Phosphate solubilizing bacteria associated with arbuscular mycorrhizal fungi, beneficial symbionts of crop plants

An important group of beneficial soil microorganisms is represented by arbuscular mycorrhizal fungi (AMF) which establish mutualistic symbioses with the roots of most food crops and play a key role in soil fertility and plant nutrition. AMF performance may be positively affected by functionally diverse bacteria associated with their spores, which differentially enhance nutrient availability to plant roots by different mechanisms, such as phosphorus (P) solubilization. In our work we utilized: i) a culture-independent approach, PCR-DGGE, to identify the diverse bacterial species associated with AMF spores; ii) a culture-dependent approach to isolate bacteria with functionally important traits, such as P-solubilizing activity. Sequence analysis of the major DGGE bands showed the occurrence of Arthrobacter, Bacillus, Paenibacillus, Pseudomonas, Rhizobium, genera including P-solubilizing strains. Isolation of the microbiota associated with Rhizophagus intraradices spores yielded 5.4±0.9-23±0.7 CFU of bacteria per spore. 374 strains were isolated in pure culture (135 actinobacteria, 189 heterotrophic, 27 N-fixing and 23 chitinolytic bacteria) and then functionally characterized for P-solubilizing, chitinase, nitrogen fixing activity and production of siderophores, indole acetic acid (IAA) and exopolysaccharides (EPS). Phosphatase and phytase activities were detected in 73% and 100% of Actinobacteria, in 74% and 83% of chitinolytic bacteria and in 44% and 52% of nitrogen-fixers. The most efficient strains, after molecular characterization, were identified as Sinorhizobium meliloti, Arthrobacter phenanthrenivorans, Nocardioides albus and Streptomyces spp. Such P-solubilising strains, alone or in combination with AMF, could be used as efficient biofertilizers and biohenancers in sustainable plant production systems.