Influence of saprobe fungi and their exudates on arbuscular mycorrhizal symbioses

The effects of saprobe fungi Aspergillus niger (341), Penicillium restrictum (512) and Trichoderma harzianum (153) isolated from soil, or their exudates on the arbuscular mycorrhizal (AM) fungi Glomus mosseae and Gigaspora rosea were studied in greenhouse and in vitro experiments. A. niger and its exudates stimulated the germination and hyphal outgrowth of G. mosseae and Gi. rosea spores and AM colonization of roots by these and native fungi in an unsterilized soil. In contrast, T. harzianum and its exudates had no effects, and P. restrictum and its exudates had no effects on spore germination or hyphal outgrowth from spores of either G. mosseae or colonization by the native AM endophytes in an unsterilized soil. Effect of A. niger were dependent on the concentration of exudates and varied according to the AM endophyte. For example, the application of 1-2.5 mul/ml of exudates of A. niger increased the outgrowth of hyphae from spores of G. mosseae, but for Gi. rosea increases only occurred in the range 1-10 mul/ml. On the other hand, the colonization of plants by Gi. rosea increased with the application of 10 ml/pot of exudates of A. niger, but the colonization of soybean by G. mosseae or by native AM endophytes increased in the presence of 20 mul/pot of exudates. The similarity in trends observed in the effects of the saprobe fungi or their exudates on spore germination and hyphal length and effects on AM colonization of plants and number of propagules of AM fungi suggest that the effects of saprobe fungi are largely due to effects on the extraradical phase of the AM fungi.