Agriculture Reference
In-Depth Information
6.2.2 Layers of the meristem
The arrangement of cells in distinct layers is another characteristic feature of the
shoot apical meristem (Fig. 6.1B). The outer or
tunica
region of the meristem usually
consists of two cell layers in most dicots, called L1 and L2, or a single layer in
monocots (L1). Cells within these layers divide anticlinally (perpendicular to the
surface), whereas cells in the underlying L3 layer divide in any orientation to form
the
corpus
. The stereotypical pattern of cell divisions within each layer means that
cells rarely move between layers and thus each layer is essentially clonally distinct.
This is elegantly demonstrated by the occurrence of periclinal chimaeras, plants
in which one or more layers of the shoot apical meristem are genetically distinct
from adjacent layers (see Szymkowiak & Sussex, 1996). Owing to the stability of
periclinal chimaeras, derivatives of each layer can be traced into the stem and lateral
organs (Satina
et al.
, 1940; Dermen, 1953; Stewart & Burk, 1970). This shows
that in most organs, the epidermis is derived from the L1 layer, the sub-epidermal
tissue from the L2 layer and the vasculature and internal tissue of the organs and
stem from the L3 layer. However this pattern may vary in some organs, for instance
sub-epidermal tissue in the margins of petals is derived from the L1 layer.
6.2.3 Symplastic fields within the meristem
A further level of organisation within the apex, which might be relevant to meristem
function, is the arrangement of plasmodesmatal connections that potentially allow
communication between cells. Plasmodesmata interconnect symplastic fields that
may include cells from the same or different lineages. They are also able to regulate
the passage of potential signalling molecules, including developmentally important
transcription factors or their RNA precursors (see Chapter 5; Haywood
et al.
, 2002)
and may be involved in determining the polarity of movement. Studies of the birch
and
Arabidopsis
meristems have shown that symplastic fields predict some aspects
of meristem fate and are highly dynamic (Rinne & van der Schoot, 1998; Gisel
et al.
,
1999). Whether symplastic signalling is important in maintaining the function within
a field or for specifying different functions remains to be determined.
6.3
Periclinal chimaeras reveal a role for signalling in plant development
If the function of cells within the meristem and lateral organs is determined by the
inheritance of information from a progenitor cell, then cells with a common ancestry
are expected to follow similar fates. Evidence that lineage does not control cell fate
comes from the analysis of periclinal chimaeras in which a cell from one layer has
been displaced into an adjacent layer following a periclinal division. Such events can
occur within the meristem as well as in the developing leaf primordium, although
the frequency of cell displacement varies between these tissues and even between
species. Regardless of when or where the displacement occurs, all descendants
