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 sol ani were reduced significantly in roots simultaneously infected with the ve sicular arbuscular mycorrhizal (AM) fungus Glomus intraradices. R. solani i nduced five- to tenfold increases in the steady-state levels of chalcone is omerase and isoflavone reductase mRNAs a doubling of root peroxidase activi ty and a marked autofluorescence in the infected tissue. These changes were inhibited by the presence of G. intraradices. Interestingly, germination o f G, intraradices spores and hyphal elongation were sensitive to low concen trations (2 muM) of medicarpin-3-O-glucoside, an isoflavonoid phytoalexin t hat accumulated both in roots colonized by the pathogenic fungus as well as in AM-treated roots receiving high P, where no colonization by the benefic ial fungus occurred. These data support the hypothesis that during early st ages of colonization by G. intraradices, suppression of defense-related pro perties is associated with the successful establishment of AM symbiosis. (C ) 2001 Elsevier Science Ireland Ltd. All rights reserved.