Agriculture Reference
In-Depth Information
7.2.3.2 Spatial regulation of flowering-time control
The question of in which tissues the separate flowering-time pathways or their
individual components act to regulate flowering was not addressed until recently.
Therefore in most cases, it remains unclear whether these pathways regulate the
function of long-distance signals expressed in the leaves or respond to these signals
in the meristem. Simply analyzing the spatial pattern of expression of these genes
did not help to address this problem since many flowering-time genes are expressed
broadly.
Classical physiological experiments suggested that vernalization acts in the
meristem to promote flowering (Michaels & Amasino, 2000). Initial observations
were based on exposing only the leaves or only the apices of celery plants to ver-
nalization treatments, and demonstrating that vernalization of the meristem was
sufficient to induce flowering. The vernalization pathway is therefore likely to act in
the meristem to reduce
FLC
expression and thereby induce flowering, and this may
also be true for the autonomous pathway. Consistent with vernalization acting in the
meristem,
FLC
is expressed specifically in the shoot and root meristems in young
seedlings, although in older plants it is also expressed in expanded leaves (Sheldon
et al.
, 2002; Noh & Amasino, 2003; Bastow
et al.
, 2004). FLC protein binds to the
promoter of the
SOC1
gene
in vitro
and the binding sites are required for repression
of
SOC1
expression by FLC (Hepworth
et al.
, 2002). The expression pattern of
FLC
and its role in repressing
SOC1
expression suggests that it may act directly in the
meristem to repress transcription of target genes. Such a role would suggest that
FLC does not repress flowering by altering the synthesis or transport of the floral
stimulus, but the response of the meristem to the stimulus. Nevertheless, analysis
of somatic sectors predicted to express FLC suggested that its repressive effect on
flowering can be overcome by non-cell autonomous signaling (Furner
et al.
, 1996).
Somatic sectors homozygous for the
fca
mutation were created in an otherwise wild-
type plant, and since this mutation impairs the autonomous pathway these sectors
would be predicted to cause localized expression of
FLC
at high levels. Mutant
fca
sectors within the L2 and L3 layers of the meristem did not affect the morphology
of the meristem or of the mutant cells, suggesting that the effect of
FLC
expression
in those cells is overcome by non-cell autonomous signaling from wild-type cells
(Furner
et al.
, 1996). This signal may originate in cells present in the L1, or in L2 or
L3 cells neighboring the mutant sector or even from other organs such as the leaves.
Existence of the floral stimulus was originally demonstrated by inducing flow-
ering with appropriate day lengths, as described in Section 7.2.1. Therefore the
photoperiod pathway of
Arabidopsis
might be expected to include a long-distance
signaling component analogous to the floral stimulus. A molecular hierarchy within
the photoperiod pathway has been defined. Two flowering-time genes specific to
this pathway are
GIGANTEA
(
GI
) and
CO
. The
GI
gene encodes a large protein of
1180 amino acids that is present in the nucleus and is highly conserved among the
angiosperms but has no animal homologues (Fowler
et al.
, 1999; Park
et al.
, 1999).
The biochemical function of GI is unknown, but
gi
mutations cause severe late flow-
ering (Redei, 1962), while overexpression of
GI
causes early flowering (L. Wright
