Growth and Nitrogen Fixation of DifferentMedicago Sativa-Sinorhizobium MelilotiAssociations Under Conditions of Mineral Elements Shortage

Sinorhizobium meliloti is an α-proteobacteria of the family Rhizobiaceae that alternates between a free-living phase in soil and a symbiotic phase within the host plant cells, where the bacteria ultimately differentiate into nitrogen-fixing organelle-like cells, called bacteroids. The present study was designed to compare the difference in responses to nitrogen and carbon limitation in free living Sinorhizobium meliloti strains and their ability to form symbiotic association with alfalfa (Medicago sativa) plants. The effectiveness of observed symbiotic associations was evaluated by number of formed nodules, nitrogen fixing activity, and plant biomass production in control environmental conditions of a growth chamber. A wild type strain Sinorhizobium meliloti 1021 and two mutants—Sinorhizobium meliloti NitR and Sinorhizobium meliloti TspO were used in this research. Both mutant strains were previously created to study the general stress response in Sinorhizobium meliloti and its regulation mechanisms. The products of the genes named tspO and nitR, act either as direct or indirect regulators of gene expression in response to various stresses, including starvation. The up- and down regulated genes under conditions of nitrogen and carbon limitation were identified in free living forms of the three strains. Comparison of the genes differentially expressed in the wild type strain to those found in the tspO and nitR mutants showed no significant difference. In both starvation conditions the most effective symbiotic system was established between alfalfa and Sinorhizobium meliloti TspO, concerning nitrogen fixing capacity and plant biomass production.