Presentation on

Applications Of

Meristematic Stem Cells

in Plant Growth

By

Poondru Sushma

(2011A3PS166G)

Under Guidance of

Dr. Vijayashree Nayak

•Meristem cells are unspecialized cells found in plants, similar

to stem cells found in animals.

•Meristem cells form much of the cell growth with in a plant.

•This growth is broken down into upward and outward.

Upward and downward growth is the result of fast growing apical

meristem cells

Out ward growth is the result of somewhat slower lateral meristem cells.

• The term meristem was first used in 1858 by Karl Wilhelm

von Nägeli (1817–1891) in his book Beiträge zur

Wissenschaftlichen Botanik. It is derived from the Greek

word merizein, meaning to divide, in recognition of its

inherent function.

Meristematic Cells

Apical Meristem

Organization of an

apical meristem

(growing tip)

1 - Central zone

2 - Peripheral zone

3 - Medullary (i.e.

central) meristem

4 - Medullary

tissue

Apical Meristems

Protoderm Procambium Ground

Meristem

Primary Meristems

lies around the

outside of the stem

and develops into

the epidermis

lies just inside of the

protoderm and

develops into

primary xylem and

primary phloem. It

also produces

the vascular

cambium, a

secondary meristem

It develops in to

the cortex and the pith.

It produces the cork

cambium, another

secondary meristem.

Apical Meristems

At the meristem summit, there is a small group of slowly dividing cells, which is commonly called the central zone. Cells of this zone have a stem cell function and are essential for meristem maintenance.

Apical meristems are composed of several layers. The number of layers varies according to plant type. In general the outermost layer is called the tunica while the innermost layers are the corpus.

In monocots, the tunica determines the physical characteristics of the leaf edge and margin. In dicots, layer two of the corpus determines the characteristics of the edge of the leaf. The corpus and tunica play a critical part of the plant physical appearance as all plant cells are formed from the meristems.

Shoot Apical Meristem

The shoot apical meristem is the site of most of the embryogenesis in flowering plants. Primordia of leaves, sepals, petals, stamens and ovaries are initiated here at the rate of one every time interval, called a plastochron. It is where the first indications that flower development has been evoked are manifested.

The shoot apical meristem consists of 4 distinct cell groups:

*Stem cells

*The immediate daughter cells of the stem cells

*A subjacent organizing Centre

*Founder cells for organ initiation in surrounding regions

Shoot Apical Meristem3 interacting CLAVATA genes are required to regulate the size of the stem

cell reservoir in the shoot apical meristem by controlling the rate of cell division.

CLV1 and CLV2 are predicted to form a receptor complex (of the LRR receptor

like kinase family) to which CLV3 is a ligand.

Another important gene in plant meristem maintenance is WUSCHEL (shortened

to WUS), which is a target of CLV signaling. WUS is expressed in the cells below

the stem cells of the meristem and its presence prevents the differentiation of the

stem cells.

Meristem structure

• SAMs are usually dome shaped and have a layered structure described as

a tunica and corpus.

• The tunica consists usually of 2 cell layers (often only 1 in monocots) where

cell division occurs primarily in the anticlinal plane (perpendicular to the

surface) and the surface to remain as monolayers.

• The cells in the L3 layer divide in random orientations to form a multilayered

structure referred to as the rib meristem (RM) or the organizing center (OC),

which provides stem cell-promoting cues.

Meristem structure

• SAMs also show cytological zonations independent of the layered structure.

• Central zone that stains lightly with histological stains. The central zone

includes both tunica and corpus cells.

• The peripheral zone is a ring of densely staining cells that surround the

central zone and also includes both tunica and corpus.

• Below the central zone is an area called the rib meristem.

• Several approaches have shown a correlation in the mitotic activity with the

zonation.

• Mitotic activity is most prevalent in the peripheral region as shown by

incorporation of tritiated thymidine which reflects DNA replication and

therefore mitotic activity.

• The central zone is the region of the apical initials. These divide infrequently

and occasionally contribute cells to the rapidly dividing peripheral zone.

Because most genetic mistakes are generated during mitosis, this is probably a

mechanism to minimize the occurrence of genetic mistakes in the permanent

cell population of the meristem.

Initiation and maintenance of meristems

• Initiation of the shoot meristem in the embryo requires the action of several genes that were identified in genetic screens for mutants that lack a SAM.

• One factor is a pair of genes called CUP-SHAPED COTYLEDONS (CUC)1 and 2. These are duplicate factors. That is that there are two different genes that function redundantly and both have to be mutated to see a mutant phenotype.

• The phenotype consists of both cotyledons forming in a continuous fused cup-shaped structure with no SAM. Occasionally, one can get a shoot to form in tissue culture and then growth is fairly normal up to flowering. This indicates that the CUC genes are required for SAM initiation but not maintenance.

