Effects of mycorrhizal-associated streptomycetes on growth ofLaccaria bicolor, Cenococcum geophilum, andArmillariaspecies and on gene expression inLaccaria bicolor

Interaction with Soil Bacteria Affects the Growth and Amino Acid Content of Piriformospora indica

Exploration of the effect of soil bacteria on growth and metabolism of beneficial root endophytic fungi is relevant to promote favorable associations between microorganisms of the plant rhizosphere. Hence, the interaction between the plant-growth-promoting fungus Piriformospora indica and different soil bacteria was investigated. The parameters studied were fungal growth and its amino acid composition during the interaction. Fungus and bacteria were confronted in dual cultures in Petri dishes, either through agar or separated by a Perspex wall that only allowed the bacterial volatiles to be effective. Fungal growth was stimulated by Azotobacter chroococcum, whereas Streptomyces anulatus AcH 1003 inhibited it and Streptomyces sp. Nov AcH 505 had no effect. To analyze amino acid concentration data, targeted metabolomics was implemented under supervised analysis according to fungal-bacteria interaction and time. Orthogonal partial least squares-discriminant analysis (OPLS-DA) model clearly discriminated P. indica-A. chroococcum and P. indica-S. anulatus interactions, according to the respective score plot in comparison to the control. The most observable responses were in the glutamine and alanine size groups: While Streptomyces AcH 1003 increased the amount of glutamine, A. chroococcum decreased it. The fungal growth and the increase of alanine content might be associated with the assimilation of nitrogen in the presence of glucose as a carbon source. The N-fixing bacterium A. chroococcum should stimulate fungal amino acid metabolism via glutamine synthetase-glutamate synthase (GS-GOGAT). The data pointed to a stimulated glycolytic activity in the fungus observed by the accumulation of alanine, possibly via alanine aminotransferase. The responses toward the growth-inhibiting Streptomyces AcH 1003 suggest an (oxidative) stress response of the fungus.

Adhesion to hyphal matrix and antifungal activity of Pseudomonas strains isolated from Tuber borchii ascocarps

Pseudomonas spp. isolates from Tuber borchii ascocarps, known to be able to produce phytoregulatory and biocontrol substances in pure culture, were used to perform studies on their possible physiological role in nature. Antimycotic activity was confirmed against fungal contaminants isolated from the ascocarps, suggesting that populations associated with Tuber borchii fruit bodies may play a role in the maintenance of ascocarp health. Fifty-five percent of strains tested were also able to release metabolites which affected T. borchii mycelial growth and morphogenesis in culture. On the contrary, growth of the arbuscular mycorrhizal fungus Glomus mosseae and the ectomycorrhizal fungus Laccaria bicolor, putative competitors of Tuber for mycorrhizal infection sites on roots, was not influenced by the presence of any bacterial strain. The possibility that these bacteria, which show antifungal activity and fungal growth modulation activities, might be incorporated in the developing ascocarp by means of their preferential adhesion to Tuber mycelium is discussed.