Differential molecular responses to abscisic acid and osmotic stress in viviparous maize embryos

Abstract

Substantial quantities of mRNA encoding the abundant “Em” polypeptide accumulate, in planta, in developing embryos of maize (Zea mays L.). By contrast, accumulation of “Em” mRNA is only barely detectable in embryos with the vp-5/vp-5 genotype [an abscisic acid (ABA)-deficient viviparous phenotype]. “Em” mRNA is not detectable within viviparous embryos of the vp-1/vp-1 genotype that are non-responsive to ABA. Culture of immature wild-type and vp-5/vp-5 embryos in the presence of exogenous ABA or of an osmotically active agent prevents precocious germination and results in expression of the “Em” genes. When vp-1/vp-1 embryos are cultured under similar conditions, only the application of osmotic stress prevents precocious germination. However, “Em” mRNA does not accumulate either in ABA-treated or stressed, arrested embryos, indicating a requirement for ABA perception through a VP-1-mediated mechanism for “Em” gene expression. Nevertheless, vp-1/vp-1 embryos do show both ABA and stress responses at the molecular level. Treatment with ABA causes the accumulation of mRNA encoding a polypeptide of approx. 30 kDa, whilst osmotic stress induces the accumulation both of a 30-kDa polypeptide and a set of approx. 20-kDa polypeptides. This indicates the existence of discrete, parallel ABA and stress response pathways in developing maize embryos.