Pleiotropic regulatory function of two component system CbrAB in rhizosphere bacterium Pseudomonas protegens FD6

• CbrAB is essential for the biocontrol activity of P . protegens FD6. • CbrAB globally regulates biocontrol-related phenotypes and stress tolerance. • CbrB directly binds to the promoters of algD , flgB , crcY , crcZ as activator. Pseudomonas protegens FD6 is an effective biocontrol agent that confers protection against tomato bacterial wilt disease caused by Ralstonia solanacearum . In a two-component system, CbrAB regulated different physiological functions to adapt to diverse environments in pseudomonads and Azotobacter spp.. Here, we explored the global regulatory function of CbrAB on biocontrol-associated traits in P. protegens FD6. CbrAB is involved in several functions, such as antibiotic production, motility, biofilm formation and stress response. Mutation in CbrAB system leads to a loss of the ability to control tomato bacterial wilt and growth defects in LB broth during the logarithmic phase. Additionally, CbrAB might regulate 2,4-diacetylphloroglucinol (2,4-DAPG) and pyoluteorin (PLT) production. The luminescence of luciferase reporter system data suggested that CbrA positively modulates the transcription of the antibiotic synthesis genes pltL and phlA. CbrB differentially regulates the expression of pltL and phlA at the transcriptional level, which is consistent with the results of the quantitative analysis of antibiotics. In this study, we also show that cbrA and cbrB contribute to the efficient colonization of tomato roots by P. protegens FD6. Compared with the wild type, both cbrA and cbrB mutants were also less competitive on tomato roots. We further revealed cbrB positively regulated transcription of algD and flgB, responsible for alginate synthesis and swimming motility , respectively, and sRNAs crcY and crcZ by directly binding to their upstream sequences. Overall, CbrAB in P. protegens contributes to its biological control activity, persistence during root colonization and adaptation to complicated environments.