Can stress response genes be used to improve the symbiotic performance of rhizobia?

Abstract

Rhizobia are soil bacteria able to form symbioses with legumes and fix atmospheric nitrogen, converting it into a form that can be assimilated by the plant. The biological nitrogen fixation is a possible strategy to reduce the environmental pollution caused by the use of chemical N-fertilizers in agricultural fields. Successful colonization of the host root by free-living rhizobia requires that these bacteria are able to deal with adverse conditions in the soil, in addition to stresses that may occur in their endosymbiotic life inside the root nodules. Stress response genes, such as otsAB, groEL, clpB, rpoH play an important role in tolerance of free-living rhizobia to different environmental conditions and some of these genes have been shown to be involved in the symbiosis. This review will focus on stress response genes that have been reported to improve the symbiotic performance of rhizobia with their host plants. For example, chickpea plants inoculated with a Mesorhizobium strain modified with extra copies of the groEL gene showed a symbiotic effectiveness approximately 1.5 fold higher than plants inoculated with the wild-type strain. Despite these promising results, more studies are required to obtain highly efficient and tolerant rhizobia strains, suitable for different edaphoclimatic conditions, to be used as field inoculants.

Keywords: biological nitrogen fixation; chaperone; improvement; legume; rhizobia; stress response; symbiotic effectiveness.

Figure 1.. Symbiotic efectiveness (SE) of strains ST2, ST2pRK415 and ST2pRKgroEL inoculated in chickpea plants grown in pots for 7 weeks. Different letters (a and b) correspond to statistically significant differences (P < 0.05) detected using the one-way ANOVA and the post hoc Tukey test, implemented in SPSS V.21 software (SPP Inc., Chicago, U.S.A). Bars represent standard deviation.

Figure 2.. Growth curves of ST2pRK415 and ST2pRKgroEL, after a heat shock at 48 °C during 15 min. There are statistical differences between the growth of the two strains from 133 hours onwards (P < 0.05), detected using T-test, implemented in SPSS V.21 software (SPP Inc., Chicago, U.S.A). Bars represent standard deviation.