Hsp70s function as molecular chaperones. The protective chaperone activities of hsp70 help to confer tolerance to heat, glucose deprivation, and drought. Overexpression of hsp70s in many organisms correlates with enhanced thermotolerance, altered growth, and development. To better understand the roles of hsp70 proteins in Arabidopsis, the molecular and physiological consequences of altered expression of the major heat shock cognate, Hsc70-1, were analyzed. Extensive efforts to achieve underexpression of Hsc70-1 mRNA using a full-length antisense cDNA resulted in no viable transgenic plants, suggesting that reduced expression is lethal. Constitutive overexpression of Hsc70-1 also appeared to be deleterious to viability, growth, and development because fewer transformants were recovered, and most were dwarfed with altered root systems. Despite being dwarfed, the overexpression plants progressed normally through four selected developmental stages. Heat treatment revealed that Hsc70-1 overexpression plants were more tolerant to heat shock (44 degrees C for 10 min). The elevated basal levels of HSC70-1 in transgenic plants led to delayed heat shock response of several heat shock genes. The data in this study suggest that tight regulation of Hsc70-1 expression is critical for the viability of Arabidopsis and that the functions of HSC70-1 contribute to optimum growth, development, thermotolerance, and regulation of the heat shock response.