Diversity and Distribution of Frankia Strains Symbiotic with Ceanothus in California

Abstract

Frankia strains symbiotic with Ceanothus present an interesting opportunity to study the patterns and causes of Frankia diversity and distribution within a particular host infectivity group. We intensively sampled Frankia from nodules on Ceanothus plants along an elevational gradient in the southern Sierra Nevada of California, and we also collected nodules from a wider host taxonomic and geographic range throughout California. The two sampling scales comprised 36 samples from eight species of Ceanothus representing six of the seven major biogeographic regions in and around California. The primary objective of this study was to use a quantitative model to test the relative importance of geographic separation, host specificity, and environment in influencing the identity of Ceanothus Frankia symbionts as determined by ribosomal DNA sequence data. At both sampling scales, Frankia strains symbiotic with Ceanothus exhibited a high degree of genetic similarity. Frankia strains symbiotic with Chamaebatia (Rosaceae) were within the same clade as several Ceanothus symbionts. Results from a classification and regression tree model used to quantitatively explain Frankia phylogenetic groupings demonstrated that the only significant variable in distinguishing between phylogenetic groups at the more local sampling scale was host species. At the regional scale, Frankia phylogenetic groupings were explained by host species and the biogeographic province of sample collection. We did not find any significant correspondence between Frankia and Ceanothus phylogenies indicative of coevolution, but we concluded that the identity of Frankia strains inhabiting Ceanothus nodules may involve interactions between host species specificity and geographic isolation.Frankia strains are gram-positive, filamentous, N-fixing bacteria that form symbiotic relationships with 24 genera of host plants representing 8 families (2, 3). These actinorhizal plants are found throughout the world and serve an important function in many ecosystems as the major source of N, commonly a limiting nutrient (33, 34). Three main groups of Frankia have been classified based on host plant family: an Alnus (Betulaceae)/Myricaceae/Casuarinaceae group, an Eleagnaceae/Rhamnaceae group, and a third group which includes Frankia symbiotic with actinorhizal plants in the families Coriariaceae, Datiscaceae, and Rosaceae and the genus Ceanothus (Rhamnaceae) (9,27,35). These groupings only loosely correspond to host plant phylogenetic relationships, but they are generally consistent with the Frankia infection mode and phenotype (35). Frankia strains within a single host group also exhibit some degree of genetic heterogeneity, yet the patterns and causes of this heterogeneity are less well understood.For several reasons, Frankia strains symbiotic with Ceanothus present an interesting opportunity to study the patterns and causes of Frankia diversity within a particular host-group. First, Frankia strains from the seven actinorhizal genera in the family Rhamnaceae all group with Fr...

Sybr Green- and TaqMan-Based Quantitative PCR Approaches Allow Assessment of the Abundance and Relative Distribution of Frankia Clusters in Soils

“…from locations in the United States and in Pakistan only. While previous studies on the diversity of cluster 2 frankiae in root nodules of different host plants also indicated low sequence diversity of these endophytes (27,28), future studies on cluster 2 frankiae should include a larger diversity of host plants and more locations to retrieve additional information on Frankia diversity. Sequences of cluster 4 frankiae were more diverse, with 3 subgroups delineated.…”

The Influence of the Host Plant Is the Major Ecological Determinant of the Presence of Nitrogen-Fixing Root Nodule Symbiont Cluster II Frankia Species in Soil

“…On the other hand, DNA from nodules collected in the southeastern United States (Tennessee) showed a substantial divergence, based on RFLP, from that of nodules from western North America (30). A study of nodules collected in a circumscribed area in southern coastal California found diversity of cluster II Frankia at the species level, using rep-PCR (15). The sampling area in the southern coastal California study (31) corresponds to a zone of biodiversity for the genus Ceanothus as documented previously (32).…”

“…Cluster II Frankia strains, however, have been characterized as highly host dependent, with only one report of nodule formation from soil devoid of a host, 18 months after inoculation (13). Previous DNA-based surveys of cluster II Frankia strain distribution have been conducted using field-collected nodules (14)(15)(16). Thus, there is only limited knowledge about the soil ecology of cluster II Frankia strains in relation to their host plants, particularly concerning the degree of association with the host presence.…”

Streptomyces thinghirensis sp. nov., isolated from rhizosphere soil of Vitis vinifera

“…Associations between actinomycetes and plant organs can be deleterious or beneficial for the host. While some actinomycetes secrete herbicidal compounds (Tanaka & Omura, 1993) or cause plant diseases (Locci, 1994), others can fix atmospheric nitrogen symbiotically (Oakley et al, 2004) or protect plants against fungal infections (Cao et al, 2005). Several descriptive reports have shown that actinomycetes are a promising group of fungus-antagonistic and root-colonizing microbes.…”