Cell expansion plays a major role in growth

Cell expansion plays a major role in growthRoot cells expand their volume 50 times by expanding lengthwise but not widthwise

In roots, cell expansion plays a major role in growthTwo competing plant hormones determine the

direction of cell expansion:GA (gibberellic acid) promotes growth along the lengthEthylene promotes growth along the width

Auxin and Cytokinin control shoot and root growth

High levels of Cytokinin and low levels of Auxin promote shoot development (stems with leaves)

High levels of Auxin and low levels of Cytokinin promote root development

The Miller-Skoog Experiment: Cloning

1. Place single cell on medium with high levels of Cytokinin and low levels of Auxin to promote shoot development (stems with leaves)

2. Place shoots on medium with high levels of Auxin and low levels of Cytokinin to promote root development at the base of the shoot

3. Transfer rooted shoots to soil and grow plants to maturity

Figure 38.2 Review of an idealized flower

Pollination is the first step of the fertilization process.

The pollen “germinates” and grows down into theovary where fertilization of the egg occurs.

Even at this one cell stage the embryo reveals polarity.The first cell division is asymmetric, producing a small

apical cell and a larger basal cell.

The apical cell will later give rise to the entire“embryo proper”. The basal cell will give rise to a small

umbilical cord-like structure called the suspensor.

The small apical cell divides several times to generate the globular embryo. All cells of this embryo appear

morphologically similar.

Several divisions later morphological asymmetry is seen in the heart shaped embryo.

Arabidopsis embryogenesis

Cotyledons (seed leaves)Shoot Apical Meristem

Hypocotyl (seedling stem)

Root Apical Meristem

Plant Stem Cells: Shoot and root meristem

Weigel and Jürgens, 2002; Bowman and Eshed, 2000; Nakajima and Benfey, 2002

- located at the tips of growing shoots

What is a shoot apical meristem?

-a group of undifferentiated “stem” cells

-stem cells renew themselves while generating lateral organs off the flanks

- 3 types: vegetative, inflorescence, floral

Gerd Jurgens searched for embryo pattern mutants.

1. Soak seeds in a mutagen

2. Grow plants to maturity

These plants would be carriers of mutations (m/+)

3. When these carriers self-fertilize, the resulting

embryos would be: +/+, m/+, m/m

Mutants similar to gap mutants in flies were identified, lacking regions of the embryo, including the apical structures, the stem (hypocotyl) and root

Embryo Pattern Mutants

Organization of the SAM

Fletcher 2003

L1 and L2 cells divide anticlinally:perpendicular to the surface

These divisions contribute to surface growth without increasing the number of cell layers

L3 cells divide inboth planes to add additional

cell layers to the shoot.

Organization of the SAM

Fletcher 2003

Shoot Apical Meristem

The shoot apical meristem can be divided into distinct zones.

-stem cells

The central zone is maintained as a pool of undifferentiated stem cells.

-peripheral zone

The peripheral zone is the site of organ initiation.

-stem cells -peripheral zone

As cell divisions occur in the central zone, the resultingcells are pushed into the peripheral zone where they areincorporated into organ primordia.

Dividing Stem Cells are Pushed into the Peripheral Zone

The central zone cells will give rise to all of the above-ground organs of the mature plant.

How is the stem cell population maintainedthroughout the life of the plant?

A feedback loop between organ initiation (peripheral zone) and the stem cell (central zone) population regulates the size of the meristem.

WUSCHEL and SHOOTMERISTEMLESS mutants fail to develop a shoot apical meristem.

Genes Controlling Meristem Development

Normal heart-stage embryo

WUS or STM mutantembryo

STM and WUS mutants do not form a shoot apical meristem

CLAVATA1 and CLAVATA3 mutants developa greatly enlarged shoot apical meristem.

Normal heart-stage embryo

WUS or STM mutantembryo

CLV1 or CLV3 mutantembryo

CLV1 mutants have a larger meristem and make more stem cells

wt clv1

Fletcher et al., 1999

CLV3 mutants make more stem cells and resemble CLV1 mutants

• STM and WUS are required to form and maintain the stem cell population

• CLV1 and CLV3 are required to prevent the over-proliferation of the undifferentiated stem cell population

• STM and WUS are homeobox genes and encode proteins that function as transcription factors

• CLV1 encodes a receptor protein

• CLV3 encodes a small protein that functions as a signaling molecule that binds to the CLV1 receptor

CLV / WUS Interactions

CLV3 is expressed in the L1 and L2 cell layers of the central zone

CLV1 and WUS are expressed in a small domain of L3 cells in the central zone

CLV3 expression is lost in WUS mutants.

Therefore, WUS activates CLV3 expression.

The expression domain of WUS is greatly enlarged in CLV1 and CLV3 mutants.

Therefore, CLV1 and CLV3 negatively regulate (repress) WUS expression.

CLV3 binds to and activates the CLV1 receptor, which then represses WUS expression.

WUS is part of an “organizing center” that promotes stem cell proliferation in overlying cells.

A feedback loop between CLV and WUS maintains the stem cell population throughout the life of a plant.

• The greatly enlarged meristems that form in clv mutants are largely suppressed when the activity of STM is reduced (for example, in stm/+ plants).

Genetic Interactions between STM and CLV

• The greatly enlarged meristems that form in clv mutants are largely suppressed when the activity of STM is reduced (for example, in stm/+ plants).

• Similarly, the loss of shoot meristems in stm mutants is restored in plants that have reduced CLV activity (for example in clv/+ plants).

Genetic Interactions between STM and CLV

Young Leaf

Shoot Tip

Axillary Bud

Internode

Phytomere

NodeLeafAxillary BudInternode

Internode

Increase in length of the stem occurs largely by internodal elongation.

Plant cells are surrounded by rigid cell walls.

Cell migration does not occur in plants.

Fertilization

Figure 38.10 The development of a dicot plant embryo

A successful fertilization will produce afertilized egg with 2X DNA.

Cell expansion plays a major role in growth

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