Name: exploring the root microbiome: extracting bacterial community data from the soil, rhizosphere, and root endosphere
Uploaded: 2018-05-02T14:30:00
Description: Watch this Scientific Journal Video about Exploring the Root Microbiome: Extracting Bacterial Community Data from the Soil, Rhizosphere, and Root Endosphere at JoVE.com

This work was funded by the USDA-ARS (CRIS 2030-21430-008-00D). TS is supported by the NSF Graduate Research Fellowship Program.

1. Collection and Separation of Root Endosphere, Rhizosphere, and Soil Samples
<ol>
	<li>Prior to entering the field, autoclave ultrapure water (at least 90 mL of water per sample) to sterilize. Prepare epiphyte removal buffer (at least 25 mL per sample) by adding 6.75 g of KH2PO4, 8.75 g of K2HPO4, and 1 mL of Triton X-100, to 1 L of sterile water. Sterilize the buffer using a vacuum filter with 0.2 &#181;m pore size.
	<ol>
		<li>For steps 1.2 to 1.5, wear clean gloves steri...

This protocol demonstrates an established pipeline for exploring root endosphere, rhizosphere, and soil microbial community compositions, from field sampling to sample processing and downstream sequencing. Studying root-associated microbiomes presents unique challenges, due in part to the inherent difficulties in sampling from soil. Soils are highly variable in terms of physical and chemical properties, and different soil conditions can be separated by as little as a few millimeters28,<...

Plant-associated microbiomes consist of dynamic and complex microbial communities comprised of bacteria, archaea, viruses, fungi, and other eukaryotic microorganisms. In addition to their well-studied role in causing plant disease, plant-associated microbes can also positively influence plant health by improving tolerance to biotic and abiotic stresses, promoting nutrient availability, and enhancing plant growth through the production of phytohormones. For this reason, particular interest exists in characterizing the taxa that associate with plant root endospheres, rhizospheres, and the surrounding soil. While some microbes can be cultured in isolation on laboratory generated media, many cannot, in part because they may rely on symbiotic relationships with other microbes, grow very slowly, or require conditions that cannot be replicated in a lab environment. Because it circumvents the need for cultivation and is relatively inexpensive and high-throughput, sequence-based phylogenetic profiling of environmental and host-associated microbial samples has become a preferred method for assaying microbial community composition.
The selection of appropriate sequencing technologies provided by various next generation sequencing (NGS) platforms1 is dependent on the users&#39; needs, with important factors including: desired coverage, amplicon length, expected community diversity, as well as sequencing error-rate, read-length, and the cost-per-run/megabase. Another variable that needs to be considered in amplicon-based sequencing experiments is what gene will be amplified and what primers will be used. When designing or choosing primers, researchers are often forced to make tradeoffs between the universality of amplification and the taxonomic resolution achievable from the resulting amplicons. For this reason, these types of studies often chose primers and markers that selectively target specific subsets of the microbiome. Evaluating the composition of bacterial communities is commonly accomplished by sequencing one or more of the hypervariable regions of the bacterial 16S rRNA gene2,3. In this study, we describe an amplicon based sequencing protocol developed for a NGS platform that targets the 500 bp V3-V4 region of the bacterial 16S rRNA gene, which allows for broad amplification of bacterial taxa while also providing sufficient variability to distinguish between different taxa. Additionally, this protocol can easily be adapted for the use with other primer sets, such as those targeting the ITS2 marker of fungi or the 18S rRNA subunit of eukaryotes.
While other approaches such as shotgun metagenomics, metatranscriptomics, and single-cell sequencing, offer other advantages including resolved microbial genomes and more direct measurement of community function, these techniques are typically more expensive and computationally intensive than the phylogenetic profiling described here4. Additionally, performing shotgun metagenomics and metatranscriptomics on root samples yields a large percentage of reads belonging to the host plant genome, and methods to overcome this limitation are still being developed5,6.
As with any experimental platform, amplicon-based profiling can introduce a number of potential biases which should be considered during the experimental design and data analysis. These include the methods of sample collection, DNA extraction, selection of PCR primers, and how library preparation is performed. Different methods can significantly impact the amount of usable data generated, and can also hinder the efforts to compare results between studies. For example, the method of removing rhizosphere bacteria7 and the use of different extraction techniques or choice of DNA extraction kits8,9 have been shown to significantly impact downstream analysis, which leads to different conclusions regarding which microbes are present and their relative abundances. Since amplicon-based profiling can be customized, making comparisons across studies can be challenging. The Earth Microbiome Project has suggested that researchers studying complex systems such as the plant-associated microbiome would benefit from the development of standardized protocols as a means of minimizing the variability caused by the application of different methods between studies10,11. Here, we discuss many of the above topics and offer suggestions as to best practices where appropriate.
The protocol demonstrates the process of collecting soil, rhizosphere, and root samples from Sorghum bicolor and extracting DNA using a well-established DNA isolation kit11. Additionally, our protocol includes a detailed amplicon sequencing workflow, using a commonly utilized NGS platform, to determine the structure of the bacterial communities12,13,14. This protocol has been validated for the use in a wide range of plant hosts in a recent published study of the roots, rhizosphere, and associated-soils of 18 monocot species including Sorghum bicolor, Zea mays, and Triticum aestivum15. This method has also been validated for use with other marker genes, as demonstrated by its successful application to studying the fungal ITS2 marker gene in studies of the agave microbiome16,17 and strawberry microbiome18.

