Agriculture Reference
In-Depth Information
Photoreversible regulation of ABA level during seed germination is mainly
mediated by phyB photoreceptor, since reduction of ABA level after red light pulse
was not observed in the
phyB
mutant (Seo et al.
2006
). When phyB is activated,
the ABA anabolic genes,
ABA
-
DEFICIENT 1
(
ABA1
),
AtNCED6
, and
AtNCED9
,
are down-regulated, whereas an ABA catabolic gene,
CYP707A2,
is induced (Kim
et al.
2008
; Oh et al.
2007
). This process is mainly controlled by the transcrip-
tion factor PIL5/PIF1, which negatively regulates phyB responses (Oh et al.
2007
,
other ref). PIL5 indirectly regulates the transcript levels of ABA metabolic genes,
including
ABA1
,
AtNCED6
,
AtNCED9,
and
CYP707A2
, and some GA metabolic
genes, such as
GA3ox1
and
GA2ox2
(Oh et al.
2007
). PIL5 can target
SOMNUS
(
SOM
) and directly activates its expression. In the
som
mutant, the expres-
sion levels of
ABA1
and
AtNCED6
are reduced, whereas the level of
CYP707A2
is increased compared to the wild type. Consistently, the
som
seeds contain low
amount of ABA and high levels of active GA
4
(Kim et al.
2008
). Therefore, PIL5
regulates endogenous ABA level largely through SOM. However, the question how
SOM regulates the expression of ABA metabolic genes is still unknown.
Besides, at the seed germination stage, the expression of
CYP707A2
is also
significantly up-regulated, whereas the transcript levels of
AtNCED
genes
(including
AtNCED2
,
3
,
5
, and
9
) are decreased by light during seedling de-etio-
lation (Charron et al.
2009
). Moreover, the expression patterns of tomato
LeZEP1
(encoding zeaxanthinepoxidase) and
LeNCED
are under circadian regulation
(Thompson et al.
2000
). In addition, the transcript level of
ABA-INSENSITIVE 3
(
ABI3
), encoding a key component in the ABA signaling pathway, is also affected
by mutations in
phyB
in Arabidopsis (Mazzella et al.
2005
), suggesting that phy-
tochrome regulates both the metabolic and signaling genes of ABA.
13.3.2 ABA Modulates the Expression
of Light-Responsive Genes
Light-harvesting chlorophyll a/b-binding proteins (LHCBs) are the apoproteins
of the photosystem II complex that absorb and transfer light energy. Expression
of these nuclear
LHCB
genes is tightly controlled by light, and therefore,
LHCB
s
serve as typical light-responsive genes (Johanningmeier
1988
; Johanningmeier
and Howell
1984
). Being a stress signal, ABA plays an important role in the regu-
lation of
LHCB
expression under environmental stress conditions. For example,
exogenously application of high concentrations of ABA inhibits
LHCB
expression
in various tissues, including tomato leaves, Arabidopsis seedlings,
Lemma gibba
cells, and developing seeds of soybean (Bartholomew et al.
1991
; Staneloni et al.
2008
; Weatherwax et al.
1996
; Chang and Walling
1991
). However, low level of
ABA enhances
LHCB1.2
transcript level in Arabidopsis seedlings, and
cab3
(
GmLHCB
) expression in soybean seeds (Voigt et al.
2010
; Chang and Walling
1991
). This is consistent with a recent study showing that physiological levels
of ABA enhance
LHCB
expression in Arabidopsis (Liu et al.
2013
). Liu and the
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