Abstract
The role of arbuscular mycorrhizas in response of plants to soilborne root pathogens is unclear. A time course study was conducted to monitor disease development and expression of mRNA for the defence-related genes phenylalanine ammonia lyase, chalcone synthase, chalcone isomerase, and hydroxyproline-rich glycoprotein in bean (Phasoelus vulgaris L.) plants colonized by the arbuscular mycorrhizal fungus Glomus intraradices Schenck & Smith and postinfected with the soilborne pathogen Rhizoctonia solani Kühn. Precolonization of bean plants by G. intraradices did not significantly reduce the severity of rot symptoms. RNA blot analysis of the defence-related genes revealed a systemic increase in the four defence genes in response to R. solani infections. On the other hand, precolonization of bean plants with G. intraradices elicited no change in phenylalanine ammonia lyase, chalcone synthase, and chalcone isomerase transcripts. A differential and systemic alteration in the expression of all four defence genes was observed in all tissues only during the pathogenic interaction of arbuscular mycorrhizal beans. Depending on the time after infection with R. solani and the tissue examined, varying responses from stimulation to suppression to no change in transcript levels were detected.Key words: induced resistance, defence-related genes, RNA analysis, Rhizoctonia solani, Glomus intraradices.
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