Identification of a drimenol synthase and drimenol oxidase from Persicaria hydropiper, involved in the biosynthesis of insect deterrent drimanes.

The sesquiterpenoid polygodial, which belongs to the drimane family, has been shown to be an antifeedant for a number of herbivorous insects. It is presumed to be synthesized from farnesyl diphosphate via drimenol, subsequent C-12 hydroxylation and further oxidations at both C-11 and C-12 to form a dialdehyde. Here, we have identified a drimenol synthase (PhDS) and a cytochrome P450 drimenol oxidase (PhDOX1) from Persicaria hydropiper. Expression of PhDS in yeast and plants resulted in production of drimenol alone. Co-expression of PhDS with PhDOX1 in yeast yielded drimendiol, the 12-hydroxylation product of drimenol, as a major product, and cinnamolide. When PhDS and PhDOX1 were transiently expressed by agro-infiltration in Nicotiana benthamiana leaves, drimenol was almost completely converted into cinnamolide and several additional drimenol derivatives were observed. In vitro assays showed that PhDOX1 only catalyses the conversion from drimenol to drimendiol, and not the further oxidation into an aldehyde. In yeast and heterologous plant hosts, the C-12 position of drimendiol is therefore likely to be further oxidized by endogenous enzymes into an aldehyde and subsequently converted to cinnamolide, presumably by spontaneous hemiacetal formation with the C-11 hydroxyl group followed by oxidation. Purified cinnamolide was confirmed by NMR and shown to be deterrent with an effective deterrent dose (ED50 ) of about 200-400 µg g-1 fresh weight against both whiteflies and aphids. The putative additional physiological and biochemical requirements for polygodial biosynthesis and stable storage in plant tissues are discussed.

Comparative antifeedant activities of polygodial and pyrethrins against whiteflies (Bemisia tabaci) and aphids (Myzus persicae).

BACKGROUND: Polygodial, a sesquiterpene dialdehyde of the drimane family, has been shown to have deterrent and antifeedant effects on various insect species, including Myzus persicae (Sulzer), Spodoptera spp. and Leptinotarsa decemlineata (Say). This compound may have potential as a broad-spectrum biocontrol agent, similar to pyrethrins, given that it was previously reported to improve yield when sprayed on barley fields. RESULTS: This study compares the deterrent effect of polygodial and pyrethrins against the silverleaf whitefly Bemisia tabaci (Gennadius) and the green peach aphid M. persicae in dual-choice assays using compound-coated tomato leaf discs. B. tabaci adults were deterred by polygodial at an ED50 (effective dose at which 50% of the insects are deterred) of about 25 µg g(-1) fresh weight (FW), and green peach aphids at about 54 µg g(-1) FW. Bioassays were benchmarked with pyrethrins that had a 20-fold lower ED50 of approximately 1.4 µg g(-1) FW against whiteflies, but only a twofold lower ED50 (about 28 µg g(-1) FW) against peach aphids. Polygodial showed moderate phytotoxic effects (score of 2 on a scale of 1-5) on tomato leaves at concentrations above the ED50 concentrations (≥ 90 µg g(-1) FW). CONCLUSION: The sesquiterpene dialdehyde polygodial is 2-20 times less deterrent than pyrethrins, depending on the insect species, but it could provide a useful complement to pyrethrin sprays as it has a different mode of action, is food grade and has low volatility. However, a formulation that reduces the risks of phytotoxic effects should be developed.