Evaluation of actinomycete strains for key traits related with plant growth promotion and mycorrhiza helping activities

Thirty actinomycete strains were isolated from the rhizosphere of field-grown plants (Trifolium repens L.) and identified by microscopic characteristics, biochemical tests and molecular techniques. The 16S rDNA sequence analysis showed that most of the isolates belong to the Streptomyces genus. These thirty isolates were tested for their capabilities of solubilizing/mineralizing sparingly phosphate sources, N2-fixation and/or siderophore production, typical traits of the so-called Plant Growth Promoting Rhizobacteria (PGPR). Phosphate solubilizing ability was widely exhibited by the isolated. All of them produced acid phosphatase and thirteen of them alkaline phosphatase. Ten strains grew in N-free media. Almost all strains produced siderophores, however the production level was in general very low and only the strain Thermobifida MCR24 released considerable amounts of this metabolite. Streptomyces MCR9, Thermobifida MCR24 and Streptomyces MCR26 were selected to test their interactions with arbuscular mycorrhizal (AM) fungi because they produced the highest plant growth beneficial effects among ten isolates preselected as promissory PGPR. The inoculation of AM fungi improved early establishment of MCR9 in clover rhizosphere, and the late establishment of MCR24. The three actinomycete strains improved AM mycelial growth in absence of the test plant, and MCR9 and MCR26 also stimulate AM fungal spore germination. Inoculation of clover plants with either of the selected actinomycetes enhanced plant growth and N acquisition. Co-inoculation of actinomycetes and Glomusmosseae produced synergic benefits on plant growth and MCR9 and MCR24 also on P acquisition. The three selected actinomycetes improve AM formation by clover plants and Glomus mosseae.