Plant Physiol, July 2000, Vol. 123, pp. 1121-1132

Acquired Thermotolerance and Expression of the HSP100/ClpB Genes of Lima Bean¹

Department of Plant and Soil Sciences (S.J.K., C.M.B., J.G.H., K.A.K., M.M.J., S.L.K.) and Department of Animal and Food Sciences (C.L.K.), University of Delaware, Newark, Delaware 19717; and E. I. Dupont de Nemours, Delaware Technology Park, Newark, Delaware 19717 (D.L.T.)

Acquired thermotolerance (AT) is the ability of cells to survive a normally lethal temperature treatment as a consequence of pretreatment at an elevated but sublethal temperature. In yeast and cyanobacteria, the expression of the HSP100/ClpB protein is required for the AT response. To determine whether the HSP100/ClpB protein is associated with this response in lima bean (Phaseolus lunatus), we have cloned an HSP100/ClpB homolog and assessed expression of the two gene copies under heat stress conditions, which induce AT. Transcription of the cytoplasmically localized HSP100/ClpB protein genes is stringently controlled by heat stress in both of the laboratory and field heat stress conditions. From a heat-induced cDNA library, we identified a clone of a putative chloroplast-targeted (cp) HSP100/ClpB protein gene sequence. The cp HSP100/ClpB protein genes are constitutively expressed, but transcript levels increase post-heat stress in laboratory heat stress experiments. In field conditions the genes for the cp HSP100/ClpB are constitutively expressed. Although we were unable to correlate differences in the timing of AT response with the expression or genetic structure of the HSP100/ClpB genes in heat-tolerant or -sensitive varieties of lima bean, we clearly demonstrate the association of expression of HSP100/ClpB proteins with heat response in this species.