New aspects on the acquisition of biotrophic status by a vesicular—arbuscular mycorrhizal fungus, Gigaspora margarita

Abstract

An in vitro system for culturing the vesicular arbuscular (VA) mycorrhizal fungus Gigaspora margarita Becker & Hall with Ri T‐DNA transformed roots of carrot has been used to investigate the progressive dependency of a VA mycorrhizal fungus on its host during mycorrhizal establishment. The growth of hyphae from germinating spores of G. margarita was measured under different combinations of the following treatments: in absence or presence of root, with or without mycorrhizal colonization, with or without spore removal. Two distinct mechanisms of hyphal growth stimulation have been identified. The first required the presence of the root hut remained spore dependent. The second is exclusively root dependent. It has been demonstrated that the second mechanism is triggered when arbuscules are forming, indicating they are necessary for establishment of biotrophy. It is proposed that roots contribute to fungal growth by an inductive action, in addition to providing nutrients. Copyright © 1989, Wiley Blackwell. All rights reserved