Identification of a novel genetically controlled step in mycorrhizal colonization: plant resistance to infection by fungal spores but not extra-radical hyphae

Vesicular arbuscular mycorrhizal fungi infect plants by means of both spore s and vegetative hyphae at early stages of symbiosis. Using 2500 M2 fast-ne utron-mutagenized seeds of the miniature tomato (Lycopersicon esculentum) c ultivar, Micro-Tom, we isolated a mutant, M161, that is able to resist colo nization in the presence of Glomus intraradices spores. The myc(-) phenotyp e of the mutant was stable for nine generations, and found to segregate as a single Mendelian recessive locus. The mutant exhibited morphological and growth-pattern characteristics similar to those of wild-type plants. Altera tions of light intensity and day/night temperatures did not eliminate the m yc(-) characteristic. Resistance to mycorrhizal fungal infection and coloni zation was also evident following inoculation with the fungi Glomus mosseae and Gigaspora margarita. Normal colonization of M161 was evident when muta nt plants were grown together with arbuscular mycorrhizal-inoculated wild-t ype plants in the same growth medium. During evaluation of the pre-infectio n stages in the mutant rhizosphere, spore germination and appressoria forma tion of G. intraradices were lower by 45 and 70%, respectively, than the ra tes obtained with wild-type plants. These results reveal a novel, genetical ly controlled step in the arbuscular mycorrhizal colonization process, gove rned by at least one gene, which significantly reduces key steps in pre-myc orrhizal infection stages.