Isoflavonoid Accumulation and Expression of Defense Gene Transcripts During the Establishment of Vesicular-Arbuscular Mycorrhizal Associations in Roots of Medicago truncatula. Maria J. Harrison. The Samuel Roberts Noble Foundation, Plant Biology Division, P.O. Box 2180, Ardmore, Oklahoma 73402, U.S.A. Richard A. Dixon. The Samuel Roberts Noble Foundation, Plant Biology Division, P.O. Box 2180, Ardmore, Oklahoma 73402, U.S.A. MPMI 6:643-654. Accepted 17 June 1993. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1993.

The levels of flavonoid and isoflavonoid secondary metabolites and mRNA transcripts encoding enzymes of their biosynthesis were measured in Medicago truncatula and Medicago sativa roots during colonization with the vesicular-arbuscular mycorrhizal fungus Glomus versiforme. Distinct qualitative and quantitative changes in secondary metabolites occurred during the establishment of the symbiosis, including transient increases in the level of the phytoalexin medicarpin during the early stages of colonization. Levels of phenylalanine ammonia-lyase (PAL) and chalcone synthase (CHS) transcripts were elevated in mycorrhizal roots, consistent with increased flavonoid biosynthesis. In contrast, isoflavone reductase (IFR) transcripts, which encode an enzyme specific for medicarpin biosynthesis, decreased below the levels in control roots during the later stages of the interaction. The decrease in IFR transcripts, and the subsequent decrease in medicarpin levels, did not occur in unsuccessful interactions between G. versiforme and a mycorrhizal minus (myc¯) line of M. sativa.