyster and that a high level of thermotolerance, i.e. > 75% survival after lethal stimulus, can be maintained for 21 days. As the recovery duration increased from 24 h to 40 days, the expression of four HSP genes decreased with various speed and extent. HSP68 decreased the fastest, while HSP27 and HSP90 decreased moderately. HSP10 decreased the slowest, and its expression was consistently higher than that of the control group. Furthermore, the expression of these four genes at 72 h after the lethal stimulus increased with the extension of recovery duration. This study presents a systematic analysis of the acquired thermotolerance of the Fujian oyster. The findings expand our understanding of oyster thermotolerance and provide a potential strategy to enhance the thermotolerance of oysters by altering the external environment. For example, the temperature could be artificially adjusted during the transfer of spat and intermediates, or the water temperatures could be adjusted by adjusting the water layer in which the oysters are cultured at sea. [ABSTRACT FROM AUTHOR]

Durum wheat is strongly affected by climatic constraints such as high temperatures and drought, which frequently lead to yield reduction. Damages due to high temperatures are related to plant thermotolerance, a trait determined by two components: basal and acquired thermotolerance. In this study, the effect of drought and heat stress imposed singularly or sequentially was investigated in ten durum wheat cultivars (cvs) at the physiological and molecular level. The traits analyzed were cell membrane stability, relative water content, proline content, and expression level of several genes for heat shock proteins (HSPs). Our results indicate that drought priming can induce the acquisition of thermotolerance in most cultivars already classified as able to acquire thermotolerance by heat pre-treatment. Proline accumulation was correlated to cell membrane stability, meaning that the most thermotolerant cvs were able to accumulate higher levels of proline. Acquired thermotolerance is also due to the activation of HSP gene expression; similarly, pre-treatment with water stress was able to activate HSPs expression. The results reported indicate that water stress plays an important role in inducing thermotolerance, comparable to mild heat stress pre-treatment. This is the first report on the effect of drought stress on the acquisition of thermotolerance. [ABSTRACT FROM AUTHOR]

Global warming is predicted to alter temperatures thereby leading to a significant loss in crop productivity and yield. One target of global warming is plant sexual reproduction, which will be deeply threatened by high temperatures because the male gametophyte (the pollen) is particularly susceptible to heat stress. As a reaction to the latter, pollen enacts many responses that involve specific reprogramming of genetic, metabolic, and cellular activities. Most published works focus on a single episode of heat stress but less is known about the effects of heat pre-exposure (priming) on acquired thermotolerance (ATT). The purpose of this review is to gain a deeper understanding of ATT mechanisms in pollen and pollen tube, primarily from available literature. In particular, we will examine many facets (cell wall, cytoskeleton, ion fluxes, reactive oxygen species, etc.) that could be both targets of heat stress and active responses to priming.

Heat stress events are major factors limiting crop productivity. During summer days, land plants must anticipate in a timely manner upcoming mild and severe temperature. They respond by accumulating protective heat-shock proteins (HSPs), conferring acquired thermotolerance. All organisms synthetize HSPs; many of which are members of the conserved chaperones families. This review describes recent advances in plant temperature sensing, signaling, and response. We highlight the pathway from heat perception by the plasma membrane through calcium channels, such as cyclic nucleotide-gated channels, to the activation of the heat-shock transcription factors (HSFs). An unclear cellular signal activates HSFs, which act as essential regulators. In particular, the HSFA subfamily can bind heat shock elements in HSP promoters and could mediate the dissociation of bound histones, leading to HSPs transcription. Although plants can modulate their transcriptome, proteome, and metabolome to protect the cellular machinery, HSP chaperones prevent, use, and revert the formation of misfolded proteins, thereby avoiding heat-induced cell death. Remarkably, the HSP20 family is mostly tightly repressed at low temperature, suggesting that a costly mechanism can become detrimental under unnecessary conditions. Here, the role of HSP20s in response to HS and their possible deleterious expression at non-HS temperatures is discussed.

Heat stress events are major factors limiting crop productivity. During summer days, land plants must anticipate in a timely manner upcoming mild and severe temperature. They respond by accumulating protective heat-shock proteins (HSPs), conferring acquired thermotolerance. All organisms synthetize HSPs; many of which are members of the conserved chaperones families. This review describes recent advances in plant temperature sensing, signaling, and response. We highlight the pathway from heat perception by the plasma membrane through calcium channels, such as cyclic nucleotide-gated channels, to the activation of the heat-shock transcription factors (HSFs). An unclear cellular signal activates HSFs, which act as essential regulators. In particular, the HSFA subfamily can bind heat shock elements in HSP promoters and could mediate the dissociation of bound histones, leading to HSPs transcription. Although plants can modulate their transcriptome, proteome, and metabolome to protect the cellular machinery, HSP chaperones prevent, use, and revert the formation of misfolded proteins, thereby avoiding heat-induced cell death. Remarkably, the HSP20 family is mostly tightly repressed at low temperature, suggesting that a costly mechanism can become detrimental under unnecessary conditions. Here, the role of HSP20s in response to HS and their possible deleterious expression at non-HS temperatures is discussed. [ABSTRACT FROM AUTHOR]

