Diversity, Function and Potential Applications of the Root-Associated Endophytes

Both mycorrhizal fungi and systemic fungal endophytes in the Order Clavicipitales have been extensively studied. Compared to these groups, root-associated fungal endophytes have received very little attention, even though they seem common in many ecosystems. Based on published reports, comparisons between host colonization by the root endophytes and mycorrhizal fungi from various habitats suggest that endophytes are possibly as abundant as mycorrhizas (Mandyam and Jumpponen 2005). As more reports that document the abundance of root endophytes in different habitats become available, a better understanding of the ecology and functions of these endophytes seems not only logical but critical.

The term ‘endophyte’ is used to describe either bacterial or fungal intracellular symbionts of plants that do not cause any visible signs of tissue damage or adverse effects on the host (Petrini 1991; Wilson 1995; Stone et al. 2000; Schulz and Boyle 2005). Fungal root endophytes are a paraphyletic group primarily occurring in the Ascomycota, although some examples also exist for Basidiomycetous endophytes (see Verma et al. 1998; Barazani et al. 2005). In this group, we usually include all root-inhabiting fungi that are considered non-mycorrhizal based on the morphology of the colonized host roots and on fungal structures produced in colonized roots typically considered indicative of dark septate endophytes (DSE). We also include fungi that produce hyaline structures when colonizing hosts intracellularly (O’Dell et al. 1993; Barrow and Aaltonen 2001; Ohki et al. 2002; Narisawa et al. 2003), but do not form typical DSE structures. These hyaline fungi can routinely be isolated from the roots of many plant species. Well-studied systemic and foliar endophytes of grasses, such as Acremonium sp., Epichoë sp. and Neotyphodium sp., will be excluded from this discussion.