Biodiversity in rhizosphere is fundamentally important to enhance plant growth rather an inoculating beneficial bacteria directly

A group from Nanjing Agricultural University, Weigang, Nanjing, China, has reported that the beneficial effects of microbial inoculation can be driven indirectly through effects on the diversity and composition of the resident plant rhizosphere microbiome.
https://pubmed.ncbi.nlm.nih.gov/34641724/

One way to increase microbiome-associated multifunctionality is the introduction of additional rhizobacteria into the soil. For a long time, microorganisms have been selected on the basis of their ability to directly express functions of interest, such as nutrient mineralization, nitrogen fixation or pathogen suppression. However, such direct effects are notoriously unstable as the introduced species fail to establish at the density needed to function in a natural microbiome context. Further, inherent trade-offs in microbial physiology will limit the expression of functions one microbial species or strain can provide to the plant.

It is so important to understand if the potential benefits were driven directly by inoculated rhizobacteria via introduction of essential functions to the microbiome, or indirectly via changes in the resident bacterial community.

In this study, 8 different Pseudomonas strains were used as inoculants at the maximum, and the plant growth enhancement was measured by using tomato.

It was found that increasing probiotic consortium richness increased inoculant colonization success, and was associated with improved plant growth, nutrient assimilation and protection from pathogen infection. Crucially, inoculants caused shifts in the resident microbiome and these effects were magnified with increasing inoculated numbers of Pseudomonas strains, leading to an increase in the abundance of rare taxa and overall microbiome biodiversity. While plant-beneficial functions of Pseudomonas strains in vitro predicted poorly the individual plant growth characteristics. The only notable exception was a positive relationship between the Pseudomonas phosphate solubilization capacity and the plant shoot phosphorus content. Instead, positive effects on the plant growth were best explained by inoculated Pseudomonas-mediated shifts in the resident microbiome, which were associated with phytohormone production and resource competition by the probiotic consortia.