Defense genes are differentially induced by a mycorrhizal fungus and Rhizobium sp in wild-type and symbiosis-defective pea genotypes

Mycorrhiza-resistant and non-nodulating pea mutants provide a model system for identifying common genes regulated during the early events in mycorrhiz a and nodule establishment. Inoculation of pea roots with Glomus mosseae or Rhizobium leguminosarum can induce overexpression of seven defense-related genes (pI 206, pI 49, pI 176, PR 10, basic A1-chitinase, transcinnamic aci d 4-hydroxylase, chalcone isomerase), depending on the plant genotype and t he time point of interaction between the plant and the microsymbiont. Expre ssion of the pI 206 gene is closely correlated with appressorium formation by the mycorrhizal fungus on both mutant and wild-type pea roots, The gene is also induced by the pathogen Aphanomyces euteiches, Transcript accumulat ion was higher in mutant than in wild-type genotypes for five and six of th e studied genes during early stages of root interactions with G. mosseae an d R. leguminosarum, respectively, and this is discussed in relation to the symbiotic-defective phenotype of Myc(-1)Nod(-) pea. The early induction of similar plant defense genes in response to arbuscular mycorrhizal fungi and rhizobia reinforces the hypothesis of common regulatory steps in both root symbioses.