Plants can obtain superinduction of defense against unpredictable challenges based on prior acclimation, but the mechanisms involved in the acclimation memory are little known. The objective of this study was to characterize mechanisms of heat acclimation memory in Rhododendron hainanense, a thermotolerant wild species of azalea. Pretreatment of a 2-d recovery (25/18°C, day/night) after heat acclimation (37°C, 1 h) (AR-pt) did not weaken but enhanced acquired thermotolerance in R. hainanense with less damaged phenotype, net photosynthetic rate, and membrane stability than non-acclimation pretreated (NA-pt) plants. Combined transcriptome and proteome analysis revealed that a lot of heat-responsive genes still maintained high protein abundance rather than transcript level after the 2-d recovery. Photosynthesis-related genes were highly enriched and most decreased under heat stress (HS: 42°C, 1 h) with a less degree in AR-pt plants compared to NA-pt. Sustainably accumulated chloroplast-localized heat shock proteins (HSPs), Rubisco activase 1 (RCA1), beta-subunit of chaperonin-60 (CPN60β), and plastid transcriptionally active chromosome 5 (pTAC5) in the recovery period probably provided equipped protection of AR-pt plants against the subsequent HS, with less damaged photochemical efficiency and chloroplast structure. In addition, significant higher levels of RCA1 transcripts in AR-pt compared to NA-pt plants in early stage of HS showed a more important role of RCA1 than other chaperonins in heat acclimation memory. The novel heat-induced RCA1, rather than constitutively expressed RCA2 and RCA3, showed excellent thermostability after long-term HS (LHS: 42/35°C, 7 d) and maintained balanced Rubisco activation state in photosynthetic acclimation. This study provides new insights into plant heat acclimation memory and indicates candidate genes for genetic modification and molecular breeding in thermotolerance improvement.

Plants can obtain superinduction of defense against unpredictable challenges based on prior acclimation, but the mechanisms involved in the acclimation memory are little known. The objective of this study was to characterize mechanisms of heat acclimation memory in Rhododendron hainanense , a thermotolerant wild species of azalea. Pretreatment of a 2-d recovery (25/18°C, day/night) after heat acclimation (37°C, 1 h) (AR-pt) did not weaken but enhanced acquired thermotolerance in R. hainanense with less damaged phenotype, net photosynthetic rate, and membrane stability than non-acclimation pretreated (NA-pt) plants. Combined transcriptome and proteome analysis revealed that a lot of heat-responsive genes still maintained high protein abundance rather than transcript level after the 2-d recovery. Photosynthesis-related genes were highly enriched and most decreased under heat stress (HS: 42°C, 1 h) with a less degree in AR-pt plants compared to NA-pt. Sustainably accumulated chloroplast-localized heat shock proteins (HSPs), Rubisco activase 1 (RCA1), beta-subunit of chaperonin-60 (CPN60β), and plastid transcriptionally active chromosome 5 (pTAC5) in the recovery period probably provided equipped protection of AR-pt plants against the subsequent HS, with less damaged photochemical efficiency and chloroplast structure. In addition, significant higher levels of RCA1 transcripts in AR-pt compared to NA-pt plants in early stage of HS showed a more important role of RCA1 than other chaperonins in heat acclimation memory. The novel heat-induced RCA1, rather than constitutively expressed RCA2 and RCA3, showed excellent thermostability after long-term HS (LHS: 42/35°C, 7 d) and maintained balanced Rubisco activation state in photosynthetic acclimation. This study provides new insights into plant heat acclimation memory and indicates candidate genes for genetic modification and molecular breeding in thermotolerance improvement. [ABSTRACT FROM AUTHOR]

Greengram is an important protein-rich food legume crop. During the reproductive stage, high temperatures cause flower drop, induce male sterility, impair anthesis, and shortens the grain-filling period. Initially, 116 genotypes were evaluated for 3 years in two locations, and based on flowering, biomass, and yield attributes, they were grouped into four major clusters. A panel of 17 contrasting genotypes was selected for their heat tolerance in high-temperature greenhouses. The seedlings of the selected genotypes were exposed to heat shock in the range 37°C–52°C and their recovery after heat shock was assessed at 30°C. The seedlings of EC 398889 turned completely green and rejuvenated, while those of LGG 460 failed to recover, therefore, EC 398889 and LGG 460 were identified as heat-tolerant and heat-sensitive genotypes, respectively. Except for EC 398889, the remaining genotypes could not survive after heat shock. Fresh seeds of EC 398889 and LGG 460 were planted in field and pollen fertility and sucrose-synthase (SuSy) activity in grains were assessed at high temperatures. The pollen germination and SuSy activity were normal even at temperatures beyond 40°C in EC 398889 and high SuSy activity enabled faster grain filling than in LGG 460. The precise phenotyping demonstrated significant differences in the light-temperature response of photosynthesis, chlorophyll fluorescence imaging of quantum yield (Fv/Fm), and electron transport rate (ETR) between heat-tolerant (EC 398889) and heat-sensitive (LGG 460) genotypes. Molecular profiling of selected accessions showed polymorphism with 11 SSR markers and the markers CEDG147, CEDG247, and CEDG044 distinguished tolerant and sensitive groups of accessions.

