Thanks to all of our collaborators who have assisted with sweet corn field collections, including Dominic Reisig and Dan Mott at North Carolina State University, and Pat Porter at Texas A&#38; M University in Lubbock, TX. Thanks to Fiona Clissold for helping to optimize the protocols and for providing edits to this manuscript. This work was supported in part by the Texas A&#38; M C. Everette Salyer Fellowship (Department of Entomology) and the Biotechnology Risk Assessment Grant Program competitive grant no. 2015-33522-24099 from the U.S. Department of Agriculture (awarded to GAS and STB).

1. Plant Collection and Processing
<ol>
	<li>Collect and process plant samples
	<ol>
		<li>After collecting plant samples, flash-freeze samples by dipping plant material into liquid nitrogen with forceps and store at -80 &#176;C. If the plant samples collected are too large to flash-freeze, quickly cool the samples using dry ice and transfer to a -80 &#176;C freezer as soon as possible. The macronutrient content of plant material can change after tissues are separated from the plant, so it is important to freeze plant ...

<ol>
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S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+mineral+nutrition+of+wild+plants.">The mineral nutrition of wild plants.</a> Annual Review of Ecology, Evolution, and Systematics. 11 (1), 233-260 (1980).</li><li>Marschner, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Marschner's Mineral Nutrition of Higher Plants. , (1956).</li><li>Stieger, P. A., Feller, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Senescence+and+protein+remobilization+in+leaves+of+maturing+wheat+plants+grown+on+waterlogged+soil.">Senescence and protein remobilization in leaves of maturing wheat plants grown on waterlogged soil.</a> Plant and Soil. 166, 173-179 (1994).</li><li>Li, R., Volenec, J. J., Joern, B. 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A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spatio-temporal,+genotypic,+and+environmental+effects+of+plant+soluble+protein+and+digestible+carbohydrate+content:+implications+for+insect+herbivores+with+cotton+as+an+exemplar.">Spatio-temporal, genotypic, and environmental effects of plant soluble protein and digestible carbohydrate content: implications for insect herbivores with cotton as an exemplar.</a> Journal of Chemical Ecology. 42 (11), 1151-1163 (2016).</li><li>Boisen, S., Bech-Andersen, S., Eggum, B. O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+critical+view+of+the+conversion+factor+6.25+from+total+nitrogen+to+protein.">A critical view of the conversion factor 6.25 from total nitrogen to protein.</a> Acta Agriculturae Scandinavica. 37, 299-304 (1987).</li><li>Ezeagu, I. E., Petzke, J. K., Metges, C. C., Akinsoyinu, A. O., Ologhobo, A. 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A reappraisal of its definition and determination. Variation according to species and seed protein content.</a> Journal of Agricultural and Food Chemistry. 38, 18-24 (1990).</li><li>Bradford, M. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+rapid+and+sensitive+method+for+the+quantitation+of+microgram+quantities+of+protein+utilizing+the+principle+of+protein-dye+binding.">A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding.</a> Analytical Biochemistry. 72 (1-2), 248-254 (1976).</li><li>Jones, C. G., Hare, J. D., Compton, S. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measuring+plant+protein+with+the+Bradford+assay.">Measuring plant protein with the Bradford assay.</a> Journal of Chemical Ecology. 15 (3), 979-992 (1989).</li><li>Dubois, M., Gilles, K. A., Hamilton, J. K., Rebers, P. A., Smith, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Colormetric+method+for+determination+of+sugars+and+related+substances.">Colormetric method for determination of sugars and related substances.</a> Analytical Biochemistry. 28, 350-358 (1956).</li><li>Clissold, F. J., Sanson, G. D., Read, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+paradoxical+effects+of+nutrient+ratios+and+supply+rates+on+an+outbreaking+insect+herbivore,+the+Australian+plague+locust.">The paradoxical effects of nutrient ratios and supply rates on an outbreaking insect herbivore, the Australian plague locust.</a> Journal of Animal Ecology. 75, 1000-1013 (2006).</li><li>Smith, D., Paulsen, G. M., Raguse, C. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Extraction+of+total+available+carbohydrates+from+grass+and+legume+tissue.">Extraction of total available carbohydrates from grass and legume tissue.</a> Plant Physiology. 39 (6), 960-962 (1964).</li><li>Cui, S. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Food carbohydrates: Chemistry, physical properties, and applications. , (2005).</li><li>Chow, P. S., Landh&#228;usser, S. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+method+for+routine+measurements+of+total+sugar+and+starch+content+in+woody+plant+tissues.">A method for routine measurements of total sugar and starch content in woody plant tissues.</a> Tree Physiology. 24 (10), 1129-1136 (2004).</li><li>Masuko, T., Minami, A., Iwasaki, N., Majima, T., Nishimura, S. I., Lee, Y. 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By combining well-established colorimetric assays with effective plant-specific extraction protocols, the assays demonstrated here provide a reasonable and accurate method for measuring plant soluble protein and digestible carbohydrate content. Our results using corn as an exemplar illustrates how these protocols can be used to obtain precise measurements across different biologically-relevant spatial scales. For example, we were able to detect differences in plant soluble protein and digestible carbohydrate content betw...

