Journal of Experimental Botany, Vol. 54, No. 382,
Regulation of Carbon Metabolism Special Issue, pp. 451±456, January 2003
DOI: 10.1093/jxb/erg049
Starch: the need for improved quality or quantityÐan
overview
M. M. Burrell1
Advanced Technologies (Cambridge) Ltd., 210 Cambridge Science Park, Milton Road, Cambridge CB4 0WA, UK
Received 9 July 2002; Accepted 24 September 2002
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 calori®c 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.
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 calori®c 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
calori®c 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 a1,4 linked glucose residues with relatively few a-1,6
linked branches whereas amylopectin is a highly branched
molecule of shorter a-1,4 linked glucose molecules and
more frequent a-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
mm in diameter and the other 10±20 mm, whereas in potato
the granules can vary between 10 and 100 mm. 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
1
Present address: Department of Animal and Plant Sciences, University of Shef®eld, Western Bank, Shef®eld S10 2TN, UK. E-mail: mmb1005@
compuserve.com
Journal of Experimental Botany, Vol. 54, No. 382, ã Society for Experimental Biology 2003; all rights reserved
452 Burrell
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). Modi®ed 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
Fig. 1. The food or industrial pipeline.
Table 1. Examples of the use of starch
Food and drinks
Animal feed
Agriculture
Plastic
Pharmacy
Building
Textile
Paper
Various
Mayonnaise
Baby food
Bread
Soft drinks
Meat products
Confectionery
Pellets
By products
Seed coating
Fertilizer
Biodegradable
plastic
Tablets
Dusting powder
Mineral ®bre
Gypsum board
Concrete
Warp
Fabrics
Yarns
Corrugated
board
Cardboard
Paper
Oil drilling
Water treatment
Glue
Source: International Starch Institute, Aarhus, Denmark web site http://home3.inet.tele.dk/starch
Table 2. UK farm income and costs
Crop
Winter wheat
Winter barley
Maincrop potatoes
Source: Nix, 1999.
Price (£ per tonne)
% Decrease
1990
1999
112
105
80
65
63
75
41.9
40
6.25
Wage rates per hour
% Rise
1990
1999
3.51
5.15
46.7
Starch: the need for improved quality or quantity 453
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
¯uid loss. In paper-making, starch is used for several
purposes. As a ®ller it bonds the cellulose ®bres 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
®nish 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 mm
in diameter. A fraction of wheat starch best meets these
criteria.
Fig. 2. World grain yield.
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 suf®cient 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 ®eld 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 ®nal product. The extent of these
losses is in part determined by the quality of the product
harvested from the ®eld. 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 dif®cult 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
Table 3. Changes in rice yield in Japan
Year
Yield (tons ha±1)
800±900
1550
1720
1878±1887
1908±1917
1938±1942
1956±1965
1.01
1.65
1.92
1.85
2.64
2.99
3.95
Comment
Systematic introduction of irrigation
Variety Improvement
Introduction of chemical fertilizers
Combined use of irrigation, fertilizers, pesticides, fungicides and new varieties
Source: Ishizuka, 1969
Table 4. Interaction of rice variety and agronomy on yield
Spacing (cm)
10310
20320
30330
40340
50350
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)
350
194
140
96
70
2.25
3.47
3.92
3.79
4.29
5744
5533
4494
3474
2803
340
250
198
167
141
2.02
2.97
3.54
3.36
3.43
6119
6444
5733
4816
4649
454 Burrell
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.
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.
Starch: the need for improved quality or quantity 455
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.
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 ®gure is slightly smaller (FAO statistics).
Much of this unused land is quite unsuitable for growing
Fig. 6. Effect of temperature on the grain yield of wheat.
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
®lling 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. coli glgA 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
¯ux 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
456 Burrell
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 modi®ed starches. Perhaps the most important of
these is waxy maize which was identi®ed early in the last
century. Waxy maize has a very characteristic temperature
pro®le 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) identi®ed a
waxy potato. Wheat is hexaploid with three genomes
which has made it dif®cult 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 ¯ux 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 ®eld 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.
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