Agriculture Reference
In-Depth Information
of a displaced cell adopt fates identical to those of the surrounding cells in the
receiving layer. This implies that cell fate is largely determined by positional rather
than inherited information, and that cells must continually assess their position
presumably by communicating with their neighbours.
Surgical experiments first conducted in the early part of the last century also
showed the importance of cell-cell interactions in meristem development. In this
case destroying part or all of central zone by surgical treatment, or more recently
by laser ablation, causes cells in the adjacent peripheral zone to adopt central zone
functions and re-establish a morphologically normal meristem (Steeves & Sus-
sex, 1989; Sussex, 1989; Reinhardt
et al.
, 2003a). This implies that cells of the
central zone impart peripheral zone identity on adjacent cells, and that in the ab-
sence of this signal, stem cell identity is re-established in the peripheral zone (see
later).
In addition to the role of signalling in determining cell fate, there is extensive
evidence that it is also required to coordinate growth between the layers of the
meristem and developing organ. This is particularly evident in leaf development,
where analysis of periclinal chimaeras has shown that the contribution of cells from
the L2 and L3 layers to the internal tissue of the blade is highly variable (Stewart &
Dermen, 1975, 1979). This implies that growth of each layer is coordinated, such
that a smaller contribution from one layer is matched by a correspondingly larger
contribution from the other layer in any particular region of the leaf.
Further evidence for interlayer communication comes from the analysis of per-
iclinal chimaeras that arise from a graft union between two related species with
different patterns of growth. As the meristem of these plants have cell layers derived
from different species, the contribution of each layer to the size and shape of an
organ can be assessed. In some instances leaf and flower morphology is determined
by the identity of the L2 layer, implying that signals from this layer regulate growth
in adjacent layers (Jorgenson & Crane, 1927). However, other studies have shown
that the L1 layer is more influential (Stewart
et al.
, 1972). The consequences of
signalling between the layers is therefore variable and is dependent on the type of
organ and the particular developmental stage.
By analogy to animal development, there are various types of signalling sys-
tems that might regulate cell function within the meristem and developing organ.
Signalling molecules, such as secreted proteins, RNAs and hormones, may be ac-
tively moved via membrane-bound transporters or through symplastic networks,
forming a concentration gradient extending over many cells. Cells might respond
according to the concentration of signal they experience, effectively converting a
gradient into discontinuous domains of gene expression and cell identity. At the
other extreme, signalling may involve interactions between adjacent cells, utilising
a receptor/ligand system. This latter system seems to be important in plants, as hun-
dreds of transmembrane receptors have been identified in the
Arabidopsis
genome
(Shiu & Bleecker, 2003). Examples of these various types of signalling systems are
considered in the following sections.
