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MYB21, MYB24 and MYB57, transcript levels of each individual MYB gene were
studied in four quadruple mutants in which only one of the four DELLA genes
remains intact. All three MYB genes were almost undetectable in the Q1 (ga1-3
gai-t6 rgl1-1 rgl2-1, wild type for RGA) and barely detectable in the Q3 (ga1-3
gai-t6 rgl1-1 rga-t2, wild type for RGL2) mutants but were detected at high levels
in the Q2 (ga1-3 rga-t2 rgl1-1 rgl2-1, wild type for GAI) and Q4 (ga1-3 gai-t6
rga-t2 rgl2-1, wild type for RGL1) mutants (Figure 2B), suggesting that RGA and
RGL2, but not GAI nor RGL1, were the more effective DELLAs in repressing
the expression of these three MYB genes. Interestingly, we showed previously that
while Q1 and Q3 mutants, as the ga1-3 mutant, were retarded in floral develop-
ment both Q2 and Q4 mutants produced normal fertile flowers (Figure 2C) [8].
Therefore, it seems there is a nice correlation between normal floral development
and the expression of MYB21, MYB24 and MYB57, suggesting that these three
MYBs are probably necessary for normal floral development.
MYB21, MYB24, and MYB57 Function Redundantly in
Controlling Stamen Filament Elongation
Expression analysis showed that MYB21 and MYB24 [32],[34] as well as MYB57
are flower-specific genes (Figure 2D). To determine if the spatial and temporal
expression patterns of MYB21 and MYB24 correlate with their proposed role
during stamen filament elongation, we examined MYB21 expression via in situ
hybridization and generated pMYB24:GUS transgenic for examining MYB24 ex-
pression. Our in situ hybridization result showed that, starting from floral stage
12 [1],[38], MYB21 is expressed in the anther vascular tissue and in cells at the
junction between anther and stamen filament where rapid filament elongation is
hypothesized to occur starting from the floral stage 13 after a successful pollina-
tion [1]. MYB21 expression is also detected in the nectaries and ovules. Similarly,
staining the young inflorescence of the pMYB24::GUS plants revealed that strong
GUS activity was detected in the vascular tissue of stamen filament and sepals
whereas only weak GUS activity was detectable in the petals starting from floral
stage 12. GUS activity was also detected in the upper part of the pistils.
To investigate their roles in GA-mediated floral organ development, we
identified T-DNA insertional mutant lines corresponding to these three MYB
genes from the Salk Institute Genomic Arabidopsis Laboratory (SIGnAL) data-
base. Mutant alleles were confirmed (data not shown) and designated as myb21-
t1 (SALK_042711) for MYB21, myb24-t1 (SALK_017221) for MYB24, and
myb57-t1 (SALK_065776) for MYB57 (Figure 3A). myb24-t1 and myb57-t1 are
both likely null alleles since MYB24 and MYB57 transcripts were undetectable
in myb24-t1 and myb57-t1 mutant flower buds, respectively (Figure 3B). On
