Physiological adaptation to low temperatures of strains of Rhizobium leguminosarum bv. viciae associated with Lathyrus spp.

Strains of Rhizobium leguminosarum bv. viciae, isolated from the legume spe cies Lathyrus japonicus and Lathyrus pratensis in northern Quebec (Canada), showed different capacities for growing at low temperature. In the present study, we investigated some mechanisms related to cold adaptation. Two col d-adapted strains (psychrotrophs) were compared to a poorly adapted strain and to a cold-sensitive strain (reference strain) for freezing survival, pr otein induction and fatty acid composition under low temperature. Following cold shocks (25 degrees C to 10, 5 and 0 degrees C), a common 6.1-kDa CSP (cold shock protein) was induced in all strains, but the total number of CS Ps synthesized at 0 degrees C was higher in cold-adapted strains than in th e cold-sensitive strain. The synthesis of CAPs (cold acclimation proteins) was observed under continuous growth at 5 degrees C in all three strains ca pable of growth at this temperature. Levels of survival after 24 h at -80 d egrees C where higher in cold- (79%) and poorly adapted (64%) strains than in the cold-sensitive strain (33%), but a 2-h acclimation period at 5 degre es C before freezing doubled the survival of the cold-sensitive strain. Low temperature conditions affected similarly the fatty acid composition of al l strains, regardless of their cold adaptation level. The proportion of uns aturated fatty acids increased significantly with the lowering of growth te mperature from 25 to 5 degrees C, but showed a tendency to decrease after a cold shock from 25 to 5 degrees C. A specific unsaturated fatty acid, cis- 12 octadecanoic acid, was produced during growth at 5 degrees C. The unsatu rated cis-vaccenic acid was the principal component under all conditions. T he cold adaptation trait was weakly reflected in symbiosis with the agronom ic legume, Lathyrus sativus, with which one cold-adapted strain showed a sl ightly higher nitrogenase activity and shoot dry matter yield than a commer cial strain under a sub-optimal temperature regime. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.