SAM typically consists of a dome of cells connected to two developing leaf primordia
(Fig. 4.1). This area contains around 100 cells inArabidopsis. The dome structure contains
the least differentiated cells and consists of three different histocytological zones (Fig. 4.5).
The central zone in the middle of the dome contains cells that divide infrequently, yet this is
the location of the self-renewing undifferentiated stem cells. Surrounding the central zone is
the peripheral zone, where the rate of cell division is higher and cells contribute to the
organs of the plant, including leaves, inflorescence meristems, and floral meristems.
Below the central zone is another region of rapidly dividing cells, called the “rib” meristem.
Division and elongation of rib meristem cells gives rise to the stem of the plant.
The SAM also consists of different cell layers. The surface layer of cells is called theL1
cell layer. Cells in L1 divide only by forming anticlinal cell walls, that is, cell division is
always perpendicular to the meristem surface. As a result, cells in the L1 layer and their
daughter cells always remain in this layer. TheL2 cell layer, below the L1 cells, divide
the same way. TheL3orcorpus cells, divide in all planes, and fill the interior of the SAM.
A major issue in plant biology concerns how shoot meristems are organized and how
molecular information in the SAM determines the precise placement/function of cells.
More recent molecular studies indicate that the maintenance of stem cell function
depends on a feedback loop involving the CLV1–3 (Clavata) gene products and WUS
(Wuschel). Inclavatamutants, the meristem is enlarged, due to excessive accumulation
of stem cells, suggesting that CLV1–3 are required to regulate the number of stem cells
in the meristem. In contrast,wusmutants contains a smaller meristem with differentiated
cells, suggesting that WUS is a positive regulator of stem cell identity. Analysis of the inter-
actions between these key regulators indicates that (1) theClvgenes repress WUS at the
transcript level and (2) WUS expression is sufficient to induce meristem cell identity and
the expression of the stem cell marker CLV3. As the different CLV genes encode a receptor
and a ligand that binds this receptor, it appears that the CLV gene products together form a
signal transduction pathway that limits the expression region of WUS. Thus the interaction
between CLV and WUS maintains stem cell function and the maintenance of the meristem
as a source of cells for the shoot.
Figure 4.5.The shoot apical meristem. Schematic of a shoot apical meristem showing the central
(CZ), peripheral (PZ), and rib meristem (RZ) zones. Proteins involved in meristem development
are also shown. [Reprinted from Lenhard and Laux (1999), with permission from Elsevier
Science Ltd.]
4.3. MERISTEMS 93