Rhamnolipids from the rhizosphere bacterium Pseudomonas sp. GRP(3) that reduces damping-off disease in Chilli and tomato nurseries

A detailed screening of bacterial isolates from the Central Himalayan region for plant growth promotion and antimycelial activity against Pythium and Phytophthora strains afforded seven isolates, of which three were particularly effective against the incidence of damping-off in field trials on chilli and tomato. In this investigation an initial spectroscopic survey of the methanolic extracts of the seven bacterial isolates showed complex mixtures except for Pseudomonas sp. GRP3, one of the most promising isolates on the basis of field studies. Strain GRP3 was selected for structural characterization of its secondary metabolites, particularly glycolipids. The extracellular secondary metabolites were enriched by Amberlite XAD-16 adsorber resin followed by separation and structural analysis using TLC, LC-MS, MS-MS, and NMR spectroscopy. Acquired data show the presence of a number of mono- and dirhamnolipids and include rhamnose (Rha)-C8-C10, Rha-C10-C8, Rha-C10-C10, Rha-C10-C12:1, Rha-C10-C12, Rha-Rha-C8-C10, Rha-Rha-C10-C10, Rha-Rha-C10-C10:1, Rha-Rha-C10-C12, Rha-Rha-C10-C12:1, Rha-Rha-C12-C12:1, and Rha-Rha-C12-C12 in strain GRP3. Since rhamnolipids are involved in the lysis of the plasma membrane of zoospores of fungi, application of such rhamnolipid-producing rhizobacteria could facilitate control of damping-off plant pathogens.