Frankia Diversity in Host Plant Root Nodules Is Independent of Abundance or Relative Diversity of Frankia Populations in Corresponding Rhizosphere Soils

Abstract

Actinorhizal plants form nitrogen-fixing root nodules in symbiosis with soil-dwelling actinobacteria within the genus , and specific taxonomic clusters nodulate plants in corresponding host infection groups. In same-soil microcosms, we observed that some host species were nodulated (, ,, ) while others were not (, ). Nodule populations were represented by eight different sequences of gene fragments. Two of these sequences characterized frankiae in nodules, and three others characterized frankiae in nodules. Frankiae in nodules were represented by five sequences, one of which was also found in nodules from and , while another was detected in nodules from Quantitative PCR assays showed that vegetation generally increased the abundance of frankiae in soil, independently of the target gene (i.e., or the 23S rRNA gene). Targeted Illumina sequencing of-specific gene fragments detected 24 unique sequences from rhizosphere soils, 4 of which were also found in nodules, while the remaining 4 sequences in nodules were not found in soils. Seven of the 24 sequences from soils represented>90% of the reads obtained in most samples; the 2 most abundant sequences from soils were not found in root nodules, and only 2 of the sequences from soils were detected in nodules. These results demonstrate large differences between detectable populations in soil and those in root nodules, suggesting that root nodule formation is not a function of the abundance or relative diversity of specific populations in soils. The nitrogen-fixing actinobacterium forms root nodules on actinorhizal plants, with members of specific taxonomic clusters nodulating plants in corresponding host infection groups. We assessed diversity in root nodules of different host plant species, and we related specific populations to the abundance and relative distribution of indigenous frankiae in rhizosphere soils. Large differences were observed between detectable populations in soil and those in root nodules, suggesting that root nodule formation is not a function of the abundance or relative diversity of specific populations in soils but rather results from plants potentially selecting frankiae from the soil for root nodule formation. These data also highlight the necessity of using a combination of different assessment tools so as to adequately address methodological constraints that could produce contradictory data sets.

In Planta Sporulation of Frankia spp. as a Determinant of Alder-Symbiont Interactions

“…These data indicate that, in agreement with findings from a previous study (50), host species in the field determined the nodule-forming populations of Frankia species. Nodule formation has been shown not to be a function of Frankia population abundance (51). Similarly, in this study, the sequences of trapped Frankia strains harbored relatively low abundances compared to those of soil-dwelling Frankia strains, possibly indicating that our metabarcoding approach did not allow for the detection of differences among soil Frankia communities beneath the different alder species.…”

Evolution and biogeography of actinorhizal plants and legumes: A comparison

“…A subgroup within this cluster (Cluster 1c, Figure 2) (Simonet et al, 1999). The remaining strains in Cluster 1 are cosmopolitan and nodulate Alnus, Comptonia and Myrica (Ben Tekaya et al, 2018;Benson & Dawson, 2007;Huguet et al, 2005). Cluster 2 Frankia spp.…”

Saline and Arid Soils: Impact on Bacteria, Plants, and Their Interaction

“…Free-living microorganisms include genera as Azospirillum and Azotobacter [ 36 ]. Among the symbiotic bacteria, the members of the family Rhizobiaceae ( Rhizobium , Bradyrhizobium , Sinorhizobium , Azorhizobium , and Mesorhizobium ) can establish symbiotic relationships with legume, while bacteria belonging to Frankia genus can be associated with other plant families among which, Betulaceae ( Alnus glutinosa for example) [ 37 ].…”

“…Free-living microorganisms include genera as Azospirillum and Azotobacter [36]. Among the symbiotic bacteria, the members of the family Rhizobiaceae (Rhizobium, Bradyrhizobium, Sinorhizobium, Azorhizobium, and Mesorhizobium) can establish symbiotic relationships with legume, while bacteria belonging to Frankia genus can be associated with other plant families among which, Betulaceae (Alnus glutinosa for example) [37]. The close dialogue between rhizobia and the leguminous plants, mediated by the bacterial lipochitooligosaccharides (Nod factors) and plant flavonoids, leads to the formation of root nodules [36,38].…”