High temperature is one of the most devastating environmental factors that severely impede plant growth multi-laterally and threatens global food security. Global warming and the predicted steady rise in temperature emphasize the urgent need to improve heat stress resilience of crop plants to meet the growing food demand. Although known for several years, a memory-based mechanism termed "priming-induced stress tolerance" or "acquired stress tolerance" has gained the attention of the plant science community in recent years. Thermopriming is one such phenomenon that enhances the plant tolerance to subsequent heat stress and promotes growth. The memory of the foregoing mild increase in temperature is captured and retained as dormant signals, which upon exposure to subsequent high temperature aids in mounting a faster, stronger, and sensitized response. Such acquired thermotolerance is more effective than the basal endurance of the plant due to altered molecular regulatory networks. Thereupon, thermopriming can be used as a convenient tool to study and improve plant response to heat stress. In this chapter, the protocol to study thermopriming-mediated short- and long-term acquired thermotolerance is described in detail., (© 2024. The Author(s), under exclusive license to Springer Science+Business Media, LLC, part of Springer Nature.)

Greengram is an important protein-rich food legume crop. During the reproductive stage, high temperatures cause flower drop, induce male sterility, impair anthesis, and shortens the grain-filling period. Initially, 116 genotypes were evaluated for 3 years in two locations, and based on flowering, biomass, and yield attributes, they were grouped into four major clusters. A panel of 17 contrasting genotypes was selected for their heat tolerance in high-temperature greenhouses. The seedlings of the selected genotypes were exposed to heat shock in the range 37°C–52°C and their recovery after heat shock was assessed at 30°C. The seedlings of EC 398889 turned completely green and rejuvenated, while those of LGG 460 failed to recover, therefore, EC 398889 and LGG 460 were identified as heat-tolerant and heat-sensitive genotypes, respectively. Except for EC 398889, the remaining genotypes could not survive after heat shock. Fresh seeds of EC 398889 and LGG 460 were planted in field and pollen fertility and sucrose-synthase (SuSy) activity in grains were assessed at high temperatures. The pollen germination and SuSy activity were normal even at temperatures beyond 40°C in EC 398889 and high SuSy activity enabled faster grain filling than in LGG 460. The precise phenotyping demonstrated significant differences in the light-temperature response of photosynthesis, chlorophyll fluorescence imaging of quantum yield (Fv/Fm), and electron transport rate (ETR) between heat-tolerant (EC 398889) and heat-sensitive (LGG 460) genotypes. Molecular profiling of selected accessions showed polymorphism with 11 SSR markers and the markers CEDG147, CEDG247, and CEDG044 distinguished tolerant and sensitive groups of accessions. [ABSTRACT FROM AUTHOR]

In plants, most DREB1s/CBFs proteins with a single AP2/ERF domain participate in responses to abiotic stresses. Here, we cloned a DREB subfamily gene from Lilium longiflorum (designated as LlDREB1G). Sequence analyses revealed that it is composed of 219 amino acids, and contains an AP2/ERF domain, a putative nuclear localization signal (NLS), a DSAW/R and LWSY motif. LlDREB1G, which was localized to the nucleus, showed strongly induced expression under low temperature, and was also stimulated by mechanical agitation, heat shock, ABA and H2O2 stresses. Moreover, the LlDREB1G gene had an obvious circadian rhythm and organ specificity in expression. The LlDREB1G transcription factor was functional in the heterologous system (Arabidopsis). Transgenic Arabidopsis plants with higher transcription of LlDREB1G gene exhibited increased acquired thermotolerance and resistance to freezing and dehydration stresses. Besides, the transgenic seeds were insensitive to exogenous ABA treatment than those of the wild type (WT) in germination experiments. Compared with the WT seedlings, the seedlings of transgenic Arabidopsis overexpressing LlDREB1G showed strongly upregulated expression of stress-related genes after cold, drought or acclimation to heat shock treatments. Furthermore, overexpression of LlDREB1G increased the proline levels and survival rates, but decreased the electrolyte leakage and H2O2 accumulation in the transgenic plants after freezing, dehydration and high temperature treatments. These results imply that LlDREB1G gene may be involved in the responses to multiple abiotic stresses in an ABA-dependent and -independent manner. Key message: LlDREB1G gene, a DREB subfamily gene from Lilium longiflorum, may be involved in the responses to multiple abiotic stresses in an ABA-dependent and -independent manner. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The heat stress priming and acclimation system for acquired and maintenance of acquired thermotolerance of tomato have been established. • Antioxidant enzymes act as indicators predicting intension of acquired and maintenance of acquired thermotolerance. • The relationships between basal, acquired and maintenance of acquired thermotolerance of tomato are illuminated. Abstract Heat stress seriously threats the crop production. Understanding different types of thermotolerance and illustrating the relationship of these types were essential. The activities of different antioxidant enzymes were determined after heat priming and acclimation for a heat resistant (R) and sensitive tomato genotype. We optimized the treatments for priming in acquired thermotolerance (AT) and acclimation in maintenance of acquired thermotolerance (MT). Then the intension of basal thermotolerance (BT), AT and MT were tested by using lethal temperature (45 °C). The peak of antioxidant enzyme activity (AEA) was identity with the priming time, indicating AEA can predict the optimal-priming time of AT and MT. Moderate high-temperature induced stronger thermotolerance than warmer high-temperature, but the priming time was longer than warmer high-temperature. Moreover, S needed longer priming time than R, due to the slowly AEA increasing and the AT of S also showed weaker than that of R. When the first-optimized-priming combined with secondary-priming for a reasonable duration, its thermotolerance further enhanced. Surprisingly, these optimized-acclimation treatments showed no difference in MT, suggesting secondary-priming could supply gaps produced by the first-optimized-priming of warmer high-temperature. In addition, R also responded the secondary-priming quickly than S, due to the quickly AEA increasing. While, after optimized-acclimations, thermotolerance of S showed no difference with R, suggesting secondary-priming can also narrow the thermotolerance-gap between S and R. Results from this study will accelerate screening of heat tolerant cultivars and provide insights into the choice of tomato cultivars in the changing temperatures environment. [ABSTRACT FROM AUTHOR]

