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Regulation of Dormancy in Barley by Blue Light and
After-Ripening: Effects on Abscisic Acid and
Gibberellin Metabolism
1[W]
Frank Gubler*, Trijntje Hughes, Peter Waterhouse, and John Jacobsen
Plant Industry, Commonwealth Scientific and Industrial Research Organisation, Canberra, Australian Capital
Territory 2601, Australia
White light strongly promotes dormancy in freshly harvested cereal grains, whereas dark and after-ripening have the opposite
effect. We have analyzed the interaction of light and after-ripening on abscisic acid (ABA) and gibberellin (GA) metabolism
genes and dormancy in barley (Hordeum vulgare ‘Betzes’). Analysis of gene expression in imbibed barley grains shows that
different ABA metabolism genes are targeted by white light and after-ripening. Of the genes examined, white light promotes
the expression of an ABA biosynthetic gene, HvNCED1, in embryos. Consistent with this result, enzyme-linked immunosor-
bent assays show that dormant grains imbibed under white light have higher embryo ABA content than grains imbibed in the
dark. After-ripening has no effect on expression of ABA biosynthesis genes, but promotes expression of an ABA catabolism
gene (HvABA8#OH1), a GA biosynthetic gene (HvGA3ox2), and a GA catabolic gene (HvGA2ox3) following imbibition. Blue
light mimics the effects of white light on germination, ABA levels, and expression of GA and ABA metabolism genes. Red and
far-red light have no effect on germination, ABA levels, or HvNCED1. RNA interference experiments in transgenic barley
plants support a role of HvABA8#OH1 in dormancy release. Reduced HvABA8#OH1 expression in transgenic HvABA8#OH1
RNAi grains results in higher levels of ABA and increased dormancy compared to nontransgenic grains.
Seed dormancy is a critical adaptive trait that is
present in many plant species. It is imposed during the
latter stages of embryo development and prevents
germination prior to the completion of seed maturation
(Baskin and Baskin, 1998). The persistence of dormancy
after seed maturity is variable among species, but, in
many species where it is retained, it offers adaptive
advantages, such as avoidance of temporary conditions
that do not support seedling establishment, and also
the formation of seed banks in soils that remain viable
for many years. In contrast to wild relatives, mo dern
cereals, such as barley (Hordeum vulgare) and wheat
(Triticum aestivum), have undergone strong selection by
breeders against dormancy to promote quick and uni-
form germination in successive rounds of breeding
(Simpson, 1990). As a consequence of the selective
pressure, modern barley and wheat cultivars have low
dormancy and are thus prone to preharvest sprouting.
In cereals and other seeds, it is well established
through physiological and genetic studies that abscisic
acid (ABA) plays an important role in the induction
and maintenance of dormancy (Finkelstein, 2004;
Gubler et al., 2005; Feurtado and Kermode, 2007).
For example, many viviparous mutants in maize (Zea
mays) have a defect in ABA biosynthesis or signaling,
indicating a role for ABA in preventing precocious
germination (McCarty, 1995). During seed develop-
ment, ABA content is low during the early stages,
increases rapidly, peaks around midmaturation, and
thereafter declines gradually during seed desiccation
(Bewley, 1997). In Arabidopsis (Arabidopsis thaliana)
and tobacco (Nicotiana tabacum), it is clear that the early
increase in ABA is derived from maternal tissue, but
the increase during the midmaturation stage is due to
ABA synthesized in the embryonic tissues (Karssen
et al., 1983; Frey et al., 2004). Genetic studies using
reciprocal crosses have ruled out the possibility that
maternal ABA is responsible for the induction of dor-
mancy. In Arabidopsis, analysis of mutants and ex-
pression patterns of ABA biosynthetic genes shows
that expression of the 9-cis-ep oxycarotenoid dioxy-
genase (NCED) genes plays a critical role in spatio-
temporal regulation of ABA synthesis in the seed
(Tan et al., 2003; Lefebvre et al., 2006). AtNCED6 and
AtNCED9 are the most highly expressed NCEDsin
developing seeds with AtNCED6 expressed specifi-
cally in the endosperm and AtNCED9 expressed both
in the endosperm and embryo until the latter stages of
maturation (Lefebvre et al., 2006). Functional analysis
of AtNCED6 and AtNCED9 mutants reveals that ABA
synthesized in the endosperm and possibly the
embryo during the early to midstages of maturation
contributes to dormancy (Lefebvre et al., 2006). The
double mutant exhibits reduced ABA content and
reduced dormancy. In contrast to NCEDs, many of the
1
This work was supported by the Grains Research and Devel-
opment Corporation and Commonwealth Scientific and Industrial
Research Organisation.
* Corresponding author; e-mail frank.gubler@csiro.au.
The author responsible for distribution of materials integral to the
findings presented in this article in accordance with the policy
described in the Instructions for Authors (www.plantphysiol.org) is:
Frank Gubler (frank.gubler@csiro.au).
[W]
The online version of this article contains Web-only data.
www.plantphysiol.org/cgi/doi/10.1104/pp.107.115469
886 Plant Physiology, June 2008, Vol . 147, pp. 886–896, www.plantphysiol.org Ó 2008 American Society of Plant Biologists
enzymes in ABA biosynthesis are encoded by single
genes and these appear to be ubiquitously expressed
during seed development. ABA catabolism has also
been shown to play an important role in dormancy
(Kushiro et al., 2004; Millar et al., 2006; Okamoto et al.,
2006). Single and double mutants of the Arabidopsis
ABA 8#-hydroxylase (ABA8#OH)genefamily,CYP707A2
and CYP707A3, are more dormant and have increased
ABA content, whereas over expression results in de-
creased ABA content and reduced dormancy.
Cross talk between ABA and other hormones, such
as GA and ethylene, is likely to be important in
dormancy regulation (Feurtado and Kermode, 2007;
Hilhorst, 2007). Application of GA can overcome seed
dormancy in many species, including barley, suggest-
ing that the ABA to GA ratio may be critical for
dormancy mai ntenance (e.g. Jacobsen et al., 2002).
Recent evidence indicates that ABA may act, at least in
part, to inhibit germination by suppressing GA bio-
synthesis in Arabidopsis seeds (Seo et al., 2006). Ex-
pression levels of GA biosynthetic genes (AtGA3ox 1
and AtGA3ox2) were elevated in seeds of an ABA-
deficient mutant, aba2-2, compared to wild type. It is
possible that the loss of dormancy in the aba2-2 mu tant
is due to loss of repression of GA biosynthesis. In
contrast, ethylene appears to act at least in part by
regulating ABA biosynthesis and signaling. Loss-of-
function mutations in the ethylene signaling pathway,
such as ein2 and etr1, result in higher ABA content and
higher dormancy (Chiwocha et al., 2003).
