Localization of the fungal symbiont (Chaetothyriales) in Ipomoea carnea

Plants belonging to the genera Astragalus, Oxytropis, Ipomoea, Sida and Swainsona often contain the toxin swainsonine (SW), which causes a neurological disorder in livestock. It has been demonstrated that SW is produced by a symbiont fungus associated with these plants. In Ipomoea carnea, the fungus belongs to the ascomycetes of the order Chaetothyriales and grows ectopically on the adaxial surface of leaves. In this study, the presence of the symbiont fungus in different organs of I. carnea is investigated using imaging techniques to understand the process of endophyte transmission. In I. carnea plants with whitish mycelia on the leaf surface, SW could be detected in seeds. Inside the seeds, the mycelium was located on the surface of the cotyledons and on the inner side of the seed coat. We corroborate that the presence of this fungus is a prerequisite for the presence of SW in I. carnea. The demonstration of the localization of the fungus within the seed strongly suggests that this endophyte is vertically transmitted. Our observations verify that the SW-producing fungus in I. carnea is an endosymbiont with epibiotic behavior.