Biomedical Engineering Reference
In-Depth Information
command the same price premium and may not be as quickly adaptable to new or
small production specialty grain types that may find profitable niche application
with food.
The genetics behind starch biosynthesis has been extensively researched in
maize for food applications and is well defined by numerous evaluations of null
mutations (Table
9.2
)[
19
,
43
-
49
]. In addition, many gene combinations have been
discussed and reviewed [
35
,
45
,
50
-
55
].
Waxy and amylose-extender are genetics on which commercial businesses have
been built; improvement on these starch types provides immediate application.
Unique gene combinations that could expand on these applications are highly
desirable and would provide the industry with new commercial grain types.
Disruption of the initial steps of the pathway results in very little or no starch
being produced. Both
shrunken-2
and
brittle-2
, code for subunits of the tetrameric
ADP-glucose pyrophosphorylase, prevent starch production, and
shrunken-2
is
used in commercially released sweet corn hybrids. The ADP translocator enzyme
coded by the
brittle-1
gene prevents transport of glucose-6-phosphate across the
amyloplast organelle membrane.
The
sugary-1
mutation, no longer greatly used in the sweet corn industry, codes
for starch debranching enzyme and results in phytoglycogen as the major endo-
sperm carbohydrate rather than starch. Phytoglycogen is more highly branched than
amylopectin; it does not form granules but is retained in the dried kernel where it is
35 % of the dry-matter weight [
56
].
Phytoglycogen molecules, with a diameter of 40-50 nm, are much smaller than
starch granules (15
m). While not a starch, phytoglycogen is a very interesting
hydrocarbon and has been shown to have many useful food and industrial applica-
tions [
56
,
57
].
Substrate availability, enzyme availability, and enzymatic activity are the pre-
dominant regulators of biosynthesis. While much of the current breeding for
specialty corn is built on null mutations, and thereby enzyme availability, the starch
pathway in maize appears also to include some more complicated aspects. The
observations and descriptions of ADP-glucose pyrophosphorylase (
shrunken-2
)
provide an interesting example and demonstrate the potential of starch genetics to
contribute to improved yield and agricultural performance [
58
-
60
].
Soluble starch synthase (
dull-1
) gene has been shown to reduce SSII and SBEIIb
[
61
]. The formation of multi-subunit protein complexes containing SSIIa, SSIII,
SBEIIa, and SBEIIb has been well described and outlines a higher-order structural
component within the pathway [
45
].
Simple starch pathway genetic combinations that are most prominent in the
literature are
waxy:sugary2
and
waxy:amylose-extender. Sugary-2
has been shown
to have a dosage effect on amylopectin chain length and produces a change in the
onset of gelatinization temperature [
62
]. The shorter amylopectin chain lengths
have been cited as contributing to reduced starch retrogradation in
waxy:sugary2
starch [
55
]. The
waxy:amylose-extender
combination changes the amylopectin
chain length in the opposite way, extending chain length [
55
].
μ
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