Biology Reference
In-Depth Information
mRNA in lines transformed with
35S::SPL3
; in particular, it was noted that
transgenic seedlings with elevated levels of the SPL3 mRNA displayed no
increase in the SPL3 protein (
Gandikota et al., 2007
). Subsequent studies
demonstrating that mutations in the microtubule-binding protein, katanin
(
Brodersen et al., 2008
), and the GW-repeat protein, SUO (
Yang,
Wu, & Poethig, 2012
), increase the abundance of the SPL3 and SPL9 pro-
teins without affecting the level of their mRNAs, provided additional evi-
dence that plant miRNAs are capable of mediating translational repression.
Although the mechanism by which katanin promotes miRNA function is
still unknown, GW-repeat proteins in animals have well-characterized roles
in miRNA-mediated translational repression and mRNA turnover (
Braun,
Huntzinger, & Izaurralde, 2013; Ding &Han, 2007
) and it may be that SUO
acts in a similar fashion. In any case, it is significant that the phenotype of
suo
mutations is largely attributable to a reduction in the activity of miR156
(
Yang et al., 2012
). This result demonstrates that translational repression
is important for the normal function of miR156, and implies that the activity
of
SPL
genes cannot be accurately predicted from the levels of their tran-
scripts in cells in which miR156 is expressed.
Evidence that the effect of miR156 on vegetative phase change is attrib-
utable to its effect on
SPL
expression comes from the phenotype of
SPL
genes with mutations in the miR156 target site. Transgenic plants constitu-
tively expressing wild-type SPL3, SPL4, and SPL5 transcripts have a nearly
normal phenotype, whereas plants transformed with miR156-resistant
versions of these transcripts flower extremely early, and express some adult
vegetative traits precociously (
Cardon, Hohmann, Nettesheim, Saedler, &
Huijser, 1997; Gandikota et al., 2007; Jung et al., 2011; Wang et al.,
2009; Wu & Poethig, 2006; Yamaguchi et al., 2009
). Similarly, transgenic
plants expressing miR156-resistant SPL9, SPL10, or SPL13 transcripts have
a much stronger phenotype than plants expressing the corresponding
wild-type versions of these transcripts (
Martin et al., 2010; Shikata et al.,
2009; Wang et al., 2008; Wu et al., 2009
). However, the best evidence that
miR156 regulates phase change through its effect on
SPL
gene expression is
provided by an Ethyl methane sulfonate-induced mutation of
SPL15
, whose
precocious phenotype is attributable to a single nucleotide change in the
miR156 target site that leads to an increase in the expression of the
SPL15 transcript (
Usami, Horiguchi, Yano, & Tsukaya, 2009
). These results
demonstrate that miR156 has a major effect on the expression of
SPL
genes
and suggest that it acts primarily, if not exclusively, by repressing the expres-
sion of this gene family.