Plant biomass forms the foundation of virtually all terrestrial food-webs. Plants acquire nutritional elements from the soil through their roots systems and utilize sunlight in their foliar tissues to synthesize biomolecules. In particular, carbon and nitrogen are used to create carbohydrates, proteins (comprised of amino acids), and lipids that are needed to build plant biomass (it should be noted that in plant physiology the term &#34;macronutrient&#34; often refers to soil elements, such as N, P, K, and S, however, throughout this paper this term will refer to biomolecules, such as proteins, carbohydrates, and lipids). When herbivores consume plant material, the macronutrients contained in plant tissues are broken down into their constituent parts and then used to drive the physiological processes of the consumer. In this way, plant macronutrients have a strong influence on consumer physiology along with important implications for higher order ecological interactions and food-web dynamics.
Across the animal kingdom, soluble protein and digestible carbohydrates are the macronutrients most closely tied to survival, reproduction, and performance1. Moreover, the majority of animals actively regulate their intake of these two macronutrients to meet their physiological demands1,2. This is particularly true for insect herbivores that detect the concentrations of sugars and amino acids in plant tissues, which in turn directs feeding behavior. As a result, plant soluble protein and digestible carbohydrate content has played a strong role in the evolution of plant-insect interactions.
While data on plant soluble protein and digestible carbohydrate content are relatively rare (but see6,7,8,9,10,11), there is a preponderance of data available on plant elemental content (carbon, nitrogen, and phosphorus). Largely this is because elements play a primary role in plant mineral nutrition3,4,5. Where elements are measured, correlations have been used to extrapolate the amount of soluble protein and digestible carbohydrate, but accurate calculations are often difficult to obtain. For instance, it is impossible to use carbon as an indicator of plant digestible carbohydrate content because carbon is ubiquitously present in all organic compounds. A stronger relationship exists between elemental nitrogen and plant soluble protein content, and generalized nitrogen-to-protein conversion factors are often utilized. However, there is strong evidence that nitrogen-to-protein conversions are highly species-specific12,13,14,15, making the use of generalized conversion likely inaccurate. Because of this, nitrogen-to-protein conversion factors often lack precision, particularly to the extent that is required for nutritional studies on herbivores. Also, the presence of N-containing plant allelochemicals,&#160;such as alkaloids and glucosinolates that are often toxic to herbivores, can confound these conversions.
Here, we offer two chemical assays for measuring the concentration of soluble proteins and digestible carbohydrates in plant tissues. These assays are presented separately, but it is suggested that they be used concurrently to analyze the same plant samples in order to achieve a more comprehensive analysis of plant macronutrients. Both employ similar methodologies, consisting of an extraction step, followed by quantification via absorbance. Plant sample prep is also identical for both protocols, making it easy to run both analyses in tandem. The utility of these assays do not stem from their novelty, as they rely on older, (Bradford, Jones, Dubois) well-established colorimetric assays16,17,18, but here we have organized a clear and easy-to-follow protocol that combines these methods with more obscure plant-specific extraction techniques17,19 in order to make the application of these assays more accessible to those in plant-relevant fields.
For both assays, plant macronutrients are first extracted physically by freezing, lyophilizing, and grinding the plant material. For the soluble protein assay, further chemical extraction is done17,19 through several rounds of vortexing and heating samples in NaOH solution. The well-known Bradford assay, utilizing Coomassie brilliant blue G-250, is then used to quantify soluble proteins and polypeptides between 3,000-5,000 Daltons16,17. This assay has a detection range between 1-20 &#181;g total proteins per microplate well or &#60;25 &#181;g/mL, but does not measure free amino acids. The extraction step of the digestible carbohydrate assay is based on the dilute acid method of Smith et al.20 and allows for the isolation of soluble sugars, starch, and fructosan &#8211; but not structural carbohydrates. A phenol-sulfuric acid quantification method is taken from Dubois et al.18 and measures all mono-, oligo-, and polysaccharides (as well as methyl derivatives). This assay is able to quantify specific sugars, but here we use it as an indicator of total digestible carbohydrate content (see Smith et al.20 for more detailed analysis). Together, these assays measure the two macronutrients that are strongly tied to plant eco-physiology and herbivore performance, providing important data on resource quality at the base of terrestrial food-webs. Presenting these protocols promotes the generation of plant macronutrient datasets in order to obtain a more thorough understanding of plant physiology, herbivore nutritional ecology, and plant-herbivore interactions.

