Induction of volatile biosynthesis in the Lima bean (Phaseolus lunatus) by leucine‐ and isoleucine conjugates of 1‐oxo‐ and 1‐hydroxyindan‐4‐carboxylic acid: evidence for amino acid conjugates of jasmonic acid as intermediates in the octadecanoid signalling pathway

One of the most intriguing plant defense reactions against herbivores is the emission of volatiles as potentially attractive signals for the natural enemies of the attacking species. Like many other low and high molecular weight chemical defenses, volatile production is under the control of the octadecanoid signalling pathway leading to jasmonic acid (2) (threshold concentration of jasmonic acid giving rise to volatile induction in Phaseolus lunatus: approximately 100 nmol.ml-1). A significantly more active compound is the phytotoxin coronatine (3) (threshold concentration: > or = 1 nmol.ml-1). Methyl esters of 1-oxo-indanoyl-isoleucine (4) or 1-oxo-indanoyl-leucine (5), designed as readily available analogues of coronatin (3), have also been shown to be active (threshold concentration: > or = 20 nmol.ml-1). Crucially, their component parts, i.e. 1-oxo-indan-carboxylic acid and the amino acids are completely inactive. The pattern of emitted volatiles, produced by plants treated with these analogues, is largely identical to that released from coronatine- or jasmonic acid-treated plants. While the reduction of the carbonyl group of jasmonic acid (2) results in an inactive molecule, namely curcurbic acid, the methyl ester of the 1-hydroxy-indanoyl-isoleucine conjugate (8) is at least as effective as the corresponding oxo-derivatives (4) and (5) (threshold concentration: > or = 20 nmol.ml-1). The results support the concept that epi-jasmonic acid (1) may be converted into a leucine or isoleucine conjugate at an early stage in the natural signal transduction pathway. Their later interaction with a macromolecular receptor apparently requires enolization of the carbonyl group in the jasmonate moiety, yielding a planar segment which is essential for successful binding with the macromolecule. The resulting hydroxy group is implicated in the formation of a hydrogen bond in the ensuing ligand/receptor complex.