Rutin, the phenolglycoside from eucalyptus root exudates, stimulates <i>Pisolithus</i> hyphal growth at picomolar concentrations

• Ectomycorrhizal hyphal growth is shown to be stimulated by a phenol compound isolated from Eucalyptus globulus ssp. bicostata root exudates, highlighting the importance of phenolics in host-fungal interaction. • HPLC analysis allowed separation and identification of phenolic compounds from Eucalyptus seedling tissues and root exudates. The activity of the flavonol, rutin, was tested on a range of mycorrhizal and saprophytic fungi. • Rutin stimulated Pisolithus hyphal growth by more than twofold, and the fungus responded significantly to concentrations as low as 1 pM; only a few strains responded. • Rutin from Eucalyptus globulus ssp. bicostata root exudates is a flavonoid signal for Pisolithus, and is the first such flavonoid signal identified. A rutin gradient could contribute to orientating hyphal elongation toward the root tip thereby favouring mycorrhizal infection, and might also influence the interaction between fungi in the rhizosphere.