REGULATION OF ARBUSCULAR MYCORRHIZAL DEVELOPMENT BY PLANT HOST AND FUNGUS SPECIES IN ALFALFA

Two cvs of alfalfa (Medicago sativa L.), Gilboa and Moapa 69, were ino culated in glasshouse pots with three arbuscular mycorrhizal (AM) fung i to investigate the efficacy of mycorrhizas with respect to the exten t of colonization and sporulation. Paspalum notatum Flugge also was in oculated to describe fungal parameters on a routine pot culture host. Percentage root length of P. notatum colonized by Glomus mosseae (Nico l. & Gerd.) Gerdemann & Trappe, Glomus intrardices Schenck & Smith, an d Gigaspora margarita Becker & Hall increased from 10 to 21 wk, and al l fungi sporulated during that period. In alfalfa, only colonization b y G. intraradices increased over that time period, and it was the only fungus to sporulate in association with alfalfa at 10 wk. Glomus moss eae did not sporulate after 16-21 wk despite having colonized 30-35% o f the root length of both alfalfa cvs. In vitro experiments in which R i T-DNA-transformed roots of alfalfa were inoculated with AM fungi sho wed normal mycorrhizal formation by G. intraradices and a hypersensiti vity-like response to Gi. margarita. Colonized cells became necrotic, and HPLC analysis indicated increased concentrations of phenolics and isoflavonoids in these root segments. These data strongly support the existence of a degree of specificity between AM fungi and host that mi ght rely on specific biochemical regulatory processes initiated in the host as a result of the attempts at colonization by the fungus.