Biomedical Engineering Reference
In-Depth Information
ultimately the number of fibers per seed, a yield component. Once the fiber cell
protrudes from the epidermal layer, it elongates rapidly beginning primary elonga-
tion. The rate and duration of this phase determines many important fiber traits,
including the length, shape, structure, and composition of the fiber cell [
75
]. Fiber
elongation occurs largely by diffuse growth that coordinates cell turgor (the driving
force of cell expansion) and cell wall loosening [
76
]. During the latter stage of
elongation, ~16 and 21 days past anthesis (dpa), there is a transition from primary
elongation to SCW synthesis [
77
] distinguished by the realignment of parallel
microtubules and cellulose microfibrils in a steeply pitched manner [
78
]. Though
once considered mutually exclusive, there is a period of overlap between phases
[
79
,
80
], since elongation can continue up to 45 dpa in longer genotypes [
81
]or
cease around 25 dpa in shorter genotypes [
80
]. These differences are due to genetic
variation between cotton species and cultivars [
82
]. During SCW synthesis, cotton
fibers increase in dry mass due to cellulose deposition which continues until the boll
sutures open. Fiber strength is directly influenced by the rate of cellulose deposition
during SCW synthesis [
83
], while the amount of cellulose partially determines yield
[
84
]. A greater understanding of the molecular physiological processes that regulate
which cells become fibers and control each of the stages of their development could
enhance the ability to either breed for or to engineer cotton plants with a higher
density of fibers which are longer, stronger, and finer, hence higher yielding.
Discovery of Genes Involved in Fiber Development and Their
Manipulation Through Genetic Engineering
The recent advances in functional genomics, genetic, and analytical tools, espe-
cially comprehensive gene expression profiling of cotton fiber cells, together with
the availability of a sequenced genome, have provided new opportunities to
improve cotton fiber traits through genetic modification. Many fiber-specific
genes involved in fiber cell initiation, fiber elongation, or cell wall biogenesis
have been identified as candidates for genetic manipulation to improve fiber yield
and/or quality (Fig.
10.4
). For example, two MYB genes,
GhMYB25
and
GhMYB25
-like, which are related to a petal epidermal cell patterning MIXTA-
MYB from
Antirrhinum majus
, and a homeodomain transcription factor (
GhHD-1
)
were identified from microarray comparisons between fiberless mutants and wild-
type cotton [
42
]. Silencing these genes in tetraploid cotton affects either the
initiation or timing of expansion of fiber initials and their overexpression under a
constitutive or seed coat-specific promoter results in an increased number of fiber
initials on the surface of the ovule [
85
-
87
]. Whether this increased fiber initiation
translates into an increase in lint percentage or yield remains to be tested in the field.
Transcript profiling and ovule culture experiments both indicate that several phy-
tohormones, including auxin, gibberellic acid, and brassinosteroids mediate cotton
fiber initiation and early growth [
88
-
90
]. Seed-specific expression of the
iaaM
gene
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