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RGL1 and RGL2 four DELLA genes). We also showed that absence of the four
DELLAs (GAI, RGA, RGL1 and RGL2) cannot suppress the short stamen phe-
notype conferred by the loss-of-function of MYB21 and MYB24. In addition,
we observed that application of exogenous JA onto the ga1-3 gai-t6 rgl1-1 rgl2-1
quadruple mutant flower buds could restore the expression of MYB21, MYB24
and MYB57 whereas application of exogenous GA onto opr3 mutant flower
buds failed to increase the expression of these three MYBs. Most importantly,
we showed that GA upregulates JA-biosynthetic genes DAD1 and LOX1 and
the JA content in the young flower buds of the GA-deficient quadruple mutant
ga1-3 gai-t6 rga-t2 rgl1-1 is much lower than that in the WT and penta mutant.
Therefore, we conclude that GA upregulates the DAD1 and LOX1 expression to
promote JA production to promote the expression of the three MYBs necessary
for stamen filament development.
Results
Identification of DELLA-Repressed Stamen-Enriched Genes
The ga1-3 mutant is retarded in floral development, suggesting that the tran-
scriptome for floral development in the ga1-3 mutant must be kept at a re-
pressive state. Conversely, the fact that the ga1-3 gai-t6 rga-t2 rgl1-1 rgl2-1
mutant (penta mutant) confers GA independent flowering suggests that the
transcriptome responsible for floral development must have been constitu-
tively activated in the penta mutant. We compared the expression profiles be-
tween ga1-3 and ga1-3 gai-t6 rga-t2 rgl1-1 rgl2-1 and identified 360 DELLA-
repressed and 273 DELLA-activated genes essential for floral development
[30]. To identify DELLA-repressed stamen-enriched genes, we examined ex-
pression of 43 DELLA-repressed genes in the sepal, petal, stamen and pistil
via semi-quantitative RT-PCR. These 43 genes were chosen based on two
criteria: 1) they are homologous to transcription factors known to regulate
GA-response (e.g MYB gene family) and 2) genes whose expression showed
drastic changes between the ga1-3 and penta mutant [30]. Only genes whose
expression are either enriched in the stamen or highly expressed in the stamen
and also in some other floral organs but not ubiquitously highly expressed in
all four floral organs were classified as the stamen-enriched genes. A total of
34 genes, including two APG-like genes (At1g75880, At1g75900) and three
genes (IRX1, IRX3, IRX5) encoding the cellulose synthase subunits which
are known to be enriched in the stamen, were identified as DELLA-repressed
stamen-enriched genes (Figure 1; Table 1).