Main conclusion: A powerful acquired thermotolerance response in potato was demonstrated and characterised in detail, showing the time course required for tolerance, the reversibility of the process and requirement for light.Potato is particularly vulnerable to increased temperature, considered to be the most important uncontrollable factor affecting growth and yield of this globally significant crop. Here, we describe an acquired thermotolerance response in potato, whereby treatment at a mildly elevated temperature primes the plant for more severe heat stress. We define the time course for acquiring thermotolerance and demonstrate that light is essential for the process. In all four commercial tetraploid cultivars that were tested, acquisition of thermotolerance by priming was required for tolerance at elevated temperature. Accessions from several wild-type species and diploid genotypes did not require priming for heat tolerance under the test conditions employed, suggesting that useful variation for this trait exists. Physiological, transcriptomic and metabolomic approaches were employed to elucidate potential mechanisms that underpin the acquisition of heat tolerance. This analysis indicated a role for cell wall modification, auxin and ethylene signalling, and chromatin remodelling in acclimatory priming resulting in reduced metabolic perturbation and delayed stress responses in acclimated plants following transfer to 40 °C. [ABSTRACT FROM AUTHOR]

Brassica juncea (also known as Indian mustard) is an important vegetable oil seed crop whose growth and productivity is severely affected by heat stress. Interestingly, pre-exposure to sublethal heat stress, referred as thermopriming can remarkably enhance the plant heat stress tolerance. However, little is known about the impact of thermopriming on heat stress response of the important oil seed crop, B. juncea. In this study, we investigated the basal and thermopriming-induced heat stress response of 16 different cultivated, agronomically important varieties of Brassica juncea. Based on their basal heat stress response, we classify the varieties as heat sensitive, moderately tolerant or tolerant. Notably, almost all the varieties displayed enhanced heat stress tolerance upon thermopriming (acquired thermotolerance), albeit to varying magnitudes. Strikingly, the high oil-yielding, drought tolerant, heat sensitive variety Pusa Bold showed remarkable acquired thermotolerance upon thermopriming. Investigations in Pusa Bold indicate that the thermopriming-induced acquired thermotolerance predominantly correlates with the rapid activation of ROS scavenging mechanisms and differential expression of Heat Shock Family of transcription factors (BjHSF s). Taken together, our study reveals the positive impact of thermopriming in alleviating the harmful effects of heat stress on Brassica juncea seedlings and paves the path to enhance heat stress tolerance for improved productivity in the important oilseed crop Indian mustard. [Display omitted] • Heat stress affects the survivability of different varieties of Indian mustard. • Thermopriming provides acquired thermotolerance even in heat-sensitive varieties. • Thermopriming regulates ROS homeostasis and the expression of heat shock factors. [ABSTRACT FROM AUTHOR]

Aldehyde dehydrogenases (ALDH) are a family of enzymes that are involved in plant metabolism and contribute to aldehyde homeostasis to eliminate toxic aldehydes. The ALDH enzymes produce NADPH and NADH in their enzymatic reactions and thus contribute to balancing redox equivalents. Previous studies showed that Arabidopsis ALDH genes are expressed in response to high salinity, dehydration, oxidative stress, or heavy metals, suggesting important roles in environmental adaptation. However, the role of ALDH genes in high temperature and stress combinations (heat stress combined with dehydration, wounding, or salt stress) is unclear. Here, we analysed expression patterns of selected ALDH genes on the transcript and protein level at different time points of heat stress, basal and acquired thermotolerance, and stress combination treatments. Our results indicate that ALDH3I1 and ALDH7B4 are strongly induced by heat stress. Higher levels of ALDH7B4 accumulated in response to dehydration-heat, heat-salt and wounding-heat combination stress than in response to single stressors. The comparison of physiological and biological parameters in T-DNA double mutants of ALDH genes and wild-type plants demonstrated that mutant lines are more sensitive to heat stress and stress combinations than wild-type plants. [ABSTRACT FROM AUTHOR]

