The Effect Of Abrb On Rhizosphere Colonization Activities Of Bacillus Amyloliquefaciens SQR9and Its Molecular Mechanisms

Bacillus species are ubiquitous, robust bacteria, which produce a variety of secondary metabolites that are involved in the biocontrol of plant pathogens. Biocontrol has gained increasing attention due to their environmentally friendly nature, and Bacillus species, as Plant Growth-Promoting Rhizobacteria (PGPR), are important biocontrol agents. Research on the behavior of PGPR on the plant root surface and the interaction between them, contribute to the understanding of the mechanisms underlying colonization, and facilitate the application of PGPR in agriculture. A key factor to the molecular understanding of PGPR is the global transcriptional regulator AbrB.In an attempt to study the effect of AbrB on the root colonization activity of Bacillus amyloliquefaciens SQR9, we analyzed the effect of AbrB on the abilities of biofilm formation, chemotaxis and swarming which were important to the root colonization activity.1. We disrupted of the abrB gene of the laboratory strain SQR9, and the abrB mutant strain demonstrated an elevated biofilm formation efficiency, chemotaxis activity toward the root exudates, as well as the root colonization activity. No significant difference in the pathogen inhibition activity between the strain SQR9and the abrB mutant was observed, even so, the abrB mutant exhibited enhanced biocontrol activity, indicating that root colonization plays an important role in biocontrol.2. We do the transcriptional analysis to the root colonization related genes by using qRT-PCR, the results showed that the expression level of the genes involved in the biofilm formation and chemotaxis was higher in the abrB mutant than the wild type. Meanwhile, the surfactin gene was expressed in a much higher level than that of the wild type, indicating that surfactin may play a pivotal role in the root colonization by Bacillus amyloliquefaciens SQR9.3. Inactivation of the srfAA gene, phenotypic observation found that the biofilm formation activity of the srfAA mutant decreased, and the biofilm formed thereby was loose and fragile. The mutant strain also showed disabled swarming activity, indicating that surfactin may play a pivotal role in the root colonization by Bacillus amyloliquefaciens SQR9.In summary, the transcription regulator AbrB can down regulate the genes involved in the biofilm formation and chemotaxis directly affecting the root colonization, and also down regulate the surfactin production indirectly influencing the root colonization ability of Bacillus amyloliquefaciens SQR9. To some extent the AbrB factor inhibited the colonization of the SQR9strain in plant roots and biocontrol potential.