TY - JOUR
AU1 - Burrell, M. M.
AB - Abstract Starch is one of the most important plant products to man. It is an essential component of food providing a large proportion of the daily calorific intake and is important in non‐food uses such as in adhesives. However, while much is known about the chemistry and pathways of synthesis for starch, there are major gaps in this knowledge so that it is not possible to modify the quantity or quality of starch produced by plants in a predictable way. While yield has improved markedly over the last century it is no longer improving faster than the growth in population and, at the same time, farmers’ incomes in Europe have been falling, especially in the UK. Thus, production, even in Europe, is not much greater than demand. In the western world an increasing amount of the harvested crop is processed and, therefore, the quality of the raw product becomes an increasingly important issue. There is, therefore, an increasing need to combine the modern mathematical modelling tools with modern biochemical tools and the modern science of genomics. Key words: Amylopectin, amylose, barley, farm income, paper, potato, quality, rice, starch, wheat, yield. Received 9 July 2002; Accepted 24 September 2002 Feeding the world The purpose of this short review is to highlight the need for further research in plant carbon metabolism. To do this the focus will be on starch. Starch is one of the most important plant products to man. Photosynthesis produces 2850 million tonnes of starch annually. The major sources of starch for man’s use are the cereals, but roots and tubers are also important. Annual starch production from cereals is approximately 2050 million tonnes and from roots and tubers 679 million tonnes (Tester and Karkalas, 2002). In the developed world starch provides at least 35% of man’s daily calorific intake, but this can be much higher. The main crops that are harvested for this are maize, rice, wheat, and potatoes. In many areas, especially Africa and the Far East, starch can provide 80% of man’s daily calorific intake and this may be only from one source such as rice. Starch has been used in both food and non‐food products for centuries. The ancient Egyptians and later the Romans used it as an adhesive, and it is still used as such today. The Greeks used it for medicinal purposes (Tester and Karkalas, 2001). Uses of starch The two major components of starch are amylose and amylopectin. Amylose consists of long linear chains of α‐1,4 linked glucose residues with relatively few α‐1,6 linked branches whereas amylopectin is a highly branched molecule of shorter α‐1,4 linked glucose molecules and more frequent α‐1,6 branches (Banks and Muir, 1980). These two molecules are assembled together to form a semi‐crystalline starch granule. The granule also contains small amounts of lipid and phosphate (Kainuma, 1988). The exact proportions of these molecules and the size of the granule vary between species. For example in wheat there are two classes of granules, one class is less than 10 µm in diameter and the other 10–20 µm, whereas in potato the granules can vary between 10 and 100 µm. The diversity of both composition and physical parameters gives rise to diverse processing properties and therefore many end‐uses for starch (Table 1). Maltodextrin and starch are important energy sources in baby foods because they have a low fermentability. It is estimated that 70% of the starch produced is converted into syrups for food use (Tester and Karkalas, 2002). Modified waxy maize starch is important in processed meat products where its gelling properties are useful as a binder to maintain the texture and stability of the processed product. Food is not the only use of starch, industrial uses are also very important. Starch is used as an additive in cement to improve the setting time and it is used to improve the viscosity of drilling muds in oil wells and so seal the walls of bore holes and prevent fluid loss. In paper‐making, starch is used for several purposes. As a filler it bonds the cellulose fibres together and improves the strength of the product. It is used as an adhesive in paper bags. It is also used as a coating to improve writing and erasing properties and to improve the finish quality. One particular use is in carbonless copy papers (Nachtergaele and Vannuffel, 1989; White, 1998). Here starch granules of a uniform size are mixed with encapsulated ink particles of a slightly smaller size. The starch keeps the sheets of paper apart, but when compressed by the tip of a pen the ink capsules break and ink is released onto the surface of the paper. For this the