Agriculture Reference
In-Depth Information
18.4 Possible Drought Independent Roles of ABA in
Flowering
Besides drought, ABA could mediate several endogenous and environmental stim-
uli that affect flowering via regulation of
FT
levels. One example is warm ambient
temperature, an important cue for flowering in several plant species.
FT
expression
is upregulated following an increase in ambient temperature and this is partially
independent of CO (Balasubramanian et al.
2006
; Kumar et al.
2012
). Warm tem-
perature causes the upregulation of several ABA-related transcripts and ABA lev-
els are elevated under warm temperature in germinating seeds (Balasubramanian
et al.
2006
; Toh et al.
2008
). These correlative observations may point to a role
for ABA in warm ambient temperature-mediated flowering response. ABA could
also play an important role in carbon signalling. Expression analysis have shown a
significant overlap between glucose- and ABA-regulated gene expression (Li et al.
2006
). Also, genes involved in trehalose metabolism are upregulated upon ABA
applications. This is interesting as trehalose metabolism plays a key role in flower-
ing (Wahl et al.
2013
). Trehalose-6-phosphate acts at two sites: it is required for
the optimal induction of
FT
in the leaves and it affects the expression of flowering
genes in the SAM independent of
FT
.
18.5 Flowering Time Phenotypes Associated with Lesions
in ABA Signalling Components
Genetic screens identified several mutants altered in both ABA and flowering
responses. Supporting a negative role for ABA in flowering, some ABA hypersen-
sitive mutants display a late flowering phenotype. For instance mutants for the
ʲ
subunit of farnesyl transferase
era1
(
enhanced response to ABA 1
) are late flower-
ing under both LDs and SDs (Yalovsky et al.
2000
). Similarly,
hyponastic leaves
1
mutants combine hypersensitivity to ABA and late flowering (Lu and Fedoroff
2000
). Genetic screens for
hos
(
high expression of osmotically regulated genes
)
mutants identified two CTD phosphatase-like (
CPL
-like) genes (dubbed
cpl1
and
cpl3
) (Koiwa et al.
2002
). Both
cpl1
(also known as
fiery2)
) and
cpl3
mutants are
characterized by increased sensitivity to ABA (Koiwa et al.
2002
; Xiong et al.
2002b
). However, whilst
cpl1
mutants are late flowering,
cpl3
are early flowering.
At
CPL1 and
At
CPL3 phosphatases are believed to be general regulators of gene
expression that modulate the phosphorylation of RNA Polymerase II.
Mutations in the gene encoding the large subunit of the nuclear mRNA cap-
binding protein complex (CBC),
ABA hypersensitive 1
(
ABH1
), result in an early
flowering phenotype, irrespective of the day length (Kuhn et al.
2007
; Bezerra
et al.
2004
). The early flowering of
abh1
mutants can be largely attributed to a dra-
matic reduction of the strong flowering repressor
FLOWERING LOCUS C
(
FLC
)
(Bezerra et al.
2004
). The
FLC
messenger RNA is heavily processed and interacts
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