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

Hsp101 is a molecular chaperone that is required for the development of thermotolerance in plants and other organisms. We report that Arabidopsis thaliana Hsp101 is also regulated during seed development in the absence of stress, in a pattern similar to that seen for LEA proteins and small Hsps; protein accumulates during mid-maturation and is stored in the dry seed. Two new alleles of the locus encoding Hsp101 (HOT1) were isolated from Arabidopsis T-DNA mutant populations. One allele, hot1-3, contains an insertion within the second exon and is null for Hsp101 protein expression. Despite the complete 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 also 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 hypocotyls 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 expresses Hsp101 in seeds, but at a tenfold reduced level, resulting in reduced thermotolerance of germinating seeds and underscoring the importance of Hsp101 to seed stress tolerance.     

AtHsfA2 modulates expression of stress responsive genes and enhances tolerance to heat and oxidative stress in Arabidopsis

In nature, plants are subject to changes of tempera-ture. 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 a wide range of organisms. In all spe-cies studied, the heat shock (HS) response is charac-terized by a rapid production and a transient accumu-lation of specific families of proteins known as heat shock proteins (Hsps) th…     

Stress and development phenotyping of Hsp101 and diverse other Hsp mutants of Arabidopsis thaliana

Heat shock proteins or Hsps are critical in mounting plant resistance against heat stress. The complex Hsp spectrum of Arabidopsis thaliana plant contains over two hundred proteins belonging to six different families namely Hsp20, Hsp40, Hsp60, Hsp70, Hsp90 and Hsp100. Importantly, the cellular function(s) of most Hsps remains to be established. We aimed at phenotyping of stress and development response of the selected, homozygous hsp mutant lines produced by T-DNA insertional mutagenesis method. The heat stress phenotype was assessed for basal and acquired heat stress response at seed and seedling stages. Distinct phenotype was noted for the hot1-3 mutant (knockout mutant of Hsp101 gene) showing higher heat sensitivity and for the salk_087844 mutant (knockout mutant of Hsc70-2 gene) showing higher heat tolerance than the wild type seedlings. The homozygous cs808162 mutant (mutant of ClpB-p gene encoding for the chloroplast-localized form of Hsp101) did not survive even under unstressed, control condition. salk_064887C mutant (mutant of cpn60β4 gene) showed accelerated development cycling. The hot1-3 mutant apart from showing different heat response, exhibited development lesions like bigger size of seeds, buds, siliques, and pollen compared to the wild type plants. In response to controlled deterioration treatment of seeds, hot1-3 seeds showed higher accumulation of reactive oxygen species molecules, higher rates of protein and lipid oxidation and a faster decline in germination rate as compared to wild type seeds. Our findings show that Hsps perform diverse metabolic functions in plant response to stress, growth, and development.