The 7[prime]-Methyl Group of Abscisic Acid Is Critical for Biological Activity in Wheat Embryo Germination

Wheat (Triticum aestivum L.) embryo germination is inhibited by natural (S)-(+)-abscisic acid (ABA). In this report we have determined critical structural features of the ABA molecule, particularly the methyl and ketone groups of the ABA ring, required for inhibitory activity. To examine the ring residues a series of new optically active ABA analogs have been synthesized in which the 4prime]-keto, 7prime]-, 8prime]-, or 8prime]- and 9prime]-carbons have been replaced with hydrogen atoms. Each of the analogs was tested over a range of concentrations as a germination inhibitor. Enantiomers of the analogs altered at the 4prime]-keto or 8prime]- and 9prime]-methyl groups were active, but less so than ABA. Both enantiomers of 7prime]-demethylABA were inactive as germination inhibitors. The results show that the 7prime]-methyl group is absolutely required for activity, but that the other residues are less critical for hormone recognition.