Some properties of arctic rhizobia

WHITE LOCOWEED TOXICITY IS FACILITATED BY A FUNGAL ENDOPHYTE AND NITROGEN-FIXING BACTERIA

Mutualistic interactions with fungal endophytes and dinitrogen-fixing bacteria are known to exert key biological influences on the host plant. The influence of a fungal endophyte alkaloid on the toxicity of a plant has been documented in Oxytropis sericea. Oxytropis sericea is a perennial legume responsible for livestock poisoning in western North America. Livestock poisoning is attributed to the alkaloid swainsonine, which is synthesized inside the plant by the fungal endophyte Embellisia sp. In this study, the ability of Oxytropis sericea to form a dinitrogen-fixing symbiosis with Rhizobium and the effects of this symbiosis on the production of swainsonine by Embellisia sp. were evaluated in a greenhouse environment. Seeds of O. sericea were grown in plastic containers. Twenty-week-old O. sericea seedlings were inoculated with four strains of Rhizobium. Twenty weeks after inoculation, plant growth and root nodulation by Rhizobium were measured. Dinitrogen fixation was confirmed using an acetylene reduction assay (ARA) on excised root nodules. Dry leaves were analyzed for swainsonine content. A second set of plants was treated with fungicide to evaluate the effect of reduced fungal endophyte infection on plant growth and swainsonine production. All inoculated plants produced indeterminate nodules. The ARA indicated that 98% of the excised nodules were fixing dinitrogen. Rhizobium-treated plants had greater swainsonine concentrations than the non-inoculated controls. Plants that established from seeds treated with fungicide had lower biomass than non-fungicide-treated controls and plants treated with foliar fungicide. Plants treated with foliar fungicide and the controls had greater swainsonine concentrations than the plants that received seed fungicide. This greenhouse study is the first report of nodulation and dinitrogen fixation in O. sericea. It also demonstrates that dinitrogen fixation increases the production of swainsonine in O. sericea plants infected with Embellisia sp. Results from this study suggest that dinitrogen fixation affects swainsonine production and has the potential to support the symbiosis between Embellisia sp. and O. sericea when soil nitrogen is limited. Oxytropis sericea competitiveness appears to be facilitated by an ability to simultaneously associate with Rhizobium and a fungal symbiont.

The genetic and biochemical basis for nodulation of legumes by rhizobia

Soil bacteria of the genera Azorhizobium, Bradyrhizobium, and Rhizobium are collectively termed rhizobia. They share the ability to penetrate legume roots and elicit morphological responses that lead to the appearance of nodules. Bacteria within these symbiotic structures fix atmosphere nitrogen and thus are of immense ecological and agricultural significance. Although modern genetic analysis of rhizobia began less than 20 years ago, dozens of nodulation genes have now been identified, some in multiple species of rhizobia. These genetic advances have led to the discovery of a host surveillance system encoded by nodD and to the identification of Nod factor signals. These derivatives of oligochitin are synthesized by the protein products of nodABC, nodFE, NodPQ, and other nodulation genes; they provoke symbiotic responses on the part of the host and have generated immense interest in recent years. The symbiotic functions of other nodulation genes are nonetheless uncertain, and there remain significant gaps in our knowledge of several large groups of rhizobia with interesting biological properties. This review focuses on the nodulation genes of rhizobia, with particular emphasis on the concept of biological specificity of symbiosis with legume host plants.

Enhanced N2-fixing ability of a deletion mutant of arctic rhizobia with sainfoin (Onobrychis viciifolia)

Mutagenesis provoked by exposure at elevated temperature of the cold-adapted, arctic Rhizobium strain N31 resulted in the generation of five deletion mutants, which exhibited loss of their smaller plasmid (200 kb), whereas the larger plasmid (> 500 kb) was still present in all mutants. Deletion mutants did not show differences from the wild type in the antibiotic resistance pattern, the carbohydrates and organic acids utilization, and the growth rate at low temperature. However, deletion mutants differed from the wild type and among themselves in the ex planta nitrogenase activity, the nodulation index, and the symbiotic effectiveness. The deletion mutant N31.6rif (r) showed higher nodulation index and exhibited higher nitrogenase activity and symbiotic efficiency than the other deletion mutants and the wild type. The process of deletion mutation resulted in the improvement of an arctic Rhizobium strain having an earlier and higher symbiotic nitrogen fixation efficiency than the wild type.