The action of exogenous abscisic and gibberellic acids on gene expression in germinating castor beans.

Exogenously applied abscisic acid inhibits isocitrate-lyase activity of the endosperm during germination of castor-bean seeds. Amounts of isocitrate-lyase mRNA have been estimated by immunoprecipitation of in-vitro-translated polypeptide products. Exogenous abscisic acid leads to an inhibition of isocitrate lyase-mRNA accumulation. A large proportion of this effect of the growth factor may be accounted for by its action in inhibiting the overall accumulation of ribosomal RNA and total mRNA. However, the effect of abscisic acid on protein synthesis is not general, as the production of some mRNAs was stimulated. The major mRNA stored in the dry seed, coding for a 25600-Mr polypeptide that normally disappears within the first 12 h of germination, exhibited high levels in abscisic-acid-treated endosperms throughout the germination period. Three complementary DNA clones, of which two clones are complementary to isocitrate lyase, have been used to measure levels of transcripts during seed germination. The accumulation of both transcripts was inhibited by exogenous abscisic acid. The data strongly indicate that the action of abscisic acid on isocitrate lyase synthesis is either to inhibit the transcription, or to increase the transcript turnover. Exogenous gibberellic acid is able to counteract the inhibitory effects of abscisic acid.