Activities and Antagonism between Natural and Synthetic Analogs

The importance of the two chiral centers at C-3 and C-7 in the molecular structure of jasmonic acid in plant responses was investigated. We separated methyl jasmonate (MeJA) into (3R)and (3S)-isomers with a fixed stereochemistry at (2-3, but epimerization at C-7 i s possible. The four isomers of the nonepimerizable analog 7-methyl MeJA were synthesized. These six esters and their corresponding acids were tested in three bioassays: (a) senescence in sunflower (Helianfbus annuus) cotyledons; (b) proteinase inhibitor I 1 gene expression in transgenic tobacco (Nicofiana fabacum) with P-glucuronidase as a biochemical reporter; and (c) seed germination in Brassica napus and wheat (Trificum aesfivum). The esters and acids had similar activities in the three assays, with the ester being more effective than i ts acid. The (3R)-stereochemistry was critical for jasmonate activity. Although activity was reduced after substituting the C-7 proton with a methyl group, the analogs with (3R,7R)or (3R,7S)-stereochemistry were active in some of the assays. Although the four isomers of 7-methyl MeJA were inactive or only weakly active in the senescence assay, they could overcome the senescence-promoting effect of (3R)-MeJA. The strongest antagonistic effect was observed with the (3R,7S)-isomer.