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

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Abstract
Summary: 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.