Contrasting effects of the inoculation time with passenger endophytic Agrobacterium sp.10C2 on the nodule functioning and growth of Medicago truncatula

Agrobacterium species are one of the most frequently isolated endophytes from root nodules of a wide range of wild and cultivated legumes. Many studies have investigated the role of endophytes in phytoprotection and phytoremediation enhancement. However, their biological significance and their implication in nodule functioning have been scarcely studied, especially, the effect of the inoculation time on nodule activity and plant growth which still unknown. Accordingly, in this study, nodule-endophytic Agrobacterium sp.10C2 was inoculated before and after nodule formation, to determine the effect of the inoculation time on the plant growth and nodule functioning in the reference line Jemalong A17 of the model legume Medicago truncatula in symbiotic association with Sinorhizobium meliloti TII7. The results revealed that nodule functioning was strongly influenced by the inoculation time of the endophytic Agrobacterium sp.10C2. In fact, the early inoculation with Agrobacterium sp.10C2 induced an increase of the highest nodules nitrogenase activity, recorded at the day 66 after the inoculation, by 66%, the leghemoglobin concentration by 26% and the total sugar content by 67%, in addition to a higher photosynthesis activity and biomass plant production compared to the control. Interestingly, when inoculation occurred after nodule formation, nodules have induced a defense response against Agrobacterium sp. 10C2 evidenced by an overproduction of guaicol peroxidase which increased from 1.04 mu mol H2O2 min-1 mg-1 proteins in the control to 2.36 mu mol H2O2 min-1 mg-1 proteins in the late inoculated plants, leading to a loss of the beneficial impact of the endophytic Agrobacterium on both biomass production and nodule functioning parameters. We showed that nodule response to endophytic agrobacteria change according to the time of inoculation. When inoculated at early stage, 10C2 plays the role of a Plant Growth Promoting Rhizobacteria (PGPR), however, after nodule formation, 10C2 could act as a pathogen agent. Thus, the inoculation time could be considered as a crucial parameter for the nodule functioning and plant growth.