ABA METABOLITES INDUCE GROUP-3-LEA MESSENGER-RNA AND INHIBIT GERMINATION IN WHEAT

In plants, metabolism of the plant hormone (+)-S-abscisic acid (ABA) o ccurs by oxidation at the 8'-carbon on the ABA ring producing (+)-8'-h ydrosy-ABA (8'OH-ABA). which undergoes ring closure to phaseic acid (P A), or at the 7'-carbon producing 7'-hydroxy-ABA (7'OH-ABA), New metho ds for synthesizing and stabilizing 8'OH-ABA have enabled us for the f irst time to compare the biological activities of the ABA metabolites, 8'OH-ABA, PA and 7'OH-ABA. Activities were determined in wheat (Triti cum aestivum, cvs Brevor and Clarks Cream) using optically pure natura l enantiomers of ABA, 8'OH-ABA, PA, and 7'OH-ABA. Comparison of wheat embryo germination inhibitory activity showed that only the metabolite 7'OH-ABA had significant activity (10 to 40%, of effective ABA activi ty) as a germination inhibitor of wheat embryos. The metabolites had d ifferential effects on ABA-responsive gene expression. Both 8'OH-ABA a nd PA were effective inducers of an ABA-responsive LEA (late embryogen esis abundant) gene, wheat group 3 LEA, in seedling roots, suggesting that ABA metabolites can maintain and prolong LEA gene expression. The metabolites had only a slight effect on accumulation of an ABA-repons ive protein kinase (PKABA1) mRNA or pMA1951. These results are consist ent with the idea that then are several control points in the ABA meta bolic pathway and indicate that assessment of ABA signalling responses should include consideration of the biological activities of ABA meta bolites.