Abstract
Plant endosymbiosis with nitrogen-fixing cyanobacteria has independently evolved in diverse plant lineages, offering a unique window to study the evolution and genetics of plant–microbe interaction. However, very few complete genomes exist for plant cyanobionts, and therefore little is known about their genomic and functional diversity. Here, we present four complete genomes of cyanobacteria isolated from bryophytes. Nanopore long-read sequencing allowed us to obtain circular contigs for all the main chromosomes and most of the plasmids. We found that despite having a low 16S rRNA sequence divergence, the four isolates exhibit considerable genome reorganizations and variation in gene content. Furthermore, three of the four isolates possess genes encoding vanadium (V)-nitrogenase (vnf), which is uncommon among diazotrophs and has not been previously reported in plant cyanobionts. In two cases, the vnf genes were found on plasmids, implying possible plasmid-mediated horizontal gene transfers. Comparative genomic analysis of vnf-containing cyanobacteria further identified a conserved gene cluster. Many genes in this cluster have not been functionally characterized and would be promising candidates for future studies to elucidate V-nitrogenase function and regulation.
Highlights
Symbiotic interactions with microbes play a pivotal role in eukaryote evolution
The bestknown example is the symbiosis between legumes and rhizobia, which is an area of active investigation aiming to transfer the symbiosis to nonlegume crop plants (Mus et al 2016)
There is another and often overlooked type of N2fixing symbiosis in which certain plants partner with heterocyst-forming cyanobacteria. They are less dependent on the plant partner—they have their own specialized cells for N2-fixation and do not require hosts to create a low-oxygen environment—and hold a promising translational potential for installing N2-fixation into crop plants
Summary
Symbiotic interactions with microbes play a pivotal role in eukaryote evolution. In plants, microbial symbionts produce growth-promoting substances, boost tolerance to environmental stress, and can greatly expand access to essential nutrients (Lugtenberg and Kamilova 2009). The bestknown example is the symbiosis between legumes and rhizobia, which is an area of active investigation aiming to transfer the symbiosis to nonlegume crop plants (Mus et al 2016) There is another and often overlooked type of N2fixing symbiosis in which certain plants partner with heterocyst-forming cyanobacteria We report here the discovery of V-nitrogenase in plant cyanobionts, and present comparative genomic analyses to shed new light on the distribution and evolution of V-nitrogenase
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