High-Throughput Analysis Reveals Stable Metabolome of Paxillus involutus Under White Light Exposure Despite Reduced Mycelium Growth

Ectomycorrhizal fungi are rarely exposed to light in their natural soil habitat, and, in consequence, the metabolic effects of light exposure on mycelia’s molecular status remain largely uncharacterized. Meanwhile, such information may fill knowledge gaps and provide baseline data to understand their ecological resilience, abiotic stress responses, and metabolic regulation. This study analyzed the impact of white light on the metabolome of the ectomycorrhizal fungus Paxillus involutus. Mycelia were grown for six weeks under darkness (control) and white light conditions (150 μmol·m−2·s−1; 16/8 day/night periods). Treated mycelia were characterized by phenotypic alterations, mainly decreased growth and formation of more compact hyphal biomass. Although treated Paxillus had increased H2O2 concentrations, P. involutus exposed to light had unchanged malondialdehyde (MDA) levels, indicating that light exposure does not lead to severe oxidative stress. Indeed, a high-throughput GC-MS study revealed that the metabolome of mycelia exposed to light did not differ significantly from that of the controls, with only a few compounds showing altered abundances, all of which were more abundant in the light-exposed treatment. Lyxose and ribitol showed significant differences between treatments based on unadjusted p-values, as confirmed by individual t-tests, whereas only xylonic acid remained significant after FDR correction (α = 0.05). This finding is consistent with the overall pattern observed in the PCA and suggests that subtle changes in metabolite abundances may contribute to the observed phenotypic modifications. This first high-throughput metabolomic analysis of ectomycorrhizal mycelium exposed to light indicates that, although light is commonly considered a stress factor for soil fungi, it does not cause significant modifications in the Paxillus involutus metabolome, despite visible phenotypic changes in the mycelium. Although this study focuses solely on mycelium and does not investigate different light spectra, it may help explain potential implications for symbiotic plant interactions under fluctuating light conditions in soil ecosystems.