Section A: Interaction of Bacterial Pathogens with Plants. Section B: Rhizobium--Plant Symbiotic Interactions. Section C: Interaction of Fungi with Plants. Section D: Isolation of Plant Disease Resistance Genes. Section E: Plant Responses to Pathogens and Resistance Mechanisms. Section F: Engineered Resistance to Plant Pathogens. Index.

Advances in molecular genetics of plant-microbe interactions : proceedings of the 7th International Symposium on the Molecular Plant-Microbe Interactions, Edinburgh, U.K., June 1994

Bacteria in Agrobiology: Plant Growth Responses

The interaction between Azospirillum brasilense and plants is not fully understood, although several bacterial surface components like exopolysaccharides (EPS), flagella, and capsular polysaccharides are required for attachment and colonization. While in other plant–bacteria associations (Rhizobium–legume, Pseudomonas–potato), lipopolysaccharides (LPS) play a key role in the establishment of an effective association, their role in the root colonization by Azospirillum had not been determined. In this study, we isolated a Tn5 mutant of A. brasilense Cd (EJ1) with an apparently modified LPS core structure, non-mucoid colony morphology, increased EPS production, and affected in maize root colonization. A 3790-bp region revealed the presence of three complete open reading frames designated rmlC, rmlB and rmlD. The beginning of a fourth open reading frame was found and designated rmlA. These genes are organized in a cluster which shows homology to the cluster involved in the synthesis of dTDP-rhamnose in other bacteria. Additionally, the analysis of the monosaccharide composition of LPSs showed a diminution of rhamnose compared to the wild-type strain.

Disruption of dTDP‐rhamnose biosynthesis modifies lipopolysaccharide core, exopolysaccharide production, and root colonization in Azospirillum brasilense

In Rhizobium etli symbiotic plasmid transfer, nodulation competitivity and cellular growth require interaction among different replicons.

Azospirillum brasilense is a plant root-colonizing bacterium that exerts beneficial effects on the growth of many agricultural crops. Extracellular polysaccharides of the bacterium play an important role in its interactions with plant roots. The pRhico plasmid of A. brasilense Sp7, also named p90, carries several genes involved in synthesis and export of cell surface polysaccharides. We generated two Sp7 mutants impaired in two pRhico-located genes, noeJ and noeL, encoding mannose-6-phosphate isomerase and GDP-mannose 4,6-dehydratase, respectively. Our results demonstrate that in A. brasilense Sp7, noeJ and noeL are involved in lipopolysaccharide and exopolysaccharide synthesis. noeJ and noeL mutant strains were significantly altered in their outer membrane and cytoplasmic/periplasmic protein profiles relative to the wild-type strain. Moreover, both noeJ and noeL mutations significantly affected the bacterial responses to several stresses and antimicrobial compounds. Disruption of noeL, but not noeJ, affected the ability of the A. brasilense Sp7 to form biofilms. The pleiotropic alterations observed in the mutants could be due, at least partially, to their altered lipopolysaccharides and exopolysaccharides relative to the wild-type.

/pdf/the-azospirillum-brasilense-sp7-noej-and-noel-genes-are-hdv240ettk.pdf

Andrew V. Scheludko

Papers

Involvement of a 120-MDa plasmid of Azospirillum brasilense Sp245 in the production of lipopolysaccharides.