Bacillus velezensis SQR9: a model biofertilizer strain for beneficial plant root-rhizobacterium interaction

Plant growth-promoting rhizobacteria (PGPR) are beneficial microorganisms residing in the rhizosphere that enhance plant growth and health. While fundamental research on established PGPR strains has focused on their biological functions and interactions with plants, in-depth investigations of root colonization mechanisms and microbial interactions within the rhizosphere remain limited. This review presents Bacillus velezensis SQR9 as a model PGPR strain, affirming its inclusion of conventional plant-beneficial mechanisms, including antimicrobial metabolite production, induced systemic resistance, resource competition, phytohormone production, along with novel mechanisms unique to SQR9, such as root development enhancement, nitrogen uptake promotion, and abiotic stress tolerance. The complex processes and molecular mechanisms of root colonization-including spore germination, chemotaxis, adhesion, immune evasion, and biofilm formation-are summarized. Furthermore, the influence of SQR9 on plant microbiomes and its interactions with other soil microorganisms are examined, paving the way for leveraging beneficial microbial interactions to enhance the functionality of PGPRs. We also assess the biotechnological potential of SQR9, supported by multiple patents and successful commercial applications in biofertilizer production. By elucidating the specific roles and benefits of B. velezensis SQR9, this review serves as a practical guide for developing innovative PGPR-based solutions aimed at enhancing crop yields and promoting sustainable agricultural practices.