Dormancy in barley and other cereals can be broken
by chan ges in environmental conditions, such as tem-
perature, light , oxygen, and nutrients, and also by
after-ripening (Simpson, 1990; Jacobsen et al., 2002;
Benech-Arnold et al., 2006). Evidence so far from hor-
mone and gene expression studies of barley indicate
that dormancy release is due at least in part to changes
in ABA metabolism and possibly ABA signaling
(Jacobsen et al., 2002; Benech-Arnold et al., 2006;
Chono et al., 2006; Millar et al., 2006). Although em-
bryos from dry dormant (D) and after-ripened (AR)
barley grains contain similar levels of ABA, after 12-h
imbibition ABA content in AR grains was 25% to 50%
lower than that measured in embryos of imbibed D
grains (Millar et al., 2006). The decline in ABA content
in AR grains appeared to be due to conversion to
phaseic acid, which is less active as a germination
inhibitor (Jacobsen et al., 2002). The increase in ABA
catabolism in embryos of imbibed AR grains corre-
lated with increased gene expression of HvABA8#OH1
compared to embryos of D grains (Chono et al., 2006;
Millar et al., 2006). Functional analyses in yeast (Sac-
charomyces cerevisiae) demonstrated that HvABA8#OH1
converted ABA to phaseic acid. Similar changes in
ABA8#OH expression have been reported in AR Arab-
idopsis seeds compared to D seeds (Millar et al., 2006;
Finch-Savage et al., 2007), indicating that ABA8#OH
may function as a key regulator of dormancy release in
seeds. Reported changes in embryo ABA sensitivity
following dormancy loss in various cereals (Walker-
Simmons, 1987; Wang et al., 1995; Benech-Arnold et al.,
1999; Corbineau et al., 2000) can be explained at least
in part by increased ABA catabolism (Benech-Arnold
et al., 2006). This is further supported by the demon-
stration that ABA8#OH overexpression in Phaseolus
seeds resulted in decreased sensitivity to ABA (Yang
and Zeevaart, 2006).
There are a number of reports of light regulation of
dormancy in cereals (Simpson, 1990); however, our
understanding of the mechanisms involved remains
poor. Typically, freshly harvested barley grains have
little or no dormancy when imbibed in the dark, but
germination is strongly inhibited by white light. The
light promotion of dormancy is lost during after-
ripening with AR grains germinating equall y well in
darkness or white light (Grahl and Thielebein, 1959;
Burger, 1965; Grahl, 1965; Chaussat and Zoppolo, 1983;
Jacobsen et al., 2002). Analysis of D grains imbibed for
24 h showed that embryo ABA content was 30% to
50% lower in grains imbibed in the dark compared to
grains imbibed in white light (Jacobsen et al., 2002).
Although dark imbibition and after-ripening both
promote dormancy release and loss of ABA, it is not
yet known whether they act independently or through
a common molec ular mechanism. In this study, we
examine the effects of light and dark on expression of
ABA and GA metabolism genes in D and AR barley
grains during imbibition. We present evidence that
white light promotion of dormancy is caused by
blue light through induction of HvNCED1 expression.
After-ripening appears to override the blue light pro-
motion of HvNCED1 expression by activating ABA
catabolism and GA biosynthesis genes in embryos of
imbibing barley grains. In addition, we use transgenic
barley lines to examine the role of HvABA8#OH1 in
dormancy release. We show that reduced HvABA8#OH1
expression results in increased dormancy, but that it
appears to have little effect on after-ripeni ng.
RESULTS
Light Regulation of Dormancy and ABA Content
in Barley
Barley plants grown under cool conditions produce
grains that are highly D when imbibed under contin-
uous white light and require extended periods of after-
ripening for dormancy to decay (Jacobsen et al., 2002;
Millar et al., 2006). Germination kinetics for D and AR
grains of Betzes barley (a huske d variety) from the
same harvest showed that 100% of the AR grains
germinated after 3-d imbibition in continuous white
light compared with 5% germination for D grains
imbibed under similar conditions (Fig. 1A). When the
D and AR barley grains are imbibed in the dark, over
95% of grains germinated by 3 d, indicating that white
light is a promoter of dormancy in barley and that
imbibition in the dark breaks dormancy.
To examine the effect of white light and dark on
ABA content, ABA was extracted from embryos of D
Blue Light and Dormancy in Barley
Plant Physiol. Vol. 147, 2008 887
and AR grains imbibed in continuous light or dark and
quantified by a competitive ELISA assay (Fig. 1B).
ABA content in embryos of D grains imbibed under
continuous white light declined from 3.7 ng per em-
bryo to 2.2 ng per embryo during the first 12-h imbi-
bition and then the ABA content stabilized before
increasing over the next 12 h to 2.9 ng per embryo. In
contrast, ABA content of D grains imbibed in the dark
and AR grains imbibed in continuous white light or
dark resulted in a steady decline from 3.7 ng ABA per
embryo in dry embryos to 1.1 ng ABA per embryo
after 24 h. These results are consistent with an earlier
study that showed that release of dormancy in barley
grains by after-ripening and dark imbibition was
associated with a steady decline in embryo ABA
content and a corresponding increase in phaseic acid
during imbibition (Jacobsen et al., 2002).
White Light and After-Ripening Target Different ABA
and GA Metabolism Genes
We have previously shown that the decline in ABA
content in imbibing AR grains is likely to be due to
increased expression of an ABA catabolic enzyme,
HvABA8#OH1, and not related to any changes in the ex-
pression of HvNCEDs encoding for ABA biosynthetic
enzymes (Millar et al., 2006). To determine whether the
decrease in embryo ABA content associated with dark
release of dormancy is also due to differential expres-
sion of HvABA8#OH1, we used quantitative reverse
transcription (RT)-PCR to quantify expression levels of
genes encoding HvABA8#OHs and HvNCEDs in em-
bryos of D and AR grains imbibed under continuous
white light and dark (Fig. 2, A–C). In barley, there are
two ABA8#OH genes, HvABA8#OH1 and HvABA8#OH2,
and two NCED genes, HvNCED1 and HvNCED2 (Millar
et al., 2006).
Both HvNCED genes were expressed in embryos of
imbibed D and AR grain, but they showed different
expression patterns in response to light and after-
ripening. Continuous white light strongly induced
HvNCED1 expression in embryos of D and AR grain
by 6-h imbibition and remained high up to 24-h
imbibition compared with grains that were imbibed
in the dark (Fig. 2A). After-ripening had little effect on
HvNCED1 expression in grains, whereas HvNCED2
expression was higher in embryos of AR grain after
6-h imbibition compared with D grain (Fig. 2B). White
light had little effect on HvNCED2 expression in
embryos of imbibing grains.
As previously shown, HvABA8#OH1 expression is
over 10-fold higher than HvABA8#OH2 in imbibing
grains (Millar et al., 2006). HvABA8#OH1 expression is
strongly promoted by after-ripening in imbibing
grains, but white light or dark had little effect on
HvABA8#OH1 expression in embryos of D and AR
grains. In imbibing grains, HvABA8#OH2 expression
was very low in all treatments (data not shown).
