Grapevine exarcerbates the virulence of Neofusicoccum parvum through an overproduction of the phytotoxin terremutin
Introduction Assessing the virulence of Neofusicoccum parvum (Np) remains challenging due to its broad host range and ability to infect numerous agriculturally and forestry important plant species. As a hemibiotrophic fungus, Np can persist as a latent endophyte before transitioning to a necrotrophic pathogen, with the timing and severity of this shift depending on strain-specific virulence. In grapevine, Np is a major causal agent of Botryosphaeria dieback, secreting phytotoxins that are strongly suspected to drive the Np lifestyle transition to necrotrophy through their tissue-necrotizing activity. However, the direct impact of a living host on Np virulence has not yet been demonstrated. Methods In this study, we employed a sterilized grapevine plantlet model artificially inoculated with distinct Np strains to evaluate the direct incidence of a living vine on Np metabolome, including phytotoxin production. Virulence is quantified based on the percentage of necrosis in infected plantlets and correlated with metabolomic profiles specific to the Np strains. Metabolites are analyzed in both culture medium and plantlet tissues by LC-MS, with an additional HPLC quantification of the main phytotoxins mellein (M) and terremutin (T). Results Necroses range from 11.7% with NpB-UV9 to 33.5% with Np-Bt67 at 8 dpi, then from 34.2% with NpB-UV9 to 93.3% with Np-Bt67 at 14 dpi. Phytotoxin M is consistently detected in both medium and plant tissues and is produced at high levels by strain NpB-UV9. In contrast, T is mainly detected in the culture medium and is predominantly produced by strain Np-Bt67, which caused the most rapid and extensive necrosis in plantlets. Notably, production of both toxins is stimulated by living grapevine. Additional metabolites of both fungal and plant origin are also identified. Discussion Collectively, these results demonstrate that living grapevine exacerbates the virulence activity of distinct Np strains, for which M and T appear as conserved virulence factors relevant for detection of Np infection in necrotic grapevine tissues.