<table>
	<tbody>
		<tr>
			<td>0.1-10/20 &#181;L filtered micropipette tips</td>
			<td>USA Scientific</td>
			<td>1120-3810</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>1.5 mL microcentrifuge tubes</td>
			<td>USA Scientific</td>
			<td>1615-5510</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>10 &#181;L multi-channel pipette</td>
			<td>Eppendorf</td>
			<td>3122000027</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>10 &#181;L, 100 &#181;L, and 1000 &#181;L micropipettes</td>
			<td>Eppendorf</td>
			<td>3120000909</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>100 &#181;L multi-channel pipette</td>
			<td>Eppendorf</td>
			<td>3122000043</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>1000 &#181;L filtered micropipette tips</td>
			<td>USA Scientific</td>
			<td>1122-1830</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>2 mL microcentrifuge tubes</td>
			<td>USA Scientific</td>
			<td>1620-2700</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>2 mm soil sieve</td>
			<td>Forestry Suppliers</td>
			<td>60141009</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>200 &#181;L filtered micropipette tips</td>
			<td>USA Scientific</td>
			<td>1120-8810</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>25 mL reservoirs</td>
			<td>VWR International LLC</td>
			<td>89094-664</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>50 mL conical vials</td>
			<td>Thermo Fisher Scientific</td>
			<td>352098</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>500 mL vacuum filters (0.2 &#181;m pore size)</td>
			<td>VWR International LLC</td>
			<td>156-4020</td>
			<td></td>
		</tr>
		<tr>
			<td>96-well microplates</td>
			<td>USA Scientific</td>
			<td>655900</td>
			<td></td>
		</tr>
		<tr>
			<td>96-well PCR plates</td>
			<td>BioRad</td>
			<td>HSP9631</td>
			<td></td>
		</tr>
		<tr>
			<td>Agencourt AMPure XP beads</td>
			<td>Thermo Fisher Scientific</td>
			<td>NC9933872</td>
			<td>Instructions for use: 
			https://www.beckmancoulter.com/wsrportal/ajax/downloadDocument/B37419AA.pdf?autonomyId=TP_DOC_150180&#38;documentName=B37419AA.pdf</td>
		</tr>
		<tr>
			<td>Aluminum foil</td>
			<td>Boardwalk</td>
			<td>7124</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Analytical scale with 0.001 g resolution</td>
			<td>Ohaus Pioneer</td>
			<td>PA323</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Bioruptor Plus ultrasonicator</td>
			<td>Diagenode</td>
			<td>B01020001</td>
			<td></td>
		</tr>
		<tr>
			<td>Bovine Serum Albumin (BSA) 20 mg/mL</td>
			<td>New England Biolabs</td>
			<td>B9000S</td>
			<td></td>
		</tr>
		<tr>
			<td>Centrifuge</td>
			<td>Eppendorf</td>
			<td>5811000908</td>
			<td>Including 50mL and 96-well plate bucket adapters</td>
		</tr>
		<tr>
			<td>Cryogenic gloves</td>
			<td>Millipore Sigma</td>
			<td>Z183490</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>DNeasy PowerClean kit (optional)</td>
			<td>Qiagen Inc.</td>
			<td>12877-50</td>
			<td>Previously MoBio</td>
		</tr>
		<tr>
			<td>DNeasy PowerSoil kit</td>
			<td>Qiagen Inc.</td>
			<td>12888-100</td>
			<td>Previously MoBio</td>
		</tr>
		<tr>
			<td>Dry ice</td>
			<td>Any</td>
			<td>NA</td>
			<td></td>
		</tr>
		<tr>
			<td>DynaMag-2 magnet</td>
			<td>Thermo Fisher Scientific</td>
			<td>12321D</td>
			<td>Do not substitute</td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>VWR International LLC</td>
			<td>89125-188</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Gallon size freezer bags</td>
			<td>Ziploc</td>
			<td>NA</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Gemini EM Microplate Reader</td>
			<td>Molecular Devices</td>
			<td>EM</td>
