Biosynthesis, herbivore induction, and defensive role of phenylacetaldoxime glucoside

Aldoximes are well-known metabolic precursors for plant defense compounds such as cyanogenic glycosides, glucosinolates, and volatile nitriles. They are also defenses themselves produced in response to herbivory; however, it is unclear whether aldoximes can be stored over a longer term as defense compounds and how plants protect themselves against the potential autotoxic effects of aldoximes. Here, we show that the Neotropical myrmecophyte tococa (Tococa quadrialata, recently renamed Miconia microphysca) accumulates phenylacetaldoxime glucoside (PAOx-Glc) in response to leaf herbivory. Sequence comparison, transcriptomic analysis, and heterologous expression revealed that 2 cytochrome P450 enzymes, CYP79A206 and CYP79A207, and the UDP-glucosyltransferase UGT85A123 are involved in the formation of PAOx-Glc in tococa. Another P450, CYP71E76, was shown to convert PAOx to the volatile defense compound benzyl cyanide. The formation of PAOx-Glc and PAOx in leaves is a very local response to herbivory but does not appear to be regulated by jasmonic acid signaling. In contrast to PAOx, which was only detectable during herbivory, PAOx-Glc levels remained high for at least 3 d after insect feeding. This, together with the fact that gut protein extracts of 3 insect herbivore species exhibited hydrolytic activity toward PAOx-Glc, suggests that the glucoside is a stable storage form of a defense compound that may provide rapid protection against future herbivory. Moreover, the finding that herbivory or pathogen elicitor treatment also led to the accumulation of PAOx-Glc in 3 other phylogenetically distant plant species suggests that the formation and storage of aldoxime glucosides may represent a widespread plant defense response.