Interactions between pea root-inhabiting fungi examined using signature fatty acids.

• Interactions are investigated between the arbuscular mycorrhizal fungus, Glomus mosseae, and the root pathogen, Aphanomyces euteiches, on pea (Pisum sativum) roots, as arbuscular mycorrhiza are known to suppress a broad range of root pathogens. •  Phospholipid (PLFA) and neutral lipid (NLFA) fatty acids were used as indicators of biomass and energy reserves, respectively, of A. euteiches and G. mosseae in inoculated roots of pot-grown pea seedlings. • Symbiosis between pea and G. mosseae had no effect on the severity of disease caused by A. euteiches, which decreased pea shoot and root dry weight. However, the presence of G. mosseae in pea roots reduced both biomass and energy reserves of A. euteiches, indicated by a reduction in PLFA 14 : 0 and both NLFAs 14 : 0 and 14 : 1ω9. Similarly, a reduction in PLFA and NLFA 16 : 1ω5 indicated reduced biomass and energy reserves of G. mosseae in A. euteiches-infected roots. •  Signature fatty acids can be used to quantify biomass and energy reserves of G. mosseae and A. euteiches simultaneously, in pea root; this appears to be a promising method for studying interactions between arbuscular mycorrhizal fungi and root pathogens in planta.