Expression of genes associated with carbohydrate metabolism in cotton stems and roots Page: 7
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and were validated using qPCR analyses of expression of
selected genes. Statistical analysis of the data indicated an
increase in genes associated with carbohydrate metabo-
lism as expected for tissues accumulating starch. Addition-
ally, expression of genes associated with transcription
factor activity also increased indicating a potential role for
changes in gene expression as stems and roots started to
store starch. Some of these transcription factors are likely
to play roles in regulating carbohydrate metabolism.
There must be an increased flow of SUC from leaves to
stems and roots as starch accumulates. Consistent with
this requirement was an increase in expression of genes
associated with sugar transport in these tissues. Tran-
scripts encoding invertase and invertase inhibitors
increase in tissues accumulating starch. Invertase converts
SUC to GLC and FRC providing glucose for starch biosyn-
thesis. We also note that the change of expression of
sucrose synthase which converts SUC to GLC-UDP and
FRC fell just below the two-fold threshold for considera-
tion. Increases in levels of transcripts encoding hexoki-
nases, phosphoglucomutase and isomerases were
consistent with the production of Glucose-l-phosphate
for starch biosynthesis (Table 2). SUC and FRC levels
peaked in both roots and stems at 4 W post anthesis. Four
weeks after first anthesis represents a time of considerable
demand for carbohydrates by the developing seeds.
Sucrose is synthesized via sucrose phosphate synthase and
SPP. Sucrose phosphate synthase synthesizes sucrose-
phosphate from GLC-UDP and FRC-P and SPP dephos-
phorylates sucrose-phosphate. As previously noted the
increase in SPP transcripts fell just below the two- fold
level in starch accumulating tissues and did not subse-
quently increase. More detailed analyses of cotton stems
after flowering will elucidate important aspects of sucrose
metabolism that would allow export of carbon to support
ADPGp expression is increased in starch storing tissues.
ADPGp catalyzes the conversion of glucose-phosphate to
glucose-ADP, which is a rate limiting step in starch bio-
synthesis . As expected, other genes associated with
starch biosynthesis such as starch synthase and starch
branching enzymes increased in expression during the
starch accumulation stage of stem and root development.
Starch phosphorylase also appeared to play a role in
starch biosynthesis in rice seed . Somewhat unex-
pected was an increase in expression of genes associated
with starch degradation in starch accumulating tissues.
QPCR of an a-amylase transcript indicated that these tran-
script levels increased in starch accumulating tissue and
stayed at about the same level as starch decreased. Amy-
lase enzyme activities have been reported to correlate with
starch levels in cotton plants . a-Amylase is not
required for starch degradation in Arabidopsis leaves indi-
cating it may have functions other than starch degradation
. Starch degrading genes, such as those encoding
starch debranching enzymes, play a role in starch matura-
tion. Expression of a glucan water dikinase (GWD) tran-
script increased in expression in starch accumulating
tissues and continued to increase in expression as starch
was utilized. GWD phosphorylates starch and is necessary
for degradation of transient starch in Arabidopsis leaves
Most of the proteins directly involved in starch metabo-
lism appeared to be targeted to the plastid as expected.
One exception was the ADPGp large subunit. The gene
model used on this microarray included the 5' end of the
coding sequence but was not predicted to be targeted to
the plastid even though other plant ADPGp large subunits
were. Failure to localize this ADPGp to the plastid may
indicate an error assembling this gene or might indicate
this gene is located in the cytosol. A cytosolic localization
has been reported for some ADPGp .
Analysis of differentially expressed genes identified an
increase in transcripts encoding enzymes for TRE biosyn-
thesis and RAF biosynthesis. Analysis of RAF levels and
the expression of genes associated with the biosynthesis of
RAF and TRE indicated that they peak in expression in
field-grown stems and roots well after starch [11,23,24].
The TRE pathway (especially trehalose-6-phosphate) is
associated with control of glycolysis, ABA signaling and
starch accumulation in Arabidopsis and has been associ-
ated with drought stress in cotton . QPCR confirmed
an increase in transcripts encoding trehalose phosphate
synthase that continued even after starch levels declined.
Therefore TRE may play a role in the starch utilization
stage of cotton stem development. RAF accumulates
prominently in maturing seeds where it is thought to act
as a compatible solute in preparation for seed desiccation
and as a storage reserve for post-germinative growth .
RAF and RFO are thought to play a role in cold and desic-
cation tolerance in plants and in some plants are promi-
nent transport sugars . The role RFO is playing in
cotton is unclear because even at peak levels, they are
below those associated with desiccation tolerance. The
increase of putative RFO anabolic and catabolic gene
expression may point to transient fluctuations in levels, or
rapid flux of carbohydrate through this pathway.
Cotton is an unusual crop because it is a perennial that is
often grown as an annual row crop. One method of deter-
mining how well cotton has been annualized is to meas-
ure reserves stored for subsequent regrowth which are
unavailable for seed production and therefore wasted in
an annual row crop. Starch accounted for about 1.5 % of
the dry weight of stems and roots late in boll develop-
ment. One goal of this research is to identify genes and
Page 7 of 10
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BMC Plant Biology 2009, 9:11
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Taliercio, Earl W.; Romano, Gabriela; Scheffler, Jodi & Ayre, Brian G. Expression of genes associated with carbohydrate metabolism in cotton stems and roots, article, January 22, 2009; [London, United Kingdom]. (https://digital.library.unt.edu/ark:/67531/metadc78286/m1/7/: accessed July 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.