Biology Reference
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proteins [8],[9]. DELLAs are a subfamily of the plant GRAS family of putative
transcription regulators [10],[11] and have been revealed to function as negative
regulators of GA response in diverse plant species including Arabidopsis, barley,
rice and wheat etc [12]-[17]. There are five DELLAs in Arabidopsis, namely GAI,
RGA, RGL1, RGL2 and RGL3 [18],[19]. Genetic studies have shown that RGA,
RGL2 and RGL1 act synergistically in repressing petal and stamen development
and GA triggers the degradation of these DELLAs to promote floral development
[8], [9], [20]-[22]. Severe JA deficient mutant opr3 and JA-signaling mutant
coi1 also displayed retarded filament elongation, delayed anther dehiscence, and
reduced pollen viability. As a consequence, the opr3 and coi1 mutants are male
sterile [6],[23]. Application of exogenous JA can fully restore the stamen develop-
ment to opr3 [6].
It is intriguing to know whether GA-mediated and JA-mediated stamen devel-
opment are via two parallel pathways or in a hierarchical way to control stamen de-
velopment. In Arabidopsis, the known GA-response genes encoding transcription
factors involved in stamen development are GAMYBs (MYB33 and MYB65), a
subset of MYB genes [24]. GAMYB is the best characterized GA-regulated tran-
scription factor and was first identified in barley. GAMYB was found to bind
to the GA-response elements (GARE) in the promoter of the ?-amylase gene in
cereals [25],[26]. Genetic studies showed that Arabidopsis GAMYBs (MYB33
and MYB65) are essential to anther maturation but not for the elongation of sta-
men filament in Arabidopsis [24]. Previous studies have shown that GA regulates
GAMYB through DELLA protein SLN1 and SLR1 in barley and rice, respectively
[27],[28]. However, several reports failed to identify MYB33 and MYB65 as GA-
inducible genes in Arabidopsis and these two MYB genes are in fact regulated at
the post-transcriptional level by miRNA159 [24], [29]-[31]. Two recent reports
showed that three MYB genes (MYB21, MYB24 and MYB108) are responsive
to JA treatment in opr3 mutant and loss-of-function of MYB21 and MYB24 re-
sulted in a short stamen phenotype [32] whereas MYB108 is involved in stamen
and pollen maturation but not stamen filament elongation [33]. Interestingly,
in an expression profiling study, we identified several MYBs including MYB21,
MYB24, and MYB57 as DELLA-downregulated genes in ga1-3 flower buds [30].
This fact prompted us to investigate if there might be a cross-talk between GA
signaling and JA signaling during stamen development.
MYB21 and MYB24 have been shown to be expressed in all four whorls of
the flower [32],[34],[35]. In this report, we showed that MYB21, MYB24, and
MYB57 are down-regulated in the ga1-3 single mutant and the sterile quadruple
mutant ga1-3 gai-t6 rga-t2 rgl1-1 (loss-of-function of GAI, RGA, RGL1 three
DELLA genes but RGL2 is normal) but restored to wild type levels in the fertile
penta mutant ga1-3 gai-t6 rga-t2 rgl1-1 rgl2-1 (loss-of-function of GAI, RGA,
