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Chemotaxis towards root exudates and subsequent biofilm formation are very important for root colonization and for providing the beneficial functions of plant growth-promoting rhizobacteria (PGPRs). In this study, in comparison with other root-secreted compounds, d-galactose in the root exudates of cucumber was found to be a strong chemoattractant at the concentration of 1 μM for Bacillus velezensis SQR9. Chemotaxis assays with methyl-accepting chemotaxis proteins (MCPs) deletion strains demonstrated that McpA was solely responsible for chemotaxis towards d-galactose. Interestingly, d-galactose significantly enhanced the biofilm formation of SQR9 in an McpA-dependent manner. Further experiment showed that d-galactose also enhanced root colonization by SQR9. In addition, the secretion of d-galactose by cucumber roots could be induced by inoculation with SQR9, indicating that d-galactose may be an important signal in the interaction between plant and SQR9. These findings suggested that the root-secreted d-galactose was a signal, the secretion of which was induced by the beneficial bacteria, and which in turn induced colonization of the bacteria. [ABSTRACT FROM AUTHOR]

Aside from applications in the production of commercial enzymes and metabolites, Bacillus amyloliquefaciens is also an important group of plant growth-promoting rhizobacteria that supports plant growth and suppresses phytopathogens. A host-genotype-independent counter-selectable marker would enable rapid genetic manipulation and metabolic engineering, accelerating the study of B. amyloliquefaciens and its development as both a microbial cell factory and plant growth-promoting rhizobacteria. Here, a host-genotype-independent counter-selectable marker pheS was constructed through a point mutation of the gene pheS, which encodes the α-subunit of phenylalanyl-tRNA synthetase in Bacillus subtilis strain 168. In the presence of 5 mM p-chloro-phenylalanine, 100 % of B. amyloliquefaciens strain SQR9 cells carrying pheS were killed, whereas the wild-type strain SQR9 showed resistance to p-chloro-phenylalanine. A simple pheS and overlap-PCR-based strategy was developed to create the marker-free deletion of the amyE gene as well as a 37-kb bmy cluster in B. amyloliquefaciens SQR9. The effectiveness of pheS as a counter-selectable marker in B. amyloliquefaciens was further confirmed through the deletion of amyE genes in strains B. amyloliquefaciens FZB42 and NJN-6. In addition, the potential use of pheS in other Bacillus species was preliminarily assessed. The expression of PheS in B. subtilis strain 168 and B. cereus strain ATCC 14579 caused pronounced sensitivity of both hosts to p-chloro-phenylalanine, indicating that pheS could be used as a counter-selectable marker (CSM) in these strains. [ABSTRACT FROM AUTHOR]