Evidence for nonpathogenic relationships ofAlternariasectionUndifilumendophytes within three host locoweed plant species

Abstract

Locoweeds are legumes of the Astragalus and Oxytropis genera that live symbiotically with the fungal endosymbionts belonging to Alternaria section Undifilum. These endophytes produce the toxin swainsonine, which causes a neurological syndrome (locosim) when the plant is ingested by grazing animals. Here we characterize the growth of the endophytes within plant tissues using confocal and scanning electron microscopy (SEM) to define the physical relationship. Microscopy results revealed the endophyte distribution to be densely networked, with the mycelia aligned parallel to the long axis of the plant stem and with no pathology to plant cell walls, xylem, or phloem. The autofluorescent mycelia were located intercellulary in the pith of stems. Stereofluoroscope observations of Alternaria oxytropis, A. cinerea, and A. fulva in phytoagar showed that mycelial growth occurred at the tip of hyphae. The growth rate of A. cinerea was significantly faster than for A. oxytropis or A. fulva. All three species of endophytes grew significantly faster at 3 days age of culture and grew slowly or not at all after 20 or 30 days. Analyses of the growth of these fungi support the hypothesis that there is a nonpathogenic symbiosis between the fungal endophytes and their host plants.