Suppression of an Isoflavonoid Phytoalexin Defense Response in Mycorrhizal Alfalfa Roots

Isoflavonoids and steady-state mRNA levels of phenylalanine ammonia-lyase, chalcone isomerase, and isoflavone reductase were followed during a rapid, nearly synchronous infection of alfalfa (Medicago sativa L.) roots by the vesicular arbuscular fungus Glomus intraradices (Schenck & Smith) to test whether previously indicated suppression of the host defense response is regulated by changes in the steady-state mRNA level. Relative amounts of steady-state phenylalanine ammonia-lyase mRNA in the mycorrhizal roots doubled between d 14 and 18 and then immediately declined by 75% to reach and maintain a value lower than the control roots through d 21. Relative levels of chalcone isomerase mRNA in the inoculated roots increased 6-fold between d 14 and 17 and then decreased rapidly to the control level. Isoflavone reductase mRNA was not induced by mycorrhizal colonization. High-performance liquid chromatography, proton-nuclear magnetic resonance, and fast atom bombardment-mass spectrometry analyses showed consistent increases in formononetin levels and transient increases in medicarpin-3-O-glycoside and formononetin conjugates in the inoculated roots when colonization began. As colonization increased, levels of formononetin conjugates declined in mycorrhizal roots below those in uncolonized controls. Medicarpin aglycone, an alfalfa phytoalexin normally associated with pathogenic infections, was not detected at any stage. These findings supply detailed evidence that, during early colonization of plant roots by symbiotic Glomus, defense transcripts are induced and then subsequently suppressed.