Biomedical Engineering Reference
In-Depth Information
(PHB) [
99
], a natural biodegradable thermoplastic with physical and chemical
properties similar to polypropylene. The fibers of the transgenic plants showed
slower rates of heat uptake and cooling compared with fibers from wild-type plants
and, although the effects were small, provide some promise for this approach. There
have been attempts at expressing this biopolymer in fiber plants such as flax and
poplar [
100
,
101
], but nothing further in cotton. Zhang et al. [
102
] produced
transgenic cotton expressing rabbit keratin-associated protein (KAP61R) genes in
the fiber from the fiber-specific E6 promoter. The fibers from the transgenic plants
were reported to have improved strength and thermal properties and were 60 %
longer than the wild-type controls. However, this GM trait has not appeared in
commercial use, presumably because its reported unique properties were not
inherited. In a similar vein, Huang et al. [
103
] have expressed a spider silk protein
in cotton fibers, but it had no statistically significant effect on fiber quality. Fiber
engineering is clearly complex, but this has been recognized for many years in
many different systems where biotechnologists have attempted to alter plant
metabolism.
Identification of QTLs Linked with Fiber Quality and Yield
Many traits of agronomic interest are monogenic, meaning they are controlled by a
single gene, but most important targets for crop improvement, such as yield and
quality, are invariably polygenic. The genes that contribute to a multigenic trait are
referred to as quantitative trait loci or QTLs. A QTL can be statistically associated
with a trait and tracked using linked molecular markers to enable identification of
superior cotton lines. The DNA markers in cotton have included a range of types
such as restriction fragment length polymorphisms (RFLPs), random amplified
polymorphic DNAs (RAPDs), simple sequence repeats (SSRs), amplified fragment
length polymorphisms (AFLPs) (reviewed in Mei et al. [
104
]), and, increasingly,
single nucleotide polymorphisms (SNPs) [
105
]. The advantages of DNA markers
over conventional phenotypic selection are that they are independent of environ-
ment, age of the plant, or the presence of a pathogen or pest. Therefore, it is possible
to select for the progeny of a cross carrying the genomic regions which contain the
desired alleles that contribute to a trait, even at a stage (ie., as seeds or seedlings)
before the trait is expected to be expressed, using the marker instead of the trait
phenotype.
QTLs associated with fiber quality and other traits have been identified in a
number of studies involving both inter- and intraspecific crosses between the
tetraploid species (see reviews [
106
-
108
]). Hybridization of
G. hirsutum
G. barbadense
through conventional breeding programs to improve fiber traits of
commercial cultivars has been difficult because of their genome incompatibilities.
QTLs and genes linked to high-quality fiber traits from
G. barbadense
have been
introgressed into
G. hirsutum
[
46
,
109
] allowing the identification of large numbers
of fiber quality-related QTLs [
110
-
113
] and the construction of many high-
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