Vegetative Phase Change and Shoot Maturation in Plants - Current Topics in Developmental Biology - page 132

SPL11 are closely related, tandemly duplicated genes, and have little or no
loss-of-function phenotype alone, or in combination with spl2 ( Shikata
et al., 2009 ). To get around this problem, Shikata and colleagues ( Shikata
et al., 2009 ) fused SPL10 to the SRDX transcription repression domain,
and expressed this protein in transgenic plants using both the constitutive
35S promoter and the endogenous SPL10 promoter. This approach was
expected to produce a phenotype similar to the loss-of-function phenotype
of SPL10 on the assumption that SPL10 normally acts to promote gene
expression. Transgenic plants had narrow rosette leaves, stunted inflores-
cences, reduced apical dominance, defective flowers and siliques, and abnor-
mally round cauline leaves with excessive numbers of trichomes, but had a
normal pattern of abaxial trichome production and flowered normally.
Unfortunately, this phenotype is largely inconsistent with the phenotype
of plants expressing miR156-resistant SPL10 transcripts, whose major fea-
ture is the accelerated expression of adult traits ( Wu et al., 2009 ). Given that
both these phenotypes are produced by mutant transgenes, it is difficult to
know which phenotype more accurately reflects the normal function of
SPL10 .
6. MOLECULAR INSIGHTS INTO THE PHENOMENOLOGY
OF VEGETATIVE PHASE CHANGE
6.1. How are vegetative phases specified?
Vegetative development is commonly divided into two phases—a juvenile
and an adult phase. However, patterns of heteroblastic variation are often
more complex than this. For example, in Eucalyptus shoot development is
typically divided into seedling, juvenile, transitional, and adult phases
( Blake, 1953; Boland et al., 2006 ), and similar categories have been
described in several species native to New Zealand ( Day, Gould, &
Jameson, 1997; Gould, 1993 ). Even in weakly heteroblastic species, such
as Arabidopsis ( Telfer, Bollman, & Poethig, 1997 ) and maize ( Bongard-
Pierce, Evans, & Poethig, 1996 ), the first two leaves differ from other juve-
nile leaves in a variety of ways, and could easily be considered a distinct leaf
type. Can these patterns be reconciled with a model of shoot maturation
based on changes in the abundance of miR156?
Northern analysis shows that miR156 is present throughout shoot devel-
opment, but is present at different levels at different times in development. In
Arabidopsis , miR156 declines rapidly within 2 weeks after germination
( Jung et al., 2012; Wu et al., 2009; Wu & Poethig, 2006 ), and then declines