Biology Reference
In-Depth Information
apex, and SPL3/4/5 transcripts increase in a reciprocal fashion. Unfortu-
nately, there is still no evidence that the decline in miR156 is responsible
for the increase in SPL3/4/6 transcripts under SD, nor is there conclusive
evidence that these genes play a role in floral induction under either LD
or SD conditions. For example, the phenotype of plants expressing
miR156-resistant versions of SPL3/4/5 transcripts under the regulation of
their endogenous promoters has yet to be described. If miR156 indeed plays
an important role in the regulation of
SPL3
/
4
/
5
, these plants should have an
early flowering phenotype. More importantly, plants with reduced levels of
miR156 do not flower early under SD (
Wang et al., 2009
). They produce
fewer leaves than wild-type plants because they initiate leaves more slowly
(
Wang et al., 2008
), but they produce flowers at the same time as wild-type
plants. Another major problem is that the loss-of-function phenotype of
SPL3
,
SPL4
, and
SPL5
is still unknown. The only existing loss-of-function
mutation of
SPL3
has no obvious phenotype (
Wu & Poethig, 2006
).
Whether this is because the mutation is hypomorphic (
Kim et al., 2012
),
because of functional redundancy with
SPL4
and
SPL5
, or because
SPL3
has only a minor role in shoot development, remains to be determined.
Viral-induced gene silencing of a homolog of
SPL3
/
4
/
5
in
Antirrhinummajus
delays flowering (
Preston & Hileman, 2010
), which is consistent with the
early flowering phenotype of plants overexpressing
SPL3
/
4
/
5
. But it will
take additional work to establish the function of these genes in
Arabidopsis
.
6.3. How is timing of vegetative phase change regulated?
A remarkably comprehensive but little known study of vegetative phase
change in
Eucalyptus tenuiramus
(
Wiltshire & Reid, 1992
) illustrates some
of the common features of this process. One long-standing question is
whether the timing of this process is dependent on the age or the size of
the shoot (
Day et al., 2002; Greenwood et al., 2010; Mencuccini et al.,
2007; Robinson & Wareing, 1969; Vanderklein, Martinez-Vilalta, Lee, &
Mencuccini, 2007
).
Wiltshire and Reid (1992)
showed that the node at
which vegetative phase change occurs is more closely correlated with the
total number of nodes produced up until the transition point than with
the height of the shoot or the duration of shoot growth, and is more strongly
correlated with the cumulative amount of light received by the shoot than
with day length or temperature. These results suggest that vegetative phase
change is controlled by a factor that changes with leaf number, and suggest
that the production of this factor is light dependent. Wiltshire and Reid
