A rhizosphere Blastobotrys vanleenenianus isolate associated with the mediterranean endemism Phlomis purpurea produces phenylacetic acid and induces cell wall stress in Phytophthora cinnamomi

Mutualistic fungi colonizing the surface of plant roots are able to enhance nutrient acquisition, increase tolerance to abiotic stress, and improve resistance to phytopathogens through mechanisms such as resource competition, induction of plant defenses, and production of antimicrobial compounds, among other beneficial effects. Between the different groups of phytopathogens, oomycetes are responsible for numerous diseases in agricultural and forest ecosystems. Dieback, a severe oomycete-induced disease affecting Quercus species and other economically important plants, remains a major challenge due to its variable symptomatology and the limited availability of rapid and selective control strategies. This study explores the protective role of the rhizosphere microbiome species associated with the Mediterranean endemic shrub Phlomis purpurea against the phytopathogenic oomycete Phytophthora cinnamomi . This shrub offers protection against this oomycete to forest ecosystems in field observations. Antimicrobial metabolites from this rhizosphere associated species were identified and characterized by HPLC, HRESIMS, and NMR, and their cytotoxic activity and underlying mechanism of action were evaluated. Our results demonstrate that the yeast Blastobotrys vanleenenianus , newly identified as a member of the P. purpurea rhizosphere microbiome, produces phenylacetic acid. This metabolite effectively inhibits the growth of P. cinnamomi by the induction of cell wall stress in this pathogen. These results suggest that B. vanleenenianus contributes to the suppression of oomycete infections in the host plant. However, further research is needed to evaluate its applicability as a biological control agent.