Biology Reference
In-Depth Information
ch1
has elevated levels of miR156, and its effect on vegetative phase
change is dependent on miR156, as demonstrated by the observation
that a reduction in the level of miR156 blocks the vegetative phase change
phenotype of
ch1
. The possibility that the phenotype of
ch1
is attributable
to reduced carbohydrates was tested by examining the effect of various
sugars on the level of miR156 and vegetative phase change in mutant
and wild-type plants. Glucose or sucrose accelerated vegetative phase
change, and reduced the abundance of miR156 in
ch1
and wild-type plants
when applied to whole seedlings, isolated leaf primordia, or to petiole
stubs on defoliated seedlings (
Yang et al., 2013; Yu et al., 2013
). These stud-
ies also showed that
MIR156A
and
MIR156C
are the major sources of
miR156 in Arabidopsis seedlings, and are specifically downregulated by
sugar. These results therefore support the long-standing hypothesis that veg-
etative phase change is mediated by an increase in the nutritional status of
the shoot.
The identity of the endogenous sugars and signaling pathways involved
in vegetative phase change is still largely unknown, although there has
been some progress on this front. HEXOKINASE1 (HXK1)—a regulator
of glucose signaling in Arabidopsis—promotes the accumulation of miR156
in seedlings grown in the absence of exogenous sugar and is required for the
effect of glucose on miR156 expression, but does not block the decline in
the abundance of this miRNA during shoot development (
Yang et al.,
2013
). This result implies that glucose promotes vegetative phase change
via the HXK1 signaling pathway, but is not completely responsible for
this developmental transition. Another sugar that may be involved in
vegetative phase change is trehalose-6-phosphate (T6P). In Arabidopsis,
T6P is synthesized from glucose-6-phosphate by TREHALOSE PHOS-
PHATE SYNTHETASE 1 (TPS1) (
Blazquez et al., 1998
). Null alleles of
TPS1
are embryo lethal, but this phenotype can be corrected by transiently
expressing TPS1 during embryo development (
van Dijken, Schluepmann, &
Smeekens, 2004
), or avoided by using hypomorphic alleles (
Gomez, Gilday,
Feil, Lunn, & Graham, 2010
).
tps1
plants are extremely late flowering due to
the reduced expression of the floral inducer FT (
Wahl et al., 2013
). In addition
to having low levels of FT,
tps1
mutants have elevated levels of miR156 and
reduced levels of at least three miR156-regulated transcripts—SPL3, SPL4,
and SPL5 (
Wahl et al., 2013
). Although the functional significance of the
effect of
tps1
on miR156 and SPL3/4/5 expression has not been determined,
this phenotype suggests that T6P may play a role in vegetative phase as well as
in floral induction.