The responses of soybean genotypes to high temperature for intrinsic tolerance was studied using Temperature Induction Response (TIR) technique in order identify the genotypes tolerant to high temperature stress. Seven soybean genotypes subjected to lethal and sub lethal temperatures showed significant variation for acquired thermotolerance. Thermotolerant genotypes ADT 1 and CoSoy1 identified by the TIR technique, demonstrated higher survival percentage, and lower growth reduction. Further the tolerant genotypes identified based on TIR also showed higher antioxidant enzymes activity implying the critical role of antioxidant in cellular thermotolerance. This clearly demonstrated that TIR can be effectively used for screening high temperature tolerance genotypes in soybean. This study also established the fact that the alternations in antioxidants during induction are vital for imparting tolerance to high temperature stress. [ABSTRACT FROM AUTHOR]

Abiotic and biotic stresses are often characterized by an induction of reactive electrophile species (RES) such as the jasmonate 12-oxo-phytodienoic acid (OPDA) or the structurally related phytoprostanes. Previously, RES oxylipins have been shown massively to induce heat-shock-response (HSR) genes including HSP101 chaperones. Moreover, jasmonates have been reported to play a role in basal thermotolerance. We show that representative HSR marker genes are strongly induced by RES oxylipins through the four master regulator transcription factors HSFA1a, b, d, and e essential for short-term adaptation to heat stress in Arabidopsis. When compared with Arabidopsis seedlings treated at the optimal acclimation temperature of 37 °C, the exogenous application of RES oxylipins at 20 °C induced a much weaker induction of HSP101 at both the gene and protein expression levels which, however, was not sufficient to confer short-term acquired thermotolerance. Moreover, jasmonate-deficient mutant lines displayed a wild-typelike HSR and were not compromised in acquiring thermotolerance. Hence, the OPDA- and RES oxylipin-induced HSR is not sufficient to protect seedlings from severe heat stress but may help plants to cope better with stresses associated with protein unfolding by inducing a battery of chaperones in the absence of heat. [ABSTRACT FROM AUTHOR]

The role of nitric oxide (NO) in thermotolerance acquired by heat acclimation (38°C) was investigated. Results showed that 38°C acclimation, on the one hand, obviously reduced hydrogen peroxide (HO) and MDA contents and ion leakage degree in rice leaves; however, on the other hand, it increased the survival of rice ( Oryza sativa L.) seedlings under 50°C heat stress. Application of nitric oxide donor, sodium nitroprusside (SNP), prior to 38°C acclimation dramatically increased the acquired thermotolerance. To elucidate the role of endogenous NO in acquired thermotolerance, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO, a specific NO scavenger) was used (scavengers are used to control the level of both exogenous and endogenous NO). Results showed that PTIO pretreatment resulted in the elimination of acquired thermotolerance induced by 38°C acclimation in rice seedlings. Nitric oxide (NO) release measurement indicated that there was indeed an abrupt elevation in the NO content in 40 min after 38°C acclimation, proving the involvement of NO in acquired thermotolerance inducement in rice seedling. [ABSTRACT FROM AUTHOR]

Acquired thermotolerance in plants refers to the ability to cope with lethal high temperatures following acclimatization at sublethal high temperatures. Acquired thermotolerance reflects an actual thermotolerance mechanism naturally occurring in plants and has been extensively used in thermotolerant line identification. In recent years, great progress has been achieved in the elucidation of biochemical, physiological, and molecular mechanisms of thermotolerance acquisition by using genomic approaches, including microarray analysis and mutation, knockout, and overexpression of related genes. Heat shock proteins (HSPs), such as Hsp101, BOBBER1, and Hsa32, have been shown to be important for inducement and maintenance of acquired thermotolerance. Downstream target genes and upstream regulation factors of HsfA2, including Hsa32, Apx2, small ubiquitin-like modifier proteins, FK506-binding proteins ROF1 (FKBP62) and ROF2 (FKBP65), and heat shock transcription factor binding protein, have been revealed to be involved in thermotolerance acquisition regulation. Moreover, the role of abscisic acid, ethylene, hydrogen peroxide, and salicylic acid in acquired thermotolerance has been demonstrated by molecular evidence from Arabidopsis mutants and transgenic lines. Most importantly, different molecular mechanisms of thermotolerance acquisition have been shown to underlie various acclimatization methods. Establishment of an experimental system similar to natural conditions is important for further exploration of natural thermotolerance mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: Plants have evolved overlapping but distinct cellular responses to different aspects of high temperature stress. These responses include basal thermotolerance, short- and long-term acquired thermotolerance, and thermotolerance to moderately high temperatures. This ‘thermotolerance diversity’ means that multiple phenotypic assays are essential for fully describing the functions of genes involved in heat stress responses. A large number of genes with potential roles in heat stress responses have been identified using genetic screens and genome wide expression studies. We examine the range of phenotypic assays that have been used to characterize thermotolerance phenotypes in both Arabidopsis and crop plants. Three major variables differentiate thermotolerance assays: (1) the heat stress regime used, (2) the developmental stage of the plants being studied, and (3) the actual phenotype which is scored. Consideration of these variables will be essential for deepening our understanding of the molecular genetics of plant thermotolerance. [Copyright &y& Elsevier]

