Efficiency of probiotic traits in plant inoculation is determined by environmental constrains

Management of the soil microbial community to increase crop productivity is one of the main challenges of modern agriculture, and bacterial inoculants can help to overcome this challenge. In this work, two plant growth-promoting (PGP) bacteria were evaluated under contrasting soil conditions, in order to test a rhizosphere ecology model. This model states that plants select for phosphate (P) solubilizers in poor nutrient soils and Indolic Compounds (ICs) producers in rich nutrient soils. Rice plants were single- or co-inoculated with strains of Burkholderia and Enterobacter genera in clayey and sandy soils. Diversity gradients were generated in each soil type using the dilution-to-extinction approach. Inoculant survival, colonization, and effect on plant biomass were evaluated, besides P solubilization and ICs production from both rhizospheric and endophytic bacterial communities. The PGP efficiency of the Burkholderia strain was highest in the sandy soil, which had the highest bacterial P solubilization potential, whereas the PGP efficiency of the Enterobacter strain was highest in the clayey soil, which had the highest bacterial ICs production potential. These behaviors occurred as hypothesized by the model, which can be useful for PGPB testing and bioprospection. We highlight a strong dependency of the Enterobacter strain on the diversity level as the most critical factor affecting PGP efficiency, possibly related to the increased influence of keystone taxa in lower diversity as indicated by network analysis.