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as well as loss-of-function mutations of
MIR156A
and
MIR156C
(
Yang, Xu, Koo, He, & Poethig, 2013; Yu et al., 2013
), produce the oppo-
site phenotype: these plants express adult leaf traits precociously, are slow
growing, and flower with many fewer leaves than normal. These results
demonstrate that miR156 is both necessary and sufficient for the expression
of the juvenile vegetative phase, but has other functions as well.
The expression pattern of miR156 is consistent with its role in promot-
ing juvenile development. In
Arabidopsis
(
Jung, Ju, Seo, Lee, & Park, 2012;
Jung, Seo, Kang, & Park, 2011; Wahl et al., 2013; Wang et al., 2009; Wu
et al., 2009; Wu & Poethig, 2006
), maize (
Chuck et al., 2007
), and rice (
Xie
et al., 2006
), miR156 is expressed at high levels in young seedlings and at
lower levels in older plants. An analysis of miR156 levels in fully expanded
leaves of species that undergo significant changes in leaf morphology during
vegetative phase change (specifically,
Acacia confusa
,
Acacia colei
,
Eucalyptus
globulus
,
Hedera helix
, and
Quercus acutissima
) demonstrated that juvenile
leaves have significantly higher levels of miR156 and miR157 than adult
leaves (
Wang et al., 2011
), and showed that variation in leaf morphology
in the transition zone between these two phases was correlated with inter-
mediate levels of these miRNAs (
Fig. 5.1
). Along with the results of the
genetic analyses mentioned above, these results strongly suggest that vege-
tative phase change is mediated by a decline in the level of miR156/
miR157. From a practical standpoint, these observations also indicate that
the relative levels of miR156/miR157 can be used as molecular markers
for shoot identity in situations in which it is difficult to identify these phases
on the basis of morphological criteria.
5. miR156 TARGETS
miR156 acts by repressing the expression of
SQUAMOSA
PROMOTER BINDING PROTEIN
(
SBP
/
SPL
) genes, which encode
plant-specific transcription factors (
Cardon et al., 1999; Klein, Saedler, &
Huijser, 1996
). Three of the 13
SBP
genes in moss (
Axtell, Snyder, &
Bartel, 2007; Riese, Hohmann, Saedler, Munster, & Huijser, 2007
),
11 of the 19
SBP
genes in rice (
Xie et al., 2006
), and 11 of the 17
SPL
genes
in the Columbia accession of
Arabidopsis thaliana
(
Gandikota et al., 2007;
Rhoades et al., 2002
) have sequences complementary to this miRNA.
miR156 represses the expression of
SPL
genes by directing the cleavage
of their transcripts and by promoting translational repression. SPL transcripts
with sequences complementary to miR156 are cleaved at the expected