Background and Aims Molecular-based studies of thermotolerance have rarely been performed on wild plants, although this trait is critical for summer survival. Here, we focused on thermotolerance and expression of heat shock transcription factor A2 (HSFA2) and its putative target gene (chloroplast-localized small heat shock protein, CP-sHSP) in two allied aquatic species of the genus Potamogeton (pondweeds) that differ in survival on land. Methods The degree of thermotolerance was examined using a chlorophyll bioassay to assess heat injury in plants cultivated under non- and heat-acclimation conditions. Potamogeton HSFA2 and CP-sHSP genes were identified and their heat-induction was quantified by real-time PCR. Key Results The inhibition of chlorophyll accumulation after heat stress showed that Potamogeton malaianus had a higher basal thermotolerance and developed acquired thermotolerance, whereas Potamogeton perfoliatus was heat sensitive and unable to acquire thermotolerance. We found two duplicated HSFA2 and CP-sHSP genes in each species. These genes were induced by heat shock in P. malaianus, while one HSFA2a gene was not induced in P. perfoliatus. In non-heat-acclimated plants, transcript levels of HSFA2 and CP-sHSP were transiently elevated after heat shock. In heat-acclimated plants, transcripts were continuously induced during sublethal heat shock in P. malaianus, but not in P. perfoliatus. Instead, the minimum threshold temperature for heat induction of the CP-sHSP genes was elevated in P. perfoliatus. Conclusions Our comparative study of thermotolerance showed that heat acclimation leads to species-specific changes in heat response. The development of acquired thermotolerance is beneficial for survival at extreme temperatures. However, the loss of acquired thermotolerance and plasticity in the minimum threshold temperature of heat response may be favourable for plants growing in moderate habitats with limited daily and seasonal temperature fluctuations. [ABSTRACT FROM PUBLISHER]

Heat can be one of the major abiotic stresses that adversely affect crop production worldwide at different stages of development. As field screening for heat tolerance can be inconsistent and seasonally-limited, it is important to develop a reliable protocol under controlled conditions that allows simultaneous screening of multiple genotypes. The objective of this research was to develop a straightforward laboratory protocol using acquired thermotolerance (ATT) in peanut seedlings as a measure of one mechanism of heat stress tolerance. Sixteen genotypes, including selected accessions of the US peanut minicore collection along with standard checks, were evaluated for acquired themotolerance in two independent experiments. A change in the temperature sensitivity of chlorophyll accumulation was used as an indicator of acquired thermotolerance. Pre-incubation at 38°C for 4 h before the 30-min 50°C challenge triggered the acquired thermotolerance system of the leaf disks, resulting in chlorophyll accumulation upon exposure to light. There was considerable variation among genotypes for ATT in both experiments. Genotypic ranking for mean ATT values were highly correlated (0.949) in both experiments. The effect of seed weight on ATT was not significant. This method is relatively simple and inexpensive and can be used to screen a large number of genotypes. [ABSTRACT FROM AUTHOR]

The non-protein amino acid beta-aminobutyric acid (BABA) enhances Arabidopsis resistance to microbial pathogens and abiotic stresses through potentiation of the Arabidopsis defence responses. In this study, it is shown that BABA induces the stress-induced morphogenic response (SIMR). SIMR is observed in plants exposed to sub-lethal stress conditions. Anthocyanin, a known modulator of stress signalling, was also found to accumulate in BABA-treated Arabidopsis. These data and a previous microarray study indicate that BABA induces a stress response in Arabidopsis. High concentrations of amino acids, except for L-glutamine, cause a general amino acid stress inhibition. General amino acid inhibition is prevented by the addition of L-glutamine. L-Glutamine was found to inhibit the BABA-mediated SIMR and anthocyanin accumulation, suggesting that the non-protein amino acid BABA causes a general amino acid stress inhibition in Arabidopsis. L-Glutamine also blocked BABA-induced resistance to heat stress and to the virulent bacterial pathogen Pseudomonas syringae pv. tomato DC3000. During bacterial infection, priming of the salicylic acid-dependent defence marker PR1 was abolished by L-glutamine treatment. These results indicate that L-glutamine removal of the BABA-mediated stress response is concomitant with L-glutamine inhibition of BABA priming and BABA-induced resistance. [ABSTRACT FROM PUBLISHER]

