Nitric oxide mediates the fungal-elicitor-enhanced biosynthesis of antioxidant polyphenols in submerged cultures of Inonotus obliquus

A fungal elicitor prepared from the cell debris of the plant-pathogenic ascomycete Alternaria alternata induces multiple responses by Inonotus obliquus cells, including an increase in generation of nitric oxide (NO), activity of phenylalanine ammonia lyase (PAL) and accumulation of total mycelial phenolic compounds (TMP), but does not trigger production of oxylipins or jasmonic acid (JA). The role of NO in TMP production was investigated via the effects of the NO-specific scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPITO) and the nitric oxide synthase (NOS) inhibitor aminoguanidine (AG). TMP profiles were assayed using (1)H NMR spectroscopy combining multivariate pattern recognition strategies. Pretreatment of I. obliquus mycelia with cPITO or AG suppressed not only elicitor-enhanced NO generation and PAL activity, but also the elicitor-induced increase in TMP production. This TMP reduction by either a NO scavenger or a NOS inhibitor was reversed by exogenous addition of either a NO donor, sodium nitroprusside, or JA separately. NMR-based metabonomic analysis of TMP profiles showed that the induced TMP were hispidin analogues including inoscavins, phelligridins, davallialactone and methyldavallialactone, which possess high antioxidant activities. Thus, NO mediates an elicitor-induced increase in production of antioxidant polyphenols in I. obliquus via a signalling pathway independent of oxylipins or JA, a mechanism which differs from those in some higher plants.