It is clear from studies of Arabidopsis that light plays
a major role in coordinating ABA and GA metabolism
in imbibed seeds. In imbibed seeds, red light promotes
germination via phytochrome B by inhibiting NCED
expression and promoting ABA catabolism and GA
synthesis (Seo et al., 2006). To determine whether light/
dark regulation of dormancy in barley also acts through
coordinated changes in ABA and GA metabolism, we
investigated whether GA 3-oxidases, which encode
enzymes that catalyze the conversion GA
20
to active
GAs, and GA 2-oxidases, wh ich encode GA deactiva-
tion enzymes, are regulated by white light in D and AR
grains. Two HvGA3ox and four HvGA2ox genes have
been identified in barley (Spielmeyer et al., 2004; Dewi,
2006), but only HvGA3ox2 and HvGA2ox3 were ex-
pressed at high levels in embryos during grain imbibi-
tion (Fig. 2, D–E). HvGA3ox2 expression increased
rapidly and continued to increase up to 24-h imbibition
in AR grains in both light and dark compared to D
grains, where expression remained low over the 24-h
imbibition period. White light had little effect on
HvGA3ox1 expression. Expression of HvGA2ox3 in-
creased in all treatments, but the increase was more
rapid in AR grains and preceded the increase in
HvGA3ox2 expression.
Figure 1. Effect of white light and dark on germination and ABA
content of D and AR barley grains during imbibition. A, Germination of
D and AR barley grains irradiated with continuous white light or in
dark: Measurements are averages of four replicates with error bars
representing the
SE of the mean. B, Changes in embryo ABA content in
D and AR grains imbibed under continuous white light or dark.
Measurements are averages of three replicates with error bars repre-
senting the
SE of the mean.
Gubler et al.
888 Plant Physiol. Vol. 147, 2008
Taken together, these results suggest that white light
stimulation of HvNCED1 expression plays a major role
in maintenance of dormancy and high ABA content in
embryos of D grains. In contrast, after-ripening coun-
teracts the white light effect by promotion of ABA
catabolism via increased HvABA8#OH1 expression
and promotion of GA synthesis through increased
HvGA3ox2 expression.
Blue Light Regulates HvNCED1 Expression and ABA
Content in D Embryos
To investigate the relationship between the light
spectrum and barley grain dormancy, D grains were
imbibed under continuous blue, red, far-red, or white
light or dark (Fig. 3A). The results show that blue light
was as effective as white light in maintaining dormancy
in imbibing D grains with ,10% of the grains germi-
nating by 3-d imbibition (compare Fig. 3Awith Fig. 1A).
Blue light did not have any effect on the germination of
AR grains with over 90% of the AR grains germinating
after 3-d imbibition under constant blue light. In con-
trast to blue light, germination of D grains imbibed
under continuous red or far-red light was similar to
dark-imbibed grains with over 80% D grains germinat-
ing after 3-d imbibition. To test whether higher fluences
of far-red had any effect on germination of D grains,
huskless D grains were imbibed under continuous
273 mmm
22
s
21
far-red light. The germination results
for the high-intensity far-red light treatment were sim-
ilar to those from dark-imbibed grains with germina-
tion over 95% after 4-d imbibition (data not shown). It is
clear from these results that blue light promotes dor-
mancy in freshly harveste d barley grains and that red
and far-red light have no effect.
We investigated whether blue light promotion of
dormancy correlated also with increases in ABA con-
tent similar to that found in white light treatments. We
measured ABA levels in embryos of D and AR grains
imbibed under continuous blue light, red, far-red, and
white light, and dark after 24-h imbibition. As shown
in Figure 3B, embryo ABA content was 4-fold higher in
Figure 2. Effect of white light and dark on
expression of ABA and GA metabolism
genes in embryos of D and AR barley
grains during imbibition. Measurements
are averages of three replicates with error
bars representing the
SE of the mean. A,
HvNCED1.B,HvNCED2.C,HvABA8#OH1.
D, HvGA3ox2.E,HvGA2ox3.
Blue Light and Dormancy in Barley
Plant Physiol. Vol. 147, 2008 889
blue light-treated D grains compared to dark, red, and
far-red light-treated grains. The blue promotion of
ABA content was similar to white light treatments. In
AR grains, ABA content remained low regardless of
the light treatment.
To test whether blue light regulation of dormancy
and ABA content is associated with increased
HvNCED1 expression, we monitored expression of
genes encoding enzymes of ABA and GA metabolism
in response to variou s light treatments after 12-h
imbibition (HvNCED1, HvABA8#OH1, HvGA2ox3)
and 24-h imbibition (HvGA3ox2). As shown in Figure
4A, HvNCED1 expression was induced 2- to 3-fold in
D and AR grains imbibed under blue or white light
compared with other light treatments. The response to
blue light was not detected in ABA and GA metabo-
lism genes (Fig. 4, B–D) that had been shown to be
regulated by after-ripening (see Fig. 2).
Reduced HvABA8#OH1 Expression in Barley Grains
Results in Increased Dormancy
It has been previously shown that reduction in
ABA8#OH1 expression in Arabidopsis is associated
with increased ABA content in seeds and increased
dormancy (Kushiro et al., 2004; Millar et al., 2006;
Okamoto et al., 2006). Analysis of ABA metabolism
and gene expression in barley grains indicates that
after-ripening alleviates dormancy by increasing the
expression of a small group of genes, including
HvABA8#OH1. To study the role of HvABA 8 #OH1 in
dormancy in barley, we constructed a hairpin RNA
interference (RNAi) construct under the control of
the maize ubiquitin promoter designed to cleave
HvABA8#OH1 transcripts (designated binary vector
pWBVec8-Ubi:HvABA8#OH1RNAi). The vector was
transformed in Golden Promise barley and, from
more than 30 trans genic lines, three single-locus lines
were selected for further analysis. Grains from homo-
zygous transgenic and null segregant plants from each
of the three lines, together with grains from wild-type
plants, were analyzed for HvABA8#OH1 expression,
ABA content, and dormancy.
HvABA8#OH1 gene exp ression was monitored by
RNA-blot analysis of embryos from RNAi and null
grains imbibed for 18 h in the dark (Fig. 5A). In all
three lines, HvABA8#OH1 transcript levels were down-
regulated in embryos of RNAi grain compared to
embryos from null and wild-type grains. Quantitative
RT-PCR analysis failed to detect any effect of the RNAi
construct on HvABA8#OH2 expression, demonstrating
the specificity of the RNAi construct (data not shown).
Quantitation of ABA content in dry D grains demon-
strated that reduction of HvABA8#OH1 transcript level
in RNAi grains correlated with increased ABA content
in embryo and embryoless half-grains compared to
null and wild-type grains (Fig. 5B). Embryos from
RNAi grains from the three lines had ABA levels at
least twice as high as the corresponding null segregant
embryos. The effect of the RNAi was also detected on
ABA content of endosperm from dry grains, but the
effect was small (Fig. 5C). Endosperm tissue from
RNAi grains had higher ABA content compared to
endosperm from null segregant grains.
