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

MostFrankia -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 ofFrankia spp. is still very limited. Several studies have focused on the distribution patterns ofFrankia spp., demonstrating a combination of host and pedoclimatic parameters in their biogeography. However, very few have considered thein planta sporulation form of the strain, although it is a unique feature among all symbiotic plant-associated microbes. Compared with Sp−Frankia 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+Frankia strains might help elucidate the ecological role ofin planta sporulation and the extent to which this trait mediates host-partner interactions in the alder-Frankia -ECM fungal symbiosis.