In vitro degradation of willow salicylates

Salicylates are defensive compounds against a great variety of generalist herbivores. Salicortin and its derivatives are labile compounds that degrade immediately when cell compartmentalization is ruptured, producing a 6-hydroxy-2-cyclohexenone (6-HCH) moiety that is a strong antifeeding cue. We studied the in vitro degradation of willow salicylates in the presence and absence of foliar enzymes at acidic, neutral, and alkaline pHs. Higher substituted salicylates were degraded in the absence of foliar enzymes at alkaline pH and in the presence of foliar enzymes at all three pHs. Salicin and its diglucoside, on the other hand, were degraded only in the presence of foliar enzymes at acidic pH, probably by beta-glucosidase activity. The main degradation products of higher substituted salicylates were salicin, 6-HCH, and catechol in both the absence and presence of enzymes, suggesting that the production of 6-HCH and catechol do not necessarily demand enzymatic activity. We propose that the degradation of salicylates begins with the cleavage of a 1-hydroxy-6-oxo-2-cyclohexen-1-carbonyl moiety by foliar esterases and/or alkaline condition. This moiety is decarboxylated in nonenzymatic reaction to an anion of 2-hydroxy-3-cyclohexenone, which is tautomerized to the enol form and further to the keto form, 6-HCH. Hydroxyketone can be also oxidized to catechol, a substrate of polyphenol oxidases.