In vitro inoculation effects and colonization pattern of Leohumicola verrucosa, Oidiodendron maius, and Leptobacillium leptobactrum on fibrous and pioneer roots of Vaccinium oldhamii hypocotyl cuttings

The fine root systems of ericaceous plants possess both absorptive fibrous roots and skeletal pioneer roots. This functional differentiation (heterorhizy) can be coupled with a specific mycorrhization pattern, in which fibrous roots form more ericoid mycorrhizas than pioneer roots. However, the behaviors of root-associated fungi on the different individual roots remain largely unknown. We investigated the colonization patterns and root modifications of Leohumicola verrucosa, Oidiodendron maius, and Leptobacillium leptobactrum strains on Vaccinium oldhamii hypocotyl cuttings in vitro. Three putative and true mycorrhizal fungal strains (one L. verrucosa and two O. maius) exhibited typical heterorhizic colonization patterns and tended to increase pioneer root branching. However, a root endophytic fungus (L. leptobactrum) uniformly exhibited a lower colonization across the different roots and did not increase the number of pioneer root branches. The colonization patterns and root modification ability of mycorrhizal fungi may be linked, thereby ensuring sufficient colonization sites.