The differential effects of cell wall-associated phenolics, cell walls, and cytosolic phenolics of host and non-host roots on the growth of two species of AM fungi

Experiments were conducted to test the hypothesis that cellular compounds, especially wall-associated compounds, released during emergence of secondary roots, stimulate the growth of arbuscular mycorrhizal (AM) fungi. Purified cell walls, crude cell-wall extracts, crude cytoplasmic extracts, and phenolic compounds previously identified as cell wall-associated, from Ri T-DNA-transformed roots of host (Daucus carota L.) and non-host (Beta vulgaris L.) were incorporated into growth medium and tested for their effects upon growth of the AM fungi Gigaspora gigantea (Nicol. & Gerd.) Gerdemann and Trappe and Gigaspora margarita Becker and Hall. Purified cell walls of both plants had little effect on G. gigantea but non-host cell walls inhibited the growth of G. margarita. Ferulic acid, a major constituent of non-host root, depressed the growth of both fungi. Nothing tested which was unique to the non-host root affected hyphal growth to the point that contact would be prohibited. Caffeic acid, found in D. carota cytoplasm, also depressed growth of both fungi. Para-hydroxybenzoic acid, a constituent of D. carota roots, stimulated growth of G. margarita hyphae, but did not affect hyphal growth of G. gigantea. Vanillic acid, unique to D. carota root cell-wall extracts, stimulated hyphal growth and branching of both fungi, and should increase the probability of contact between fungus and host root.