ABA metabolites induce group 3 LEA mRNA and inhibit germination in wheat

In plants, metabolism of the plant hormone (+)-S-abscisic acid (ABA) occurs by oxidation at the 8'-carbon on the ABA ring producing (+)-8'-hydrosy-ABA (8'OH-ABA). which undergoes ring closure to phaseic acid (PA), or at the 7'-carbon producing 7'-hydroxy-ABA (7'OH-ABA), New methods for synthesizing and stabilizing 8'OH-ABA have enabled us for the first time to compare the biological activities of the ABA metabolites, 8'OH-ABA, PA and 7'OH-ABA. Activities were determined in wheat (Triticum aestivum, cvs Brevor and Clarks Cream) using optically pure natural 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 activity) as a germination inhibitor of wheat embryos. The metabolites had differential effects on ABA-responsive gene expression. Both 8'OH-ABA and PA were effective inducers of an ABA-responsive LEA (late embryogenesis 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-reponsive protein kinase (PKABA1) mRNA or pMA1951. These results are consistent with the idea that then are several control points in the ABA metabolic pathway and indicate that assessment of ABA signalling responses should include consideration of the biological activities of ABA metabolites.