Heat shock proteins and chilling sensitivity of mung bean hypocotyls

Abstract

Excised mung bean (Vigna radiata L.) hypocotyl sections were exposed to 40 °C for up to 4 h in the presence or absence of 50 μM cycloheximide (CHX) before being held at a non-chilling (20°C) or chilling (2.5°C) temperature. Mung bean hypocotyl tissue is chilling sensitive, and the rate of solute leakage is highly correlated with the extent of chilling injury. A 3 heat shock at 40°C reduced chilling-induced solute leakage by up to 40%, but leakage was similar to non-heat-shocked hypocotyls when CHX was present. Specific proteins were labelled when hypocotyls were exposed to [ 35 S] methionine during the last hour of heat shock. The nine most intense bands on the autoradiographs of SDS-PAGE gels of extracted protein corresponded to molecular weights of 114, 79, 73, 70, 60, 56, 51, 46, and 18 kDa. The 18 kDa band reached a maximum after 1 h at 40 °C and then rapidly decreased in intensity as the heat shock continued, becoming undetectable at 4 h. The four most intense bands after 3 h at 40°C corresponded to molecular weights of 79, 70, 51, and 46 kDa. The synthesis of these four hsps was markedly reduced when the hypocotyl sections were exposed to CHX during heat shock. During chilling for 6 d, the levels of hsps 79 and 70 remained significantly higher in tissue that was heat shocked prior to chilling than in tissue that was not heat shocked. In contrast, the levels of hsps 51 and 46 were similar in both heat-shocked and control tissues. Heat-shock-induced chilling tolerance was lost between 6 and 9 d of storage at 2.5°C; this loss coincided with the decay of hsps 79 and 70 to control levels. These results suggest that heat shock induces an increase in both chilling tolerance and the de novo synthesis of specific heat shock proteins; namely hsps 79 and 70. This is the first report showing a relationship between heat-shock-induced chilling tolerance and specific heat-shock-induced proteins.