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Stomata and Trichome Development

Authors:
Dr. V. Vadivel
Introduction
Stomata are specialized epidermal structures.
Stomata act as turgor-operated valves for gas
exchange
Stomata are essential for plant survival. Why?
Control the entry of co2assimilation in
photosynthesis
Optimize water use efficiency.
Stomata - found in epidermis of leaves, young parts of
stem, stamen & carpel.
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Stomata - Structure
Stomatal pore connects
intercellular spaces found inside
the plant to the atmosphere.
Stomata consists of a pair of
guard cells (GCs).
Guard cells are surrounded by
subsidiary cells (SCs).
Stomata are separated from
one another by at least one SCs
because proper opening and
closing of stomata requires H2O
and ions from surrounding cells
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Stomata occurrence
Bifacial leaf highest
in lower epidermis;
few in upper epidermis
Suberrect leaves
stomata occur both
surfaces
In floating leaves
stomata occur on the
upper leaf.
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Guard cell - Position
Same level as the
neighboring cells
Slightly raised above
neighboring cells
(Solanum tuberosum)
May be sunken
(Musa)
May be in shallow
[Nerium (xerophytes)]
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Guard cells
Living cells
chloroplasts
mitochondria
Endoplasmic
reticulum
Sphaerosomes
Peroxisomes
Shape
Kidney
Dumbell -
maize
Cell wall
Inner wall -
thick
Out wall - thin
Protuberance
Upper & lower
sides Normal
Upper side Solanum
tuberosum
Absent Pinus
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Types of stomata
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Stomata
Development
3. Mesogenous
2. Mesoperigenous
1. Perigenous
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1. Preigenous Stomata Development
[grass]
First stage (i)
Asymmetric divisions occur in
the middle cells.
Small stomatal initial cell
towards the leaf tip and larger
cell towards leaf base.
New transverse cell walls are
formed.
This results an alternation of
stomatal initials.
This initials develop into a
guard mother cell.
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1. Preigenous Stomata Development
[grass]
Second stage (ii)
Adjacent cells found on both
sides of the guard mother cell
undergo asymmetric
divisions.
Later develops cell walls
Third stage (iii)
Guard mother cell divides
symmetrically to form two
guard cells that surround a
pore.
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2. Mesoperigenous Stomatal
Development
Amborella leaves
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3. Mesogenous stomatal formation
Arabidopsis leaves
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Stomata Development in
Amborella trichopoda
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Introduction
Trichome meaning hair.
Trichomes on plants are epidermal outgrowths
They are of diverse structure and function.
Examples - hairs, glandular hairs, scales, and papillae.
Trichomes are unicellular or multicellular
They may be living or dead and may persist throughout the life
of a plant part or remain only for a short time.
Trichomes are present on the leaves, stems, sepals and seeds.
They are absent from the roots, hypocotyl, cotyledons, petals,
stamens and carpels.
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Trichome - Function
Foliar trichomes - regulats moisture exchange
with the atmosphere.
Covering hairs - plant defence, especially
against phytophagous insects.
Secretory trichomes - chemical defence against
insects
Stinging hairs - protect plants from animals.
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Trichome Development
Branched unicellular trichomes differentiate from undistinguished precursor
cells in the protoderm.
These precursor cells initiate the differentiation pathway by undergoing
deoxyribonucleic acid (DNA) synthesis without accompanying cytokinesis,
so that trichome precursors typically have eight or sixteen times the amount
of DNA of adjacent pavement cells.
Next, trichome precursors begin cell expansion in the plane perpendicular
to the epidermis, forming a tubular extension.
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Trichome Development
Once this stalk is formed, the nucleus migrates from the base of the stalk to its
tip, using the cell's cytoskeleton to pull it to anew location.
The trichome then undergoes an unusual pattern of cell wall growth, in which the
cell wall balloons out at three locations, forming the three trichome branches.
When the trichome cell has reached its full size and shape, it adds thickness to
its cell wall and deposits sharp crystals of calcium oxalate on the surface of the
trichome, adding to its effectiveness in defense against herbivores.
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Trichome Development
(molecular level)
Trichome development starts with cell fate specification of some
protodermal cells in the epidermis by expression of the
homeobox gene GL2.
