Cells Fate Is Determined by Its Position

In both the root and shoot meristem, a small number of stem cells are the ultimate source of any particular tissue, and most of the cells in a given tissue are clonal, having arisen

OUTSIDE OF CELL

2. The binding of the CLV3 multimer to the extracellular domain of the CLV1/CLV2 heterodimer induces autophosphorylation of the cytoplasmic domain of CLV1.

3. Phosphorylated CLV1 binds to the downstream effector molecules: kinase-associated protein phosphatase (KAPP) and rho-GTPase (ROP).

Plasma membrane

Plasma membrane

Wus Clv3 Expression

CLV2

1. WUS gene expression promotes the expression of the CLV3 gene.

CLV2

3. Phosphorylated CLV1 binds to the downstream effector molecules: kinase-associated protein phosphatase (KAPP) and rho-GTPase (ROP).

4. KAPP is a negative regulator of CLV1.

Clv1 Clv3 Clv2

MAPKs?

4. KAPP is a negative regulator of CLV1.

MAPKs?

1. WUS gene expression promotes the expression of the CLV3 gene.

CYTOPLASM

5. ROP may act through a mitogen-activated protein kinase (MAPK) cascade to repress WUS gene expression, forming a negative feedback loop.

FIGURE 16.24 Model of the CLAVATA1/CLAVATA2 (CLV1/CLV2) receptor kinase signaling cascade, forming a negative feedback loop with the WUS gene. See Chapter 14

on the web site for further information about receptor kinase signaling pathways. (After Clark 2001.)

from the same stem cell. However, most evidence supports the view that cell fate does not depend on cell lineage, but instead is determined by positional information (Scheres 2001).

In the vast majority of cases, shoot epidermal cells are derived from a small number of stem cells in the L1 layer. However, the derivatives of the L1 layer are committed to become epidermal cells because they occupy the outermost layer and lie on top of the cortical cell layer, not because they were clonally derived from the stem cells in the L1 layer.

The plane in which a cell divides will determine the position of its daughter cells within a tissue, and this positioning in turn plays the most significant role in determining the fate of the daughter cells. The strongest evidence for the importance of position in determining a cell's ultimate fate comes from an examination of the fate of cells that are displaced from their usual position, such that they come to occupy a different layer.

The vast majority of the divisions in the L1 and L2 layers of the meristem are anticlinal, and anticlinal division is responsible for generating the layers in the first place. Nevertheless, occasional periclinal divisions occur, causing one derivative to occupy the adjacent layer. This periclinal division does not alter the composition of the tissue derived from this layer. Instead, the derivatives assume a function that is appropriate for a cell occupying that layer.

Further support for the importance of position in determining cell fate has been obtained through observations of cell differentiation in leaves of English ivy (Hedera helix), which have a mixture of mutant and wild-type cells. When a mutation occurs in a stem cell in the shoot apical meristem, all the cells in the plant derived from that stem cell will carry the mutation. Such a plant is said to be a chimera, a mixture of cells with a different genetic makeup. The analysis of chimeras is useful for studies on the clonal origin of different tissues.

When the mutation affects the ability of chloroplasts to differentiate, the presence of albino sectors shows that these sectors were derived from the stem cells carrying the mutation. In the ivy plant shown in Figure 16.25, the L2 layer carried a mutation causing albinism, and the L1 and L3 layers had a wild-type copy of the same gene. The L1 layer gives rise to the leaf and stem epidermis, but it is colorless because chloroplasts do not differentiate in most epidermal cells. Mesophyll tissue typically is derived from the L2 layer, so the leaves should be white because the L2 stem cells carried the mutant gene and passed it on to their derivatives.

All Leaves Names English

FIGURE 16.25 Periclinal chimeras demonstrate that the mesophyll tissue has more than a single clonal origin in English ivy (Hedera helix). These variegated leaves provide clues on the clonal origins of different tissues. A mutation in a gene essential for chloroplast development occurred in some of the initial cells of the meristem, and cells derived from these mutated stem cells lack chloroplasts and are white, while cells derived from other stem cells have normal chloroplasts and appear green. (Courtesy of S. Poethig.)

FIGURE 16.25 Periclinal chimeras demonstrate that the mesophyll tissue has more than a single clonal origin in English ivy (Hedera helix). These variegated leaves provide clues on the clonal origins of different tissues. A mutation in a gene essential for chloroplast development occurred in some of the initial cells of the meristem, and cells derived from these mutated stem cells lack chloroplasts and are white, while cells derived from other stem cells have normal chloroplasts and appear green. (Courtesy of S. Poethig.)

Although a few of the leaves are white, or nearly so, most of the leaves show green patches. They are variegated. The green tissue in these leaves was derived from the cells originally in the L1 or L3 layer; the colorless regions were derived from the L2 layer. The variegation occurs because occasional periclinal divisions in the L1 or L3 layer early in leaf development establish clones of cells that can differentiate as green mesophyll cells. This is further evidence that cell differentiation is not dependent on cell lineage. The fate of a cell during development is determined by the position it occupies in the plant body.

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  • jennifer
    What is the cell's of leaf primordia?
    6 years ago
  • Hanna-Mari Hanski
    How does cell position determine fate?
    6 years ago
  • abbondanzio
    What is the function of.leaf primordia?
    5 years ago
  • james
    Is cell fate is determined by its cell lineage in plants?
    3 years ago

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