apical meristem

Establish Apical Meristems 

in Plant Embryos

Stages of Plant Embryogenesis

Uneven Cell Division Determines Polarity 

Mechanism of Uneven Cell Division 

in Plant

Radial pattern of tissue meristems are 

established in a globular embryo 

a. protoderm epidermis 

b. ground meristem cortex 

c. procambium vascular tissues 

Apical growth zones, meristems 

are established in the heart-shaped embryo 

Root Apical 

Meristems (RAM) 

remain dormant 

until seed 

germination 

First lateral organs 

are formed 

- embryonic 

leaves, cotyledons

Positional references provided by the 

early embryo pattern 

Auxin Determines Embryo Axis 

Formation (Positional Effect)

Cuc1 and cuc2 

determine the 

expression area 

for STM 

STM determines 

the area of SAM 

Wuc maintains the 

source of stem 

cells in SAM 

AS1 promote 

development of 

lateral organs 

Cell Fate Is Determined in 

L2 Layer of Meristem

Primary Growth of The Plant Body 

Primary growth: 

Apical meristems extend roots 

and shoots by giving rise to the 

primary plant body 

• Primary growth produces the primary plant 

body - the parts of the root and shoot 

systems produced by apical meristems. 

• An herbaceous plant and the youngest parts 

of a woody plant represent the primary plant 

body. 

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Primary Growth of The Shoot 

Structure of The Shoot Apical Meristem

There are three apical meristems in terminal buds 

• apical meristem : The apical meristem of a shoot is a 

dome-shaped mass of dividing cells at the terminal bud. 

• leaf primordia (derived from apical meristem) : Leaves 

arise as leaf primordia on the flanks of the apical 

meristem. 

• Axillary buds (derived from apical meristem) develop 

from islands of meristematic cells left by apical meristems 

at the leaf primordia base. 

leaf primordia 

Axillary buds 

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 

Organization of SAM 

apical meristem 

tunica – layered due to cell division planes at right angle to 

surface 

corpus – random cell division plane 

central zone – low rate of cell division 

peripheral zone – high rate of cell division

The low frequency of cell 

division in the central zone 

A pulse feeding of plants with 

3 H-thymidine result in 3 Hthymidine 

being incorporated 

in cells undergoing DNA 

synthesis and mitosis 

Sectioning and tissue sections 

are exposed x-ray films to 

visualize highly mitotic area 

(black) 

Interpretation: 

Rarely diving initial cells in the central zone. 

Frequently dividing daughter cells in the peripheral zone. 

Three Layers of Cells in SAM 

L1, anticlinal division epidermis 

L2, anticlinal divisions internal tissues 

L3, divisions internal tissues

Clonal analysis in chimeras – marking individual cells and 

tracing their progeny through successive cell divisions into 

the mature stages of plants 

chemical -(colchicine) 

induced polyploidy 

radiation-induced chloroplast 

pigmentation mutants. 

Screen functional genes that regulate formation of SAM 

Screening Mutants of Plants

The Genes that control SAM

CLV1,3 and WUS 

Maintain The Size of SAM 

CLV1,3 Restrict Expression 

of WUC at SAM

The Model of CLV/WUC Pathway 

Evidence 

1. In situ 

2. wus mutants lack CZ 

3. wus mutants have 

reduced/absent CLV3 expression 

WUS overexpression leads to 

enlarged domain domain of CLV3 

expression 

4. Immunolocalization 

5. CLV3 and CLV1 interaction 

leads to phosphorylation of CLV1 

intra cellular domain 

when expressed in yeast cells 

6. clv mutants have an expanded 

domain of WUS expression 

7. clv mutants have an expanded 

CZ region 

Apical meristems form primary meristerms. 

Primary meristems produce primary tissues


Fig. 35.18 

• Unlike their central position in a root, the 

vascular tissue runs the length of a stem in 

strands called vascular bundles. 

– At the transition zone, the stem’s vascular 

bundles converge as the root’s vascular 

cylinder. 

• Each vascular bundle of the stem is 

surrounded by ground tissue. 


• In most dicots, the vascular bundles are arranged in a ring, with pith on the 

inside and cortex outside the ring. 

– The vascular bundles have their xylem facing the pith and their phloem facing the 

cortex. 

– Thin rays of ground tissue between the vascular bundles connect the two parts of 

the ground tissue system, the pith and cortex. 


