Influence of arbuscular mycorrhizae and a genetically modified strain of Sinorhizobium on growth, nitrate reductase activity and protein content in shoots and roots of Medicago sativa as affected by nitrogen concentrations

Abstract

This study examined the effect of nitrogen fertilization (0, 4 or 8 mmol N added kg −1 soil) on plant growth, nitrate reductase activity (NR) (EC 1.6.6.1) and protein content in Medicago sativa. Plants were inoculated with two strains of Sinorhizobium meliloti [the wild type (WT) strain GR4 and its genetically modified (GM) derivative GR4(pCK3)]. In combination with these inoculations, the arbuscular mycorrhizal (AM) fungus, Glomus mosseae, was compared to a non-mycorrhizal control supplemented with phosphate. The effects of AM on plant growth were greatest when no nitrogen was added to the soil. Nitrogen fertilization reduced these effects according to the S. meliloti strain involved. Growth responses of mycorrhizal plants coinoculated with the GM strain were affected less negatively than those of mycorrhizal plants associated with the WT strain. These results were not related to differential colonization by AM. Increasing nitrogen concentrations reduced mycorrhizal infection and nodule formation was drastically inhibited in mycorrhizal plants by the addition of 4 and 8 mmol N kg −1 soil. Nevertheless, AM symbiosis increased nodule formation in the absence of N fertilization. In non-mycorrhizal plants, however, N fertilizer application did not significantly affect shoot and root growth or nodule formation. Although the P content was higher in P-fertilized, non-mycorrhizal plants than in mycorrhizal ones AM colonization significantly improved P use efficiency. The N content and use efficiency were also highest in mycorrhizal plants. However, the most relevant result regarding NR activity was the varying distributions in mycorrhizal plant shoots and roots as affected by the particular Sinorhizobium strain. The root portion was enhanced by the GM strain, while the WT strain increased the shoot portion. This change in the distribution pattern of root and shoot NR activities was unaffected by N concentrations in the soil. Protein content was substantially higher in mycorrhizal plants. The protein distribution pattern in shoot and root was also highly influenced by mycorrhizal colonization, which enhanced the root portion. Increased nitrogen supply lowered the protein content in both shoot and root tissues. This effect was greatest in AM colonized plants. These results suggest that high N fertilization levels are detrimental to mycorrhizal legume plants. However, under stress conditions (e.g. high N levels) mycorrhizal legumes nodulated by the GM Sinorhizobium strain displayed a physiological response which was better than those nodulated by the WT strain.