Biology Reference
In-Depth Information
Table
2.
Glycans in developing and mature nectaries. Nectaries from stage 9 and stage 12
flowers were isolated and compared for the presence of both free sugars and polysaccharides.
Concentration found (mg/g fresh weight)
Stage 9
Stage 12
Free glucose
0
0.5
Total glucose
2.7
3.8
Free fructose
>0.01
>0.01
Total fructose
2.4
4.6
Sucrose
1.0
0.18
Starch
20.8
4.7
In addition, the chemical nature of starch was characterized at different
stages of nectary development with respect to both composition and struc-
ture. The amylose/amylopectin ratio did not change dramatically throughout
nectary development, although there was a general trend towards the more
complex starch forms at later stages of floral development. We also evalu-
ated the structure of starch from various nectary stages using FACE (fluo-
rescent-assisted carbohydrate electrophoresis). These analyses demonstrated
that there was an increase in the overall chain length of both short and long
side chains as the nectary develops up to stage 9, when the maximum chain
lengths are attained. In the later stages of floral nectary development, starch
is significantly reduced in complexity as would be expected during starch
degradation.
Using starch metabolism in
Arabidopsis
foliage, maize kernels, and
potato tubers as a model, we identified a list of 26 target genes that partici-
pate in starch anabolism and catabolism. We used a variety of strategies to
isolate cDNAs that encode 18 of the 26 targeted starch metabolic genes (Ren
et al., unpublished). We have analysed the timing of expression of these
genes within the nectary using real-time RT-PCR. These analyses define
three distinct patterns of expression: one characteristic of the anabolic starch
metabolism genes, a second pattern characteristic of the catabolic starch
metabolism genes, and a third in which there is general expression through-
out nectary development. In cases where clones encoding multiple isoforms
of starch metabolic enzymes were identified (e.g., Starch Synthase), most of
the isoforms showed little expression at the mRNA level; however, single
unique isoforms were found to be dramatically upregulated at the transcript
level during nectary development.
As the flower nears anthesis, the nectary starch is dramatically degraded
to liberate large quantities of sugar for nectar production. Starch is mobilized
by a combination of phosphorolytic and hydrolytic mechanisms (Beck &
