Biology Reference
In-Depth Information
quadruple mutants but partially restored to a relatively high level in the penta
mutant, suggesting that GA is required for the expression of DAD1 to control
the production of JA via repression of DELLA proteins. In flowers of dad1 null
mutant, the JA levels were only 22% of that of WT [40], demonstrating that lim-
ited initial substrate generation by DAD1 reaction acts as a control point for JA
biosynthesis in flowers. Therefore, it is highly possible that reduced expression of
DAD1 in ga1-3 gai-t6 rga-t2 rgl1-1 or ga1-3 mutant may result in relative low JA
production. This hypothesis is strongly supported by the observation that JA con-
tent was greatly reduced in the young flower buds of the GA-deficient quadruple
mutant ga1-3 gai-t6 rga-t2 rgl1-1 (Q3 mutant). Furthermore, the fact that the
induction of DAD1 expression happens prior to the expression of MYBs by GA
in the ga1-3 gai-t6 rga-t2 rgl1-1 mutant strongly support our hypothesis that GA
may regulate the MYBs' expression via mobilization of the biosynthesis of JA. A
recent report showed that DAD1 expression is directly controlled by AGAMOUS
(AG) [45]. Interestingly, Yu et al reported that AG expression was downregulated
in the GA-deficient mutant ga1-3 and exogenous GA application promoted the
AG expression [20]. It will be interesting to study if there is a relationship among
DELLAs, AG and DAD1 in the future. High level of JA would induce the ex-
pression of the three MYB genes essential for stamen development. In addition to
DAD1, we also observed that expression of LOX1 was down-regulated in ga1-3
mutant and restored to the WT level in the penta mutant. On the other hand,
another JA biosynthesis gene AOC2 was up-regulated in ga1-3 mutant. These
observations suggested that GA may be one of the endogenous signal involved in
the regulation of JA biosynthesis genes.
Genetic studies have shown that MYB21, MYB24 and MYB57 are indis-
pensable for stamen development. The stamen phenotype of myb21-t1 myb24-
t1 myb57-t1 triple mutant is similar to that of JA-deficient mutants including
opr3 and dad1 mutants. Overexpression of MYB21 restored the stamen filament
elongation and fertility to the opr3 flowers, strongly suggesting that JA-mediated
stamen filament growth is mainly through the MYB pathway. Both ga1-3 single
and ga1-3 gai-t6 rga-t2 rgl1-1 quadruple mutants showed a more severe flower
phenotype than myb21-t1 myb24-t1 myb57-t1 triple mutant. The fact that ex-
pression of these MYBs in ga1-3 gai-t6 rga-t2 rgl1-1 plants was not enough to
rescue the mutant flower phenotype indicates that these MYBs are necessary but
not sufficient for GA-mediated floral development. These data also indicate that
modulation of JA pathway may be only one of the branches of GA function in
regulating stamen development.
Active cross-talk between different hormone signaling pathways have been
revealed in many developmental processes [46]. For example, it was reported that
auxin was necessary for GA-mediated Arabidopsis root growth by promoting GA-
dependent degradation of DELLA proteins [47]. In contrast, ethylene inhibits
