GFP transformation sheds more light on a widespread mycoparasitic interaction

Powdery mildews (PMs), ubiquitous obligate biotrophic plant pathogens, are often attacked in the field by mycoparasitic fungi belonging to the genus Ampelomyces. Some Ampelomyces strains are commercialized biocontrol agents of crop pathogenic PMs. Using Agrobacterium tumefaciens-mediated transformation (ATMT), we produced stable Ampelomyces transformants that constitutively expressed the green fluorescent protein (GFP), to (i) improve the visualization of the PM-Ampelomyces interaction; and (ii) decipher the environmental fate of Ampelomyces before and after acting as a mycoparasite. Detection of Ampelomyces structures, and especially hyphae, was greatly enhanced when diverse PM, leaf and soil samples containing GFP transformants were examined with fluorescence microscopy compared to brightfield and DIC optics. We showed for the first time that Ampelomyces can persist up to 21 days on PM-free host plant surfaces, where it can attack PM structures as soon as these appear after this period. As a saprobe in decomposing, PM-infected leaves on the ground, and also in autoclaved soil, Ampelomyces developed new hyphae, but did not sporulate. These results indicate that Ampelomyces occupies a niche in the phyllosphere where it acts primarily as a mycoparasite of PMs. Our work has established a framework for a molecular genetic toolbox for Ampelomyces using ATMT.