Ion channel-forming alamethicin is a potent elicitor of volatile biosynthesis and tendril coiling. Cross talk between jasmonate and salicylate signaling in lima bean.

Alamethicin (ALA), a voltage-gated, ion channel-forming peptide mixture from Trichoderma viride, is a potent elicitor of the biosynthesis of volatile compounds in lima bean (Phaseolus lunatus). Unlike elicitation with jasmonic acid or herbivore damage, the blend of substances emitted comprises only the two homoterpenes, 4,11-dimethylnona-1,3,7-triene and 4,8,12-trimethyltrideca-1,3,7,11-tetraene, and methyl salicylate. Inhibition of octadecanoid signaling by aristolochic acid and phenidone as well as mass spectrometric analysis of endogenous jasmonate demonstrate that ALA induces the biosynthesis of volatile compounds principally via the octadecanoid-signaling pathway (20-fold increase of jasmonic acid). ALA also up-regulates salicylate biosynthesis, and the time course of the production of endogenous salicylate correlates well with the appearance of the methyl ester in the gas phase. The massive up-regulation of the SA-pathway (90-fold) interferes with steps in the biosynthetic pathway downstream of 12-oxophytodienoic acid and thereby reduces the pattern of emitted volatiles to compounds previously shown to be induced by early octadecanoids. ALA also induces tendril coiling in various species like Pisum, Lathyrus, and Bryonia, but the response appears to be independent from octadecanoid biosynthesis, because inhibitors of lipoxygenase and phospholipase A(2) do not prevent the coiling reaction.

Differential signalling and plant-volatile biosynthesis.

At least two different signalling pathways have been identified that result in clearly distinguishable volatile profiles in response to pathogens and herbivores in the lima bean Phaseolus lunatus. Alamethicin, a voltage-gated ion-channel-forming peptide from Trichoderma viride, is a potent inducer of volatile biosynthesis in the lima bean. Unlike elicitation with cellulysin or herbivore damage, which act through the jasmonic acid pathway and result in a complex pattern of volatile compounds, the emitted blend comprises only the two homoterpens, 4,11-dimethylnona-1,3,7-triene and 4,8,12-trimethyltrideca-1,3,7,11-tetraene, and methyl salicylate. Both pathways, represented by jasmonic acid and alamethicin, depend on lipid-derived signalling compounds, set off by the activation of a phospholipase A and further processing by lipoxygenase activity. The alamethicin-induced signal-transduction pathway interferes with the octadecanoid cascade, probably due to increased salicylic acid levels, resulting in an inhibition of the typical jasmonic acid-induced volatile profile.

Differential induction of plant volatile biosynthesis in the lima bean by early and late intermediates of the octadecanoid-signaling pathway.

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.

Gadolinium-sensitive, voltage-dependent calcium release channels in the endoplasmic reticulum of a higher plant mechanoreceptor organ.

The lipid bilayer technique was adapted to the functional reconstitution of ion channels from the endoplasmic reticulum of a higher plant. This was obtained at high purity from touch-sensitive tendrils of Bryonia dioica. In this preparation, a calcium-selective strongly rectifying channel is prevailing whose single-channel properties have been characterized. The single-channel conductance is 29 pS in 50 mM CaCl2. The Ca2+: K+ selectivity was determined to be approximately 6.6. The channel is voltage-gated and, more importantly, the gating voltage is strongly shifted towards more negative voltages when a transmembrane Ca2+ gradient is applied. Thus, at physiological voltages across the endoplasmic reticulum membrane, the channel's open probability will be governed largely by the chemical potential gradient of Ca2+, generated by the Ca(2+)-ATPase in that same membrane. The calcium release channel described here is effectively blocked by Gd3+ which also completely suppresses a tendril's reaction to touch, suggesting that this channel could be a key element of calcium signaling in higher plant mechanotransduction. Its molecular characteristics and inhibitor data show it to be the first known member of a hitherto unrecognized class of calcium channels.