			<td>Can use another fluorometer that reads 96-well plates from the top.</td>
		</tr>
		<tr>
			<td>K2HPO4</td>
			<td>Sigma-Aldrich</td>
			<td>P3786</td>
			<td></td>
		</tr>
		<tr>
			<td>KH2PO4</td>
			<td>Sigma-Aldrich</td>
			<td>P5655</td>
			<td></td>
		</tr>
		<tr>
			<td>Lab coat</td>
			<td>Workrite</td>
			<td>J1367</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Liquid N2</td>
			<td>Any</td>
			<td>NA</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Liquid N2 dewar</td>
			<td>Thermo Fisher Scientific</td>
			<td>4150-9000</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Milli-Q ultrapure water purification system</td>
			<td>Millipore Sigma</td>
			<td>SYNS0R0WW</td>
			<td></td>
		</tr>
		<tr>
			<td>Mini-centrifuge</td>
			<td>Eppendorf</td>
			<td>5404000014</td>
			<td></td>
		</tr>
		<tr>
			<td>Molecular grade water</td>
			<td>Thermo Fisher Scientific</td>
			<td>4387937</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Mortars</td>
			<td>VWR International LLC</td>
			<td>89038-150</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Nitrile gloves</td>
			<td>Thermo Fisher Scientific</td>
			<td>19167032B</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Paper towels</td>
			<td>VWR International LLC</td>
			<td>BWK6212</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>PCR plate sealing film</td>
			<td>Thermo Fisher Scientific</td>
			<td>NC9684493</td>
			<td></td>
		</tr>
		<tr>
			<td>PCR strip tubes</td>
			<td>USA Scientific</td>
			<td>1402-2700</td>
			<td></td>
		</tr>
		<tr>
			<td>Pestles</td>
			<td>VWR International LLC</td>
			<td>89038-166</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Plastic spatulas</td>
			<td>LevGo, Inc.</td>
			<td>17211</td>
			<td></td>
		</tr>
		<tr>
			<td>Platinum Hot Start PCR Master Mix (2x)</td>
			<td>Thermo Fisher Scientific</td>
			<td>13000014</td>
			<td></td>
		</tr>
		<tr>
			<td>PNAs - chloroplast and mitochondrial</td>
			<td>PNA Bio</td>
			<td>NA</td>
			<td>Make sure to verify sequence bioinformatically</td>
		</tr>
		<tr>
			<td>Protective eyewear</td>
			<td>Millipore Sigma</td>
			<td>Z759015</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Qubit 3.0 Fluorometer</td>
			<td>Thermo Fisher Scientific</td>
			<td>Q33216</td>
			<td></td>
		</tr>
		<tr>
			<td>Qubit dsDNA HS assay kit</td>
			<td>Thermo Fisher Scientific</td>
			<td>Q32854</td>
			<td></td>
		</tr>
		<tr>
			<td>Rubber mallet (optional)</td>
			<td>Ace Hardware</td>
			<td>2258622</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Shears or scissors</td>
			<td>VWR International LLC</td>
			<td>89259-936</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Shovel</td>
			<td>Home Depot</td>
			<td>2597400</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Soil core collector (small diameter: &#60;1 inch)</td>
			<td>Ben Meadows</td>
			<td>221700</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Spray bottles</td>
			<td>Santa Cruz Biotechnology</td>
			<td>sc-395278</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Standard desalted barcoded primers (10 &#181;M) (Table 1)</td>
			<td>IDT</td>
			<td>NA</td>
			<td>4 nmole Ultramer DNA Oligo with standard desalting. NGS adapter and sequencing primer (Table 1) are designed for use with Illumina MiSeq using v3 chemistry.</td>
		</tr>
		<tr>
			<td>Thermocycler</td>
			<td>Thermo Fisher Scientific</td>
			<td>E950040015</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Triton X-100</td>
			<td>Sigma-Aldrich</td>
			<td>X100</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
		<tr>
			<td>Weigh boats</td>
			<td>Spectrum Chemicals</td>
			<td>B6001W</td>
			<td>Can substitute with equivalent from other suppliers.</td>
		</tr>
	</tbody>
</table>

