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

The genus forms symbiosis with the actinobacteria spp. and ectomycorrhizal fungi. Two types of lineages can be distinguished based on their ability to sporulate Spore-positive (Sp+) strains are predominant on and in highlands, while spore-negative (Sp-) strains are mainly associated with in lowlands. Here, we investigated whether the Sp+ predominance in nodules is due to host selection of certain genotypes from soil communities or the result of the ecological history of the alder stand soil, as well as the effect of the sporulation genotype on the ectomycorrhizal (ECM) communities. Trapping experiments were conducted using , , and plantlets on 6 soils, differing in the alder species and the frequency of Sp+ nodules in the field. Higher diversity of spp. and variation in Sp+ frequencies were observed in the trapping than in the fields. Both indigenous and trapping species shape community structure in trapped nodules. Nodulation impediments were observed under several trapping conditions in Sp+ soils, supporting a narrower host range of Sp+ species. and were able to associate equally with compatible Sp+ and Sp- strains in the greenhouse. Additionally, no host shift was observed for -specific ECM, and the sporulation genotype of spp. defined the ECM communities on the host roots. The symbiotic association is likely determined by the host range, the soil history, and the type of species. These results provide an insight into the biogeographical drivers of alder symbionts in the Holarctic region. Most -actinorhiza plant symbioses are capable of high rates of nitrogen fixation comparable to those found on legumes. Yet, our understanding of the ecology and distribution of spp. is still very limited. Several studies have focused on the distribution patterns of spp., demonstrating a combination of host and pedoclimatic parameters in their biogeography. However, very few have considered the sporulation form of the strain, although it is a unique feature among all symbiotic plant-associated microbes. Compared with Sp- strains, Sp+ strains would be obligate symbionts that are highly dependent on the presence of a compatible host species and with lower efficiency in nitrogen fixation. Understanding the biogeographical drivers of Sp+ strains might help elucidate the ecological role of sporulation and the extent to which this trait mediates host-partner interactions in the alder--ECM fungal symbiosis.