A group from Technical University of Denmark, Kongens Lyngby, Denmark, etc. has reported that Bacillus velezensis recruited Pseudomonas stutzeri for plant through metabolic cross-feeding
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8483172/
Generally, Bacillus spp. and Pseudomonas spp. are the most extensively studied beneficial microorganisms in the rhizosphere. Plant beneficial Bacillus, including B. velezensis SQR9 are known for plant growth promotion, disease suppression, and enhanced salt stress tolerance. Root colonization and plant growth-promoting properties require efficient biofilm formation (including exopolysaccharide EPS and TasA protein fibers) on the roots.
To explore the effects of B. velezensis SQR9 on rhizosphere microbiota, two-weeks-old cucumber seedlings were inoculated with strain SQR9 and the rhizosphere soil samples were collected after sixteen days. It was found that members of the genera Pseudomonas, Vogesella, Pseudoxanthomonas, Chryseobacterium, Pseudoduganella, Lysobacter, Klebsiella, and Cellvibrio were increased after SQR9 application. Remarkably, 38% of increased rhizobacteria mapped to the Pseudomonas genus, suggesting that B. velezensis SQR9 may recruit and then synergistically interact with Pseudomonas spp.
The rhizobacteria consortium had a stronger promoting effect in paddy soil, as the dual-species (Bacillus and Pseudomonas) consortium significantly increased the shoot height, shoot dry weight, and chlorophyll content of plants in comparison to plant inoculated with one species. In the case of salt treated paddy soil, Pseudomonas stutzeri XL272 protects the plant against salt stress at the most comparing with non-inoculated control plants. The protective effect of the rhizobacteria consortium was higher than that of P. stutzeri XL272 alone in comparison to control plants. These results support the suggestion that the PGPR can induce the assemblage of the indigenous beneficial microbiome, leading to the promotion of plant health and resistance to salt stress.
A big question is how B. velezensis SQR9 recruited Pseudomonas spp. etc.? Authors suggested that the metabolic cross-feeding such as branched-chain amino acids would be a key to understand the mechanism through analysis transcriptional alterations of bacillus and pseudomonas.
The image is like this; B. velezensis SQR9 is attracted by root exudates and colonizes the rhizosphere. After establishing biofilm on plant roots, it secretes metabolites that increase the abundance of indigenous plant beneficial genera (such as Pseudomonas spp.). Forming tightly associated biofilm, they share extracellular matrix and essential metabolites that increase their fitness in the rhizosphere, and help plant growth and increase resistant to salt stress.