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DELLA proteins, and GA triggers DELLA degradation to promote stamen
development. Deficiency in plant hormone jasmonate (JA) also causes male
sterility. However, little is known about the relationship between GA and JA
in controlling stamen development. Here, we show that MYB21, MYB24,
and MYB57 are GA-dependent stamen-enriched genes. Loss-of-function of
two DELLAs RGA and RGL2 restores the expression of these three MYB
genes together with restoration of stamen filament growth in GA-deficient
plants. Genetic analysis showed that the myb21-t1 myb24-t1 myb57-t1 tri-
ple mutant confers a short stamen phenotype leading to male sterility. Fur-
ther genetic and molecular studies demonstrate that GA suppresses DELLAs
to mobilize the expression of the key JA biosynthesis gene DAD1, and this is
consistent with the observation that 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. We conclude that GA promotes JA biosynthesis to
control the expression of MYB21, MYB24, and MYB57. Therefore, we have
established a hierarchical relationship between GA and JA in that modula-
tion of JA pathway by GA is one of the prerequisites for GA to regulate the
normal stamen development in Arabidopsis.
Introduction
Arabidopsis flowers are organized into four concentric whorls of distinct organs
(sepals, petals, stamens and pistils). Stamens, the male reproductive organs of
flowering plants, form the third whorl. Processes of stamen filament elongation
and anthesis are precisely controlled so that they coincide with the pistil develop-
ment to determine the fertility [1]. Mutations that impair stamen development
such as filament elongation, pollen maturation or anther dehiscence will result
in male sterility [2],[3]. Many genes have been found to control stamen develop-
ment [4],[5]. Stamen development is also subjected to hormonal control. For ex-
ample, mutations affecting biosynthesis of two plant hormones gibberellin (GA)
(e.g ga1-3 mutation) and jasmonate (JA) (e.g opr3 mutation) both confer male
sterile phenotype due to failure of stamen filament elongation and of completion
of anthesis and anther dehiscence [6],[7].
A severe Arabidopsis GA-deficient mutant, ga1-3 exhibits retarded growth at
both vegetative and reproductive stages [7]. The development of floral organs, es-
pecially petals and stamens, is impaired in the ga1-3 mutant. Detailed anatomical
analysis showed that the male sterile phenotype of ga1-3 is due to the arrestment
of stamen filament cell elongation and failure of completion of anthesis [8]. Ap-
plication of exogenous GA can restore all the floral defects of ga1-3 [7]. Further
studies revealed that the arrested floral development in ga1-3 is mediated by DELLA
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