Agriculture Reference
In-Depth Information
A
B
Figure 6.1
Shoot apical meristem structure. (A) View of an
Antirrhinum
shoot apical meristem (SAM)
showing progressively older leaves (p1-p4). Two opposing leaves arise at each node and are displaced
by 90
◦
from the previous node, an arrangement called descussate phyllotaxis. In this image one of the
p2 and p4 organs has been removed to allow a view of the meristem. (B) A cross section through the
centre of an
Antirrhinum
apex shown in (A), highlighting the morphologically distinct zones and layers
of the SAM. The epidermal L1 layer (upper layer of dark cells) and the sub-epidermal L2 layer (layer
of light cells) consists of anticlinally dividing cells that are collectively termed the tunica. Underlying
the tunica is the corpus, which is derived from the L3 layer and is composed of cells dividing in any
orientation (lower layer of dark cells). Black lines separate the meristem into functionally distinct
zones, although often the boundary between these zones is not well defined. Cells in the peripheral
zone (PZ) either form lateral organs or the outer tissue of the stem. The inner stem tissue is derived
from the rib zone (RZ). Within the central zone (CZ), pluripotent stem cells continually divide,
producing cells that move into the PZ and RZ, as well as maintaining the stem cell population.
6.2.1 Zones of the meristem
The arrangement of cells within the shoot apical meristem of flowering plants has
traditionally been described in one of two ways (Steeves & Sussex, 1989; Lyndon,
1998). The first divides the shoot apical meristem into three zones on the basis of
cytoplasmic densities and cell division rates, although the boundaries between these
zones are often unclear (for example see Laufs
et al.
, 1998b). Cells at the summit of
the shoot apical meristem form the central zone and are distinguished by having a
large, vacuolated appearance and a lower rate of cell division than that of surround-
ing tissue. Based on cell lineage studies in several species there are approximately
six to nine pluripotent stem cells within the central zone that continually divide,
producing cells that both maintain the integrity of the central zone and replace cells
that have been consumed in organ formation (Stewart & Dermen, 1970; Furner
& Pumfrey, 1992; Irish & Sussex, 1992). Flanking the central zone is the periph-
eral zone, which is composed of smaller, more rapidly dividing cells. Arising from
this zone are lateral organs, such as leaves, and the outer tissue of the stem. The rib
zone lies beneath the central zone and generates the central (pith) tissue of the stem
(Fig. 6.1B).
