Since the advent of the CTAB-based extraction method from plant leaves by Doyle and Doyle in 1987, many different iterations have been published, each with modifications to contend with the co-extractives of polyphenolics and polysaccharides present in the leaves of many plant species [3 ,5 (link)-8 (link),15 (link)]. While having demonstrated their effectiveness for isolating DNA that is suitable for PCR amplification or restriction digests, all methods currently published in the literature require long incubations, and multiple precipitation steps and ethanol washes to produce RNA-free genomic DNA of high purity. As next-generation sequencing requires large amounts of high quality DNA, each additional precipitation and wash increases handling time and lowers overall yield. Commercial column based extraction kits, such as DNeasy (Qiagen, Australia) or Wizard (Promega, Australia), are effective for isolating contaminant free DNA from recalcitrant plant species, including eucalypts [4 (link),16 (link)]. However, commercial kits can be expensive and carry the risk of losing DNA on the column, which in turn necessitates several extractions followed by pooling of DNA.
To test the modifications made to the extraction method (NGS protocol) against the well-established original CTAB method (used routinely in our laboratory to reliably extract high quality DNA from rice, sugarcane, barley and wheat for sequencing [17 ,18 (link)]), six grams of frozen Corymbia citriodora subsp. variegata leaf tissue was ground and aliquoted evenly into the extractions described below. The quality of DNA from each extraction was verified spectrophotometrically using a NanoDrop instrument and agarose gel electrophoresis. The NanoDrop absorbance profile is useful for detecting contamination such as protein, salts or polysaccharides, all of which can inhibit NGS library preparation. High quality DNA is characterized as having a 260/280 nm absorbance ratio of approximately 1.8, with a single absorbance peak at 260 nm. The spectrophotometric profile is also useful for detecting phenolic oxidation, as the aromatic structure will absorb at 230 and 270 nm [1 ]. If oxidation is suspected to have occurred, endonuclease digestion can be used to further assess DNA quality before library preparation as phenolics, which inhibit polymerases, also inhibit restriction enzymes [8 (link),9 ].
Visualization of DNA on an agarose gel provides evidence of band shearing and RNA and polysaccharide contamination. Mechanical disruption, such as vortexing, causes DNA strands to shred apart, indicated by a wide DNA band with poor resolution. NGS library submission requires intact, high molecular weight genomic DNA, so all solution mixing steps were done by gentle inversion. Gel electrophoresis is also beneficial for visualizing RNA and polysaccharides, both of which contaminate sequencing reactions. RNA is evident as a distinct banding pattern at various sizes throughout the gel, whereas polysaccharides will migrate quickly and conglomerate at the bottom of the gel as a non-distinct fluorescent structure. Yield was determined through relative band intensity approximation with 100 and 200 ng λ DNA standards, as the NanoDrop concentration readings can inflate yield of genomic DNA.
To test the modifications made to the extraction method (NGS protocol) against the well-established original CTAB method (used routinely in our laboratory to reliably extract high quality DNA from rice, sugarcane, barley and wheat for sequencing [17 ,18 (link)]), six grams of frozen Corymbia citriodora subsp. variegata leaf tissue was ground and aliquoted evenly into the extractions described below. The quality of DNA from each extraction was verified spectrophotometrically using a NanoDrop instrument and agarose gel electrophoresis. The NanoDrop absorbance profile is useful for detecting contamination such as protein, salts or polysaccharides, all of which can inhibit NGS library preparation. High quality DNA is characterized as having a 260/280 nm absorbance ratio of approximately 1.8, with a single absorbance peak at 260 nm. The spectrophotometric profile is also useful for detecting phenolic oxidation, as the aromatic structure will absorb at 230 and 270 nm [1 ]. If oxidation is suspected to have occurred, endonuclease digestion can be used to further assess DNA quality before library preparation as phenolics, which inhibit polymerases, also inhibit restriction enzymes [8 (link),9 ].
Visualization of DNA on an agarose gel provides evidence of band shearing and RNA and polysaccharide contamination. Mechanical disruption, such as vortexing, causes DNA strands to shred apart, indicated by a wide DNA band with poor resolution. NGS library submission requires intact, high molecular weight genomic DNA, so all solution mixing steps were done by gentle inversion. Gel electrophoresis is also beneficial for visualizing RNA and polysaccharides, both of which contaminate sequencing reactions. RNA is evident as a distinct banding pattern at various sizes throughout the gel, whereas polysaccharides will migrate quickly and conglomerate at the bottom of the gel as a non-distinct fluorescent structure. Yield was determined through relative band intensity approximation with 100 and 200 ng λ DNA standards, as the NanoDrop concentration readings can inflate yield of genomic DNA.
Full text: Click here