Most heat-tolerant rhizobia show high induction of major chaperone genes upon stress.

The aims of this study were to evaluate the temperature stress tolerance of chickpea rhizobia and to investigate whether tolerance is related to the species or the site of origin of the isolates. Additionally, the molecular bases of temperature stress tolerance in rhizobia were investigated, by comparing the expression of chaperone genes dnaKJ and groESL in thermotolerant and thermosensitive isolates. Tolerance to cold, heat and heat shock was evaluated for 53 mesorhizobia obtained from several provinces of Portugal and assigned to different species. Associations between isolates' tolerance phenotype and several provinces of origin were found. Some species groups were found to differ significantly in their ability to tolerate temperature stress. Analysis of the dnaK and groESL expression by Northern hybridization, using isolates from three species groups, showed an increase in the transcripts levels with heat, but not with cold stress. Interestingly, a higher induction of chaperone genes was detected in heat-tolerant isolates when compared with that of sensitive isolates of the same species. To our knowledge, this is the first analysis of chaperone genes' expression comparing tolerant and sensitive strains. The present study suggests a relationship between higher transcriptional induction of major chaperone genes and higher tolerance to heat in rhizobia.