Agriculture Reference
In-Depth Information
Leaf primordium
Leaf primordium
Stem cells
L1
L2
L3
Figure 2.3
Schematic drawing of organization of an apical meristem. The meristem is organized into
three layers: L1, L2, and L3. The stem cells are indicated as the gray area. The arrows indicate the flow
of cells as a result of stem cell proliferation. Two leaf primordia are formed on the flanks of the
meristem. After Clark (2001).
self-renewing source of undifferentiated stem cells whose descendents become in-
corporated into organ and tissue primordia and acquire different fates, leading to
formation of the above-ground organs such as leaves, flowers, vasculature and other
tissues of the stem, whereas the root meristem is responsible for development of the
root system (Sharma
et al.
, 2003).
In order for the shoot meristem to form organs continuously, different regions
of the shoot meristem have to establish constant communication so that a balance
between stem cell proliferation through cell division and cell departure from the
meristem to form lateral organs is maintained (see also Chapter 6). Cells of SAM
consist of three clonally distinct cell layers (Fig. 2.3) (Clark, 2001). Cells of the L1
layer (the epidermal layer) and the L2 layer (the subepidermal layer) divide in an
anticlinal fashion, and therefore they are single cell thick. Cells of the underlying
L3 layer divide in various orientations. Since all the differentiated cell types of the
adult plant are derived from a small number of stem cells, cell lineage patterns do
not play a critical role in regulating cell fate. Instead, cell fate is determined by
a positional effect that requires communication within and between the different
layers of cells.
Genetic screens for
Arabidopsis
mutants with an altered SAM morphology
have identified three
CLAVATA
genes (
CLV1
,
CLV 2
, and
CLV3
) whose loss-of-
function mutations result in an enlarged meristem (Clark
et al.
, 1993, 1995; Kayes
& Clark, 1998). Some of the
clv
mutants accumulate over 1000-fold more undif-
ferentiated cells in SAM than do wild-type plants. Genetic analysis revealed that
these three genes function in the same pathway by preventing unrestricted stem cell
