Mutants of Arabidopsis thaliana defective in the acquisition of tolerance to high temperature stress

The ability of organisms to acquire thermotolerance to normally lethal high temperatures is an ancient and conserved adaptive response. However, knowl edge of cellular factors essential to this response is limited. Acquisition of thermotolerance is likely to be of particular importance to plants that experience daily temperature fluctuations and are unable to escape to more favorable environments. We developed a screen, based on hypocotyl elongati on, for mutants of Arabidopsis thaliana that are unable to acquire thermoto lerance to high-temperature stress and have defined four separate genetic l oci, hot1-4, required for this process. hot1 was found to have a mutation i n the heat shock protein 101 (Hsp101) gene, converting a conserved Glu resi due in the second ATP-binding domain to a Lys residue, a mutation that is p redicted to compromise Hsp101 ATPase activity. In addition to exhibiting a thermotolerance defect as assayed by hypocotyl elongation, 10-day-old hot1 seedlings were also unable to acquire thermotolerance, and hot1 seeds had g reatly reduced basal thermotolerance. Complementation of hot1 plants by tra nsformation with wild-type Hsp101 genomic DMA restored hot1 plants to the w ild-type phenotype. The hot mutants are the first mutants defective in ther motolerance that have been isolated in a higher eukaryote, and hot1 represe nts the first mutation in an Hsp in any higher plant. The phenotype of hot1 also provides direct evidence that Hsp101, which is required for thermotol erance in bacteria and yeast, is also essential for thermotolerance in a co mplex eukaryote.