ABSTRACT
Plants under attack by a herbivore may emit characteristic volatiles that are implicated in the attraction of the natural enemies of the herbivore. The signal cascade between leaf damage and the volatile production is stimulated by high- or low-molecular-weight elicitors from the secretions of the herbivore. Besides compounds from the octadecanoid signalling pathway, several structurally non-related amino acid conjugates such as the bacterial phytotoxin coronatine, the synthetic indanoyl-isoleucine, or amino acid conjugates of linolenic acid likewise induce volatile biosynthesis. Minor changes in the amino acid moiety may result in different volatile profiles (sesqui- and diterpenoids), attributing to the amino acid substructure a specific role for the recognition and the selective induction. Volatile terpenoids (mono- and diterpenoids) are synthesised de novo along the novel deoxy-D-xylulose (DOX) pathway, while the biosynthesis of sesquiterpenes may be fuelled from both the DOX- and the mevalonate pathway. This finding may be of importance for the plant defence in case of introduction of inhibitors together with the salivary secretion of herbivores into the leaf tissue.
Subject(s)
Insecta/metabolism , Pheromones/biosynthesis , Plants/metabolism , Animals , Gene Expression Regulation, Plant , Models, Chemical , Signal TransductionABSTRACT
Plants are able to respond to herbivore damage with de novo biosynthesis of an herbivore-characteristic blend of volatiles. The signal transduction initiating volatile biosynthesis may involve the activation of the octadecanoid pathway, as exemplified by the transient increase of endogenous jasmonic acid (JA) in leaves of lima bean (Phaseolus lunatus) after treatment with the macromolecular elicitor cellulysin. Within this pathway lima bean possesses at least two different biologically active signals that trigger different biosynthetic activities. Early intermediates of the pathway, especially 12-oxo-phytodienoic acid (PDA), are able to induce the biosynthesis of the diterpenoid-derived 4,8, 12-trimethyltrideca-1,3,7,11-tetraene. High concentrations of PDA result in more complex patterns of additional volatiles. JA, the last compound in the sequence, lacks the ability to induce diterpenoid-derived compounds, but is highly effective at triggering the biosynthesis of other volatiles. The phytotoxin coronatine and amino acid conjugates of linolenic acid (e.g. linolenoyl-L-glutamine) mimic the action of PDA, but coronatine does not increase the level of endogenous JA. The structural analog of coronatine, the isoleucine conjugate of 1-oxo-indanoyl-4-carboxylic acid, effectively mimics the action of JA, but does not increase the level of endogenous JA. The differential induction of volatiles resembles previous findings on signal transduction in mechanically stimulated tendrils of Bryonia dioica.
Subject(s)
Fabaceae/metabolism , Oils, Volatile/metabolism , Plants, Medicinal , Signal Transduction/drug effects , Stearic Acids/pharmacology , Amino Acids/metabolism , Amino Acids/pharmacology , Cellulase/pharmacology , Cucurbitaceae , Cyclopentanes/antagonists & inhibitors , Cyclopentanes/metabolism , Cyclopentanes/pharmacology , Fabaceae/drug effects , Fatty Acids, Unsaturated/antagonists & inhibitors , Fatty Acids, Unsaturated/metabolism , Fatty Acids, Unsaturated/pharmacology , Indenes/pharmacology , Mevalonic Acid/metabolism , Oxylipins , Physical Stimulation , Plant Growth Regulators/antagonists & inhibitors , Plant Growth Regulators/metabolism , Plant Growth Regulators/pharmacology , Plant Leaves/drug effects , Plant Leaves/metabolism , Stearic Acids/antagonists & inhibitors , Stearic Acids/metabolism , Terpenes/metabolism , Time Factors , Volatilization , alpha-Linolenic Acid/antagonists & inhibitors , alpha-Linolenic Acid/metabolism , alpha-Linolenic Acid/pharmacologyABSTRACT
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.