<table border="0" cellpadding="0" cellspacing="0" width="670">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:273px;">microplate reader (spectrophotometer)</td>
			<td style="width:111px;">Bio-Rad</td>
			<td style="width:119px;">Model 680 XR</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:273px;">Bio-Rad Protein Assay Dye Reagent concentrate</td>
			<td style="width:111px;">Bio-Rad</td>
			<td style="width:119px;">#5000006</td>
			<td>450mL</td>
		</tr>
	</tbody>
</table>

quantifying plant soluble protein and digestible carbohydrate content, using corn (zea mays) as an exemplar

Elemental data are commonly used to infer plant quality as a resource to herbivores. However, the ubiquity of carbon in biomolecules, the presence of nitrogen-containing plant defensive compounds, and variation in species-specific correlations between nitrogen and plant protein content all limit the accuracy of these inferences. Additionally, research focused on plant and/or herbivore physiology require a level of accuracy that is not achieved using generalized correlations. The methods presented here offer researchers a clear and rapid protocol for directly measuring plant soluble proteins and digestible carbohydrates, the two plant macronutrients most closely tied to animal physiological performance. The protocols combine well characterized colorimetric assays with optimized plant-specific digestion steps to provide precise and reproducible results. Our analyses of different sweet corn tissues show that these assays have the sensitivity to detect variation in plant soluble protein and digestible carbohydrate content across multiple spatial scales. These include between-plant differences across growing regions and plant species or varieties, as well as within-plant differences in tissue type and even positional differences within the same tissue. Combining soluble protein and digestible carbohydrate content with elemental data also has the potential to provide new opportunities in plant biology to connect plant mineral nutrition with plant physiological processes. These analyses also help generate the soluble protein and digestible carbohydrate data needed to study nutritional ecology, plant-herbivore interactions and food-web dynamics, which will in turn enhance physiology and ecological research.

To show the usefulness of these methods, we analyzed the soluble protein and digestible carbohydrate content of four different field and sweetcorn tissues that serve as distinct potential nutritional resources for insect herbivores. We collected ears of corn from three agricultural regions in the United States (Minnesota, North Carolina, and Texas), encompassing five different varieties of sweet corn (i.e., genotypes) and one variety of field corn as an outgroup. Table 3<...

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Quantifying Plant Soluble Protein and Digestible Carbohydrate Content, Using Corn Zea mays As an Exemplar

The protocols described herein provide a clear and approachable methodology for measuring soluble protein and digestible (non-structural) carbohydrate content in plant tissues. The ability to quantify these two plant macronutrients has significant implications for advancing the fields of plant physiology, nutritional ecology, plant-herbivore interactions and food-web ecology.

Quantifying Plant Soluble Protein and Digestible Carbohydrate Content, Using Corn (Zea mays) As an Exemplar

Elemental data are commonly used to infer plant quality as a resource to herbivores. However, the ubiquity of carbon in biomolecules, the presence of ...

