Preformed Messenger RNAs and Early Wheat Embryo Germination.

Wheat (Triticum aestivum L.) embryo homogenates have been fractionated into three cell fractions from which RNA was extracted and assayed for mRNA content by in vitro translation and by [(3)H]polyuridylic acid hybridization. In dry embryos the preformed mRNAs are distributed equally between a rapidly sedimenting "pellet" fraction and a cytoplasmic "ribosomal/subribosomal" fraction. During germination 25 to 40% of the total mRNA becomes polyribosomal. The remaining 60 to 75% is retained in the pellet and ribosomal/subribosomal fractions.To compare the nucleotide sequences of the different mRNA fractions, cDNAs were transcribed from polyribosomal (A+) RNA of 40-minute imbibed embryos and from total A(+) RNA of dry embryos, and the ability of these cDNAs to hybridize with the more prevalent classes of mRNA from the different cell fractions was analyzed. The results suggest that there is no significant difference between the preformed mRNAs that move into polyribosomes and those remaining in the nonpolyribosomal fractions. In addition there appears to be no difference between the mRNAs of polyribosomes from embryos germinated for 45 minutes and 5 hours. Between 5 hours and 2 days, however, there is a considerable change in the mRNA composition of the embryos. We conclude for the prevalent classes of message, that the preformed mRNAs of the wheat embryo are not involved in temporal regulation of early development but that they function primarily to allow a rapid resumption of growth upon exposure of the embryo to water.