To determine the effect of reduced HvABA8#OH1
expression on grain dormancy, freshly harvested and
AR grains from wild-type, RNAi, and null plants,
together with grains from wild-type plants, were im-
bibed for 3 d under continuous white light or dark-
ness. As shown in Figure 6, freshly harvested RNAi
grains in the three lines studied were more D when
dark imbibed. Line 26 had the highest level of ABA in
dry embryos and had the highest dormancy with only
21% of the RNAi grains germinated compared with
over 75% of the null segregant grains germinated. To
test whether RNAi grains had a longer after-ripening
period compared to null segregants, grains were AR
for 1 month at 37°C and assayed for dormancy (Fig. 6,
B, D, F, and H). AR grains from all the lines showed
almost no dormancy in the dark, with germination
levels of 95% and higher. When imbibed under con-
tinuous wh ite light, AR grains from RNAi and null
Figure 3. Effect of light quality on dormancy and embryo ABA content in barley grains during imbibition. A, Germination of D
barley grains in dark and irradiated with continuous far-red, red, and blue light. Measurements are averages of four replicates
with error bars representing the
SE of the mean. B, ABA content in embryos from D and AR grains imbibed for 24 h in dark and
under continuous blue, red, far-red, and white light. Measurements are averages of three replicates with error bars representing
the SE of the mean.
Gubler et al.
890 Plant Physiol. Vol. 147, 2008
plants were partially D with lower germination com-
pared to the dark-imbibed grains.
To investigate the effect of after-ripening and light
on the ABA content in embryos of the RNAi grains,
null and transgenic grain from line 26 and wil d-type
plants was AR for 1 month and ABA content measured
in embryos of dry and imbibed grains (Fig. 7). After-
ripening had no effect on ABA content of embryos
from dry RNAi grains (Fig. 7C) similar to what had
been shown in Betzes barley (Fig. 1B). Following 24-h
imbibition, embryo ABA content remained high in the
dormant RNAi grains imbibed in the light, but a small
decrease was observed in dark-imbibed grains. After-
ripening had a major effect on embryo ABA content in
imbibed RNAi grains, with a decrease .50% after 24-h
imbibition both in the light and dark. Although the
ABA content in the null and wild-type grains (Fig. 7, A
and B) was overall much lower, decreases in embryo
ABA content were also detected in AR grains imbibed
in the light and dark compared with D grains imbibed
under similar conditions. We do not know whether
this decrease in ABA content in the RNAi grains is
catalyzed by any residual HvABA8#OH1 enzyme or
due to alternative ABA catabolic or conjugation en-
zymes or by an alternative pathway.
DISCUSSION
We have shown that freshly harve sted grains of
Betzes barley grown under the conditions described
here are highly D when imbibed under white light, but
their dormancy can be rapidly alleviated by imbibing
the grains in the dark. The white light-induced dor-
mancy can also be broken by after-ripening the dry
grains at 37°C for 4 months, by which time the grains
germinated equally well in the light or dark. These
results are consistent wi th earlier studies that showed
that white light promoted dormancy in barley and a
number of other cereals (Grahl and Thielebein, 1959;
Burger, 1965; Grahl, 1965; Chaussat and Zoppolo,
1983). In cases wh ere the grain dormancy is deep,
dark imbibition may not be effective in releasing
dormancy until the grains have been AR for a short
time. Grahl (1965) showed that barley grains from
cultivars with high dormancy were highly D after
1 week after-ripening when imbibed in the dark. After
5 to 13 weeks after-ripening at 20°C, the barley grains
germinated in the dark, but remained D under con-
tinuous white light. After 38 weeks after-ripening, the
grains germinated close to 100% in the dark or light. It
is clear from these results that the light response varies
with the depth of dormancy and that further work is
required to understand the molecular changes associ-
Figure 4. Effect of light quality on expression of ABA and GA metab-
olism genes in embryos of D and AR grains during imbibition. Gene
expression was measured in embryos from grains that had been
imbibed for 24 h under continuous blue, red, far-red, and white light
and dark. Measurements are averages of three replicates with error bars
representing the SE of the mean. A, HvNCED1. B, HvABA89OH1. C,
HvGA3ox2. D, HvGA2ox3.
Blue Light and Dormancy in Barley
Plant Physiol. Vol. 147, 2008 891
ated with the after-ripening that accompany light
repression of germination in barley and other cereals.
Analysis of ABA content and expression of ABA
metabolism genes indicates that the white light effect
on dormancy is correlated with increased ABA content
and HvNCED1 expression in the embryos of imbibing
grains compared to dark-im bibed grains. In contrast,
no differences were observed in the expression of
HvNCED2, HVABA8#OH1, and HvABA8#OH2 in em-
bryos of D grains imbibed in white light or dark,
indicating the white light effect on ABA content may
be specific to HvNCED1. Embryo ABA content of D
grains declined from 3.7 ng/embryo in dry seeds to 1.1
ng/embryo after 24-h imbibition in the light compared
with 2.9 ng/embryo when imbibed in the dark in
agreement with an earlier study (Jacobsen et al., 2002).
Our results indicate that the maintenance of a high
ABA content in embryos in light-imbibed grains may
be due to increased ABA biosynthesis rather than a
decrease in ABA catabolism. HvNCED1 expression
increased in response to white light and it reached
maximal expression after 18-h imbibition, which cor-
related with stabilization of ABA content in embryos
compared to the further decline observed in dark-
imbibed grains. The increase in ABA content as a
result of increased HvNCED1 expression may facilitate
white light-imposed dormancy, although it cannot be
ruled out that the effect may also be due to a reduction
in ABA sensitivity in response to white light.
Light stimulation of NCED mRNA expression has
also been observed in tomato (Lycopersicon esculentum)
leaves. Analysis of plants grown in a 12-h-light/12-h-
dark cycle showed a diurnal pattern of LeNCED1
expression in leaves with the peak of expression at
the end of the light period (Thom pson et al., 2000).
Switching from light/dark cycling to continuous dark
resulted in LeNCED1 expression dropping to low
levels, indicating that the diurnal cycli ng was due to
positive regulation by light and not due to a circadian
oscillator. Although there are no reports in plants that
ABA content is regulated diurnally, diurnal redistri-
bution between the chloroplast and cytosol has been
reported (Slovik and Hartung, 1992). Interestingly,
light stimulation of ABA biosynthesis has also been
observed in the fungus Botrytis cinerea (Marumo et al.,
1982) and hydroids, members of the animal phylum
Eumetazoa (Puce et al., 2004). The light -stimulated
increase in ABA content during hydroid regeneration
can be blocked by fluridone, an ABA biosynthesis
inhibitor.
