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

Abstract

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.

Proposal of 'Candidatus Frankia alpina', the uncultured symbiont of Alnus alnobetula and A. incana that forms spore-containing nitrogen-fixing root nodules

“…However even if Sp+ strains associated with alpine Alnus species are very close genetically, large-scale sampling highlighted two well supported clades corresponding to A. alnobetula and A. incana symbionts respectively [29,30] and within A. alnobetula strains a division between North American-East Asian and European strains. The host-specificity observed in cross-inoculation experiments was narrower for Sp+ than that of Sp− strains and was different for A. alnobetula and A. incana Sp+ strains [33,43]. All these results indicate the influence of host biogeography and genetics on the symbiont phylogeny and ecology and are consistent with the possible obligate symbiont hypothesis proposed for these strains [34].…”

Draft genome sequences for three unisolated Alnus-infective Frankia Sp+ strains, AgTrS, AiOr and AvVan, the first sequenced Frankia strains able to sporulate in-planta

“…In contrast to Sp- strains, up to date, Sp+ strains are still totally culture recalcitrant (none are available in pure culture despite many isolation attempts) 5. Furthermore, we recently described their narrower host specificity 6, suggesting a strong host dependence. It was hypothesized that Sp+ strains could have evolved towards an obligatory symbiont status with spores representing their only form of survival outside the host.…”

“…Three Alnus -infective Sp+ Frankia uncultured strains from Cluster 1, AgTrS, AiOr and AvVan, were selected from nodules collected in 3 distinct alder stands, colonized by A. glutinosa (Le Tremblay, Savoie, France), A. incana (Ornon, Isère, France) and A. alnobetula (Vanoise, Savoie, France), respectively 6. AgTrS, AiOr and AvVan genomes were sequenced using DNA extracted from spores directly isolated from nodules.…”

On specialists and generalists: niche range strategies across the tree of life

“…Nevertheless, annotated biome definitions are arbitrarily delineated and may be subject to human error. For example, the plants biome includes both freshwater plants [36][37][38] and seagrasses 39 , as well as macro algae from kelp forests 40 (Supp. table 1).…”