Different roles of inducible and constitutive HSP70 and HSP90 in tolerance of Arabidopsis thaliana to high temperature and water deficit

Abstract

Considering global warming, it is of critical interest to understand in detail how plants withstand elevated temperatures and drought. Heat shock proteins (HSP) are involved in cellular response to both factors. HSP70 and HSP90 are multigene families in plants that function as molecular chaperones throughout the organism's tolerance range. However, the roles of individual members under water deficit are much less understood compared to heat shock response. In this study, transcriptional profiling of 12 HSP70s and 7 HSP90s in Arabidopsis thaliana (L.) Heynh. seedlings within wide dose ranges of high temperature and dehydration was conducted by RT-PCR, and roles of distinct inducible and constitutive members in tolerance were evaluated using loss-of-function mutants. The cellular responses to both treatments included strong factor-specific induction of inducible members and mild up-regulation of most constitutive members of various subcellular compartments. Differences in their kinetics, including baseline, time course and magnitude of changes, indicate specific functions of individual members. A significant complement to the cytosolic HSP transcript pool was created through differential expression of three inducible genes: AtHSP70-4 was highly induced in both cases, whereas induction of AtHSP70-5 and AtHSP90-1 was strong at heat shock and barely detectable or absent during dehydration. Nevertheless, analysis of knockout plants showed that AtHSP70-5 and AtHSP90-1 not only significantly contributed to the seed and seedling thermotolerance, but also substantially supported the growth activity under water deficit, when their expression remained negligible. These results suggested that inducible HSP70 and HSP90 at different levels of their expression perform specialized functions in protecting cells under adverse conditions.