INDUCTION OF VOLATILE BIOSYNTHESIS IN THE LIMA-BEAN (PHASEOLUS-LUNATUS) BY LEUCINE AND ISOLEUCINE CONJUGATES OF 1-OXOINDAN-4-CARBOXLIC AND 1-HYDROXYINDAN-4-CARBOXYLIC ACID - EVIDENCE FOR AMINO-ACID CONJUGATES OF JASMONIC ACID AS INTERMEDIATES IN THE OCTADECANOID SIGNALING 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 hig h molecular weight chemical defenses, volatile production is under the control of the octadecanoid signalling pathway leading to jasmonic ac id (2) (threshold concentration of jasmonic acid giving rise to volati le induction in Phaseolus lunatus: approximate to 100 nmol . ml(-1)). A significantly more active compound is the phytotoxin coronatine (3) (threshold concentration: greater than or equal to 1 nmol . n(-1)). Me thyl esters of 1-oxo-indanoyl-isoleucine (4) or 1-oxo-indanoyl-leucine (5), designed as readily available analogues of coronatin (3), have a lso been shown to be active (threshold concentration: greater than or equal to 20 nmol . ml(-1)). Crucially, their component parts, i.e. 1-o xo-indan-carboxylic acid and the amino acids are completely inactive, The pattern of emitted volatiles, produced by plants treated with thes e analogues, is largely identical to that released from coronatine- or jasmonic acid-treated plants. While the reduction of the carbonyl gro up of jasmonic acid (2) results in an inactive molecule, namely cucurb ic acid, the methyl ester of the 1-hydroxy-indanoyl-isoleucine conjuga te (8) is at least as effective as the corresponding ore-derivatives ( 4) and (5) (threshold concentration: greater than or equal to 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 st age in the natural signal transduction pathway. Their later interactio n with a macromolecular receptor apparently requires enolization of th e carbonyl group in the jasmonate moiety, yielding a planar segment wh ich is essential for successful binding with the macromolecule. The re sulting hydroxy group is implicated in the formation of a hydrogen bon d in the ensuing ligand/receptor complex.