Dormancy and germination in cereals has been shown
to be dependent on light spectral quality (Chaussat
and Zoppolo, 1983; Simpson, 1990). Our results show
that the white light repression of germination is at least
in part caused by the blue light component. By using
narrow wavelength light-emitting diodes (LEDs), we
showed that blue light was as effective as white light in
promoting dormancy and that red light and far-red
light had no effect compared to dark-imbibed grains.
Our data are in agreement with an earlier study, which
Figure 5. Effect of RNAi-directed silencing of HvABA8#OH1 on gene
expression and ABA content of barley grains. A, RNA-blot analysis of
HvABA8#OH1 expression in embryos of wild-type Golden Promise
(GP), and null (N) and transgenic (T) grains from HvABA8#OH1 RNAi
lines 6, 18, and 26 imbibed for 18 h. The HvABA8#OH1 transcripts
have a slower mobility (black arrow) than the RNAi transcripts (white
arrow). B, ABA content in embryos from dry wild-type (GP), and null
(N) and transgenic (T) grains from HvABA8#OH1 RNAi lines 6, 18, and
26. Measurements are averages of three biological replicates with error
bars representing the
SE of the mean. C, ABA content in endosperm half-
grains from dry wild-type (GP), and null (N) and transgenic (T) grains
from HvABA8#OH1 RNAi lines 6, 18, and 26. Measurements are
averages of three biological replicates with error bars representing the
SE of the mean.
Gubler et al.
892 Plant Physiol. Vol. 147, 2008
investigated the effect of the light spectrum from a
1,600-W xenon arc light on barley dormancy (Chaussat
and Zoppolo, 1983). Only the blue region of the
spectrum (435–455 nm) inhibited germination with
longer wavelengths up to 700 nm having no effect. Our
data extend these obse rvations and indicate that blue
light is also responsible for the white light-induced
increase in HvNCED1 expression and ABA content in
embryos of imbibed grains. Similarly, the component
of white light that stimulated ABA content in myce-
lium of B. cinerea (see above; Marumo et al., 1982) has
been shown to be blue light. Studies of wild oats
(Avena fatua) have shown photoreversible germination
by red and far-red light, indicating the presence of
active phytochromes in the embryo (for review, see
Simpson, 1990). The absence of any detectable effect
of red and far-red light on barley germination and
Figure 6. Effect of after-ripening on dormancy release of wild-type and
HvABA8#OH1 RNAi transgenic grains. D (A, C, E, and G) and AR (B, D,
F, and H) grains from wild-type (A and B) and RNAi lines (C–H) were
imbibed for 4 d under continuous white light or dark. The AR grains had
been AR for 1 month at 37° C. Measurements are averages of four
replicates with error bars representing the
SE of the mean. A and B,
Wild-type Golden Promise grains (GP). C and D, Null (N) and trans-
genic (T) grains from line 6. E and F, Null (N) and transgenic (T) grains
from line 18. G and H, Null (N) and transgenic (T) grains from line 26.
Figure 7. Effect of after-ripening and light on ABA content of D and
1-month AR grains from wild-type and HvABA8#OH1 RNAi plants. The
embryos were isolated from dry grains (0 h) and grains imbibed for 24 h
under white light (L) and dark (Dk). ABA measurements are averages of
four biological replicates with error bars representing the
SE of the
mean. A, Golden Promise (GP). B, Null grains from HvABA8#OH1
RNAi line 26. C, Transgenic grains from HvABA8#OH1 RNAi line 26.
Blue Light and Dormancy in Barley
Plant Physiol. Vol. 147, 2008 893
HvNCED1 expression indicates that phytochromes
play no part in dormancy in barley. We note that the
red/far-red reversibility of germination in wild oats
was demonstrated using grains that were dehulled
(Hou and Simpson, 1992), but we were unable to
observe reversibility in huskless Betzes barley.
Plants possess several classes of photoreceptors that
absorb in the blue region of the spectrum. Photo-
tropins, cryptochromes, and the ZTL/FKF/LPK2 re-
ceptors are classified as blue light receptors, but it is
well known that the red/far-red light receptors, phy-
tochromes, also absorb and respond to the blue region
of the spectrum (Banerjee and Batschauer, 2005; Wang,
2005). Although we do not know which photoreceptor
is involved in the blue light regulation of ABA content
and HvNCED1 expression in barley embryos, recent
progress made in the understanding of photoregula-
tion of ABA metabolism in Arabidopsis seeds may pro-
vide useful insights (O h et al., 2006; Seo et al., 2006). In
Arabidopsis seeds, AtNCED6 expression is regulated
in a red/far-red photoreversible manner in imbibed
seeds and this response was absent in the phyB mutant
(Seo et al., 2006). The change in ABA content corre-
lated positively with the photoreversible red/far-red
light regulation of AtNCED6. AtNCED9 may be regu-
lated in a similar manner (Oh et al., 2006). In con-
trast, expression of other ABA biosynthetic genes did
not show any response to red/far-red light. PIL5, a
phytochrome-interacting protein, has been sho wn to
mediate the phytochrome B regulation of AtNCED6,
AtNCED9, and GA biosynthetic genes (Oh et al., 2006).
Our results indicate that after-ripening overrides the
white light-induced dormancy by enhancing ABA
catabolism and GA biosynthesis in imbibed grains. It
has been shown previously that after-ripening in-
creases the expression of HvABA8#OH1 in embryo s
by more than 2-fold compared to embryos from D
grains imbibed in the light (Chono et al., 2006; Millar
et al., 2006). The decrease in ABA content observed in
embryos of AR grains imbibed in white light is con-
sistent with the increase in HvABA8#OH1 exp ression
and indicates that increases in the expression of
HvNCED1 and HvNCED2 are not able to reverse the
decline in ABA content. In addition to HvABA8#OH1,
expressions of HvGA3ox2 and HvGA2ox3 were also
higher in AR grains compared to D grains imbibed in
the light or dark. Analysis of expression kinetics
reveals that HvABA8#OH1 expression began to in-
crease after 6-h imbibition, whereas HvGA3ox2 began
to increase after 12-h imbibition. There is evidence to
support the proposal that HvGA3ox2 expression in
barley may be repressed by ABA in D grains in a
similar way to that observed in Arabidopsis seeds (Seo
et al., 2006). Also, imbibition of barley grains in the
presence of 5 m
M ABA reduced HvGA3ox2 expression
by 45% compared to control treatments (Dewi, 2006).
In addition, we found that expression of HvGA3ox2
was higher in D grains imbibed in the dark compared
to grains imbibed in the light, correlating positively
with changes in ABA content. Nevertheless, the de-
cline in ABA content in the dark-im bibed D grains did
not correlate wi th the large increase in HvGA3ox2
expression observed in AR grains. These results sug-
gest that changes in ABA content in AR grains may
only be partly responsible for the dramatic increase in
HvGA3ox2 expression in AR grains compared to D
grains.
