Nitrogen Fixing Endophytes in Forest Trees

Abstract

Nitrogen (N) is the most growth-limiting nutrient in most terrestrial and aquatic ecosystems, with new nitrogen is brought in primarily through biological nitrogen fixation (BNF) performed by bacteria and archaea. In addition to the well-studied nodulating symbioses between bacteria and legumes or actinorhizal plants, many plants, from grasses to trees, appear to meet some of their N demand by hosting N2-fixing endophytes above- or belowground. Most studies on endophytic N2 fixation come from grasses, but knowledge about endophytic N2 fixation in forest trees, including both conifers and woody angiosperms, is emerging. Studies of how the diazotroph Paenibacillus polymyxa strain P2b-2R, interacts with its host lodgepole pine as well as other plants, suggest that diazotrophs can colonize their host intracellulary; that conifers can derive a significant part of N from the atmosphere; and that the association can take months to establish and may depend on N soil content. P. polymyxa strain P2b-2R has also been shown to colonize, promote growth and fix N2 in crops, demonstrating that endophytic diazotrophs can be generalists. Culture independent studies suggest that conifers growing in N limited high altitude environments consistently host foliar endophytes related to the diazotroph Gluconacetobacter in their needles, and that nitrogenase is active within pine foliage, suggesting that endophytes may represent an N2-fixing strategy for long-living conifers to meet their N demand in N limited subalpine ecosystems. Diazotrophs have also been isolated from poplar and willow growing in N limited riparian ecosystems. These strains are also generalists and can promote growth and provide N to both poplar clones and crops. Direct evidence of N2 fixation and incorporation by native endophytes has been demonstrated in poplar using labelling with the stable nitrogen isotope 15N2. Enrichment of 15N was highly variable among samples, potentially as a result of differences in the endophyte community structure and abundance. We provide suggestions for research efforts that can take advantage of this new knowledge with the goal of reducing the use of chemical fertilizer in agriculture and forestry.