Primary Growth of The Leaf

Meristems of Leaves Are Derived from Shoot Apical Meristem

Knox Family Controls Formation of Leaf Initiation 

Determination of Area for 

Initiation of Leaf Primordia 

Meristem–leaf signalling 

Panel b shows a transverse view of a shoot 

apex.SHOOTMERISTEMLESS (STM) encodes a 

class I KNOX homeodomain transcription factor 

that is expressed throughout the SAM (purple) but 

is absent from leaf founder cells (indicated as a 

green domain in the SAM). ASYMMETRIC 

LEAVES1 (AS1) encodes a MYB transcription 

factor and AS2 is a member of the LATERAL 

ORGAN BOUNDARIES family of putative 

transcription factors. Expression of these two 

genes is excluded from the SAM and restricted to 

leaf primordia (green). Genetic analyses indicate 

that STM negatively regulates AS1 and AS2 

function in the SAM and down regulation of STM in 

leaves allows AS1 and AS2 expression. AS1 and 

AS2, in turn, negatively regulate other class I 

KNOX genes so that KNAT1,KNAT2 and KNAT6 

are ectopically expressed in the leaves of as1 and 

as2mutants. The additional loss of KNAT1 function 

in stm;as1 double mutants results in loss of a SAM, 

indicating that KNAT1 functions redundantly with 

STM in maintaining a SAM.

Determination of Leaf 

Polarity 

Compound leaves: equal to the sum of their 

parts? 

Connie Champagne and Neelima Sinha* 

Section of Plant Biology, University of 

California, 1 Shields Avenue, Davis, CA 95616, 

USA 

*Author for correspondence (e-mail: 

nrsinha@ucdavis.edu) 

Development 131, 4401-4412 

Determination of Axial 

Polarity of Leaf Primordia 

Leaf–meristem signalling 

Panel c shows a transverse view of a 

shoot apex.PHABULOSA (PHB) and 

PHAVOLUTA (PHV) encode class III 

homeodomain zipper (HD-ZIP) 

transcription factors that are expressed 

throughout the SAM,with high expression 

in rays extending from the SAM to the 

youngest leaf primordia, and in the 

adaxial domain of older leaf primordia 

(purple). YABBY (YAB) and KANADI 

(KAN) gene families encode putative 

transcription factors:YAB proteins contain 

zinc finger and HIGH MOBILITY GROUP 

(HMG) DOMAINS, and KAN genes 

belong to a larger gene family of 

transcriptional regulators that contains a 

GARP DOMAIN109.They are expressed 

in the abaxial domain of leaf primordia 

(green) and YAB members repress STM 

and KNAT1 expression in the leaf.

Knox 1 Determines Whether 

Leaves Become Compound 

Compound leaves: equal to the sum of their parts? 

Connie Champagne and Neelima Sinha* 

Section of Plant Biology, University of California, 1 Shields Avenue, Davis, CA 95616, USA 

*Author for correspondence (e-mail: nrsinha@ucdavis.edu) 

Development 131, 4401-4412 

Knox-1 Promotes Formation of 

Leaflets 

Overexpression of this gene causes the compound leaves of a 

tomato plant to become “supercompound”.

• The vascular tissue of a leaf is 

continuous with the xylem and 

phloem of the stem. 


• Axillary buds have 

the potential to form 

branches of the shoot 

system at some later 

time. 

– While lateral roots 

originate from deep 

in the main root, 

branches of the 

shoot system 

originate from 

axillary buds, at the 

surface of a main 

shoot. 

– Because the 

vascular system of 

the stem is near the 

surface, branches 

can develop with 

connections to the 

Copyright vascular © 2002 Pearson tissue Education, Inc., publishing as Benjamin Cummings

Leaves Separated by Elongation of 

Internodes 

• Within a bud, leaf primordia are crowded close together because internodes are 

very short. 

– Most elongation of the shoot occurs by growth in length of slightly older internodes below the shoot 

apex. 

– This growth is due to cell division and cell elongation within the internode. 

– In some plants, including grasses, internodes continue to elongate all along the length of the shoot 

over a prolonged period. 

• These plants have meristematic regions, called intercalary meristems, at the base of each internode. 


Fig. 35.19 


MSU-DOE Plant Research Laboratory 

Michigan State University 

East Lansing, MI 48824-1312 USA 

Phone: (517) 353-7865 

Fax: (517) 353-9168 

e-mail: hkende@msu.edu 

Hans Kende 

Professor of Plant Biology 

University Distinguished Professor 

U.S. National Academy of Sciences 

German Academy of Natural Scientists 

Ph.D. (University of Zurich)

Primary Growth of The Root

The Structure of The Root Tip 

Fig. 35.14 


• The root tip is covered by a 

thimblelike root cap, which 

protects the meristem as the 

root pushes through the 

abrasive soil during primary 

growth. 

– The cap also secretes a 

lubricating slime. 

