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Open Access Publications from the University of California

Disentangling the rhizosphere community through stable isotope informed genome-resolved metagenomics and assembled metatranscriptomes

  • Author(s): Starr, Evan P
  • Advisor(s): Banfield, Jillian F
  • Firestone, Mary K
  • et al.

The functioning, health, and productivity of soil is intimately tied to the complex network of interactions in the rhizosphere. Because of this, the rhizosphere has been rigorously studied for over a century, but due to technical limitations many aspects of soil biology have been overlooked. In order to better understand rhizosphere functioning, my work has focused on the less explored organisms and interactions in microbial communities, this includes unculturable bacteria along with viruses and eukaryotes. Only by considering soil biology more holistically can we better understand the functioning of this enigmatic yet critical ecosystem. Knowledge about these interactions could direct how we think about plant-microbe relationships, soil carbon stabilization and the roles of understudied organisms in biogeochemical cycling.

The transformation of plant photosynthate into soil organic carbon and its recycling to CO2 by soil microorganisms is one of the central components of the terrestrial carbon cycle. There are currently large knowledge gaps related to which soil-associated microorganisms take up plant carbon in the rhizosphere and the fate of that carbon. Additionally, understanding about obligate symbionts such as members of the Candidate Phyla Radiation (CPR) in soil is severely limited, both from the perspective of their genomic potential and their interactions with the greater soil community. We conducted an experiment in which common wild oats (Avena fatua) were grown in a 13CO2 atmosphere and the rhizosphere and non-rhizosphere soil was sampled for genomic analyses. Density gradient centrifugation of DNA extracted from soil samples enabled distinction of microbes that did and did not incorporate the 13C into their DNA. A 1.45-Mbp genome of a Saccharibacteria (TM7) was identified and, despite the microbial complexity of rhizosphere soil, curated to completion. The genome lacks many biosynthetic pathways, including genes required to synthesize DNA de novo. Rather, it acquires externally derived nucleic acids for DNA and RNA synthesis. Given this, we conclude that rhizosphere-associated Saccharibacteria recycle DNA from bacteria that live off plant exudates and/or phage that acquired 13C because they preyed upon these bacteria and/or directly from the labeled plant DNA. Isotopic labeling indicates that the population was replicating during the 6-week period of plant growth. Interestingly, the genome is ~ 30% larger than other complete Saccharibacteria genomes from non-soil environments, largely due to more genes for complex carbon utilization and amino acid metabolism. Given the ability to degrade cellulose, hemicellulose, pectin, starch, and 1,3-β-glucan, we predict that this Saccharibacteria generates energy by fermentation of soil necromass and plant root exudates to acetate and lactate. The genome also encodes a linear electron transport chain featuring a terminal oxidase, suggesting that this Saccharibacteria may respire aerobically. The genome encodes a hydrolase that could breakdown salicylic acid, a plant defense signaling molecule, and genes to interconvert a variety of isoprenoids, including the plant hormone zeatin. We propose that isotopically labeled CO2 is incorporated into plant-derived carbon and then into the DNA of rhizosphere organisms capable of nucleotide synthesis, and the nucleotides are recycled into Saccharibacterial genomes.

We collected paired rhizosphere and non-rhizosphere soil at six and nine weeks of plant growth and extracted DNA that was separated by density gradient centrifugation. The separate fractions were sequenced, assembled, and binned to generate 55 unique microbial genomes that were >70% complete. Evidence for close interaction between bacteria, micro-eukaryotes and plant roots includes the ability to modulate plant signaling hormones, abundant plant pathogenicity factors and production of cyanide and insecticidal toxins. We reconstructed eukaryotic 18S rRNA sequences and identified micro-eukaryotic bacterivores and fungi in the rhizosphere soil. In addition, we reconstructed two complete genomes for phage that were among the most highly 13C-enriched entities in our study. CRISPR locus targeting connected a phage to a Burkholderiales host predicted to be a plant pathogen and a possible plant growth promoting Catenulispora may serve as the host for another phage. Thus, 13C could be tracked from the atmosphere into plant roots, soil and through the rhizosphere food web.

Viruses impact nearly all organisms on Earth, with ripples of influence in agriculture, health and biogeochemical processes. We previously investigated DNA phage, however, very little is known about RNA viruses in an environmental context, and even less is known about their diversity and ecology in the most complex microbial system, soil. Here, we assembled 48 individual metatranscriptomes from four habitats within a soil sampled over a 22-day time series: rhizosphere alone, detritosphere alone, a combination of the two, and unamended soil (four time points and three biological replicates per time point). We resolved the RNA viral community, uncovering a high diversity of viral sequences. We also investigated possible host organisms by analyzing metatranscriptome marker gene content. Based on viral phylogeny, much of the diversity was Narnaviridae that parasitize fungi or Leviviridae that infect Proteobacteria. Both host and viral communities appear to be highly dynamic, and rapidly diverged depending on experimental conditions. The viral communities were structured based on the presence of litter, while putative hosts appeared to be impacted by both the presence of litter and roots. A clear time signature from Leviviridae and their hosts indicated that viruses were replicating. With this time-resolved analysis, we show that RNA viruses are diverse, abundant and active in soil. Their replication causes host cell death, mobilizing carbon in a process that represents a largely overlooked component of carbon cycling in soil.

By combining state of the art techniques, stable isotope probing, genome-resolved metagenomics and assembled metatranscriptomics, we advanced knowledge about the interplay between understudied players in the rhizosphere and provided some clues for the fate of plant derived carbon in the soil microbial ecosystem. The use of genome resolved metagenomics is the only current way to determine the lifestyle of uncultured microbes. Complete genomes are still difficult to reconstruct however, they contain extensive of information, both regarding the presence and the absence of capabilities. Stable isotope probing allowed us to follow plant fixed carbon into the microbial community and in several cases across multiple trophic levels. This study demonstrates the power of stable isotope-informed genome-resolved metagenomics to resolve aspects of the complex rhizosphere food web. The approach will find broad application for study of other soils and different ecosystems.

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