These cells arrest cell division and switch to the endoreplication
programme and genetically controlled by different genes.
Late during trichome development, GTL1 accumulates leading
to termination of the endocycle programme and to arrest of the
endoreplicationindependent trichome growth.
Red and green fonts indicate activators and repressors of the
pathway, respectively.
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Trichome development
(Arabidopsis)
Trichomes are present on the leaves, stems, and sepals.
They are absent from the roots, hypocotyl, cotyledons, petals,
stamens, and carpels.
Trichome morphology
Unbranched found on the stems and sepals.
Branched (2 to 5 branches) found on leaves.
Different trichomes have different morphologies but their
development is controlled by the same genes.
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Trichome development proceeds as awave across the
epidermal surface on the leaves.
The first trichome initiates on the tip of the adaxial surface of the
first primordium after it achieves a length of approximately
100μm.
As trichomes mature at the leaf tip, new triochomes emerge
progressively toward base.
In addition, new trichomes initiate in between developing
trichomes.
Trichomes are separated from one another by dividing
epidermal cells.
Trichome initiation is found only in regions where epidermal cell
division is occuring.
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The first detectable step in the commitment to the trichome cell
fate is acessation of cell division.
However, nuclear DNA synthesis continues,and the committed
cell undergoes at least two rounds of endoredupication,
reaching at least 8N.
Cells surrounding a committed cell continue to divide normally.
After the committed cell radially expands to a diameter that is
approximately twofold greater than the surrounding cell, it
begins to expand preferentially on its outer surface to form stalk.
As the stalk forms, diffuse growth throughout the cell results in
continued radial expansion.
The nucleus migrates into the aerial portion of the stalk shortly
before secondary protuberances, which subsequently expand
into the branches, emerge from the aerial tip.
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During branch formation, the nucleus undergoes another round
of endo-reduplication and migrates to the base of the last branch
that forms.
Expansion ceases when a trichome reaches a height of 200-
300μmand a base diameter of approximately 50μm.
During trichome maturation the cell wall thickens to
approximately 5μm.
Finally crystals of calcium oxalate are deposited on the surface
of the trichome, adding to its effectiveness in defense against
herbivores.
And, the epidermal cells around the base of the trichome acquire
a distinct rectangular shape.
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Trichome development in
Connarus subcrosus
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Glandular Trichomes of the Inner Calyx
surface of Adenocalymma magnificum.
Calyx has capitate glandular trichomes.
It has a long peduncle composed of two
to nine cells, including two apical cells
(collar cells).
It has a multicellular secretory head,
composed of up to 10 columnar-shaped
cells, arranged in a disk.
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Trichomes are randomly
distributed, occupying a large
inner surface area of the
calyx along its entire length
and is also visible on the
flower buds.
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Trichome development in
Adenocalymma magnificum
Capitate trichome originates from the protodermis on the adaxial
face of the calyx.
Trichome precursor cell is larger than the adjacent ones, whose
projection is slightly lateral with a spherical nucleus (Fig. C).
Initially, this cell increases in volume and later undergoes a
periclinal division (Fig. D), becoming a bicellular structure,
formed by a peduncle cell and a conical apical cell, where it
undergoes successive periclinal divisions (Fig. E).
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It is composed of a uniseriate row of nine cells (Fig. F),
which corresponds to eight cells of the peduncle and the
initial cell of the secretory head.
This apical cell passes through several anticlinal divisions,
which extends forming a columnar multiseriate secretory
head, containing up to 10 columnar cells arranged in a
disk.
After this stage, the collar cell, which is under the secretory
head, divides anticlinally (Fig. G) and arranges itself side by
side with the mother cell (Fig.H).
As they develop, they will bend, with some of them
becoming adpressed to the calyx (Fig. I).
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C -I. Capitate glandular
trichome development.
C. Precursor cell with
lateral projection.
D. First periclinal division.
E. Successive periclinal
divisions.
F. Uniseriate row. G.
Collar cell evidencing the
anticlinal division.
H. Peduncle apical cells
side by side (collar
cells).
I. Capitate glandular
trichome adpressed.
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