Evidence of non-pathogenic relationship of Alternaria section Undifilum endophytes within three host locoweed plants species

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 that 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 reveal 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. oxy...