starch granules need to be rounded and be approximately 10 µm in diameter. A fraction of wheat starch best meets these criteria. Sources of starch The main sources of starch are the cereal crops, rice, maize, wheat, and the root crop potatoes although in various parts of the world many other crops are used. One can consider two aspects of the production process, the need for sufficient quantity and the need for the correct quality. The producer is the farmer, but quality is divided between the farmer and the processing chain, which may be represented as the food or industrial pipeline (Fig. 1). The producer, the farmer, has to produce a crop that will survive the processing chain and reach the market whether or not this involves extensive processing. From the field to the market losses occur due to pest, disease and handling. To reduce this, control measures have to be implemented which add cost to the final product. The extent of these losses is in part determined by the quality of the product harvested from the field. However, the competitive market tends to drive prices as low as possible. The driving force for the farmer to produce the crop is clearly his ability to generate a sustainable income. This has become increasingly difficult during the last 20 years. A comparison of income with labour costs shows that, over the last 10 years, income from wheat and barley has fallen by at least 40% while wage costs have risen by 46% (Table 2). The situation with potatoes is not quite as bad, but income has not risen while production costs have. Thus the only real way that the farmer can maintain income is by increasing yield. Plant breeders and agronomists have made substantial improvements in crop yields during the last century. On a global scale, crop production in the middle half of the last century went ahead of population growth, but as is indicated in Fig. 2, which is composed of data compiled by the FAO (Food and Agriculture Organization at: www.fao.org), over the last 20 years production has failed to keep up with demand. The scale of yield improvement during the twentieth century has dwarfed anything seen in previous centuries. For example, between 800 and 1900 rice yields improved from 1.01 to 1.85 tons ha–1 (Table 3) while between 1900 and 1960 yields increased to 3.95 tons ha–1 (Ishizuka, 1969). The combination of variety improvement and correct agronomy has been a major factor. The distribution of biomass in rice can be observed by comparing two varieties IR154 and IR8 (Table 4). In this case it is clear that while the panicle weight in IR8 is lower than IR154 the increase in tillering per plant leads to an increase in yield per hectare (Chandler, 1969). The other improvement is that IR8 is less affected by spacing than IR154. In IR154, as the spacing increases panicle number and yield decreases rapidly whereas this effect is less severe in IR8. In wheat, the picture is similar (Bingham, 1971; Siddique and Whan, 1993; Tahir and Ketata, 1993). There has been a gross redistribution of biomass in the production of higher yielding varieties (Fig. 3). The straw length has been halved which has resulted in two major gains. One is an increased yield per plant and the other is reduced losses due to the extreme weather. The short strawed wheat varieties are much less liable to lodging, which reduces the harvestable crop. Apart from the improved varieties and improved agronomy, i.e. improved production per plant, there has been an increased use of marginal land for production. However, there is not a great deal of extra land available in Europe. In the UK over 70% of the available landmass is already used for agriculture while in other European countries the figure is slightly smaller (FAO statistics). Much of this unused land is quite unsuitable for growing crops. A lot of the land already in use is marginal land for crops and requires almost continuous irrigation while an even larger area requires some irrigation to provide reasonable yields. To improve crop production in Europe to meet the forecast demand requires an increase in the water supply of 80%, but the FAO estimate that only a 12% increase is possible with the available water. Certainly an increased European food production to meet the forecast demand is not possible by increasing the area of land farmed or by increasing the amount of irrigation. Clearly there is a need to increase production in Europe as well as the rest of the world. European wheat production in 2001 was 129 110 886 metric tonnes. If one adds to this the surplus exports and intervention stock there is approximately a 10% surplus of production over