AUTHOR=Erlacher Armin , Cernava Tomislav , Cardinale Massimiliano , Soh Jung , Sensen Christoph W. , Grube Martin , Berg Gabriele 

TITLE=Rhizobiales as functional and endosymbiontic members in the lichen symbiosis of Lobaria pulmonaria L.

JOURNAL=Frontiers in Microbiology

VOLUME=6

YEAR=2015

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2015.00053

DOI=10.3389/fmicb.2015.00053

ISSN=1664-302X

ABSTRACT=<p><italic>Rhizobiales</italic> (<italic>Alphaproteobacteria)</italic> are well-known beneficial partners in plant-microbe interactions. Less is known about the occurrence and function of <italic>Rhizobiales</italic> in the lichen symbiosis, although it has previously been shown that <italic>Alphaproteobacteria</italic> are the dominating group in growing lichen thalli. We have analyzed the taxonomic structure and assigned functions to <italic>Rhizobiales</italic> within a metagenomic dataset of the lung lichen <italic>Lobaria pulmonaria</italic> L. One third (32.2%) of the overall bacteria belong to the <italic>Rhizobiales</italic>, in particular to the families <italic>Methylobacteriaceae, Bradyrhizobiaceae</italic>, and <italic>Rhizobiaceae</italic>. About 20% of our metagenomic assignments could not be placed in any of the <italic>Rhizobiales</italic> lineages, which indicates a yet undescribed bacterial diversity. SEED-based functional analysis focused on <italic>Rhizobiales</italic> and revealed functions supporting the symbiosis, including auxin and vitamin production, nitrogen fixation and stress protection. We also have used a specifically developed probe to localize <italic>Rhizobiales</italic> by confocal laser scanning microscopy after fluorescence <italic>in situ</italic> hybridization (FISH-CLSM). Bacteria preferentially colonized fungal surfaces, but there is clear evidence that members of the <italic>Rhizobiales</italic> are able to intrude at varying depths into the interhyphal gelatinous matrix of the upper lichen cortical layer and that at least occasionally some bacteria also are capable to colonize the interior of the fungal hyphae. Interestingly, the gradual development of an endosymbiotic bacterial life was found for lichen- as well as for fungal- and plant-associated bacteria. The new tools to study <italic>Rhizobiales</italic>, FISH microscopy and comparative metagenomics, suggest a similar beneficial role for lichens than for plants and will help to better understand the <italic>Rhizobiales</italic>-host interaction and their biotechnological potential.</p>