The defense response elicited by the pathogen Rhizoctonia solani is suppressed by colonization of the AM-fungus Glomus intraradices.

Defense responses of alfalfa roots to the pathogenic fungus Rhizoctonia solani were reduced significantly in roots simultaneously infected with the vesicular arbuscular mycorrhizal (AM) fungus Glomus intraradices. R. solani induced five- to tenfold increases in the steady-state levels of chalcone isomerase and isoflavone reductase mRNAs a doubling of root peroxidase activity and a marked autofluorescence in the infected tissue. These changes were inhibited by the presence of G. intraradices. Interestingly, germination of G. intraradices spores and hyphal elongation were sensitive to low concentrations (2 µM) of medicarpin-3-O-glucoside, an isoflavonoid phytoalexin that accumulated both in roots colonized by the pathogenic fungus as well as in AM-treated roots receiving high P, where no colonization by the beneficial fungus occurred. These data support the hypothesis that during early stages of colonization by G. intraradices, suppression of defense-related properties is associated with the successful establishment of AM symbiosis.