We have used an RNAi approach to study the role of
HvABA8#OH1 in barley dormancy. RNAi silencing of
HvABA8#OH1 expression resulted in approximately
2-fold higher ABA content in embryos of transgenic
grains compared to null segregant grains. The increase
in ABA content correlated positively with increased
depth of dorman cy associated with dark-imbibed
RNAi grains that had not been AR. Interestingly, de-
creased HvABA8#OH1 expression in the RNAi grains
had only a small effect on after-ripening time com-
pared to wild-type and null grai ns, indicating that
increased ABA catabolism is not solely responsible for
loss of dormancy by after-ripening. It has been re-
ported that AR grains have reduced sensitivity to ABA
compared to D grains, sugge sting that after-ripening
regulates ABA signaling components in addition to
ABA metabolism (Walker-Simmons, 1987; Corbineau
et al., 2000). A decrease in ABA sensitivity following
after-ripening may explain in part the high germina-
bility of AR HvABA8#OH1 RNAi grains even though
the embryos have a high ABA content compared to
wild-type grains. The expression of VP1, a member of
the ABI3 family of transcription factors, has been
positively correlated with the level of dormancy in
wild oats (Jones et al., 1997) and sorghum (Sorghum
bicolor; Carrari et al., 2001), but we could not detect any
differences in HvVP1 expression in D and AR barley
grains (F. Gubler, unpublished data). Transcriptome
analysis of imbibed Arabidopsis seeds has shown that
LIPID PHOSPHATE PHOSPHATASE2 (LPP2), which
negatively regulates ABA signaling, is more highly
expressed in AR seeds compared to D seeds, indicat-
ing support that loss of dormancy by after-ripening is
associated with changes in ABA signaling and de-
creased ABA sensitivity (Carrera et al., 2008).
In conclusion, our results show that manipulation of
HvABA8#OH1 provides an attractive opportun ity to
increase grain dormancy without unduly increasing
after-ripening time. This is particularly attractive in
cereal grains, which are prone to preharvest sprouting,
such as wheat ( Triticum aestivum). Alternative strate-
gies that increase after-ripening time may result in
lengthy delays before replanting and thus disadvan-
tage breeders and farmers.
MATERIALS AND METHODS
Plant Material
Barley (Hordeum vulgare ‘Betzes’) plants were grown in naturally lit phy-
totron glasshouses with air temperature set at 17°C/9°C day/night cycle as
previously described (Jacobsen et al., 2002; Millar et al., 2006). Heads were
harvested at maturity, dried for 7 d, and threshed by hand to prevent damage
Gubler et al.
894 Plant Physiol. Vol. 147, 2008
to the husk and embryo. One-half of the threshed grains was stored at 220°C
to preserve dormancy and the other half was incubated at 37°C for 4 months to
after-ripen.
Germination Assays
For germination assays, quadruplicate sets of 20 grains were placed on
9-cm plastic petri dishes containin g two 9-cm Whatman Number 1 filter
papers and 6 mL of water. The plates were sealed with parafilm and incub ated
at 20°C under continuous white light at 130 mmol m
22
s
21
(Philips TLD 36W/
865 fluorescent tubes) or wrapped in two layers of aluminum foil for darkness.
Grains with emerged coleorhizae were scored as germinated.
Imbibitions under different light quality regimes were performed using
monochromatic LEDs (for spectra, see Supplemental Fig. S1) in a light-tight
box with temperature maintained at 20°C 6 0.5°C. Blue light was provided by
NSPB510S-W/ST LEDs (Nichia Chemical Pty), far-red by L735-03AU LEDs,
and red by 660-04U LEDs (both from EPITEX). White and blue light intensities
were measured with an Apogee QMSS Quantum Meter, and red and far-red
intensities with a Licor LI-1800 spectroradiometer. Intensities of blue, red, and
far-red light were 26, 8, and 63 mmol m
22
s
21
, respectively.
Gene Expression Analyses
RNA was prepared from embryos isolated from dry and imbibed grains
using a method adapted from the hexadecyltrimethylammonium procedure
described by Chang et al. (1993). Twenty embryos were ground in liquid
nitrogen and the powder added to 1.8 mL of hot RNA lysis buffer containing
2% hexadecyltrimethylammonium (w/v). Following purification, the RNA
was used for RNA-blot analysis or quantitative real-time PCR. For RNA-blot
analysis, 20 mg of RNA was fractionated on 1.2% agarose gel containing
formaldehyde and blotted onto a nylon membrane. The blot was hybridized
with
32
P-labeled dUTP HvABA8#OH1 riboprobes. The riboprobes were tran-
scribed from PCR templates that spanned the 5# and 3# regions of the
HvABA8#OH1 cDNA (578–859 bp and 1,144–1,311 bp; accession no.
DQ145932).
For quantitative real-time PCR analysis, 50 mg RNA was treated with
RNAse-free DNAse (Promega) and further purified on a Qiagen RNeasy
column (Qiagen). Two micrograms of DNAse-treated RNA were used to
synthesize cDNA using SuperScript III (Invitrogen Life Sciences). The result-
ing cDNA was diluted 50-fold and 10 mL were used in 20-mL quantitative PCR
reactions with Platinum Taq (Invitrogen Life Sciences) and SYBR Green
(Invitrogen). Specific primers used were: HvABA8#OH1,5#-GGACACTGA-
CGGATGGAGAAC-3#,5#-CCATGACCTTCACCCGCAAG-3# (Millar et al.,
2006); HvABA8#OH2,5#-GAGATGCTGGTGCTCATC-3#,5#-ACGTCGTCGC-
TCGATCCAAC-3# (Millar et al., 2006); HvNCED1,5#-CCAGCACTAATCG-
ATTCC-3#,5#-CCAGCACTAATCGATTCC-3# (Millar et al., 2006); HvNCED2,
5#-CATGGAAAGAGGAAGTTGC-3#,5#-GAAGCAAGTGTGAGCTAAC-3#
(Millar et al., 2006); HvGA3ox1 (Spielmeyer et al., 2004); HvGA3ox2 (Spielmeyer
et al., 2004); HvGA2ox1 (EST sequence CB76549); HvGA2ox3 (EST sequence
BU972476); HvGA2ox4,5#-TCCTAGCCAGCCAGCAACT-3#,5#-GGCATGGA-
CAGGACACAGA-3# (Dewi, 2006); HvGA2ox5,5#-ACAAGAGCAGCACC-
CACAA-3#,5
#-AACCACAGGACCAGGACGA-3# (Dewi 2006); and HvActin,
5# -GCCGTGCTTTCCCTCTATG-3#,5#-GCTTCTCCTTGATGTCCCTTA-3#
(Trevaskis et al., 2006). Reactions were run on a Rotor-gene 3000A real-time
PCR machine (Corbett Research) and data analyzed with Rotor-gene software.
The expression of Actin (AY145451) was used as a control to normalize gene
expression in the various treatments. Three replicates were carried out for
each experiment. All experiments showed similar trends in separate biological
repeats.
