EFFECTS OF SINORHIZOBIUM MELILOTI STRAINS (1021 AND NitR) ON NITROGEN ASSIMILATION OF ALFALFA PLANTS UNDER CONDITIONS OF MINERAL ELEMENTS SHORTAGE

Two Sinorhizobium meliloti strains (1021 and NitR) were used for inoculation of alfalfa plants to study nitrogen assimilation under nutrient deficiency conditions in hydroponics experiments. The wild type Sinorhizobium meliloti 1021 was compared with a mutant strain – S. meliloti NitR. NitR protein was found to be a regulator of S. meliloti hmgA expression under nitrogen deprivation conditions, suggesting the presence of a ntr-independent nitrogen deprivation regulatory system. nitR insertion mutations were shown not to affect bacterial growth, nodulation of Medicago sativa plants, or symbiotic nitrogen fixation under the physiological conditions examined. The relationship between free living and symbiotic bacterial forms was revealed indirectly by the changes of nitrogen fixation and assimilation under conditions of nutrient deficiencies. Before seeds inoculation, bacteria were cultivated in Vincent minimal media with limited nitrogen source. The alfalfa plants were grown on a nutrient solution in the presence or absence of molybdenum. The differences between the two symbiotic systems were established by the variations of nodule formation and enzyme activities participated in nitrogen fixation and