Heat shock proteins and chilling sensitivity of mung bean hypocotyls

Excised mung bean (Vigna radiata L.) hypocotyl sections wereexposed to 40 C for up to 4 h in the presence or absence of50 µM cycloheximide (CHX) before being held at a non-chilling(20 C) or chilling (2.5 C) temperature. Mung bean hypocotyltissue is chilling sensitive, and the rate of solute leakageis highly correlated with the extent of chilling injury. A 3h heat shock at 40 C reduced chilling-induced solute leakageby up to 40%, but leakage was similar to non-heat-shocked hypocotylswhen CHX was present. Specific proteins were labelled when hypocotylswere exposed to [³⁵S] methionine during the last hour of heatshock. The nine most intense bands on the autoradiographs ofSDS-PAGE gels of extracted protein corresponded to molecularweights of 114, 79, 73, 70, 60, 56, 51, 46, and 18 kDa. The18 kDa band reached a maximum after 1 h at 40 C and then rapidlydecreased in intensity as the heat shock continued, becomingundetectable at 4 h. The four most intense bands after 3 h at40 C corresponded to molecular weights of 79, 70, 51, and 46kDa. The synthesis of these four hsps was markedly reduced whenthe hypocotyl sections were exposed to CHX during heat shock.During chilling for 6 d, the levels of hsps 79 and 70 remainedsignificantly higher in tissue that was heat shocked prior tochilling than in tissue that was not heat shocked. In contrast,the levels of hsps 51 and 46 were similar in bothheat-shockedand control tissues. Heat-shock-induced chilling tolerance waslost between 6 and 9 d ofstorage at 2.5 C; this loss coincidedwith the decay of hsps 79 and 70 to control levels. These resultssuggest that heat shock induces an increase in both chillingtolerance and the de novo synthesis of specific heat shock proteins;namely hsps 79 and 70. This is the first report showing a relationshipbetween heat-shock-induced chilling tolerance and specific heat-shock-inducedproteins. Key words: Ion leakage, protein synthesis, Vigna radiata