Heat Shock Proteins and Their mRNAs in Dry and Early Imbibing Embryos of Wheat

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Abstract
Two-dimensional gels of in vitro translation products of mRNAs isolated from quiescent wheat (Triticum aestivum) embryos demonstrate the presence of mRNAs encoding heat shock proteins (hsps). There were no detectable differences in the mRNAs found in mature embryos from field grown, from 25.degree.C growth chamber cultivated, or from plants given 38.degree.C heat stresses at different stages of seed development. The mRNAs encoding several developmentally dependent (dd) hsps were among those found in the dry embryos. Stained two-dimensional gels of proteins extracted from 25.degree.C growth chamber cultivated wheat embryos demonstrated the presence of hsps, including dd hsps. A study of the relationship or preexisting hsp mRNA and the heat shock response during early imhibition was undertaken. Heat shocks (42.degree.C, 90 minutes) were administered following 1.5, 16, and 24 hours of 25.degree.C imbibition. While the mRNAs encoding the low molecular weight hsps decayed rapidly upon imbibition, the mRNAs for dd hsps persisted longer and were still detectable following 16 hours of imbibition. After 1.5 hours of imbibition, the mRNAs for the dd hsps did not accumulate in response to heat shock, even though the synthesis of the proteins was enhanced. Thus, an applied heat shock appeared to lead to the preferential translation of preexisting dd hsp mRNAs. The mRNAs for the other hsps, except hsp 70, were newly transcribed at all of the imbibition times examined. The behavior of the hsp 70 group of proteins during early imbibition was examined by RNA gel blot analysis. The mRNAs for the hsp 70 groups were detectable at moderate levels in the quiescent embryo. The relative level of hsp 70 mRNA increased after the onset of imbibition at 25.degree.C and remained high through 25.5 hours of prior imbibition. The maximal levels of these mRNAs at 25.degree.C was reached at 17.5 hours of imbibition. Heat shock caused modest additional accumulation of hsp70 mRNA at later imbibition times.