EFFECTS OF TEMPERATURE STRESS ON BEAN-NODULATING RHIZOBIUM STRAINS

High soil temperatures in tropical areas limit nodulation and dinitrog en fixation by strains of Rhizobium. Several heat-tolerant bean-nodula ting Rhizobium strains have been isolated previously. However, the bas is of their resistance to heat remains unknown. In this study, we comp ared the effects of heat on symbiotic nitrogen fixation, cell survival , amino acid uptake, and protein synthesis in a heat-tolerant (CIAT899 ) and a heat-sensitive (CNPAF512) bean-nodulating Rhizobium strain. Ac etylene reduction activity of nodulated roots excised from unstressed plants was strongly diminished at 35 or 40-degrees-C when plants were nodulated either by CIAT899 or by CNPAF512. When these strains were te sted under free-living conditions, survival at 40-degrees-C as well as the kinetics of L-[S-35]methionine uptake and protein synthesis at 35 and 40-degrees-C indicated the higher tolerance of CIAT899 than of CN PAF512 to thermal stress. The synthesis of heat shock proteins was det ected in both strains, although at different temperatures. Increased s ynthesis of 14 heat shock proteins in CNPAF512 and of 6 heat shock pro teins in CIAT899 was observed at 40 and 45-degrees-C, respectively. A heat shock protein of approximately 21 kDa, of which the synthesis was strongest in both Rhizobium strains upon a temperature shift up, was also conserved in several other bean-nodulating rhizobia. Acquired the rmotolerance in CIAT899 was shown to depend on protein synthesis.