Hsp101 is necessary for heat tolerance but dispensable for development andgermination in the absence of stress

Hsp101 is a molecular chaperone that is required for the development of the rmotolerance in plants and other organisms. We report that Arabidopsis thal iana Hsp101 is also regulated during seed development in the absence of str ess, in a pattern similar to that seen for LEA proteins and small Hsps; pro tein accumulates during mid-maturation and is stored in the dry seed. Two n ew alleles of the locus encoding Hsp101 (HOT1) were isolated from Arabidops is T-DNA mutant populations. One allele, hot1-3, contains an insertion with in the second axon and is null for Hsp101 protein expression. Despite the c omplete absence of Hsp101 protein, plant growth and development, as well as seed germination, are normal, demonstrating that Hsp101 chaperone activity is not essential in the absence of stress. In thermotolerance assays hot1- 3 shows a similar, though somewhat more severe, phenotype to the previously described missense allele hot1-1, revealing that the hot1-1 mutation is al so close to null for protein activity. The second new mutant allele, hot1-2 , has an insertion in the promoter 101 bp 5' to the putative TATA element. During heat stress the hot1-2 mutant produces normal levels of protein in h ypocotyls and 10-day-old seedlings, and it is wild type for thermotolerance at these stages. Thus this mutation has not disrupted the minimal promoter sequence required for heat regulation of Hsp101. The hot1-2 mutant also ex presses Hsp101 in seeds, but at a tenfold reduced level, resulting in reduc ed thermotolerance of germinating seeds and underscoring the importance of Hsp101 to seed stress tolerance.