Sinorhizobium medicae 419 vs S. meliloti 1021: differences in root nodules induced by these two strains on the Medicago truncatula host

Symbiosis between plants of family Fabaceae and bacteria genus Rhizobium is the most widespread interaction, in which atmospheric nitrogen is reduced into ammonia. This highly oxygen-sensitive process is performed by endosymbiotc forms of rhizobia called bacteroids, which occupy newly formed plant organs called root nodules. The goal in this paper was to explore the differences in the (ultra)structure and Reactive Oxygen Species (ROS) localization in two symbiotic interactions between legume model plant Medicago truncatula Gaertn. and bacteria from genus Sinorhizobium. Two bacterial strains-Sinorhizobium meliloti 1021 and Sinorhizobium medicae 419 have the ability of inducing root nodules on the Medicago truncatula, however, such nodules differ in effectiveness of biological nitrogen fixation. We demonstrated that root nodules made of S. medicae 419 [fully effective (F++)] induce a lower number of nodule per plant in comparison to S. meliloti 1021-(partially effective F +/-). Although the differences in the structure are scant, there are some discrepancies in the ultrastructure. In the meristem of F +/- nodules, there are Cajal bodies and a lower number of vacuole profiles by larger size in comparison to the F++. Nodules F++ have a thicker interzone and store less starch than F+/, while they have higher esterase activity. Localization of different ROS seems to be comparable in the two type of nodules but interestingly, there is not any Nitric Oxide (NO) in the infection thread of F +/- root nodules.