Maize Starch for Industrial Applications - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 176

Several of the alleles found to have arisen spontaneously out of breeding
populations were shown to be the result of the action of transposable elements
[ 21 ]. Waxy alleles have also been recovered out of induced mutation populations
using ethyl methanesulfonate or gamma radiation induction.
The availability of several independent waxy alleles provided a unique resource
for early research into the organization of the gene. Using 22 of the waxy alleles, a
structural gene map was constructed and provided seminal insight into the nature of
molecular change at a level of detail not previously possible in an agricultural
crop [ 22 ].
The amylose mutation has been rediscovered less frequently than waxy; how-
ever several independently derived alleles are listed with the Maize Genetics
Cooperative Stock Center. Mark Campbell, of Truman State University, has
released a public domain amylose breeding line, GEMS-0067 [ 23 ]. In addition to
amylose-extender , GEMS-0067 has been shown to possess a mutation of the gene
for starch branching enzyme I (SBEI) [ 24 ]. The mutant SBEI gene serves as a
significant amylose modifier, elevating grain amylose content in the evaluation
population to above 70 %. Amylopectin branching has also been shown to be
impacted by SBEI and to alter starch granule structure [ 25 ].
Specialty Starch Hybrid Breeding Advancements
Breeding for specialty starches routinely involves backcross conversion of waxy or
amylose genetics into elite performing dent or flinty breeding lines. The line to be
converted, in addition to yield potential, should be considered for its ability to
confer agronomic stability, kernel starch content, ear disease resistance, good fall
dry down, kernel size, and kernel test-weight characteristics. In contrast to dent
breeding with its focus on capturing top-end yield, with specialties, performance
longevity and environmental stability are often of superior premium.
Comparisons of hybrids produced using waxy and dent isolines reported an
average yield reduction of 3.5 % [ 26 ]. A portion of the waxy yield drag may be
an artifact of linkage drag resulting from the use of less productive donor materials.
Improved waxy donor germplasm that is as closely related to the elite recurrent as
possible should be used for conversions. Marker-assisted selection, highly useful in
transgenic inbred conversions also can be effective in minimizing linkage drag
around the specialty genes of interest.
A biological or biochemical causal effect for waxy yield drag is not fully
apparent although waxy breeding lines have, in some evaluations, been observed
to have lower starch content in comparison to dent [ 27 ]. Waxy ears have also been
found to have slower dry down in the fall, a characteristic that could contribute in
some growing conditions to reduced yield and increased susceptibility to fungal
infection [ 28 , 29 ].
Genetically engineered lines with transgenes for protection against herbicide and
insect damage have been available in waxy hybrid seed since 2007 [ 30 ]. Engineered