ABA Measurements
The content of isolated embryos and endosperm half-grains was measured
using a Phytodetek Competitive ELISA kit (Agdia). Ten embryos were
isolated from dry and imbibed barley grains and frozen on dry ice. The
remaining half-grains, which included the starchy endosperm, aleurone,
glumes, and seed coat, were cut into small pieces and frozen on dry ice. The
frozen plant material was transferred to plastic tubes containing 80% meth-
anol and two stainless steel ball bearings and homogenized in a Qiagen tissue
lyser at 30 cycles s
21
. The homogenate was mixed overnight at 4°Cand
centrifuged at 2,000 rpm to pellet the plant debris. The pellet was extracted
five times with 80% methanol and the supernatants combined and concen-
trated in a SpeedyVac (Savant) until the methanol was removed. The aqueous
extract (approximately 100 mL) was diluted to 1 mL by addition of Tris-
buffered saline (25 m
M Trizma base, 100 mM sodium chloride, 1 mM magne-
sium chloride, 3 m
M sodium azide, pH 7.5) and ABA content was measured in
the competitive ELISA assay as described by the Phytodetek protocol. Three
biological replicates were carried out for each experiment.
Transformation of Barley with Hairpin RNAi Construct
A hairpin RNAi construct targeting theHvABA8#OH1 RNAi gene was made
by inserting a PCR product spanning the region 578 to 859 bp of the
HvABA8#OH1 cDNA (Millar et al., 2006) in both orientations into the hairpin
RNAi vector pStarling. The region from 578 to 859 bp was chosen as the optimal
target for RNAi because it is not conserved between HvABA8#OH1 and
HvABA8#OH2. The hairpin RNAi HvABA8#OH-1 construct was subcloned
into the NotI site of the binary vector, pWBVec8 (Wang et al., 1998) before
being transferred into Agrobacterium tumefaciens strain AGL0 by triparental
mating. Transformation of Golden Promise barley was performed using the
Agrobacterium-mediated technique as described by Jacobsen et al. (2006).
Supplemental Data
The following materials are available in the online version of this article.
Supplemental Figure S1. Spectra of blue, red, and far-red LED lights.
ACKNOWLEDGMENTS
We thank Ingrid Venables for assistance with transformation of barley;
Professor Jim Reid and Ian Cummings, School of Plant Science, University of
Tasmania, for introducing us to LED technology; Mike Hauptman for
building the light cabinet; and Dr. John Evans, School of Biological Sciences,
Australian National University, for assistance with light intensity measure-
ments.
Received December 21, 2007; accepted April 3, 2008; published April 11, 2008.
LITERATURE CITED
Banerjee R, Batschauer A (2005) Plan t bl ue-light receptors. Planta 220:
498–502
Baskin CC, Baskin JM (1998) Seeds; Ecology, Biogeography, and Evolution
of Dormancy and Germination. Academic Press, San Diego
Benech-Arnold RL, Giallorenzi MC, Frank J, Rodriguez V (1999) Termi-
nation of hull-imposed dormancy is correlated with changes in embry-
onic ABA content and sensi tivity. Se ed Sci Re s 9: 39–47
Benech-Arnold RL, Gualano N, Leymarie J, Co
ˆ
me D, Corbineau F (2006)
Hypoxia interferes with ABA metabo lism and increases ABA sensitivity
in embryos of dorman t barley grains. J Exp Bot 57: 1423–1430
Bewley JD (1997) Seed germinati on and dormancy. Plant Cell 9: 1055–1066
Burger WC (1965) Effect of light on the germination of barley an d its
relation to dormancy. J Inst Brew 71: 244–250
Carrari F, Perez-Flores L, Lijavetzky D, Enciso S, Sanchez R, Benech-
Arnold R, Iusem N (2001) Clo ning and expression of a sorghum gene
with homology t o mai ze vp1. Its potential involvem ent in pre-harvest
sprouting resistance. Plant Mol Biol 45: 631–640
Carrera E, Holman T, Medhurst A, Dietrich D, Footitt S, Theodoulou FL,
Holdsworth MJ (2008) Seed after-ripening is a discrete developmental
pathway associated with specific gene networks in Arabidopsis . Plant J
53: 214–224
Chang S, Puryear J, Cairney K (1993) A simple and efficient method for
isolating RNA from pine trees. Plant Mol Biol Rep 11: 113–116
Chaussat R, Zoppolo J (1983) Lumie
`
re et germination de l’orge. Bios 14:
30–32
Chiwocha SDS, Abrams SR, Ambrose SJ, Cutler AJ, Loewen M, Ross
ARS, Kermode AR (2003) A met hod for profiling cl asses of plant hor-
mones and their metabolites using liquid chromatography-electrospray
ionization tandem mass spectrometry: an analysis of horm one regula-
tion of thermodorm ancy of l ettuce (La ctuc a sa tiva L.) seeds. Plant J 35:
405–417
Blue Light and Dormancy in Barley
Plant Physiol. Vol. 147, 2008 895
Chono M, Honda I, Shinoda S, Kushiro T, Kamiya Y, Nambara E,
Kawakami N, Kaneko S, Watanabe Y (2006) Field studies on the
regulation of absc isic acid conten t and germinability during grain
development of barley: molecular and chemical analysis of pre-harvest
sprouting. J Exp Bot 57: 2421–2434
Corbineau F, Benamar A, Come D (2000) Changes in sensitivity to ABA of
the developin g and maturing embry o of whe at cult ivars with different
sprouting susceptibility. Isr J Plant Sci 48: 189–197
Dewi K (2006) The role of gibberellins in early growth regulation and
dormancy breakage in barley (Hordeum vulgare L. ‘Himalaya’). PhD
thesis. Australian National University, Canberra, Australi a
Feurtado JA, Kermode AR (2007) A merging of paths: abscisic acid and
hormonal cross-talk in the control of seed dormancy mainte nance and
alleviation. In K Bradford, H Nonogaki, eds, Seed Development, Dor-
mancy and Germination. Annual Plant Reviews, Vol 27. Blackwell
Publishing, Oxford, pp 176–223
Finch-Savage WE, Cadman CSC, Toorop PE, Lynn JR, Hilhorst HWM
(2007) Seed dormancy release in Ara bidopsis Cvi by dry after-ripening,
low temperature, nitrate and lig ht shows c ommon q uantitative patterns
of gene expression directed by environmentally specific sensing. Plant J
51: 60–78
Finkelstein RR (2004) The role of hormones during seed development and
