Presence of Heat Shock mRNAs in Field Crown Soybeans

Our laboratory has extensively defined many parameters of the heat shock (HS) response in etiolated soybean (Glycine max L.] Merr.) hypocotyls, including the identification of cDNA clones for mRNAs encoding several low molecular weight HS proteins. We have now investigated the response of mature plants to a HS in a growth chamber and to high temperature stress under field conditions. Soybean plants show induction of HS mRNAs when the temperature of the chamber is rapidly shifted from 28°C to 45°C. This temperature of induction is significantly higher than the optimal induction temperature for etiolated hypocotyls, probably reflecting the ability of mature plants to lower their leaf temperatures below the ambient air temperature through transpirational cooling. Samples of soybean leaves were taken from an irrigated and a nonirrigated field during a 24-h period when midday temperatures reached 40°C. Several HS mRNAs were present in samples from both fields, although the levels of these mRNAs were much higher in nonirrigated leaves. This differential response of HS mRNA steady state levels was not a response to water stress, since water-stressed plants at 28°C did not induce HS mRNAS. Rather, these quantitative differences are probably due to differences in actual leaf temperatures between irrigated and nonirrigated leaves. The presence of these HS mRNAS in field-grown plants suggests that HS proteins are produced as part of the normal plant response to high temperature.