Host species and habitat affect nodulation by specific Frankia genotypes in two species of Alnus in interior Alaska

Abstract

Alders (Alnus spp.) are important components of northern ecosystems due to their ability to fix nitrogen (N) in symbiosis with Frankia bacteria. Availability of optimal Frankia may be a contributing factor in limiting the performance and ecological effects of Alnus, but the factors underlying distribution of Alnus-infective Frankia are not well understood. This study examined the genetic structure (nifD-K spacer RFLP haplotypes) of Frankia assemblages symbiotic with two species of Alnus (A. tenuifolia and A. viridis) in four successional habitats in interior Alaska. We used one habitat in which both hosts occurred to observe differences between host species independent of habitat, and we used replicate sites for each habitat and host to assess the consistency of symbiont structure related to both factors. We also measured leaf N content and specific N-fixation rate (SNF) of nodules ((15)N uptake) to determine whether either covaried with Frankia structure, and whether Frankia genotypes differed in SNF in situ. Frankia structure differed between sympatric hosts and among habitats, particularly for A. tenuifolia, and was largely consistent among replicate sites representing both factors. Leaf N differed between host species and among habitats for both hosts. SNF did not differ among habitats or host species, and little evidence for differences in SNF among Frankia genotypes was found, due largely to high variation in SNF. Consistency of Frankia structure among replicate sites suggests a consistent relationship between both host species and habitat among these sites. Correlations with specific environmental variables and possible underlying mechanisms are discussed.

FrankiaAssemblages Associated withAlnus rubraandAlnus viridisAre Strongly Influenced by Host Species Identity

“…The nonrandom distribution of genotypes between hosts is consistent with the results of previous studies covering a variety of host taxa and geographic locations (Van Dijk et al 1988;Rouvier et al 1996;Clawson et al 1999;Navarro et al 1999;Huguet et al 2001) as well as for Frankia assemblages captured on different actinorhizal species exposed to the same soil inoculum (Mirza et al 2009). In combination with host-associated Frankia distribution patterns at Alnus sites in Alaska (Anderson et al 2009) and Ceanothus sites in California (Oakley et al 2004), these results implicate host species identity as a key influence on the structure of Frankia assemblages in western North America.…”

“…Second, a number of prior studies examining host species effects in combination with other factors have found significant interactions. For example, many field-based studies have documented coincident effects of host species identity and Huguet et al 2001Huguet et al , 2004Oakley et al 2004;Anderson et al 2009). Other studies have found geography alone rather than host species identity was the best predictor of Frankia assemblage structure (Jeong and Myrold 1999;Ritchie and Myrold 1999).…”

“…Distinct Frankia assemblages have been observed among allopatric hosts across different species (Rouvier et al 1996;Clawson et al 1999;Navarro et al 1999) and genera (Rouvier et al 1996;Simonet et al 1999). More strikingly, studies have also found that distinct Frankia assemblages associate with different sympatric host species both within (Anderson et al 2009;Pokharel et al 2010) and across (Huguet et al 2001) genera. While some studies have found no evidence for a correlation between Frankia genotypes and host plant species (Vanden Huevel et al 2004), there appears to be general support for the importance of host species in influencing both the composition and diversity of Frankia assemblages (Oakley et al 2004;Mirza et al 2009).…”

Growth responses of indigenous Frankia populations to edaphic factors in actinorhizal rhizospheres

“…While a considerable amount of information is available on nodule-forming populations, and the interaction of frankiae with host plant species [5,9], information on frankiae in soil, its second ecological niche, is relatively scarce [8]. Most of the information on soil Frankia populations focuses on their potential to form root nodules [19,35,36,39], diversity as a function of host plant species [1,2,23,35] and the effects of environmental characteristics on nodule-forming capacity [16,31,37,40]. Environmental characteristics such as soil organic matter and matric potential, for example, have been shown to affect the development of specific Frankia populations [17,18,20,21], with organic matter such as leaf litter supporting growth of one specific population, i.e., subgroup II of the Alnus host infection group represented by strain Ag45/Mut15 only [17].…”

Responses of symbiotic N2 fixation in Alnus species to the projected elevated CO2 environment

“…Symbiotic Frankia assemblages can differ widely between sympatric Alnus spp. and between successional habitats occupied by a given host species (Anderson et al 2009). Phylogenetic Tables 1 and 2 Trees specificity is a significant factor in the Alnus tenuifoliaFrankia interaction, and significant habitat-based differentiation may exist among A. tenuifolia-infective genotypes .…”