No abstract

The relationships between symbiotic performance and nodular antioxidant enzymes were studied for the associations between three Medicago truncatula lines and three Sinorhizobium meliloti strains. The results showed that the variability in symbiotic efficiency was dependent on the bacterial partner, host plant and their interaction. The contribution of each factor to the total amount of variance differed with the measured parameter. The aerial biomass production and nitrogen‐fixing capacity were affected similarly by the three factors, whereas root and nodule biomass and catalase (CAT, E.C. 1.11.1.6), guaiacol peroxidase (POX, E.C. 1.11.1.7) and ascorbate peroxidase (APX, E.C. 1.11.1.11) antioxidant activities were mainly influenced by the M. truncatula line. The nodule number was dependent on the bacterial strain, and superoxide dismutase (SOD, E.C. 1.15.1.1) was dependent mainly on the plant–rhizobium interaction. A highly significant correlation was found between nitrogen‐fixing activity, shoot biomass production, total nodule protein content and catalase activity. The other nodular antioxidant enzymes were differentially expressed between associations and showed no clear correlation with symbiotic efficiency.

Thirty-six symbiotic associations involving six chickpea cultivars against six rhizobial strains were evaluated for symbiotic performance and responses to osmotic stress applied by mannitol (50 mM) in aerated hydroponic cultures. Analyses in different symbioses were focused on biomass production, nodulation, nitrogen fixation, and their modulation under osmotic stress conditions, as well as expression of nodular antioxidant enzymes. Mesorhizobium ciceri reference (835) and local (CMG6) strains, as well as the local (C 11 ) M. mediterraneum allowed the best symbiotic efficiency for all chickpea cultivars. The osmotic stress induces severe decrease ranging 30-50% in aerial biomass and 50-70% for nitrogen fixation. Nevertheless, plants inoculated with M. ciceri (835) and M. mediterraneum (C 11 ) preserve a relatively high growth (4 g plant -1 ) with nitrogen-fixing activity (25 lmols h -1 plant -1 ). The bacterial partner was the most important factor of variance of the analysed parameters in osmotic stress or physiological conditions where it gets to 60-85%. The strains allowing the best competent symbioses were proposed for field assays. Under osmotic stress, nodular peroxidase (POX) and ascorbate peroxidase (APX) activities were significantly enhanced. The increase of POX and APX was inversely correlated with the inhibition of aerial biomass production (p = 0.05) and nitrogen-fixing capacity (p = 0.01), suggesting a protective role of these enzymes in nodules. Superoxide dismutase (SOD) was also activated in stressed nodules. However, the spectacular decrease in catalase (CAT) activity discounts its involvement in osmotic stress response.

Antioxidant responses and nodule function of Medicago truncatula genotypes differing in salt tolerance were studied. Salinity effects on nodules were analysed on key nitrogen fixation proteins such as nitrogenase and leghaemoglobin as well as estimating lipid peroxidation levels, and were found more dramatic in the salt-sensitive genotype. Antioxidant enzyme assays for catalase (CAT, EC 1.11.1.6), superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11) and guaiacol peroxidase (EC 1.11.1.7) were analysed in nodules, roots and leaves treated with increasing concentrations of NaCl for 24 and 48 h. Symbiosis tolerance level, depending essentially on plant genotype, was closely correlated with differences of enzyme activities, which increased in response to salt stress in nodules (except CAT) and roots, whereas a complex pattern was observed in leaves. Gene expression responses were generally correlated with enzymatic activities in 24-h treated roots in all genotypes. This correlation was lost after 48 h of treatment for the sensitive and the reference genotypes, but it remained positively significant for the tolerant one that manifested a high induction for all tested genes after 48 h of treatment. Indeed, tolerance behaviour could be related to the induction of antioxidant genes in plant roots, leading to more efficient enzyme stimulation and protection. High induction of CAT gene was also distinct in roots of the tolerant genotype and merits further consideration. Thus, part of the salinity tolerance in M. truncatula is related to induction and sustained expression of highly regulated antioxidant mechanisms.