Seasonal changes in Undifilum colonization and swainsonine content of locoweeds

Locoweeds (Astragalus and Oxytropis) are leguminous plants that are toxic due to a symbiotic association with the endophytic fungus Undifilum oxytropis. The fungus produces the alkaloid swainsonine, an α-mannosidase-inhibitor that causes serious damage to mammals when consumed. A real-time PCR technique was developed to quantify the colonization extent of Undifilum in locoweeds and to compare it to the swainsonine concentration in the plants. Amplification of the endophyte nuclear ITS region allowed reliable quantification of Undifilum DNA from field plants and in vitro cultures. Swainsonine concentration was highly correlated (ρ = 0.972, P < 0.001) with the proportion of Undifilum DNA during the first 4 weeks of in vitro culture growth. Species of Astragalus and Oxytropis were sampled seasonally in New Mexico and Colorado for two years. High swainsonine concentration in plant samples was associated with high levels of endophyte DNA, except in plant reproductive tissues.