Abstract
Biological nitrogen fixation is one of the most critical processes contributing to ecosystem productivity and stability on a global scale. In temperate climates of the northern hemisphere, plant-root associated bacteria of the genus Frankia are the major nitrogen fixers in forest environments. Trees belonging to the genus Alnus are the most widespread hosts of Frankia in the Pacific Northwest, and a myriad of biotic and abiotic factors can influence the robustness of this symbiosis. Host identity and bacterial strain are important features that can impact Alnus-Frankia association, but little is known about the interplay of intrageneric hosts that co-occur in natural settings. In this study we investigated the genetic diversity and host specificity of Frankia bacteria associated with sympatrically occurring populations of Alnus rubra (red alder) and Alnus rhombifolia (white alder) in Oregon. Based on sequence analysis of the nifH gene recovered from root nodules we found low overall bacterial diversity. One dominant Frankia genotype was associated with both host species, indicating a lack of strong host specificity in this system. Our results suggest that certain intrageneric plant hosts with overlapping distributions show cross-compatibility with symbiotic actinorhizal bacteria, and that low strain diversity of these bacteria can persist across mixed host populations.
Highlights
Nitrogen-fixing bacteria associated with plant roots are one of the primary sources of biological nitrogen inputs in terrestrial ecosystems
Of the 58 nodules processed we successfully generated nifH sequences for 47, which were comprised of five Operational taxonomic units (OTUs) at 97% sequence similarity across both tree species
The goal of the current study was to assess genetic diversity and host specificity of actinorhizal Frankia bacteria associated with the roots of two co-occurring Alnus taxa
Summary
Nitrogen-fixing bacteria associated with plant roots are one of the primary sources of biological nitrogen inputs in terrestrial ecosystems. Bacteria of the genus Frankia are the major group that forms such symbioses with 24 actinorhizal angiosperm genera worldwide (Schwintzer and Tjepkema 1990, Benson and Silvester 1993). Members of the genus Alnus (alders) are the only known trees that form associations with Frankia bacteria, and Alnus rubra Bong. (red alder) is one of the most common tree species in the northwest (Franklin and Dyrness 1973). A. rubra represents a critical host for actinorhizal bacteria in the Pacific Northwest owing to its widespread distribution and relatively long lifespan compared to non-tree actinorhizal hosts in this region. Alders are pioneer tree species that can enhance soil nitrogen content, The performance of the actinorhizal symbiosis can vary significantly depending on host plantbacterial genotype combinations (Wheeler et al 1986, Weber et al 1987). While a number of different factors can influence the genotypic diversity of Frankia including edaphic (Sheppard et al 1989, Crannell et al 1994, Nickel et al 1999) and biogeographic (Khan et al 2007, Põlme et al 2014) factors, the effect of host identity is of particular interest and has been widely studied
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