• Mutations in the SHOOT MERISTEMLESS (STM) also result in embryos without a SAM. Occasionally, in weak stm alleles, adventitious shoots will form but the shoots progressively lose their meristem indicating that STM is required for both the initiation and maintenance of SAMs. STM encodes a homeodomain transcription factor.

Initiation and maintenance of meristems

• Maintenance of shoot apical meristems requires regulating the balance between cell proliferation and differentiation.

• Several genes have been identified that regulate this balance. As mentioned, STM is required to maintain the SAM and in mutants the SAM dwindles away because of either too rapid cell differentiation or insufficient cell proliferation.

• Accordingly, the proposed function of STM is to either promote proliferation (i.e., promote the meristematic state) or to inhibit differentiation. Consistent with this, ectopic expression of STM-like genes, such as the maize KNOTTED gene, in leaves causes the formation of ectopic meristems.

• Another gene called WUSHEL (WUS) has a similar function (ie, similar mutant phenotype) and also encodes a homeodomain transcription factor.

• Three genes called CLAVATA (CLV)1,2 and 3 have the opposite effect in that mutations cause an over proliferation of cells in the SAM (the SAM gets too big). Therefore the function of these genes is to restrict cell proliferation or to promote cell differentiation.

Initiation and maintenance of meristems

• CLV1 is a receptor kinase and CLV2 is another receptor-like protein. These 2

are proposed to form a heterodimeric receptor complex.

• CLV3 is a small protein that is predicted to be secreted and is hypothesized to

function as the activating signal ligand for the CLV1/2 receptor complex.

• CLV1 is expressed mostly in the corpus region of the SAM and CLV3 is

expressed predominantly in the tunica region of the SAM, highlighting the

interaction among layers of the SAM.

• Because cells in both the tunica and corpus proliferate abnormally

in clv1 mutants, it is concluded that CLV1 acts non-autonomously.

• It is possible that activation of the CLV1 receptor results in the production of a

secondary signal that diffuses throughout the meristem.

• The balance between the activity of the proliferation promoting

(differentiation inhibiting) and the proliferation inhibiting (differentiation

promoting) pathways is required for the long-term function of the SAM.

Initiation and maintenance of meristems

• The CLV and STM pathways are antagonistic to each other

because clv and stm mutants partially suppress one another.

• This also indicates that these represent independent pathways

because each still functions in the absence of the other.

• On the other hand, the WUSHEL gene acts downstream of CLV

because clv / wus double mutants look just like wus.

• The CLV pathway acts to inhibit the expression of WUS which

acts to promote cell proliferation. In clv mutants there is nothing

to restrict WUS, resulting in overproliferation. However, if wus is

mutant, then it doesn’t matter whether CLV is around to restrict it

or not.

WUSHEL

• Cell–cell communication between distinct cell types within stem cell niches is critical for stem cell maintenance in both plants and animals, although they differ in their niche architecture and cell behaviors.

• In the shoot apical meristem (SAM) stem cell niche not all stem cells make contact with the niche, they also do not exhibit oriented and asymmetric cell divisions to regulate stem cell numbers.

• For example, the Arabidopsis SAM stem cell niche is a collection of ∼500 cells located at the growing tip of each shoot.

• The CZ of the SAM harbors stem cells. The stem cell progeny that are displaced into the adjacent peripheral zone (PZ) proliferate before differentiating.

• Visually, the SAM stem cell niche is a multilayered structure consisting of three clonally distinct layers of cells, and stem cells are found in each of these layers.

• WUSCHEL (WUS), a homeodomain-containing transcription factor, is both necessary and sufficient for stem cell specification.

• WUS RNA is found in a few cells of the RM/OC located just beneath the CZ .

WUSHEL

• Restriction of WUS transcription to cells of the OC is critical for maintaining a

constant number of stem cells, and this is mediated by the CLAVATA (CLV)

signaling pathway.

• CLAVATA3 (CLV3), expressed in the CZ, encodes a small peptide that is

secreted into the extracellular space and binds to CLAVATA1 (CLV1), a

leucine-rich repeat receptor kinase predominantly expressed in cells of the

RM.

• Activation of CLV1 and related receptor kinases has been shown to mediate

repression of WUS transcription through a signaling cascade that is not well

understood.

• WUS, which is expressed in cells of the RM/OC, not only specifies stem cell

fate in overlying cells of the CZ, but also activates its own negative regulator,

CLV3, in a non-cell-autonomous manner.

• Thus, the WUS–CLV feedback system forms a self-correcting mechanism for

maintaining a constant number of stem cells and the SAM size.

WUSHEL

*WUS protein migrates from OC/niche into adjacent cells

*WUS protein migration is required for shoot meristem

function

*WUS binds to CLV3 regulatory regions to activate its

transcription