Growth-Promoting Effects and Mechanisms of Synthetic Plant Growth-Promoting Rhizobacteria on Maize Seedlings

With the development of microbial fertilizers, efforts have been made to enrich the strain resources of plant growth-promoting rhizobacteria (PGPR) in maize and to compare the growth-promoting effects of synthetic microbial communities (SynComs) with those of single strains. To achieve this, phenotypic measurements and RNA sequencing (RNA-seq) were performed on maize roots treated with SynComs and single-strain bacterial suspensions, aiming to investigate the regulatory influence of PGPR on differential gene expression and key metabolic pathways in maize roots. In this study, 59 PGPR strains were selected, representing genera including <i>Bacillus</i>, <i>Pseudomonas</i>, <i>Burkholderia</i> sp., <i>Curtobacterium pusillum</i>, <i>Acidovorax</i>, <i>Sphingobium</i>, <i>Mitsuaria</i>, <i>Bacterium</i>, <i>Rhodanobacter</i>, <i>Variovorax</i>, <i>Ralstonia</i>, <i>Brevibacillus</i>, <i>Terrabacter</i>, <i>Flavobacterium</i>, <i>Comamonadaceae</i>, <i>Achromobacter</i>, <i>Paraburkholderia</i>, and <i>Massilia</i>. Based on the growth-promoting effects observed in pot experiments, optimal bacterial strains were selected according to the principles of functional complementarity and functional superposition to construct the SynCom. The selected strains included <i>Burkholderia</i> sp. A2, <i>Pseudomonas</i> sp. C9, <i>Curtobacterium pusillum</i> E2, and <i>Bacillus velezensis</i> F3. The results demonstrated that individual strains exerted measurable growth-promoting effects on seedlings; however, the growth-promoting capability of the SynCom was significantly stronger than that of single strains. The synthetic microbial community ALL group markedly increased root length, shoot fresh weight, shoot dry weight, number of branches, and number of root tips in maize seedlings. RNA-seq analysis of maize roots treated with the SynCom (ALL group) was conducted in comparison with CK, A2, C9, E2, and F3 treatment groups. A total of 5245 differentially expressed genes (DEGs) were identified, of which only 133 were common across treatments. GO and KEGG analyses revealed that DEGs were enriched in multiple biological processes, including cellular amide biosynthetic and metabolic processes, flavonoid biosynthetic and metabolic processes, carbohydrate metabolism, amino acid metabolism, lipid metabolism, and translation. The majority of enriched pathways were associated with primary and secondary metabolism, indicating that these bacterial strains promote plant growth by modulating a wide range of metabolic pathways in plant cells. Overall, this study provides a molecular framework for understanding the mechanisms underlying the growth-promoting effects of SynComs on maize roots and offers valuable insights for future research aimed at identifying key regulatory genes.