Cryotolerance strategies of Pseudomonads isolated from the rhizosphere of Himalayan plants

The cold stress biology of psychrotrophic Pseudomonas strains isolated from the rhizosphere of Himalayan plants have been explored to evaluate their cryotolerance characteristcs. Pseudomonas strains were examined for stress metabolites, viz., exopolysaccharide (EPS) production, intracellular sugar, polyols and amino acid content, ice nucleation activity, and their freezing survival at -10 and -40°C, respectively. High freezing survival was observed for the Pseudomonas strains that were grown at 4°C prior to their freezing at -10 or -40°C. Increased EPS production was noticed when Pseudomonas strains were grown at lower temperatures, i.e., 4 and 15°C, in comparison with their optimal growth temperature of 28°C. All Pseudomonas strains showed low level of type-III class ice nucleation activity at -10°C after 96 h. Considerable differences were noticed in accumulated contents of various intracellular sugars, polyols, amino acids for all Pseudomonas strains when they grown at two different temperatures, i.e., 4 and 28°C, respectively. The unusual complement of stress protectants especially, raffinose, cysteine and aspartic acid that accumulated in the bacterial cells at low temperature was novel and intriguing finding of this study. The finding that raffinose is a key metabolite accumulated at low temperature is an exciting discovery, and to the best of our information this is first report ever signifying its role in bacterial cold tolerance.