Biology Reference
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the size of the shoot (
Day et al., 2002; Mencuccini et al., 2007
). A third type
of variation, termed “seasonal heterophylly” (
Godley, 1985
), is represented
by the reproducible changes in shoot morphology that occur during a
growing season along the primary axis of the shoot and in newly formed
branches of some herbaceous and woody perennials (
Kozlowski, 1971;
Moriuchi &Winn, 2005; Winn, 1999
). These changes sometimes resemble
those that occur early in the growth of the shoot (
Critchfield, 1960
), but
differ in that they occur regularly, rather than only once. Changes in veg-
etative morphology may also arise from environmental heterogeneity, such
as variation in light quality, temperature, growth substrate, and humidity
(
Bruni, Young, & Dengler, 1995; Cutri et al., 2013; Deschamp &
Cooke, 1985; Fisher, Posluszny, & Lee, 2002; Goliber & Feldman, 1990;
Jones, 1995; Lee & Richards, 1991; Ray, 1987
), and may also be induced
by damage from herbivory or disease (
Boege & Marquis, 2005
). Changes
produced by these environmental conditions can resemble the changes that
occur during phase change, but it is unclear if they are mediated by the same
regulatory mechanism.
In higher plants, the transition from vegetative to reproductive develop-
ment is marked by the production of a completely new structure specialized
for gamete production (e.g., a flower or cone). In contrast, vegetative phase
change involves heterogeneous and sometimes quite subtle changes in the
character of leaves, stems, and buds. These changes are species-specific
and include leaf shape and size, branching patterns, epicuticular wax, pat-
terns of trichome production, cell shape, vascular patterns, histological
staining, phyllotaxy, the capacity for adventitious root production, and
the presence or absence of anthocyanin or other phytochemicals, as well
as disease or insect resistance (
Allsopp, 1967; Boege & Marquis, 2005;
Greenwood, 1995; Hackett, 1985; Kerstetter & Poethig, 1998; Poethig,
1990; Whalen, 2005
). Depending on the species and the trait under inves-
tigation, vegetative transitions may occur quickly and encompass only a few
nodes, or occur gradually and encompass many nodes. This greatly compli-
cates the analysis of vegetative phase change because different temporal pat-
terns and traits may be controlled by different mechanisms in different species
(
Borchert, 1976; Jones, 1999
). Fortunately, this problemhas been resolved by
the discovery of major regulators of vegetative phase change—the micro-
RNAs, miR156, and miR157, and their direct targets, the SBP/SPL family
of transcription factors. Recent studies demonstrating that these evolution-
arily conserved genes regulate vegetative phase change in a range of plants,
and are differentially expressed in juvenile and adult tissues of herbaceous
