Mycorrhizal colonization of transgenic aspen in a field trial.

Mycorrhizal colonization of genetically modified hybrid aspen (Populus tremula x P. tremuloides Michx.) was investigated over 15 months in a field experiment. The aspen carried the rolC gene from Agrobacterium rhizogenes under control of either the constitutive cauliflower mosaic virus 35S promoter or the light-inducible rbcS promoter. Arbuscular mycorrhizas (AMs) were rare in all root samples, while fully developed ectomycorrhizas (EMs) were found in all samples. No significant differences in the degree of mycorrhizal colonization between aspen lines were seen with either AMs or EMs. The EM community on the release area was dominated by four fungal species that formed more than 90% of all mycorrhizas, while eleven EM types were found occasionally. Mycorrhizal diversity did not differ between transgenic and non-transgenic trees. The structure of mycorrhizal communities was similar for most aspen lines. The sole significant difference was found in the abundance and development of one of the four common EM morphotypes, which was rare and poorly developed on roots from the transgenic aspen line Esch5:35S-rolC-#5 compared with non-transgenic controls. This effect is clone specific as the formation of this EM type was not affected by the transgene expression in the other transgenic line, Esch5:35S-rolC-#1. This is the first demonstration of a clonal effect influencing the ability of a transgenic plant to form a mycorrhizal symbiosis with a potential fungal partner.