Most of the starch consumed by man is supplied by three
cereals: rice, wheat and maize. Although they are all
members of the family Poacea, they differ in their genetic
structure (Feuillet and Keller, 2002). Rice is diploid and
has a relatively small genome (4108 bp). It is easily
transformable and there are concerted efforts in many
countries to produce mutants in each of the genes
(e.g. http://signal.salk.edu). The maize genome is much
larger, about 2.5109 bp. It is considered to be a partial
allotetraploid, i.e. portions of its genome represent related
sequences from two ancient progenitor species. Many
mutants of the kernel phenotype have been described for
maize because of the ease of screening hundreds of kernels
per ear. The wheat genome is larger again, about 50 times
the size of the rice genome, about 1.61010 bp. It is
hexaploid and so there are three sets of duplicated
chromosomes in somatic cells. Wheat is difficult to
transform and phenotypic mutants are rare because
mutations for the three homeologous genes usually need
to be combined before the function can be affected. It is
useful to keep these differences in mind as genetic
engineering and natural variation are considered as routes
to obtaining the desired grain quality.
Starches are homopolymers in which the glucose residues
are linked predominantly by a-1,4 bonds although about
3–6% are a-1,6 linkages (see for example Tester et al.
(2004) for a detailed discussion of starch structure). Starch
accumulates as insoluble granules in the plastid of all
higher and lower plants. This plastid can be the chloroplast
of the plant leaf cells or the non-photosynthetic plastid of a
starch storage organ cell such as the wheat endosperm.
Starch is synthesised during the day in leaves and other
photosynthetic tissues and broken down at night; this type
of starch is known as transitory starch. Starch is also laid
down in seeds, tubers and other reserve tissues by the plant
during one stage of its life-cycle to be used in another stage;