Comparative Analysis of Six DNA Extraction Methods in Cowpea ( Vigna unguiculata L . Walp )

High quality DNA extractions are a prerequisite for genetic studies of a variety of plants including cowpea (Vigna unguiculata). Nowadays, there are a great number of plant DNA extraction methods, and commercially available extraction kits are also becoming more and more popular. It appears that different procedures work best for different plant groups. Thus in the genetic studies of cowpea, which DNA extraction method to choose becomes a concern. To solve this problem, five classic plant DNA isolation methods, including three CTAB methods and two SDS methods, were compared and evaluated while isolation using a commercial kit was also undertaken. The DNA extracted by these six methods from two-week-old cowpea seedlings were analyzed according to their cost and time, yield, purity, integrity, and functionality in restriction endonuclease digestion and PCR (polymerase chain reaction) based downstream analysis. After the evaluation, one most suitable method, described by Dellaporta et al. (1983) was selected and chosen for isolating DNA from young leaves of cowpea seedlings. The cost and time required in this method was relatively low. In addition, the quantity and the quality of the DNA extracted by this method were high enough to perform hundreds of PCR-based reactions.


Introduction
Cowpea, Vigna unguiculata (L.) Walp, Leguminosae (2n = 2x = 22), is an essential food crop in developing countries of the tropics and subtropics, especially in sub-Saharan Africa, Asia, and Central and South America (Singh et al., 1997).The planting area is more than 10.9 million ha, with a production of more than 5.5 million metric tons world-wide in 2010 (FAO, 2010).Cowpea has a wide range of uses and is grown primarily for human consumption as a dry grain legume, fresh shelled 'peas', fresh pods ('snap beans'), and fresh and dried leaves (Diouf et al., 2005).Due to its good protein quality and high nutritional value, cowpea is often referred to as the "poor man's meat" (Jackai et al., 1986).Additionally, cowpea is a drought-tolerant crop that thrives in dry environments, thus makes it the crop of choice in the semi-arid/arid zones of West and Central Africa (Agbicodo et al., 2009).Also, the crop fixes 80% of its nitrogen requirement for growth from the atmosphere (Asiwe et al., 2009), thereby reducing nitrogen fertilizer demand and costs of crop production.In a word, cowpea is a versatile crop that plays an important role in the development of agriculture.
For genetic studies of cowpea, such as gene mapping, genetic fingerprinting, population studies, and phylogenetic analyses, extraction of DNA presents the first essential step for all subsequent genetic analysis, which are frequently PCR based methods.In the past thirty years，plant DNA isolation methods have been described by numerous authors, each contributing a different method to overcome the problems that arise when extracting DNA.The cetyl trimethylammonium bromide (CTAB) method was first presented by Murry and Thompson (1980), who employed cesium chloride density gradients to eliminate enzyme inhibiting polysaccharides.But this method needs expensive equipments like ultracentrifuge and includes some hazardous reagents like ethidium bromide.What's more, the process is complex，time-consuming, and not well suited for assaying large number of samples.Then Saghai-Maroof et al. (1984) reported a modification of the method of Murry and Thompson without the use of cesium chloride density gradients, and simplified the procedure.The protocol was based on lyses and purification with CTAB that selectively precipitates DNA while maintaining the solubility of many polysaccharides (Ribeiro et al., 2007).In 1985, Rogers et al. (1985) developed a technique based on the CTAB nucleic acid extraction procedures of Murray and Thompson (1980) and Taylor and Powell (1982), which made it possible to extract purified high molecular weight plant DNA without the use of expensive equipment and/or time-consuming procedures.Afterwards, Doyle et al. (1987) made a simple modification of the DNA isolation procedure described by Saghai-Maroof et al. in 1987.They did some further cleaning to the DNA yield by the protocol of Saghai-Maroof et al., and obtained high quality and pure DNA.Since then, the Doyle method has become a popular plant DNA isolation method.
The first SDS method applied in plant DNA isolation was reported by Dellaporta et al. (1983), which was adapted from a procedure commonly used for yeast DNA preparation (Davis et al., 1980).This protocol used sodium dodecyl sulfate (SDS) as detergent and the addition of potassium acetate resulted in the removal of some polysaccharides and proteins as a complication with the potassium-SDS precipitate (Ribeiro et al., 2007).Later in 1995, Jobes et al. (1995) made an improvement of the standard proteinase K-SDS method.The procedure alleviated the time and expense of CTAB and caesium chloride methods by utilizing polyvinylpyrrolidone (PVP) to bind the polyphenolic compounds.And in 1997，Aljanabi et al. reported a universal and rapid salt-extraction of high quality genomic DNA from different organisms.The method was also based on SDS, and did not require expensive and environmentally hazardous reagents and equipment.
It appears that different procedures work best for different plant groups.The reason might be the great diversity of plant secondary compounds that may interfere with a particular method of DNA isolation (Doyle et al., 1987).Another reason to compare is for the following step to use the extracted DNA.The residues of chemicals used in an extraction might affect what you will do next, e.g., PCR reaction.Therefore, it is necessary to compare and optimize some widely used DNA isolation methods in a specific species.In this study, five classic DNA extraction methods reported by Dellaporta et al., Saghai-Maroof et al., Rogers et al., Doyle et al. and Aljanabi et al., respectively, were compared while isolation using a commercial kit was also undertaken.All the DNA extractions were evaluated according to their cost and time, yield, purity, integrity and functionality, in order to find an ideal method which is suitable for isolating high yield and high quality DNA from cowpea.

