Flavonoids Are Not Necessary Plant Signal Compounds in Arbuscular Mycorrhizal Symbioses. Guillaume Becard. Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Philadelphia, PA 19118, U.S.A. Loverine P.Taylor(2), David D. Douds, Jr.(1), Philip E. Pfeffer(1), and Landis W. Doner (1) (1)Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Philadelphia, PA 19118, U.S.A. and (2) horticulture Dept. and Program in Plant Physiology, Washington State University, Pullman, WA 99164 U.S.A. MPMI 8:252-258. Accepted 1 November 1994. 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, 1995.

Some common flavonoids have been shown to greatly stimulate growth of arbuscular mycorrhizal (AM) fungi. Although exudates of Ri T-DNA transformed roots of carrot similarly stimulate growth of AM fungi, HPLC (retention time), UV spectral and 1H NMR analyses of tissue detected no flavonoids but rather various derivatives of caffeic acid. A pollen germination rescue bioassay also detected no flavonoids in root extracts. The addition of polyvinypolypyrrolidone to the culture medium to sequester putative exuded flavonoids did not prevent successful mycorrhizal establishment. Maize plants deficient in chalcone synthase activity, necessary for the biosynthesis of flavonoids, were equally colonized with AM fungi as maize with chalcone synthase. These results suggest that root metabolites in addition to flavonoids may stimulate AM fungal growth and that flavonoids are not necessary for the establishment of mycorrhizal symbiosis.