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To investigate the importance of different processes to heat stress tolerance, 45 Arabidopsis (Arabidopsis thaliana) mutants and one transgenic line were tested for basal and acquired thermotolerance at different stages of growth. Plants tested were defective in signaling pathways (abscisic acid, salicylic acid, ethylene, and oxidative burst signaling) and in reactive oxygen metabolism (ascorbic acid or glutathione production, catalase) or had previously been found to have temperature-related phenotypes (e.g. fatty acid desaturase mutants, uvh6). Mutants were assessed for thermotolerance defects in seed germination, hypocotyl elongation, root growth, and seedling survival. To assess oxidative damage and alterations in the heat shock response, thiobarbituric acid reactive substances, heat shock protein 101, and small heat shock protein levels were determined. Fifteen mutants showed significant phenotypes. Abscisic acid (ABA) signaling mutants (abi1 and abi2) and the UV-sensitive mutant, uvh6, showed the strongest defects in acquired thermotolerance of root growth and seedling survival. Mutations in nicotinamide adenine dinucleotide phosphate oxidase homolog genes (atrbohB and D), ABA biosynthesis mutants (aba1, aba2, and aba3), and NahG transgenic lines (salicylic acid deficient) showed weaker defects. Ethylene signaling mutants (ein2 and etr1) and reactive oxygen metabolism mutants (vtc1, vtc2, npq1, and cad2) were more defective in basal than acquired thermotolerance, especially under high light. All mutants accumulated wild-type levels of heat shock protein 101 and small heat shock proteins. These data indicate that, separate from heat shock protein induction, ABA, active oxygen species, and salicylic acid pathways are involved in acquired thermotolerance and that UVH6 plays a significant role in temperature responses in addition to its role in UV stress.Plants and other organisms have both an inherent ability to survive exposure to temperatures above the optimal for growth (basal thermotolerance) and an ability to acquire tolerance to otherwise lethal heat stress (acquired thermotolerance). Acquired thermotolerance is induced by a short acclimation period at moderately high (but survivable) temperatures or by treatment with other nonlethal stress prior to heat stress (Kapoor et al., 1990;Vierling, 1991;Flahaut et al., 1996;Burke et al., 2000;Hong and Vierling, 2000;Massie et al., 2003;Larkindale et al., 2005). The ability to withstand and to acclimate to supra-optimal temperatures results from both prevention of heat damage and repair of heat-sensitive components. Organisms must also maintain metabolic homeostasis during stress or be able to reestablish homeostasis subsequent to the stress period. Although plants are frequently subjected to dramatic heating to above the optimal growth temperature, relatively little is known about the critical genes controlling either basal or acquired thermotolerance in plants.Heat stress has a complex impact on cell function, suggesting that many processes are involved in th...

Plants, in common with all organisms, have evolved mechanisms to cope with the problems caused by high temperatures. We examined specifically the involvement of calcium, abscisic acid (ABA), ethylene, and salicylic acid (SA) in the protection against heat-induced oxidative damage in Arabidopsis. Heat caused increased thiobarbituric acid reactive substance levels (an indicator of oxidative damage to membranes) and reduced survival. Both effects required light and were reduced in plants that had acquired thermotolerance through a mild heat pretreatment. Calcium channel blockers and calmodulin inhibitors increased these effects of heating and added calcium reversed them, implying that protection against heat-induced oxidative damage in Arabidopsis requires calcium and calmodulin. Similar to calcium, SA, 1-aminocyclopropane-1-carboxylic acid (a precursor to ethylene), and ABA added to plants protected them from heat-induced oxidative damage. In addition, the ethylene-insensitive mutant etr-1, the ABA-insensitive mutant abi-1, and a transgenic line expressing nahG (consequently inhibited in SA production) showed increased susceptibility to heat. These data suggest that protection against heat-induced oxidative damage in Arabidopsis also involves ethylene, ABA, and SA. Real time measurements of cytosolic calcium levels during heating in Arabidopsis detected no increases in response to heat per se, but showed transient elevations in response to recovery from heating. The magnitude of these calcium peaks was greater in thermotolerant plants, implying that these calcium signals might play a role in mediating the effects of acquired thermotolerance. Calcium channel blockers and calmodulin inhibitors added solely during the recovery phase suggest that this role for calcium is in protecting against oxidative damage specifically during/after recovery.In nature, plants are subject to changes of temperature, both during changes in season and more rapidly over the course of individual days. The temperature of an individual plant cell can change much more rapidly than other factors that cause stress (e.g. water levels or salt levels). Thus, like other organisms, plants have evolved strategies for preventing damage caused by rapid changes in temperature and for repairing what damage is unavoidable.Heat stress responses have been well documented in wide range of organisms. In all species studied, heat stress results in the production of specific families of proteins known as heat shock proteins (HSPs; Howarth and Ougham, 1993). These proteins have been classified into a number of families based on their molecular mass, and most have chaperonin function (Jaenicke and Creighton, 1993). All organisms produce HSPs from all of the major families (HSP90s, HSP70s and small HSPs), but plants are unique in the number of different small HSPs that they produce (Jakob and Buchner, 1994). Most studies investigating heat stress in plants have focused on HSPs (Howarth and Ougham, 1993; Sullivan and Green, 1993;Park et al., 1996;Schoffl et al.,...

Plants can acclimate rapidly to environmental conditions, including high temperatures. To identify molecular events important for acquired thermotolerance, we compared viability and transcript profiles of Arabidopsis thaliana treated to severe heat stress (45°C) without acclimation or following two different acclimation treatments. Notably, a gradual increase to 45°C (22°C to 45°C over 6 h) led to higher survival and to more and higher-fold transcript changes than a step-wise acclimation (90 min at 38°C plus 120 min at 22°C before 45°C). There were significant differences in the total spectrum of transcript changes in the two treatments, but core components of heat acclimation were apparent in the overlap between treatments, emphasizing the importance of performing transcriptome analysis in the context of physiological response. In addition to documenting increases in transcripts of specific genes involved in processes predicted to be required for thermotolerance (i.e. protection of proteins and of translation, limiting oxidative stress), we also found decreases in transcripts (i.e. for programmed cell death, basic metabolism, and biotic stress responses), which are likely equally important for acclimation. Similar protective effects may also be achieved differently, such as prevention of proline accumulation, which is toxic at elevated temperatures and which was reduced by both acclimation treatments but was associated with transcript changes predicted to either reduce proline synthesis or increase degradation in the two acclimation treatments. Finally, phenotypic analysis of T-DNA insertion mutants of genes identified in this analysis defined eight new genes involved in heat acclimation, including cytosolic ascorbate peroxidase and the transcription factors HsfA7a (heat shock transcription factor A7a) and NF-X1.