exploring the root microbiome: extracting bacterial community data from the soil, rhizosphere, and root endosphere

The intimate interaction between plant host and associated microorganisms is crucial in determining plant fitness, and can foster improved tolerance to abiotic stresses and diseases. As the plant microbiome can be highly complex, low-cost, high-throughput methods such as amplicon-based sequencing of the 16S rRNA gene are often preferred for characterizing its microbial composition and diversity. However, the selection of appropriate methodology when conducting such experiments is critical for reducing biases that can make analysis and comparisons between samples and studies difficult. This protocol describes in detail a standardized methodology for the collection and extraction of DNA from soil, rhizosphere, and root samples. Additionally, we highlight a well-established 16S rRNA amplicon sequencing pipeline that allows for the exploration of the composition of bacterial communities in these samples, and can easily be adapted for other marker genes. This pipeline has been validated for a variety of plant species, including sorghum, maize, wheat, strawberry, and agave, and can help overcome issues associated with the contamination from plant organelles.

Performing the recommended protocol should result in a dataset of indexed paired-end reads that can be matched back to each sample and assigned to either a bacterial operational taxonomic units (OTU) or exact sequence variant (ESV, also referred to as amplicon sequence variant (ASV) and sub-operational taxonomic unit (sOTU)), depending on downstream analysis. In order to obtain high-quality sequence data, care must be taken at each step to maintain consistency between samples and minimize...

Watch this Scientific Journal Video about Exploring the Root Microbiome: Extracting Bacterial Community Data from the Soil, Rhizosphere, and Root Endosphere at JoVE.com

Exploring the Root Microbiome: Extracting Bacterial Community Data from the Soil, Rhizosphere, and Root Endosphere

Here, we describe a protocol to obtain amplicon sequence data of soil, rhizosphere, and root endosphere microbiomes. This information can be used to investigate the composition and diversity of plant-associated microbial communities, and is suitable for the use with a wide range of plant species.

The intimate interaction between plant host and associated microorganisms is crucial in determining plant fitness, and can foster improved tolerance to ...

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