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these sequences, they have been deposited in a mySQL database. The se-
quences of these individual ESTs and the clustered unigenes are freely available
to the scientific community at the Nectary Gene Expression (NecGEx) website
(http://www.bb.iastate.edu/necgex/ests/db). These sequences have also been
deposited in the GenBank as Accession # EB688597 to EB701044.
Preliminary analysis of these unigenes has revealed several novel fea-
tures of nectary development and function. These features include:
The identification of specific transcription factors (TFs) that may regulate
nectary development and function as well as downstream target genes that
are driven by these TFs.
The identification of cDNA clones encoding the entire metabolic pathway
for β-carotene, a compound that accumulates to very high levels in the nec-
tary (see “Conversion of chloroplasts into chromoplasts” on page 270).
The discovery that the nectary is preprogrammed for the ethylene response
that occurs in flowers following fertilization.
The discovery that many of the most highly expressed ESTs function in
in stress responses, suggesting novel functions for the nectary (see
Macroarray analysis identifies defence genes” on page 274).
Additional in-depth analysis will certainly yield increased information
about the nectary. One goal is to produce nectary-specific microarrays that
will permit us to probe deeply into the nectary transcriptome to identify the
factors that affect nectary development and function.
5.3
Nectary-specific gene expression
The expression of genes in the nectary has also received recent attention.
Nectarin I, the major protein secreted into the nectar of tobacco plants, is a
novel manganese-containing superoxide dismutase ( Carter et al., 1999;
Carter & Thornburg, 2000). We have isolated the Nec1 gene and analysed the
expression of the Nec1 promoter in transgenic plants (Carter & Thornburg,
2003). The tissue specificity of marker gene expression demonstrated that
this promoter was expressed uniquely in the floral nectary. Other floral or
additional plant organs did not express the Nec1 promoter. Further, the Nec1
promoter was expressed only when nectar was actively being secreted from
flowers. The promoter is not active in presecretory nectaries, and it is also
not active following fertilization of the flower, when nectar secretion ceases.
This analysis suggests that anthesis and protein secretion into nectar are
coordinated events.
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