Evidence for non-ciradian light/dark-regulated expression of Hsp70s in spinach leaves

Expression of six Hsp70s in spinach (Spinacia oleracea cv Longstanding Bloo msdale) leaves grown under isothermal conditions is regulated by a light/da rk (L/D) mechanism distinctly different from the light-regulated mechanism for the chlorophyll a/b-binding protein (cab) or small subunit of ribulose- 1,5-bisphosphate carboxylase oxygenase (rbcS). Subjecting entrained plants to two or three L/D cycles within a 24-h period resulted in an equal number of oscillations in expression for five out of six 70-kD heat shock protein s (Hsp70s). Three cycles appear to be the maximum, as shorter L/D treatment s do not consistently increase the number of cycles in a 24-h period. The e xpression response of Hsp70s to L/D is overridden by heat shock. Protein di sulfide isomerase, a second molecular chaperone of the endoplasmic reticulu m, has an expression pattern in entrained plants that is similar to hsc70-2 , the endoplasmic reticulum luminal Hsp70 binding protein. The parallel exp ression patterns for the various Hsp70s and protein disulfide isomerase ind icate a likely general coordinate L/D regulation for molecular chaperones i n plants. Multiple inductions in response to successive L/D treatments with in a 24-h period in entrained plants for five of six Hsp70s support the con clusion that expression is not a consequence of circadian control, but inst ead is independently cued by non-circadian-mediated L/D signals where peak Hsp70 expression precedes the daily thermoperiod maximum.