Material Preparation
The cowpea cultivar (Cheng-jiang 7), which belonged to Vigna unguiculata ssp.sesquipedalis, was provided by the Chengdu Academy of Agriculture and Forestry Sciences, China.Seeds were grown under greenhouse conditions and leaves were harvested from two-week-old seedlings for DNA isolation.100 milligram of fresh leaf tissue was weighed in an electronic balance and ground into a fine powder in liquid nitrogen by a pestle and mortar, then transferred into a 1.5 ml microfuge tube and temporarily stored at -20°C.
In addition, total DNA was also isolated using E.Z.N.A. ® Plant DNA Kit (Omega Bio-Tek, Inc., Norcross, GA, USA) according to the manufacturer's protocol.At the end of each method, DNA was air dried for 30 min (except for the commercial kit) and diluted in 100 µl of TE buffer (10 mM Tris-cl, pH 7.4, 1 mM EDTA, pH 8.0).Each method was replicated ten times.

Cost and Time Estimation
The method to evaluate the cost and time in the DNA extraction was followed by Chen et al. (2010).The cost for each method was estimated based on the price of chemicals, enzymes, and disposable items (including microfuge tubes and pipette tips) consumed in one extraction from 100 mg leaf tissue.The minimum time required to finish one extraction from 100 mg leaf tissue using each method was estimated based on the procedures used in this study, including the time for incubation, centrifugation and 30 min for DNA drying if necessary.The time spent grounding samples in liquid nitrogen and for solution preparation in all the methods was excluded.

DNA Analysis
DNA concentration was measured photometrically at 260 nm wavelength (OD 260 ) with an Eppendorf Bio Photometer (BioPhotometer Plus, Eppendof Co. Ltd., Germany).Total DNA yield was calculated as the DNA amount in µg in the complete extraction volume.
Purity was determined by measuring additionally at 280nm wavelength (OD 280 ) and computing the ratio between the two values (OD 260/280 ).
To evaluate the DNA integrity and functionality, the extracted DNA were digested with Hind Ⅲ, which were performed using approximately 1 µg of DNA and 15 units of enzyme in a total volume of 20 µl and incubated at 37°C for 2 hours.Afterwards 5 µl undigested DNA and 5 µl digested DNA were subjected to electrophoresis on a 0.8% agarose gel at 90 V for 100 min, then stained in ethidium bromide for 20 min, and photographed in GeneGenius Imaging System (Syngene, A Division of Synoptics Ltd., UK).
The amplification was performed in a Peltier Thermal Cycler (BIO-RAD DNAEngine).Cycling conditions consisted of a 5 min initial denaturation at 95°C followed by 1 min denaturing at 95°C, 1 min annealing at 55°C and 1 min extension at 72°C repeated for 40 cycles and with 5 min extension at 72°C.Subsequently, 6 µl PCR products and 1 µl 6×loading buffer were subjected to electrophoresis on a 1.5% agarose gel at 75 V for 75 min, stained in ethidium bromide for 20 min, and photograped in GeneGenius Imaging System (Syngene, A Division of Synoptics Ltd., UK).

