Colonization characteristics of dark septate endophytes in roots of invasive Ipomoea cairica.
Dark septate endophytes (DSE) widely colonize invasive Ipomoea cairica in southern China. It was observed under light microscope that I. cairica roots were colonized by a large number of lipid-filled vesicles, and the most striking feature was that a great quantity of lipids were accumulated and decomposed into small lipid bodies which were transferred through hyaline or dark hyphae, demonstrating that carbon transfer between I. cairica and DSE. Lipid-filled vesicles could produce hyaline hyphae and self-duplication, indicating that vesicles had reproductive capacity. Atypical structures such as lipid-filled vesicle and hyaline hyphae without obvious wall or with transparent wall were considered to be active fungal structures. Hyaline hyphae and lipid-filled vesicles were mainly colonized in physiologically active roots. Dark septate hyphae and microsclerotia were typical structures of DSE, mainly colonized in relatively inactive roots. DSE colonizes the vascular cylinder and cortical and epidermal cells of I. cairica and extends to the soil, indicating that DSE forms a continuous and systematic fungal network between host plant and soil. The potential symbiotic relationship between DSE and I. cairica was discussed in the light of the polymorphic nature of DSE, the huge fungal network and accumulation and distribution of lipids.