Characterization of facultative yet endohyphal interaction between Umbelopsis and Paraburkholderia

The generalist and facultative symbioses are more common in nature and usually precede obligate and specialized ones. Although bacterial-fungal interactions are probably not an exception, little is known about the role that facultative bacterial-fungal symbioses may play in adaptation to occupying specific niches and changing environmental conditions. While Mucoromycota fungi are known to form close relationships with endohyphal bacteria, examples of more facultative interactions have been recently reported in this lineage as well. One example of facultative, yet endohyphal interaction is Umbelopsis-Paraburkholderia consortium which seems to be highly prevalent in deadwood. In this study we aimed to characterize this relationship as, so far, in-depth characterizations are available only for strictly endohyphal bacteria and their hosts.

We compared functionally annotated genomes of both partners with genomes of their free-living and symbiotic relatives. Analysis showed that the genome of Paraburkholderia does not differ greatly from genomes of free-living Burkholderiaceae, as it is not reduced and possesses a similar enzymatic potential. The genome of the fungal partner is also similar to the genomes of its close relatives, however Umbelopsis belongs to a relatively understudied taxonomic group, thus conclusions about its genomic content are less certain. In the genomes we identified some features needed for establishment and maintenance of endohyphal interaction, such as genes coding for diacylglycerol kinase (DGK) and secretion systems. Finally, we detected potential metabolic intertwinement in wood degrading capacities of partners, which, combined with the detection of genes encoding plant growth promotion factors in bacteria, allows us to assume that the consortium is well adapted to develop in the decaying wood ecosystem from which it was isolated.

While presence of Paraburkholderia decreases growth rate of Umbelopsis in the simplified laboratory system, it is possible that together they may occupy niches not available to each partner individually. Paraburkholderia, unlike strictly endohyphal symbionts, has a non-reduced genome, with extensive metabolic capacities which probably play an important role in adaptation to colonizing otherwise inaccessible niches by their fungal host.