Data Analyses
The general linear model (GLM) was applied to test the effect of extraction method on the DNA yield and on the OD 260/280 ratio.Tukey's pairwise comparisons with the confidence interval of 95% were used to compare the yield or the ratios between the methods.The statistical analyses were accomplished by using the MINITAB ® software Release 16 (http://www.minitab.com).

Choice of the Material
Proper choice of the leaf tissue is very important for DNA extraction (Lodhi et al., 1994).In this research, leaf tissue harvested from two-week-old cowpea seedlings was used for DNA extraction because fresh, young leaf tissue was preferable since it may contain less polyphenolic and terpenoid compounds than older tissue (Rosenthal et al., 1979).Generally, mature plant tissues are not preferred for DNA extraction due mainly to the presence of high concentrations of polysaccharides, polyphenols, and other secondary metabolites (Dabo et al., 1993;Zhang et al., 2000).

Cost and Time Consumed
In order to perform the PCR technique routinely, low cost and time per assay is very important.Studies based on DNA markers require large amounts of quality genomic DNA, emphasizing the need for inexpensive, rapid and simple DNA extraction methods (Weishing et al., 1995).
The estimated cost in US dollar (USD) and time in hours for each method to extract DNA from 100mg leaf tissue of cowpea are presented in Table 1.The expenses of the laboratory-prepared SDS and CTAB buffers in the five classic methods were much lower (approximately, 0.0003-0.003USD per sample).As a result, these methods were less costly than the commercial kit.The E.Z.N.A. ® Plant DNA Kit was the most expensive among the six methods but the least extraction time was required.

DNA Yield
Yield of the DNA extracted by the six methods were listed in Table 1.The extraction method had a significant effect (F = 29.20,df = 5, P<0.01) on the DNA yield.The DNA yield by the SDS methods (Aljanabi et al., 1997;Dellaporta et al., 1983) were significantly higher than those obtained by the CTAB methods (Saghai-Maroof et al., 1984;Rogers et al., 1985;Doyle et al. 1987) and E.Z.N.A. ® plant DNA Kit (Tukey's, P<0.05).But according to Doosty et al. (1994), DNA extracted with the SDS method described by Dellaporta et al. had not good quantity and quality.This extraction method for Satureja khuzistanica did not show acceptable results because the SDS buffer attached to the secondary metabolite and prevented extraction DNA with high quality.Furthermore these results were in agreement with the findings of Ziegenhagen and Scholz (1993).In our research, a high yield of DNA was obtained from cowpea using the Dellaporta method, probably because the young cowpea seedlings contain less secondary metabolite.

Purity
The assessment of the purity of a nucleic acid sample is often performed by a procedure commonly referred to as In this study, the RNA residue in the DNA isolated by the four methods described by Saghai-Maroof et al., Aljanabi et al., Rogers et al. and Dellatporta et al., did not inhibit the digestion with restriction enzyme Hind Ⅲ and the amplification of the target gene fragment, suggesting that in most cases, the RNase treatment is not certainly needed.

Conclusion
In this study, five classic methods and a commercial kit for isolating DNA from cowpea (Vigna unguiculata) were compared and analyzed from the following perspectives: cost and time demands, yield of DNA, the purity of DNA acquired, intactness, and functionality.All the six methods compared in this study turned out to be suitable to extract DNA from cowpea.In summary, the conclusions in this research are as follows: 1).The cowpea DNA yield by the SDS methods (Aljanabi et al., 1997;Dellaporta et al., 1983) were significantly higher than those obtained by the CTAB methods (Saghai-Maroof et al., 1984;Rogers et al., 1985;Doyle et al., 1987) and E.Z.N.A. ® plant DNA Kit (Tukey's, P<0.05).
3).When extracting DNA for restriction enzymes digestion or PCR based downstream molecular manipulations, the RNase treatment is not needed.4).After evaluating the cost and time, yield, purity, integrity and functionality among the six methods, the SDS method described by Dellaporta et al., was considered an ideal protocol to isolate DNA from Vigna unguiculata.The cost and time required in this method was relatively low.Besides, the quantity and the quality of the DNA extracted by this method were high enough to perform hundreds of PCR-based reactions and also to be used in other DNA manipulation techniques such as restriction digestion, Southern blot and cloning.In addition, it had the added advantage of not requiring any phenol or chloroform extraction.

Table 1 .
DNA yield, OD 260/280 ratios, and estimated cost and time used for one cowpea extraction from 100 mg leaf tissue by six extraction methods (The results from each method were from 10 replications of each extraction method)