Structure of surface polysaccharides from Aeromonas sp. AMG272, a plant-growth promoting rhizobacterium isolated from rice rhizosphere.

Aeromonas sp. AMG272 is a Gram-negative bacterium that has been isolated from agricultural soil and studied for its plant growth-promoting activities. Structures of the O-specific polysaccharide chain of the AMG272 lipopolysaccharide and its capsular polysaccharide were elucidated using GLC-MS and NMR spectroscopy. The structure of the O-specific polysaccharide, →4)-α-l-Rhap-(1 → 3)-ß-d-GlcpNAc-(1→, has been found in other Aeromonas strains and related bacteria, whereas the structure of the capsular polysaccharide has not been reported before: →6)[ß-d-Fucp3NAc4Ac-(1 → 3)]-α-d-GlcpNAc-(1 → 4)-α-d-Galp-(1 → 3)-α-d-GalpNAc-(1 → 4)-α-d-Galp-(1 → .

Rice and bean AHL-mimic quorum-sensing signals specifically interfere with the capacity to form biofilms by plant-associated bacteria.

Many bacteria regulate their gene expression in response to changes in their population density in a process called quorum sensing (QS), which involves communication between cells mediated by small diffusible signal molecules termed autoinducers. n-acyl-homoserine-lactones (AHLs) are the most common autoinducers in proteobacteria. QS-regulated genes are involved in complex interactions between bacteria of the same or different species and even with some eukaryotic organisms. Eukaryotes, including plants, can interfere with bacterial QS systems by synthesizing molecules that interfere with bacterial QS systems. In this work, the presence of AHL-mimic QS molecules in diverse Oryza sativa (rice) and Phaseolus vulgaris (bean) plant-samples were detected employing three biosensor strains. A more intensive analysis using biosensors carrying the lactonase enzyme showed that bean and rice seed-extract contain molecules that lack the typical lactone ring of AHLs. Interestingly, these molecules specifically alter the QS-regulated biofilm formation of two plant-associated bacteria, Sinorhizobium fredii SMH12 and Pantoea ananatis AMG501, suggesting that plants are able to enhance or to inhibit the bacterial QS systems depending on the bacterial strain. Further studies would contribute to a better understanding of plant-bacteria relationships at the molecular level.

Structure of the O-antigen of the lipopolysaccharide isolated from Pantoea ananatis AEP17, a rhizobacterium associated with rice.

The lipopolysaccharide of a Gram-negative bacterium having a putative plant-growth promoting activity (Pantoea ananatis AEP17) has been isolated and subjected to partial hydrolysis. The O-antigen has been studied by mass spectrometry and NMR experiments. On the basis of these experiments it is concluded that the following repeating unit is present in the polysaccharide: →3)-ß-d-GlcpNAc-(1→3)[α-d-GalpAN-(1→2)]-α-l-Rhap-(1→2)-α-l-Rhap-(1→3)-α-l-Rhap-(1→2)-α-l-Rhap-(1→ The occurrence of d-galacturonamide (GalAN) is unusual in bacterial O-polysaccharides. It has only been reported in Escherichia coli O65 [Perry, M. B.; MacLean, L. L. Carbohydr. Res.1999, 322, 57-66].