Cold-adapted rhizobia for nitrogen fixation in temperate regions

Abstract

Rhizobia from Canadian soils were selected for cold adaptation with the aim of improving productivity of legumes that are subjected to cool temperatures during the growing season. One approach was to use rhizobia associated with legume species indigenous to arctic and subarctic regions: (i) Mesorhizobium sp. isolated from Astragalus and Oxytropis spp. and (ii) Rhizobium leguminosarum from Lathryrus spp. The majority of these rhizobia are considered psychrotrophs because they can grow at 0 °C. The advantages of cold adaptation of arctic Mesorhizobium to improve legume symbiosis were demonstrated with the temperate forage legume sainfoin (Onobrychis viciifolia). In laboratory and field studies, arctic rhizobia were more efficient than temperate (commercial) rhizobia in improving growth of sainfoin and were more competitive in forming nodules. Biochemical studies on cold adaptation showed higher synthesis of cold shock proteins in cold-adapted than in nonadapted arctic rhizobia. Since arctic Mesorhizobium cannot nodulate agronomically important legumes, the nodulation genes and the bacterial signals (Nod factors) were characterized as a first step to modifying the host specificity of nodulation. Another valuable approach was to screen for cold adaptation, that is, rhizobia naturally associated with agronomic legumes cultivated in temperate areas. A superior strain of Sinorhizobium meliloti adapted for nodulation of alfalfa at low temperatures was selected and was the most efficient for improving growth of alfalfa in laboratory and field studies. This strain also performed well in improving regrowth of alfalfa after overwintering under cold and anaerobic (ice encasement) stresses, indicating a possible cross-adaptation of selected rhizobia for various abiotic stresses inherent to temperate climates.Key words: cold adaptation, legumes, symbiotic efficiency, cold shock protein, nodulation genes, anaerobiosis.