supply, although the actual reserve supply is considerably less. An increase in temperature of 5 °C can lead to a 10–15% decrease in yield in some varieties. Obviously such comparisons cannot be exact because of the many interacting factors, but clearly an increase in temperature in Europe could easily lead to a shortage of wheat. It is feasible to engineer varieties to cope with these stresses. It is believed that one of the limitations to endosperm filling in wheat at temperatures above 20 °C is the temperature sensitivity of starch synthase (Jenner et al., 1995). ATC Ltd. and Biogemma produced transgenic wheat containing the E. coliglgA gene encoding glycogen synthase. The resulting progeny was analysed by Sit (personal communication) and Hedley, 2000. Measurements of total starch synthase activity showed that between 8 and 15 d post‐anthesis (dpa) the transgenic lines (79.42a and 72.11b) had an increased activity (Fig. 4). This increased activity was associated with an increased flux of carbon into starch (Fig.5). Although at 20 °C this increased activity did not lead to an increase in starch stored the decrease in seed weight at the higher temperature was not as large in the transgenic lines (Fig. 6) as in the controls. However, it is also clear from these data that, while small adjustments in productivity under certain environmental regimes might be obtained, there is not a simple relationship between starch made and starch stored. This is not surprising since starch synthesis is a consequence of the interaction of many enzymes and, as described above, the starch granule consists of more than one type of molecule. The quality of the starch is important in its end use (Table 1). Engineering the quality of the starch by plant breeding and molecular biology has been achieved. Maize is the dominant crop for starch production and there are many mutants, which have been successfully used to provide modified starches. Perhaps the most important of these is waxy maize which was identified early in the last century. Waxy maize has a very characteristic temperature profile and is important in food use. Until recently, waxy wheat and waxy potatoes had not been found. In the early 1980s (Hovenkamp‐Hermelink et al., 1987) identified a waxy potato. Wheat is hexaploid with three genomes which has made it difficult to identify mutants. However, both varieties with all three waxy alleles mutated and genetically engineered varieties have been produced. ATC and Biogemma have introduced a wheat GBSS1 sequence in antisense on a high molecular weight glutenin promoter and produced some lines, which contain a waxy phenotype. It is therefore possible to use both classical plant breeding and more recent plant biotechnological approaches to improve both the quality and the quantity of the crop harvested by the farmer. However, the pathways of starch biosynthesis are complex and control does not reside at a few steps (Fell, 1999). Prior to the mid‐1970s biochemists viewed metabolic pathways as individual entities and considered the control of flux to reside at a single point. More recently, molecular biologists have approached modifying metabolism by altering one gene and looking for a large effect. This approach has been largely unsuccessful. Metabolic control analysis (Heinrich and Rapoport, 1974; Kacser, 1987; Thomas and Fell, 1998) has illustrated not only how control is distributed along a pathway but also that pathways cannot be considered as unique entities. Similarly, control across the pathway will alter as the environment alters. Changing one parameter, such as an enzyme activity, at a time is clearly a necessary experimental approach if one is to understand how metabolism is controlled but this poses a dilemma for the biotechnologist. If the aim is to develop a crop that stores more starch in the field under many environmental conditions when many enzyme activities and substrate concentrations are changing, how do you perform an interpretable experiment simultaneously changing many parameters. A framework for this has been provided by Thomas and Fell (1998). Clearly there is much work to do. View largeDownload slide Fig. 1. The food or industrial pipeline. View largeDownload slide Fig. 1. The food or industrial pipeline. View largeDownload slide Fig. 2. World grain yield. View largeDownload slide Fig. 2. World grain yield. View largeDownload slide Fig. 3. Change in straw length in wheat. Wheat varieties from 1910 to today grown at the Botanic Garden Cambridge, UK, to show how the length of the stem has decreased over the century. View largeDownload slide Fig. 3. Change in straw length in wheat. Wheat