Dual-mycorrhizal colonization is determined by plant age and host identity in two species of Populus

Plants have evolved symbioses with mycorrhizal and endophytic fungi that are essential for their growth and survival. While most plants associate with a single guild of mycorrhizal fungi, a select group termed “dual-mycorrhizal plants” associate with both arbuscular mycorrhizal and ectomycorrhizal fungi. Although a shift from arbuscular mycorrhizal to ectomycorrhizal colonization with plant development has been demonstrated on other dual-mycorrhizal hosts, it is not known how mycorrhizal colonization shifts with plant age in Populus species. We performed an in planta mycorrhization experiment to test for the occurrence of mycorrhizal switching in response to plant age and for host-specific patterns of fungal colonization in two species of Populus ( P. tremuloides and P. trichocarpa ). We found that only P. trichocarpa displayed dual-mycorrhizal colonization, while P. tremuloides associated with ectomycorrhizal fungi, but not arbuscular mycorrhizal fungi. On P. trichocarpa , both guilds of mycorrhizal fungi increased in abundance with plant age, while root endophytic fungal colonization decreased. Many of the early-colonizing endophytic fungi that we documented have strong saprotrophic capabilities, which may be an important trait for fast colonization. Dark septate endophytes were more abundant than either guild of mycorrhizal fungi, and are likely functionally important members of the Populus root fungal community. Our findings represent a novel pattern in the development of dual-mycorrhizal colonization and illustrate that Populus species vary in their association with arbuscular mycorrhizal fungi. Our results also stress the importance of a “third guild” of root fungal symbionts – the dark septate endophytes – on dual-mycorrhizal plants.