– Calyptrogen: meristem for root 

cap 


•Growth in length is concentrated near the 

root’s tip, where three zones of cells at 

successive stages of primary growth are 

located. 

•the zone of cell division, 

•the zone of elongation 

•the zone of maturation 

• Root apical meristem: 

1. The apical meristem produces the cells of the 

primary meristems and also replaces cells of 

the root cap that are sloughed off. 

2. Produce primary meristems 

3. Near the middle is the quiescent center, 

cells that divide more slowly than other 

meristematic cells. 

• These cells are relatively resistant to damage 

from radiation and toxic chemicals. They may 

act as a reserve that can restore the meristem if 

it becomes damaged. 

• Primary meristems. 

1. Protoderm will produce dermal tissues 

(including root hair 

2. Procambium will produce vascular tissues 

3. ground meristem will produce ground 

tissues 

The zone of cell division 


• The zone of cell division 

blends into the zone of 

elongation where cells 

elongate, sometimes to 

more than ten times their 

original length. 

– It is this elongation of cells 

that is mainly responsible 

for pushing the root tip, 

including the meristem, 

ahead. 

– The meristem sustains 

growth by continuously 

adding cells to the youngest 

end of the zone of 

elongation. 


• In the zone of maturation, cells begin to 

specialize in structure and function. 

– In this root region, the three tissue systems 

produced by primary growth complete their 

differentiation, their cells becoming 

functionally mature. 


• Three primary 

meristems give rise to 

the three primary 

tissues of roots. 

– The epidermis 

develops from the 

dermal tissues. 

– The ground tissue 

produces the 

endodermis and cortex. 

– The vascular tissue 

produces the stele, the 

pericycle, pith, xylem, 

and phloem. 


• The protoderm, the outermost primary meristem, 

produces the single cell layer of the epidermis. 

– Water and minerals absorbed by the plant must enter 

through the epidermis. 

– Root hairs enhance absorption by greatly increasing 

the surface area. 


• An established root may 

sprout lateral roots from the 

outermost layer of stele, the 

pericycle. 

– Located just inside the 

endodermis, the pericycle is 

a layer of cells that may 

become meristematic and 

begin dividing. 

– Through mitosis in the 

pericycle, the lateral root 

elongates and pushes 

through the cortex until it 

emerges from the main root. 

– The stele of the lateral root 

maintains its connection to 

the stele of the primary root. 

Fig. 35.16 


Fig. 35.15 


Secondary Growth of The Plant Body 


• The stems and roots, but not 

the leaves, of most dicots 

increase in girth by secondary 

growth. 

– The secondary plant body 

consists of the tissues produced 

during this secondary growth in 

diameter. 

– The vascular cambium acts as 

a meristem for the production of 

secondary xylem and secondary 

phloem. 

– The cork cambium acts as a 

meristem for a tough thick 

covering for stems and roots that 

replaces the epidermis. 


• Vascular cambium is a cylinder of 

meristematic cells that forms secondary 

vascular tissue. 

– The accumulation of this tissue over the years 

accounts for most of the increase in diameter 

of a woody plant. 

– Secondary xylem forms to the interior and 

secondary phloem to the exterior of the 

vascular cambium. 

Fig. 35.20 


Fig. 35.21 


lenticels

Thickening of Perennial Monocot

• While the pattern of growth and 

differentiation among the primary and 

secondary tissues of a woody shoot 

appears complex, there is an orderly 

transition of tissues that develop from the 

initial apical meristem of the stem. 


Fig. 35.24

Phase Change 

Turn Vegetative into Reproductive 

Phase changes mark major shifts in 

development 

juvenile state mature state

Local example: Garland chrysauthemum 

juvenile state mature state 

• The leaves of juvenile versus mature shoot 

regions differ in shape and other features. 

– Once the meristem has laid down the juvenile 

nodes and internodes, they retain that status 

even as the shoot continues to elongate and 

the meristem eventually changes to the mature 

phase. 

Fig. 35.34 


Flower meristem identity genes 

form flower meristems 

Fig. 35.35 


Meristem Identity mutants 

A. lfy flower 

B. lfy inflorescence 

C. ap1 flower 

D. lfy ap1 inflorescence 

E. tfl flower & whole plant 

Organ identity genes regulate positional information and function 

in the development of the floral pattern.

• Organ identity genes code for transcription 

factors. 

– Positional information determines which organ identity 

genes are expressed in which particular floral-organ 

primordium. 

– In Arabidopsis, three 

classes of organ identity 

genes interact to produce 

the spatial pattern of floral 

organs by inducing the 

expression of those genes 

responsible for building 

an organ of specific 

structure and function. 


Fig. 35.36

apical meristem

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