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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.

Three genotypes of the model legume Medicago truncatula were assessed for symbiotic effectiveness in cross inoculation with two strains of Sinorhizobium meliloti under mannitol‐mediated osmotic stress. Symbioses showed different tolerance levels revealed on plant growth, nitrogen‐fixing capacity and indices of nodule functioning and protection. The variability of stress response was essentially correlated with performance at non‐stressful conditions. Symbiosis attitude depended on bacterial partner, host‐plant genotype and their interaction. Plant genotype manifested the highest contribution to symbiotic efficiency indices under osmotic stress, even for nodulation and nitrogen fixation where the bacterial strain effect is highly pronounced. Contrasting (tolerant/sensitive) associations were identified for tolerance behaviours, involving the same plant genotype with different rhizobial strains and vice versa. In nodules, osmotic stress leads to accumulation of oxidized lipids and decrease in total protein and leghaemoglobin contents. Antioxidant responses were manifested as induction of guaiacol peroxidase (POX, E.C. 1.11.1.7) and superoxide dismutase (E.C. 1.15.1.1). POX induction was higher in tolerant symbioses and both enzymes were suggested as contributors to the protection of nodule integrity and functioning under osmotic stress. In conclusion, symbiotic efficiency in M. truncatula–S. meliloti combinations under osmotic stress is determined by each symbiont’s input as well as the plant–microbe genotype interaction, and POX induction could prove a sensitive marker of tolerant symbioses.

The biochemical processes underlying the expression of resistance in the roots of Medicago truncatula against Aphanomyces euteiches infection was investigated, with emphasis on oxidative stress. The levels of H 2 O 2 , superoxide dismutase, peroxidase, ascorbate peroxidase, catalase, soluble phenolics and lignin were measured in the roots of two lines, A17 partially resistant and F83005.5 susceptible to A. euteiches at three infection stages; penetration of the epidermis (1 dpi), colonization of the cortex (3 dpi) and invasion of the root stele (6 dpi). A rapid and large decrease of the H 2 O 2 levels in A17 roots occurred. However, in F83005.5 roots, the decrease in H 2 O 2 levels was delayed until 3 dpi. In A17 roots, the activities of ascorbate peroxidase, peroxidase and catalase were induced as early as 1 dpi, whereas a general decrease in the activity of the four antioxidant enzymes was observed in F83005.5 roots. The levels of soluble phenolics and lignin were increased in A17 roots at 3 and 6 dpi, respectively. The H 2 O 2 levels were negatively correlated to ascorbate peroxidase, catalase and lignin production at 1, 3 and 6 dpi, respectively in A17 roots. Physiological concentrations of H 2 O 2 found in M. truncatula infected roots had no detrimental effect on the in vitro growth of this oomycete. Our data suggest that H 2 O 2 does not have a direct antimicrobial effect on M. truncatula resistance to A. euteiches, but is involved in cell wall strengthening around the root stele, preventing pathogen invasion of the vascular tissues.

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Agrobacterium sp. 10C2 is a nonpathogenic and non-symbiotic nodule-endophyte strain isolated from root nodules of Phaseolus vulgaris. The effect of this strain on nodulation, plant growth and rhizosphere bacterial communities of P. vulgaris is investigated under seminatural conditions. Inoculation with strain 10C2 induced an increase in nodule number (+54 %) and plant biomass (+16 %). Grains also showed a significant increase in phosphorus (+53 %), polyphenols (+217 %), flavonoids (+62 %) and total antioxidant capacity (+82 %). The effect of strain 10C2 on bacterial communities was monitored using terminal restriction fragment length polymorphism of PCR-amplified 16S rRNA genes. When the initial soil was inoculated with strain 10C2 and left 15 days, the Agrobacterium strain did not affect TRF richness but changed structure. When common bean was sown in these soils and cultivated during 75 days, both TRF richness and structure were affected by strain 10C2. TRF richness increased in the rhizosphere soil, while it decreased in the bulk soil (root free). The taxonomic assignation of TRFs induced by strain 10C2 in the bean rhizosphere revealed the presence of four phyla (Firmicutes, Actinobacteria, Bacteroidetes and Proteobacteria) with a relative preponderance of Firmicutes, represented mainly by Bacillus species. Some of these taxa (i.e., Bacillus licheniformis, Bacillus pumilus, Bacillus senegalensis, Bacillus subtilis, Bacillus firmus and Paenibacillus koreensis) are particularly known for their plant growth-promoting potentialities. These results suggest that the beneficial effects of strain 10C2 observed on plant growth and grain quality are explained at least in part by the indirect effect through the promotion of beneficial microorganisms.