Can seed-borne endophytes promote grass invasion by reducing host dependence on mycorrhizas?

Symbiotic interactions between plants and microorganisms have recently become the focus of research on biological invasions. However, the interaction between different symbionts and their consequences in host-plant invasion have been seldom explored. Here, we propose that vertically transmitted fungal endophytes could reduce the dependency of invasive grasses on mycorrhizal fungi allowing host establishment in those environments where the specific mutualist may be not present. Through analyzing published studies on nine grass species, we evaluated the effect of seed-borne Epichloe endophytes on the relationship of invasive and noninvasive grasses with arbuscular mycorrhizal fungi (AMF), a symbiosis known to be fundamental for plant fitness and invasion success. The endophyte effect on AMF colonization differed between invasive and noninvasive grasses, reducing mycorrhization only on invasive species but with no impact on their biomass. These results allowed us to propose that Epichloe endophytes could reduce the dependency of host plants on the mutualism with AMF, promoting host grass establishment and subsequent invasion. Simultaneous interactions with different types of mutualists may have profound effects on the host-plant fitness facilitating its range expansion. Our findings suggest that some specific mutualistic fungi such as epichloid endophytes facilitate host invasion by reducing the requirements of the benefits derived from other mutualisms.