1. Presentation on
Applications Of
Meristematic Stem Cells
in Plant Growth
By
Poondru Sushma
(2011A3PS166G)
Under Guidance of
Dr. Vijayashree Nayak

2. Meristematic Cells
•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.

3. Apical Meristem
Organization of an
apical meristem
(growing tip)
1 - Central zone
2 - Peripheral zone
3 - Medullary (i.e.
central) meristem
4 - Medullary
tissue

4. Apical Meristems
Primary Meristems
Protoderm Procambium Ground
Meristem
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.

5. 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.

6. 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

7. Shoot Apical Meristem
3 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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 .

15. 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.

16. 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