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or within the shoot apical meristem (SAM), or operate more generally on
organs and tissues throughout the shoot.
This question has been addressed in maize, where adult shoots can be
readily rejuvenated. Adult shoot apices with one or two leaf primordia
undergo complete rejuvenation
in vitro
(
Irish & Karlen, 1998
). The plants
that develop from these cultured shoot apices produce the same number
of juvenile leaves and flower with the same number of leaves as seed-derived
plants. In contrast, explanted shoot apices with five or more leaf primordia
undergo partial rejuvenation (
Orkwiszewski & Poethig, 2000
). The leaf
primordia present on the apex at the time it was explanted into culture,
develop juvenile tissue at their base, and these partially rejuvenated leaves
are followed by one or two completely juvenile leaves before the shoot
reverts to the adult phase and produces flowers. Importantly, the number
of leaves produced by explanted shoots is identical to the number of leaves
they would have produced
in situ
. That is, the fate of the SAM is unaffected
by this treatment. These results indicate that the phase identity of a
leaf is specified independently of the phase identity of the SAM, and implies
that the factors that regulate juvenile versus adult leaf identity act directly
on leaf primordia, and can modify their identity even after they have
been initiated.
To identify the source of these factors, Yang and colleagues deleted the
root system, cotyledons, and leaf primordia of juvenile
Arabidopsis
seedlings,
and monitored the effect of these treatments on leaf identity and the expres-
sion of miR156 (
Yang, Conway, & Poethig, 2011
). They found that phase
change occurred normally in the absence of the root system and in plants
lacking cotyledons, but was delayed by leaf ablation. This effect was associ-
ated with an increase in miR156 expression, and was dependent on
miR156. miR156 levels also increased significantly in adult shoot apices
of maize cultured
in vitro
, which is consistent with the effect of this treatment
on shoot identity (
Irish & Karlen, 1998; Orkwiszewski & Poethig, 2000
),
and with the observation that
SPL
gene expression decreases in rejuvenating
maize shoots (
Strable, Borsuk, Nettleton, Schnable, & Irish, 2008
). The
conclusion of this study is that vegetative phase change is promoted by a
factor or factors produced by leaves. This conclusion is consistent with
earlier studies demonstrating that defoliation and severe pruning prolongs
the production of juvenile leaves in both herbaceous (
Njoku, 1956
) and
woody plants (
Libby & Hood, 1976; Schaffalitzky de Muckadell, 1954
).
Efforts to identify the endogenous factors that regulate vegetative phase
change have focused on carbohydrates and on the hormone, GA. Depending
