Biology Reference
In-Depth Information
phenotype is attributable to the increased transcription of
MIR172B
, which
is a direct target of SPL9 (
Wu et al., 2009
). A reasonable hypothesis is that
high levels of miR156 during the juvenile phase reduce the sensitivity of
plants to a photoperiodic stimulus (i.e., reduce reproductive competence)
by repressing the expression of
SPL9
and other
SPL
genes that promote
the transcription of
MIR172B
. This hypothesis is supported by the observa-
tion that
spl9 spl15
double mutants are late flowering and display a delayed
sensitivity to photoperiod (
Schwarz et al., 2008
).
Three other targets of miR156 that have been implicated in flowering
are
SPL3
,
SPL4
, and
SPL5
. Indeed, the first evidence that the miR156-
SPL interaction plays a role in shoot maturation was the observation that
plants overexpressing miR156-resistant forms of these genes flower early
(
Cardon et al., 1997; Gandikota et al., 2007; Wu & Poethig, 2006
). In addi-
tion to being directly repressed by miR156,
SPL3
/
4
/
5
are transcriptionally
repressed by the AP2-like proteins targeted by miR172, and are thus indi-
rectly repressed by miR156 through its effect on
miR172B
expression (
Jung
et al., 2011; Wu et al., 2009
;
Fig. 5.2
).
SPL3
/
4
/
5
are expressed at relatively
low levels in SD, and at much higher levels in LD (
Cardon et al., 1999;
Schmid et al., 2003
). This response to photoperiod is independent of
miR156 because miR156 levels are identical in plants grown in LD and
SD (
Jung et al., 2012
); furthermore, mutations in genes required for the
photoperiodic response do not affect miR156 levels (
Wang et al., 2009
).
Photoperiodic regulation of
SPL3
/
4
/
5
is mediated by FT (
Schmid et al.,
2003
) through its effect on the transcription of
SOC1
and
FD
, both of which
directly promote the transcription of
SPL3
(
Jung et al., 2012
). SPL3, in turn,
directly regulates the transcription of several genes involved in flowering,
including
FUL
,
LFY
,
AP1
, and
FT
(
Wang et al., 2009; Yamaguchi
et al., 2009
).
Given that photoperiod is capable of inducing
SPL3
/
4
/
5
expression
independently of miR156, what role does miR156 play in the regulation
of these genes during shoot development? One possibility is that miR156
overrides the inductive effects of LD during the juvenile phase, and this
hypothesis is supported by the observation that plants overexpressing
miR156-sensitive SPL3/4/5 transcripts flower only slightly earlier than nor-
mal in LD (
Wu & Poethig, 2006
). Apparently, miR156 levels are sufficiently
high at this stage that even elevated levels of SPL3/4/5 transcripts cannot
overcome repression by this miRNA. miR156 has also been postulated
to play a role in floral induction in SD (
Wahl et al., 2013; Wang et al.,
2009
). Under these conditions, miR156 levels decline gradually at the shoot
