Thermotolerance is the ability of an organism to survive high temperatures. An organism's natural tolerance of heat is their ''basal thermotolerance''. Meanwhile, ''acquired thermotolerance'' is defined as an enhanced level of thermotolerance after exposure to a heat stress.
In plants
Multiple factors contribute to thermotolerance including signaling molecules like
abscisic acid
Abscisic acid (ABA or abscisin II) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to ...
,
salicylic acid
Salicylic acid is an organic compound with the formula HOC6H4COOH. A colorless (or white), bitter-tasting solid, it is a precursor to and a active metabolite, metabolite of acetylsalicylic acid (aspirin). It is a plant hormone, and has been lis ...
, and pathways like the
ethylene signaling pathway and
heat stress response pathway.
The various heat stress response pathways enhance thermotolerance.
The heat stress response in plants is mediated by heat shock transcription factors (
HSF) and is well conserved across eukaryotes. HSFs are essential in plants’ ability to both sense and respond to stress. The HSFs, which are divided into three families (A, B, and C), encode the expression of heat shock proteins (
HSP). Past studies have found that transcriptional activators HsfA1 and HsfB1 are the main positive regulators of heat stress response genes in
Arabidopsis thaliana
''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small plant from the mustard family (Brassicaceae), native to Eurasia and Africa. Commonly found along the shoulders of roads and in disturbed land, it is generally ...
. The general pathway to thermotolerance is characterized by sensing of heat stress, activation of HSFs, upregulation of heat response, and return to the non-stressed state.
In 2011, while studying heat stress ''A. thaliana'', Ikeda et al. concluded that the early response is regulated by HsfA1 and the extended response is regulated by HsfA2. They used
RT-PCR to analyze the expression of HS-inducible genes of mutant (ectopic and nonfunctional HsfB1) and wild type plants. Plants with mutant HsfB1 had lower acquired thermotolerance, based on both lower expression of heat stress genes and visibly altered phenotypes. With these results they concluded that class A HSFs positively regulated the heat stress response while class B HSFs repressed the expression of HSF genes. Therefore, both were necessary for plants to return to non-stressed conditions and acquired thermotolerance.
In animals
In bacteria
In bacteria, thermotolerance is the resistance of cells to the lethal effects of higher temperatures. ''Escherichia coli'' bacteria can be induced to undergo thermotolerance by exposure to a brief period of heat shock, i.e. 15 minutes at 42 degrees.
As a result of such exposure the ''E. coli'' cells become resistant to the lethal effects of higher temperatures, such as 50 degrees. Thermotolerance in ''E. coli'' depends on the expression of the gene ''dnaK'' (that encodes a heat shock protein) so that thermotolerance does not develop in ''dnaK'' mutant cells.
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References
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Biological concepts