Inoculation of Phaseolus vulgaris, Medicago laciniata and Medicago polymorpha with Agrobacterium sp. strain 10C2 may enhance nodulation and shoot dry weight but does not affect host range specificity

The aim of this work was to assess the effect of inoculation with the non-symbiotic Agrobacterium strain 10C2, previously isolated from root nodules of Phaseolus vulgaris, on nodulation, effectiveness and host range specificity. Two rhizobial strains, Ensifer meliloti 2011 and Ensifer medicae A321, were used in this study on the basis of their differential sensitivity to the in vitro antagonistic activity exercised by Agrobacterium sp. 10C2. Three host legumes, P. vulgaris, Medicago laciniata and M. polymorpha, were also selected for this experiment on the basis of their variable symbiotic specificity towards strains 2011 and A321. Cross inoculation experiments were conducted with or without co-inoculation with Agrobacterium sp. 10C2. Results showed that both rhizobial strains induced ineffective nodules on P. vulgaris. Nevertheless, inoculation with Agrobacterium sp. 10C2 slightly increased shoot dry weight with both strains and enhanced nodule number with strain 2011 only. Inoculation of M. polymorpha with strain 10C2 significantly increased the nodule number induced by E. medicae A321 but did not affect biomass production. However, inoculation of M. laciniata by strain 10C2 enhanced nodule number and shoot dry weight with strain 2011 only. Agrobacterium sp. 10C2 did not affect the non-inoculated control, nor the contrasted host range of both rhizobial strains towards M. polymorpha and M. laciniata. The potential negative effect of the in vitro antagonistic activity of strain 10C2 on reduction of nodule number or symbiotic effectiveness was not proved. No evidence of symbiotic gene transfer from the rhizobial strains to Agrobacterium sp. 10C2 was also proved. The effect of strain 10C2 is probably mediated by nodules since no result was observed in the non-inoculated control or when the rhizobial strain failed to induce nodules. It would be interesting to investigate the implications on nodule functioning, mainly the impact on nitrogenase activity, antioxidant enzymes and nodule senescence.