This method allows researchers in the field of plant sciences and nutritional ecology to accurately measure concentrations of soluble proteins and digestible carbohydrates in plant tissue. The main advantage of these techniques is that they offer a quick and easy method for quantifying these two incredibly important macronutrients with high accuracy. These techniques have strong implications for the field of ecology because plant proteins and carbohydrates make up the base of terrestrial food webs.Generally, individuals new to this technique may struggle because there are several steps and some techniques which are not commonly performed in a general-use lab. To begin, weigh out replicate samples of each tissue into labeled 1.5 milliliter microcentrifuge tubes. Next, record the precise mass of each sample.Using a micropipetter, add 500 micro-liters of 0.1 molar sodium hydroxide to each tube. Close the lids tightly and sonic heat the tubes for 30 minutes. Next, place the tubes in a preheated hot-water bath for 15 minutes.After this, centrifuge the tubes at 15, 000 G for 10 minutes. Pipette the supernatant into new, labeled microcentrifuge tubes, using a new pipette tip for each sample. Add 300 microliters of 0.1 molar sodium hydroxide to the pellet, and repeat the centrifugation for 10 minutes.After this, remove the supernatant and transfer it to the tubes containing the supernatant from the first centrifugation. To neutralize the pH of the supernatant, add 11 microliters of 5.8 molar hydrochloric acid. Use litmus paper to confirm the pH is approximately seven.Next, add 90 microliters of 100%trichloroacetic acid to each tube. Then incubate the tubes on ice for 30 minutes. Centrifuge the samples at 13, 000 G for 10 minutes at four degrees Celsius.Carefully use a vacuum line and glass micropipette tip to remove the trichloroacetic acid. Be very careful not to disturb the pellet with the suction tip. Next, wash the pellet with 100 microliters of acetone cooled to 20 degrees Celsius, then allow the acetone to evaporate in a fume hood.Dissolve the protein pellet with one milliliter of sodium hydroxide. Additional rounds of heating, vortexing and sonic heating may be required. Add 160 microliters of each IGG standard solution to a 96-well plate in triplicate, starting with the A1 position Next, in a new 1.5 milliliter tube add 50 microliters of each sample solution to 950 microliters of distilled water.Then add 60 microliters of each diluted sample to the well plate in triplicate, starting with the H1 position. Add 100 microliters of distilled water to all of the blank and unknown sample wells. Using a multi-channel pipette, add 40 microliters of Coomassie Brilliant Blue G-250 protein dye to each well of the plate.Using a needle, pop any bubbles present in the wells. After this, allow the plate to incubate at room temperature for five minutes. Then use a microplate spectrophotometer to record the absorbence values for each well at 595 nanometers.First, weigh out replicates from each tissue sample into glass 15 milliliter tubes. Label the tubes and record the exact mass of each sample. Next, add one milliliter of 0.1 molar sulfuric acid to each tube and close the caps tightly.Then place the tubes in a boiling-water bath for one hour. Transfer the tubes to a tepid water bath to cool. Then pour the contents of the tubes into labeled 1.5 milliliter microcentrifuge tubes.After this centrifuge tubes at 15, 000 G for 10 minutes. Use a micro pipette to transfer the supernatant to new labeled 1.5 milliliter tubes. Using a pipette transfer 15 microliters of each unknown sample to its own test tube.Then add 385 microliters of distilled water to each tube. In a fume hood add 400 microliters of 5%phenol to each standard and unknown sample test tube. Immediately thereafter add two milliliters of sulfuric acid to each tube.Make sure to add the sulfuric acid to the surface of the solution. After incubating the tubes for 10 minutes, vortex the tubes. Then incubate the tube for an additional 30 minutes.Transfer 800 microliter from each sample tube to three polystyrene 1.5 milliliter semi-microcuvettes. Then set the spectrophotometer to read at 490 nanometers and calibrated with a blank cuvette. Finally, run each cuvette through the spectrophotometer and record the absorbence.Using this protocol, the soluble protein and digestible carbohydrate content of four different field and sweet corn tissues from three geographical regions were analyzed. Significant differences in soluble protein content were observed between the regions, thus necessitating separate analyses for each region. Across regions there were several differences in soluble protein and digestible carbohydrate content between tissues.In Minnesota, tissues only varied in soluble protein content while North Carolina tissues varied in both soluble protein and digestible carbohydrates. Variability in tissue macronutrient content in Texas depended on the variety. This technique will allow researchers in the field of plant sciences and nutritional ecology to precisely measure soluble plant protein and digestible carbohydrates, rather than extrapolating from elemental measures.Don't forget that working with liquid nitrogen, concentrated acids and bases, can be extremely hazardous. So always use precautions such as gloves, safety goggles, aprons and closed-toed shoes whenever working with these chemicals.

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19.4K visualizações.  Texas A&M University. This method allows researchers in the field of plant sciences and nutritional ecology to accurately measure concentrations of soluble proteins and digestible carbohydrates in plant tissue. The main advantage of these techniques is that they offer a quick and easy method for quantifying these two incredibly important macronutrients with high accuracy. These techniques have strong implications for the field of ecology because plant proteins and carbohydrates make up the base of terrestrial food...