Abstract: We previously isolated a combination of high-light and heat-shock stress (HL+HS)-inducible genes, including a suppressor of the G2 allele of skp1 (Sgt1a) from Arabidopsis. Arabidopsis contains two Sgt1 genes (Sgt1a and Sgt1b). Here we explored the physiological function of Arabidopsis Sgt1a or Sgt1b under stressful conditions. The transcript and protein levels of Sgt1a markedly increased under not only HL+HS stress, but also HS stress or H2O2 treatment. There was no significant difference in acquired thermotolerance between the wild-type, Sgt1a-overexpressing or Sgt1b-overexpressing plants or Sgt1b-disrupted mutants (edm1). However, the acquired thermotolerance was lower in the knockout Sgt1a mutants (KO-Sgt1a) than the wild-type plants. Furthermore, the transcript levels of some heat shock proteins (Hsp) were significantly lower in the KO-Sgt1a mutants than the wild-type plants and edm1 mutants. The present findings indicated Sgt1a to be important for the acquired thermotolerance. [Copyright &y& Elsevier]

Abstract: Plants are strongly affected by heat stress, which they overcome by modifying several physiological and biochemical mechanisms. At the cellular and molecular levels, the synthesis of heat shock proteins (HSPs) is essential in preventing or minimising the deleterious effect of high temperature. Plant responses to high temperatures are mediated by both their inherent ability to survive known as basal thermotolerance, and their ability to acquire thermotolerance after acclimation. A major aspect of the acclimation response involves the expression of HSP genes. Different Triticum durum cultivars were characterised for their response to high temperature at the physiological and molecular levels. Determination of cell membrane stability for both basal and acquired thermotolerance, and HSP gene expression analysis were performed. The two genotypes which contrasted most in their ability to acquire thermotolerance were exposed to different stress conditions and the expression of HSP101C and four small HSP genes was analysed. Differences in HSP transcripts accumulation were observed during the acclimation treatments. There is substantial evidence that induction of HSP gene expression has a role in the acquisition of thermotolerance; moreover, the accumulation of mitochondrial HSP transcripts appears to be related to the acquisition of thermotolerance. [Copyright &y& Elsevier]

Abstract: Hsp70B′ is the only major human isoform in the hsp70 family that is strictly stress-inducible, and therefore available to function only in stressed cells. Since Hsp70B′ is evolutionarily closely related to human Hsp72, they are thought to function similarly, but direct evidence of Hsp70B′ function in stressed cells has been lacking. Here we showed that both Hsp70B′ and Hsp72 are essential relatively early after heat stress in the acquisition of cytoprotection by two human colon cell lines. Using flow cytometry to count viable cells, we also showed that cytoprotection is more pronounced in cultures grown at low cell number (LCN), where there is an ample amount of both Hsp70s. siRNA knock-down of either Hsp70B′ or Hsp72 severely handicapped the ability of cells to acquire cytoprotection. Hsp70B′ and Hsp72 were found to form a complex following stress that included the co-chaperone HOP. These results taken together support the hypothesis that Hsp70B′ and Hsp72 play cooperative roles in cell survival of proteotoxic stress. In addition there are implications for chemotherapy protocols and for pathological conditions in which the contributions to cytoprotection of both Hsp70B′ and Hsp72 are modulated by cell numbers or density. [Copyright &y& Elsevier]

The non-protein amino acid β-aminobutyric acid (BABA) primes Arabidopsis to respond more quickly and strongly to pathogen and osmotic stress. Here, we report that BABA also significantly enhances acquired thermotolerance in Arabidopsis. This thermotolerance was dependent on heat shock protein 101, a critical component of the normal heat-shock response. BABA did not enhance basal thermotolerance under a severe heat-shock treatment. No roles for the hormones ethylene and salicylic acid in BABA-induced acquired thermotolerance were identified by mutant analysis. Using global gene expression analysis, transcript levels for several transcription factors and DNA binding proteins regulating responses to the stress hormone abscisic acid (ABA) were found to be elevated in BABA-treated plants compared with water-treated plants. The role of ABA in BABA-induced thermotolerance was complex. BABA-enhanced thermotolerance was partially compromised in the ABA-insensitive mutant, abi1-1, but was augmented in abi2-1. In an unrelated process, BABA, like ABA, inhibited root growth, and the level of inhibition was roughly additive in roots treated with both compounds. Root growth of both abi1-1 and abi2-1 was also inhibited by BABA. Unexpectedly, abi1-1 and abi2-1 root growth was inhibited more strongly by combined ABA and BABA treatments than by BABA alone. Our results, together with previously published data, suggest that BABA is a general enhancer of plant stress resistance, and that cross-talk occurs between BABA and ABA signalling cascades. Specifically, the BABA-mediated accumulation of ABA transcription factors without concomitant activation of a downstream ABA response could represent one component of the BABA-primed state in Arabidopsis. [ABSTRACT FROM AUTHOR]

