Nonlegume Parasponia andersonii deploys a broad rhizobium host range strategy resulting in largely variable symbiotic effectiveness

The non-legume genus Parasponia has evolved the rhizobium symbiosis independent from legumes and has done so only recently. We aim to study the promiscuity of such newly evolved symbiotic engagement and determine the symbiotic effectiveness of infecting rhizobium species. It was found that Parasponia andersonii can be nodulated by a broad range of rhizobia belonging to four different genera, and therefore, we conclude that this non-legume is highly promiscuous for rhizobial engagement. A possible drawback of this high promiscuity is that low-efficient strains can infect nodules as well. The strains identified displayed a range in nitrogen-fixation effectiveness, including a very inefficient rhizobium species, Rhizobium tropici WUR1. Because this species is able to make effective nodules on two different legume species, it suggests that the ineffectiveness of P. andersonii nodules is the result of the incompatibility between both partners. In P. andersonii nodules, rhizobia of this strain become embedded in a dense matrix but remain vital. This suggests that sanctions or genetic control against underperforming microsymbionts may not be effective in Parasponia spp. Therefore, we argue that the Parasponia-rhizobium symbiosis is a delicate balance between mutual benefits and parasitic colonization.