germination. In PJ Davies, ed, Plant Hormo nes: Biosynthesis, Signal
Transd uction, Action! Kluwer Academi c Publi shers, Dordrecht, The
Netherlands, pp 223–247
Frey A, Godin B, Bonnet M, Marion-Poll A (2004) Maternal syn thesis of
abscisic acid controls seed development and yield in Nicotiana plumba-
ginifolia. Planta 218: 958–964
Grahl A (1965) Lichteinfluss auf die keimung des getreides in abhangigkeit
von der keimruhe. Landbauforschung Volkenrode 2: 97–106
Grahl A, Thielebein M (1959) Einfluss von licht auf die keimung der
gerste. Naturwissenschaften 46: 336–337
Gubler F, Millar AA, Jacobsen JV (2005) Dormancy release, ABA and pre-
harvest sprouting. Curr Opin Plant Biol 8: 183–187
Hilhorst HWM (2007) Definitions and hypotheses of seed dormancy. In
K Bradford, H Nonogaki, ed s, Se ed Development, Dormancy and
Germination. Annual Plant Reviews, Vol 27. Blackwell Publishing,
Oxford, pp 50–71
Hou JQ, Simpson GM (1992) After-ripening and phytochrome action in
seeds of dormant lines of wild oat (Avena fatua). Physiol Plant 86: 427–432
Jacobsen JV, Pearce DW, Poole AT, Pharis RP, Mander LN (2002) Abscisic
acid, phaseic acid and gibberellin contents associ ated with dormancy
and germination in barley. Physiol Plant 115: 428–441
Jacobsen JV, Venables I, Wang MB, Matthews P, Ayliffe M, Gubler F
(2006) Barley (Hordeum vulgare L.). In K Wang, ed, Agrobacterium
Protocols, Vol 1, Ed 2. Humana Press, Totawa, NJ
Jones HD, Peters NCB, Holdsworth MJ (1997) Genotype and environ-
ment interact to con trol dormancy and differential expression of the
VIVIPAROUS 1 homologue in embryos o f Avena fatua. Plant J 12: 911–920
Karssen CM, Brinkhorst-van der Swan DLC, Breekland AE, Koornneef
M (1983) Induction of dormancy dur ing seed development by en dog-
enous abscisic acid: studies on abs cisic acid-deficient genotypes of
Arabidopsis thaliana
(L.) Heynh. Planta 157: 158–165
Kushiro T, Okamoto M, Nakabayashi K, Yamagishi K, Kitamura S, Asami
T, Hirai N, Koshiba T, Kamiya Y, Nambara E (2004) The Arabidopsis
cytochrome P450 CYP707A encodes ABA 8#-hydroxylases: key enzymes
in ABA catabolism. EMBO J 23: 1647–1656
Lefebvre V, North H, Frey A, Sotta B, Seo M, Okamoto M, Nambara E,
Marion-Poll A (2006) Function al analysis of Arabidopsis NCED6 and
NCED9 genes indicates that AB A sy nthesi zed in the endosperm is
involved in the induc tion of seed d ormancy. Pl ant J 45: 309–319
Marumo S, Katayama M, Komori E, Ozaki Y, Natsume M, Kondo S (1982)
Microbial production of abscisic acid by Botrytis cinerea. Agr ic Biol
Chem 46: 1967–1968
McCarty DR (1995) Genet ic con trol and in tegration of maturation and
germination pathways in seeds developm ent. Annu Rev Plant Biol 46:
71–93
Millar AA, Jacobsen JV, Ross JJ, Helliwell CA, Poole AT, Scofield G, Reid
JB, Gubler FG (2006) Seed dormancy and ABA metabolism in Arabi-
dopsis and barley: the role of ABA 8#-hydroxylase. Plant J 45: 942–954
Oh E, Yamaguchi S, Kamiya Y, Bae G, Chung W, Choi G (2006) Light
activates the degradation of PIL5 protein to promote seed germination
through gibberellin in Arab idopsis. Plant J 47: 124–139
Okamoto M, Kuwahara M, Seo M, Kushiro M, Asami T, Iria N, Kamiya Y,
Koshiba T, Nambara E (2006) CYP707A1 and CYP707A2, which encode
abscisic acid 8# -hydroxylases, are indi spensable fo r proper control
of seed dormancy and ger mination in Arabi dopsis. Plant Physi ol 141:
97–107
Puce S, Basile G, Bavestrello G, Bruzzone S, Cerrano C, Giovine M, Arillo
A, Zocchi E (2004) Abscisic acid signaling through cyclic ADP-ribose in
hydroid regeneration. J Bi ol Chem 279: 3 9783–39788
Seo M, Hanada A, Kuwahara A, Endo A, Okamoto M, Yamauchi Y, North
H, Marion-Poll A, Sun T, Koshiba T, et al (2006) Regulation of ho rmone
metabolism in Arabidopsis seeds: phy tochrome regulation of abscisic
acid metabolism and abscisic acid regulation o f gibberellin metabolism.
Plant J 48: 354–366
Simpson GM (1990) Seed Dormancy in Grasses. Cambridge University
Press, Cambridge, UK
Slovik S, Hartung W (1992) Compartmental distribution and redistribu-
tion of ab scisic acid in intact leaves. II. Model analysis. Planta 187: 26–36
Spielmeyer W, Ellis M, Robertson M, Shahjahan A, Lenton JR, Chandler
PM (2004) Isolat ion of gibberellin metabolic pathway genes from barl ey
and comparative mapping in barley, wheat and rice. Theor Appl Genet
109: 847–855
TanBC,JosephLM,DengWT,LiuL,LiQB,ClineK,McCartyDR(2003)
Molecular char acteri zation of the Arabidopsis 9-cis epoxycarotenoid
dioxygenase gene family. Plant J 35: 44–56
Thompson AJ, Jackson AC, Parker RA, Morpeth DR, Burbridge A, Taylor
IB (2000) Abscisic acid biosynthesis in tomat o: regulation of zeaxanthin
epoxidase and 9-cis-epoxycarotenoiddioxygenasemRNAsbylight/
dark cycles, water stress and abscisic acid. Plant Mol Biol 442:
833–845
Trevaskis B, Hemming MN, Peacock WJ, Dennis EJ (2006) HvVRN2
responds to daylength, whereas HvVRN1 is regulated by vernalization
and developmental status. Plant Physi ol 140: 1397–1405
Walker-Simmons MK (1987) ABA levels and sensitivity in developing
wheat embryos of sprouting resistant and susceptible cultivars. Plant
Physiol 84: 61–66
Wang H (2005) Signaling mechanisms of higher plant phot oreceptors: a
structure-function perspective. Curr Top Dev Biol 68: 227–261
Wang M, Heimovaara-Dijkstra S, Van Duijn B (1995) Modulation of
germination of embryos isolated from dorman t and nondormant grains
by manipulation of endogenous abscisic acid. Planta 195: 586–59 2
Wang MB, Matthews PR, Upadhyaya NM, Waterhouse PM (1998) Im-
proved vectors for Agrobacterium tumefaciens-mediated transforma-
tion of monocot plants. Acta Hortic 461: 401–407
Yang SH, Zeevaart JAD (2006) Expression of ABA 8#-hydroxylases in
relation to l eaf water relations and seed development in bean. Pla nt J 47:
675–686
Gubler et al.
896 Plant Physiol. Vol. 147, 2008
