A novel transcription regulator of Azospirillum brasilense involved in Plant-Microbe Interactions

Background and Aims For Azospirillum to promote plant growth, successful colonization of the associated plant is required. However, the factors governing the interaction between Azospirillum and plants remain unknown. Methods To examine these associations, a deletion mutant strain of Azospirillum brasilense FP2 was constructed by targeting the narL-like gene, which is highly expressed in bacteria colonizing wheat roots. We then characterized its role in the interaction with grass. We compared the phenotype of the new mutant with that of a ΔflcA strain, since flcA is located near the narL-like gene, in terms of biological nitrogen fixation, flocculation, binding to Congo red, root colonization, and plant growth promotion. Results The ΔflcA - and ΔnarL -like strains showed a lower ability to bind Congo red than FP2, suggesting an altered surface polysaccharide composition in these mutants. Interestingly, ΔnarL -like showed a higher epiphytic population in wheat and Setaria , and a higher population adhered to maize roots than the wild-type strain. Conclusions The results support the possible involvement of the putative NarL-like response regulator in the interaction of Azospirillum with wheat, maize, and Setaria , and its importance in the Azospirillum plant-interactions. This is the first report describing a functional characterization of the narL-like gene.