The diversity and community structure of symbiotic cyanobacteria in hornworts inferred from long‐read amplicon sequencing

Abstract

Premise: Nitrogen-fixing endosymbioses with cyanobacteria have evolved independently in five very different plant lineages. Expanding knowledge of these symbioses promises to improve the understanding of symbiosis evolution and broaden the toolkit for agricultural engineering to reduce artificial fertilizer use. Here we focused on hornworts, a bryophyte lineage in which all members host cyanobacteria, and investigated factors shaping the diversity of their cyanobiont communities. Methods: We sampled hornworts and adjacent soils in upstate New York throughout the hornwort growing season. We included all three sympatric hornwort species in the area, allowing us to directly compare partner selectivity. To profile cyanobacteria communities, we established a metabarcoding protocol targeting rbcL-X with PacBio long reads. Results: The hornwort cyanobionts detected were phylogenetically diverse, including clades that do not contain other known plant symbionts. We found significant overlap between hornwort cyanobionts and soil cyanobacteria, a pattern not previously reported in other plant-cyanobacteria symbioses. Cyanobiont communities differed between host plants only centimeters apart, but we did not detect an effect of sampling time or host species on the cyanobacterial community structure. Conclusions: This study expands the phylogenetic diversity of known symbiotic cyanobacteria. Our analyses suggest that hornwort cyanobionts have a tight connection to the soil background, and we found no evidence that time within growing season, host species, or distance at the scale of meters strongly govern cyanobacteria community assembly. This study provides a critical foundation for further study of the ecology, evolution, and interaction dynamics of plant-cyanobacteria symbiosis.

Dynamic plastid and mitochondrial genomes in Chaetopeltidales (Chlorophyceae) and characterization of a new chlorophyte taxon

“…As part of our ongoing effort to isolate symbiotic cyanobacteria from hornworts (Nelson et al, 2019(Nelson et al, , 2021, we inadvertently obtained an unknown chlorophyte species. The hornwort, Notothylas orbicularis, was collected from a roadside drainage trench at Potato Hill State Forest (Tompkins County, New York; 42°20′37.8″N 76°16′09.1″W, 488 m a.s.l.…”

Phylogenetic structure of specialization: A new approach that integrates partner availability and phylogenetic diversity to quantify biotic specialization in ecological networks

“…The availability proxy using interaction frequencies might be especially problematic for the Peltigera–Nostoc dataset, which was sampled at a global scale in a non‐systematic way. In this case, the interaction frequencies may lead to highly inaccurate estimates of the partner availabilities, particularly since Nostoc symbionts can be free‐living (Nelson et al, 2021 ).…”

“…The thresholds for the shaded areas (random phylogenetic structure) were defined as the mean of the 95% confidence interval of the null distributions generated for each taxon in the datasets. Points on the diagonal red line indicate identical PSS values interaction frequencies may lead to highly inaccurate estimates of the partner availabilities, particularly since Nostoc symbionts can be free-living (Nelson et al, 2021).…”

What can hornworts teach us?

“…The cyanobionts hosted by hornworts are usually from the genus Nostoc spp. ( Adams and Duggan, 2008 ) but can be phylogenetically diverse ( Nelson et al., 2021 ; Rahmatpour et al., 2021 ). Hornworts host the cyanobacteria in mucilage cavities that can be accessed through ventral mucilage clefts, which superficially resemble stomata ( Renzaglia et al., 2000 ; Villarreal A and Renzaglia, 2006 ).…”