EFFECT OF ABSCISIC-ACID ON EMBRYO-SPECIFIC GENE-EXPRESSION DURING NORMAL AND PRECOCIOUS GERMINATION IN NORMAL AND VIVIPAROUS MAIZE (ZEA-MAYS) EMBRYOS

Numerous studies have demonstrated that the phytohormone abscisic acid (ABA) plays an important role in many processes during seed developme nt. Abscisic acid prevents precocious germination of the developing se ed and regulates the expression of specific genes during development. The availability of maize (Zea mays L.) viviparous mutants, which are defective in ABA synthesis or responsiveness to this phytohormone, all owed the investigation of varying physiological ABA levels on expressi on patterns of specific ABA-responsive genes. The present work indicat ed that the expression of several ABA-responsive genes is variable amo ng the different viviparous mutants. Among the genes examined, Em show ed the highest correlation between endogenous ABA content and transcri pt levels. Results from ABA studies strongly support the positive role of this phytohormone in inducing Globulin (Glb1 and Glb2) gene expres sion in maize embryos. In the absence of ABA, no Glb1 transcripts are detected within 24 h of embryo germination. On the other hand, inducti on of Glb1 is observed in germinating embryos within 18 h in the prese nce of ABA. Although Glb1 is positively induced by ABA, the endogenous ABA levels obtained from sibling normal and viviparous mutant embryos do not correspond to the increase in Globulin expression in these mut ants. Neither Glb1, Glb2, nor Em transcripts are detected in vp1 mutan t embryos suggesting that a functional Vp1 gene product is required fo r Globulin and Em gene expression. These studies indicate that Globuli n gene expression in maize embryos is under the control of different f actors which require at least a combination of specific endogenous ABA levels and the presence of a functional Vp1 gene product.