STRUCTURAL REQUIREMENTS OF JASMONATES AND MIMICS FOR NICOTINE INDUCTION IN NICOTIANA-SYLVESTRIS

Jasmonic acid (JA) is strongly implicated in the long-distance signal transduction cascade increasing nicotine synthesis in the roots of pla nts after leaf wounding. In order to explore the structural requiremen ts of the inducing signal, we examined jasmonates, mimics, and a biosy nthetic precursor for nicotine-inducing activity (NIA). We examine the importance of the keto group on the five-membered ring and the double bond in the n-pentenyl chain by comparing the NIA of methyl jasmonate (MJ) with that of cucurbic acid, 1,3-dithiolane-MJ, 1,3-dioxolane-MJ, methyl dihydrojasmonate (DHMJ), 1,3-dioxolane-DHMJ, 1-oxo-indan-4-car boxylic acid ILE-methyl ester, and 1-hydroxyl-indan-4-carboxylic acid ILE-methyl ester. We found that: 1,3-dioxolane MJ, cucurbic acid, and 1,3-dioxolane DHMJ were less active than MJ and that the isoleucine (I LE) conjugates of l-oxo-and 1-hydroxyindanon-4-carboxylic acid had the same NIA as MJ. The activities of these indanon amino acid conjugates may be due to the structural similarity of their keto or hydroxyl gro ups on the five-membered ring to MJ or to the keto-enolized MJ. These results support the hypothesis that the enolization of the keto group during or prior to its interaction with the putative JA receptor is re quired for activity. We explore the importance of the esterification o f the carboxyl functional group by comparing the NIAs of cucurbic acid and cucurbic acid methyl ester, 1-oxo-indan-4-carboxylic acid, 1-oxo- indan-4-carboxylic acid methyl ester, and 1-oxo-indan-4-carboxylic aci d ILE-methyl ester. In all cases, the esters were more active than the free acids. We compared the NIA of MJ of different epimeric compositi on (8% and 20% 3R,7S-MJ); 12-oxophytodienoic acid (12-oxo-PDA) methyl ester, an important precursor of JA; and coronatine (a well-known phyt otoxin and putative structural mimic of 12-oxo-PDA). We found that: (1 ) the epimeric composition of MJ did not affect its NIA; (2) 12-oxo-PD A methyl ester had lower NIA than MJ; and (3) coronatine significantly inhibited plant growth but did not increase nicotine biosynthesis. In summary, JA, rather than its biosynthetic precursor, 12-oxo-PDA, is l ikely the endogenous signal in Nicotiana sylvestris, and the keto func tional group on the five-membered ring and the double bond in the n-pe ntenyl side chain are crucial components of JA for MA.