varieties from 1910 to today grown at the Botanic Garden Cambridge, UK, to show how the length of the stem has decreased over the century. View largeDownload slide Fig. 4. The expression of glycogen synthase in transgenic wheat. Starch synthase activity was determined in cell‐free extracts of endosperm isolated from 8–28 dpa from wheat plants grown at two temperatures after anthesis. Each bar represents the mean of extracts prepared from three developing endosperm from three plants. For 8 dpa and 15 dpa the experiment was duplicated in a different greenhouse compartment. View largeDownload slide Fig. 4. The expression of glycogen synthase in transgenic wheat. Starch synthase activity was determined in cell‐free extracts of endosperm isolated from 8–28 dpa from wheat plants grown at two temperatures after anthesis. Each bar represents the mean of extracts prepared from three developing endosperm from three plants. For 8 dpa and 15 dpa the experiment was duplicated in a different greenhouse compartment. View largeDownload slide Fig. 5. The effect of expression of glycogen synthase in transgenic wheat on the synthesis of starch. [U‐14C] sucrose (37 kBq) was supplied to isolated endosperm in a medium of 10 mM Mes‐NaOH (pH 5.6), 310 mM sorbitol, 20 mM sucrose, 60 mM KCl, and 6 mM MgCl2 for 3 h. The endopserm was washed, frozen in liquid nitrogen, the tissue ground in 10% trichloroacetic acid and the starch extracted. The starch was degraded to glucose and the radioactivity determined. Units are expressed as nmoles of hexose incorporated into starch per minute per gram fresh weight of endosperm. View largeDownload slide Fig. 5. The effect of expression of glycogen synthase in transgenic wheat on the synthesis of starch. [U‐14C] sucrose (37 kBq) was supplied to isolated endosperm in a medium of 10 mM Mes‐NaOH (pH 5.6), 310 mM sorbitol, 20 mM sucrose, 60 mM KCl, and 6 mM MgCl2 for 3 h. The endopserm was washed, frozen in liquid nitrogen, the tissue ground in 10% trichloroacetic acid and the starch extracted. The starch was degraded to glucose and the radioactivity determined. Units are expressed as nmoles of hexose incorporated into starch per minute per gram fresh weight of endosperm. View largeDownload slide Fig. 6. Effect of temperature on the grain yield of wheat. View largeDownload slide Fig. 6. Effect of temperature on the grain yield of wheat. Table 1. Examples of the use of starch Food and drinks  Animal feed  Agriculture  Plastic  Pharmacy  Building  Textile  Paper  Various   Mayonnaise  Pellets  Seed coating  Biodegradable  Tablets  Mineral fibre  Warp  Corrugated  Oil drilling  Baby food  By products  Fertilizer  plastic  Dusting powder  Gypsum board  Fabrics  board  Water treatment  Bread          Concrete  Yarns  Cardboard  Glue  Soft drinks              Paper    Meat products                  Confectionery                  Food and drinks  Animal feed  Agriculture  Plastic  Pharmacy  Building  Textile  Paper  Various   Mayonnaise  Pellets  Seed coating  Biodegradable  Tablets  Mineral fibre  Warp  Corrugated  Oil drilling  Baby food  By products  Fertilizer  plastic  Dusting powder  Gypsum board  Fabrics  board  Water treatment  Bread          Concrete  Yarns  Cardboard  Glue  Soft drinks              Paper    Meat products                  Confectionery                  Source: International Starch Institute, Aarhus, Denmark web site http://home3.inet.tele.dk/starch View Large Table 2. UK farm income and costs Crop  Price (£ per tonne)  % Decrease  Wage rates per hour  % Rise    1990  1999    1990  1999    Winter wheat  112  65  41.9        Winter barley  105  63  40  3.51  5.15  46.7  Maincrop potatoes  80  75  6.25        Crop  Price (£ per tonne)  % Decrease  Wage rates per hour  % Rise    1990  1999    1990  1999    Winter wheat  112  65  41.9        Winter barley  105  63  40  3.51  5.15  46.7  Maincrop potatoes  80  75  6.25        Source: Nix, 1999. View Large Table 3. Changes in rice yield in Japan Year  Yield (tons ha–1)  Comment  800–900  1.01    1550  1.65    1720  1.92  Systematic introduction of irrigation  1878–1887  1.85    1908–1917  2.64  Variety Improvement  1938–1942  2.99  Introduction of chemical fertilizers  1956–1965  3.95  Combined use of irrigation, fertilizers, pesticides, fungicides and new varieties  Year  Yield (tons ha–1)  Comment  800–900  1.01    1550  1.65    1720  1.92  Systematic introduction of irrigation  1878–1887  1.85    1908–1917  2.64  Variety Improvement  1938–1942  2.99  Introduction of chemical fertilizers  1956–1965  3.95  Combined use of irrigation, fertilizers, pesticides, fungicides and new varieties  Source: Ishizuka, 1969 