Actinobacteria are involved in many soil processes related to plant growth, including the secretion of various bioactive products and nutrient cycling. The aim of this study was to characterize the plant growth promoting traits (phosphate solubilizing activity, phytate degradation ability and indolic compounds production, and organic acids secretion) of seven actinobacteria isolates and evaluate the effect of two selected rhizospheric Streptomyces on growth and phosphorus content of maize in presence of different phosphorus (P) sources (tri-calcium phosphate, triple super phosphate and calcium phytate). Phylogenetic analysis of partial 16S rRNA sequences showed that the Streptomyces isolates were clustered in two distinct separated groups. All Streptomyces isolates produced 2-ketogluconic acid and oxalic acid in fermentation medium supplemented with inorganic P sources, while in the presence of phytate, all isolates secreted 2-ketogluconic acid and malic acid. All the isolates were able to release inorganic P from insoluble phosphate sources and this behavior was negatively correlated with pH reduction. Phytate degradation raged from 1.65% to 95.2%, and total indole compound production ranged from 6.3 to 27.8 µg ml−1. The shoot dry weight, root dry weight, plant height, chlorophyll content and tissue P concentration were significantly (p < 0.05) increased by both inoculations with Streptomyces isolates and by the addition of different phosphorus sources, compared to their appropriate controls. The inoculation of Streptomyces isolates resulted in a significant increase in plant phosphorus content from different phosphate sources. Streptomyces sp. UTMC 1478 (Streptomyces sp. 63) and Streptomyces sp. 47 were suitable inoculants for increasing maize growth and P from soil amended with different phosphorus sources, indicating their potential use as plant growth-promoting phospho-bacteria.
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