Plants are constantly challenged with various abiotic stresses in their natural environment. Elevated temperatures have a detrimental impact on overall plant growth and productivity. Many plants increase their tolerance to high temperatures through an adaptation response known as acquired thermotolerance. To identify the various mechanisms that plants have evolved to cope with high temperature stress, we have isolated a series of Arabidopsis mutants that are defective in the acquisition of thermotolerance after an exposure to 38 °C, a treatment that induces acquired thermotolerance in wild-type plants. One of these mutants, atts02, was not only defective in acquiring thermotolerance after the treatment, but also displayed a reduced level of basal thermotolerance in a 30 °C growth assay. The affected gene in atts02 was identified by positional cloning and encodes digalactosyldiacylglycerol synthase 1 (DGD1) (the atts02 mutant was, at that point, renamed dgd1-2). An additional dgd1 allele, dgd1-3, was identified in two other mutant lines displaying altered acquired thermotolerance, atts100 and atts104. Expression patterns of several heat shock proteins (HSPs) in heat-treated dgd1-2 homozygous plants were similar to those from identically treated wild-type plants, suggesting that the thermosensitivity in the dgd1-2 mutant was not caused by a defect in HSP induction. Lipid analysis of wild-type and mutant plants indicated a close correlation between the ability to acquire thermotolerance and the increases in digalactosyldiacylglycerol (DGDG) level and in the ratio of DGDG to monogalactosyldiacylglycerol (MGDG). Thermosensitivity in dgd1-2 and dgd1-3 was associated with (1) a decreased DGDG level and (2) an inability to increase the ratio of DGDG to MGDG upon exposure to a 38 °C sublethal temperature treatment. Our results suggest that the DGDG level and/or the ratio of DGDG to MGDG may play an important role in basal as well as acquired thermotolerance in Arabidopsis. [ABSTRACT FROM AUTHOR]

Abstract: If plants are pre-exposed to moderate heat stress, they can acquire enhanced tolerance to otherwise lethal high temperatures. To elucidate gene regulatory events involved in the acquisition of thermotolerance, we here conducted a comprehensive transcriptomic analysis. Chinese cabbage microarrays (Brassica rapa EST 6.4K) were used to compare gene expression of 7-day old seedlings exposed to permissive high temperatures over a time course (0, 0.25, 1, 3, and 12h at 37°C). Expression profiling showed that heat treatment triggered significant accumulation of heat shock proteins over time and that some cell wall-modificatory (CWM) genes were up-regulated at the later time points. This up-regulation of CWM genes was verified by reverse-transcription (RT)-PCR and histochemical analysis also provided evidence that cell wall thickness is increased by permissive heat treatment. These observations imply that the activation of CWM-related genes by permissive high temperature stress is an important response for the acquisition of thermotolerance. [Copyright &y& Elsevier]

Abstract: Treatment of Escherichia coli with non-lethal doses of heat or benzyl alcohol (BA) causes transient membrane fluidization and permeabilization, and induces the rapid transcription of heat-shock genes in a σ32-dependent manner. This early response is followed by a rapid adaptation (priming) of the cells to otherwise lethal elevated temperature, in strong correlation with an observed remodeling of the composition and alkyl chain unsaturation of membrane lipids. The acquisition of cellular thermotolerance in BA-primed cells is unrelated to protein denaturation and is not accompanied by the formation of major heat-shock proteins, such as GroEL and DnaK. This suggests that the rapid remodeling of membrane composition is sufficient for the short-term bacterial thermotolerance. [Copyright &y& Elsevier]

An ability of plants memorizing past heat exposure to modulate the expression of stress response transcripts during recovery is essential for efficient acquired thermotolerance. In this study, we demonstrated that expression of heat response transcripts spiked at 30 min or 1 h, but dramatically declined at 3 h during recoveries following exposure to 5-min heat stress in Arabidopsis. In contrast, expression of transcripts up-regulated by 45-min heat stress was sustained for 30 min or 1 h then declined during recovery. These results suggest that heat memory can be differently modulated depending on the duration of heat exposure, and indicate that plants can memorize even 5-min heat stress to regulate acclimatory responses during recovery. Later hypothesis can be supported by the finding that accumulation of heat response proteins was also modulated during recovery following 5-min heat stress. In addition, 5-min heat stress followed by 3 h recovery was efficient to activate acquired thermotolerance of plants, although spike of transcript expression was observed at 1 h during recovery. These results suggest that plants possess the ability to quickly memorize heat stress and reset cellular states during recovery to adapt to subsequent severe heat stress.

Because of their sessile lifestyle, plants cannot escape from heat stress and are forced to alter their cellular state to prevent damage. Plants, therefore, evolved complex mechanisms to adapt to irregular increases in temperature in the natural environment. In addition to the ability to adapt to an abrupt increase in temperature, plants possess strategies to reprogram their cellular state during pre-exposure to sublethal heat stress so that they are able to survive under subsequent severe heat stress. Such an acclimatory response to heat, i.e., acquired thermotolerance, might depend on the maintenance of heat memory and propagation of long-distance signaling. In addition, plants are able to tailor their specific cellular state to adapt to heat stress combined with other abiotic stresses. Many studies revealed significant roles of reactive oxygen species (ROS) regulatory systems in the regulation of these various heat responses in plants. However, the mode of coordination between ROS regulatory systems and other pathways is still largely unknown. In this review, we address how ROS regulatory systems are integrated with other signaling networks to control various types of heat responses in plants. In addition, differences and similarities in heat response signals between different growth stages are also addressed. [ABSTRACT FROM AUTHOR]