View Large Table 4. Interaction of rice variety and agronomy on yield Spacing (cm)  Variety IR154 (low tillering)  Variety IR8 (high tillering)    Panicle (m–2)  Panicle wt plant–1  Yield (t ha–1)  Panicle (m2)  Panicle wt plant–1  Yield (t ha–1)  10×10  350  2.25  5744  340  2.02  6119  20×20  194  3.47  5533  250  2.97  6444  30×30  140  3.92  4494  198  3.54  5733  40×40  96  3.79  3474  167  3.36  4816  50×50  70  4.29  2803  141  3.43  4649  Spacing (cm)  Variety IR154 (low tillering)  Variety IR8 (high tillering)    Panicle (m–2)  Panicle wt plant–1  Yield (t ha–1)  Panicle (m2)  Panicle wt plant–1  Yield (t ha–1)  10×10  350  2.25  5744  340  2.02  6119  20×20  194  3.47  5533  250  2.97  6444  30×30  140  3.92  4494  198  3.54  5733  40×40  96  3.79  3474  167  3.36  4816  50×50  70  4.29  2803  141  3.43  4649  View Large References BanksW, Muir DD. 1980. Structure and chemistry of the starch granule. In: Preiss J, ed. The biochemistry of plants, Vol. 3. New York: Academic Press, 321–369. Google Scholar BinghamJ. 1971. Physiological objectives in breeding for grain yield in wheat. In: Lupton FGH, Jenkins G, Johnson RR, eds. The way ahead in plant breeding: Sixth congress of Eucarpia. Cambridge, 15–29. Google Scholar ChandlerRF. 1969. Plant morphology and stand geometry in relation to nitrogen. In: Dinauer RC, ed. Physiological aspects of crop yield. Madison, Wisconsin: American Society of Agronomy, 265–289. Google Scholar FellDA. 1999. Traditional concepts of metabolic control mislead more than enlighten. The Biochemist February, 13–16. Google Scholar HedleyC. 2000. The relationship between starch synthesis and respiration during wheat endosperm (Triticum aestivum) development. PhD thesis, Department of Biological Sciences, University of Manchester, and Advanced Technologies (Cambridge) Ltd. Google Scholar HeinrichR, Rapoport TA. 1974. A linear steady‐state treatment of enzymatic chains. Critique of the cross‐over theorem and a general procedure to identify interaction sites with an effector. European Journal of Biochemistry  42, 97–105. Google Scholar Hovenkamp‐HermelinkJHM, Jacobsen F, Ponstein AS, Visser RGF, Vos‐Scheperkeuter GH, Ijmolt FW, de Vries JN, Witholt B, Feenstra WJ. 1987. Isolation of an amylose‐free starch mutant of the potato (Solanum tuberosum L.). TAG  75, 217–221. Google Scholar IshizukaY. 1969. Engineerng for higher yields. In: Dinauer RC, ed. Physiological aspects of crop yield. Madison, Wisconsin: American Society of Agronomy, 15–25. Google Scholar JennerCF, Denyer K, Guerin J. 1995. Thermal characteristics of soluble starch synthase from wheat endosperm. Australian Journal of Plant Physiology  22, 703–709. Google Scholar KacserH. 1987. Control of metabolism. In: Davies DD, ed. Biochemistry of plants, Vol. 11. Academic Press, 39–67. Google Scholar KainumaK. 1988. Structure and chemistry of the starch granule. In: Preiss J, ed. The biochemistry of plants, Vol. 14. New York: Academic Press, 141–180. Google Scholar NachtergaeleW, Vannuffel J. 1989. Starch as a stilt material in carbonless copy paper—new developments. Starch  41, 386–392. Google Scholar NixJ. 1999. Farm management pocketbook. Ashford, Kent: Wye College Press. Google Scholar SiddiqueKHM, Whan BR. 1993. Ear:stem ratios in breeding populations of wheat. In: Lin ZS, Xin ZY, eds. Eighth international wheat genetics symposium. Peking: China Agricultural Scientific Press, 1139–1144. Google Scholar TahirM, Ketata H. 1993. Rate and duration of grain filling in Triticum species and its breeding implications. In: Lin ZS, Xin ZY, eds. Eighth international wheat genetics. Peking: China Agricultural Scientific Press, 1145–1148. Google Scholar TesterRF, Karkalas J. 2001. The effects of environmental conditions on the structural features and physico‐chemical properties of starches. Starch  53, 513–519. Google Scholar TesterRF, Karkalas J. 2002. Polysaccharides. II. Polysaccharides from eukaryotes. In: Vandamme EJ, De Baets S, Steinbuchel A, eds. Starch in biopolymers, Vol. 6. Weinheim: Wiley‐VCH, 381–438. Google Scholar ThomasS, Fell DA. 1998. The role of multiple enzyme activation in metabolic flux control. Advances in Enzyme Regulation  38, 65–85. Google Scholar WhiteMA. 1998. The chemistry behind carbonless copy paper. Journal of Chemical Education  75, 1119–1120. Google Scholar
TI - Starch: the need for improved quality or quantity—an overview
JF - Journal of Experimental Botany
DO - 10.1093/jxb/erg049
DA - 2003-01-01
UR - https://www.deepdyve.com/lp/oxford-university-press/starch-the-need-for-improved-quality-or-quantity-an-overview-HZKEbJcYy7
SP - 451
EP - 456
VL - 54
IS - 382
DP - DeepDyve
ER -