POLLEN WALL MORPHOGENESIS

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POLLEN WALL MORPHOGENISIS

• Presented by Anjali Krishnan
• Roll No 13

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CONTENTS

• Pollen wall
• Layers of pollen
• Intine
• Exine
• Development of Pollen Wall
• Pollenkitt
• Sporopollenin
• References

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POLLEN wall

• The wall of the pollen is stratified.
• It comprises of two principal layers the inner
  one called intine outer one called exine.
• The exine consists of many sub layers

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Wall Layers of pollen

• Pollen Wall

Exine
Intine
Ektexine
Endexine
Tectum
Baculum
Foot-layer
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INTINE

• The intine is pecto-cellosic in nature.
• The cellulose component is microfibrillar, with
  the microfibrills oriented in a plane parallel to
  the surface. A special feature of the intine is
  the presence of beads, ribbons, plate of
  enzymatic proteins, particularly in the vicinity
  of the germ-pores.

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Exine

• The exine is composed of a class of material
  called Sporopollenin.
• It is derived from carotenoids by oxidative
  polymerisation. Sporopollenin is resistant to
  physical biological decomposition. Because of
  this property the pollen grain walls are often
  preserved for long periods in fossil deposits.
• The pollen wall also fulfills the important
  protective function during the journey of pollen
  from anther to the stigma.

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Development Of Pollen Wall

• At the end of meiosis 4 haploid microspores are
  enclosed in a common callose wall. The individual
  spore lacks a wall of its own is separated from
  the other spores of the tetrad by callose
  partitions.
• While they are still enclosed in the callose wall
  the individual spore starts forming its wall.
• There are 2 phases in the formation of pollen
  wall.

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• In the first phase the wall material is
  contributed by the cytoplasm of the spore alone,
  this is during the tetrad stage.
• In the second phase, starts after the release of
  the spores through the enzymatic degradation of
  the callose wall, the wall materials are
  contributed by the tapetal cells in addition to
  the spore cytoplasm.

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• The first layer of the pollen wall to be formed
  is cellulosic is known as primexine. Cellulose
  microfibrills of this layer are deposited in
  between the convoluted plasma lemma of the spore
  the callose wall.
• The cellulosic primexine is discontinuous in
  these origins the gaps thus created mark the
  position of germ-pores.
• When the cellulosic primexine has reached a
  certain thickness additional gaps appear in it,
  columns of convoluted lamellae are deposited in
  these gaps at the surface of the plasma lemma.

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Formation of Ubisch bodies Development of
pollen wall
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• These columns are called probacula. The cytoplasm
  of the spore starts synthesising the precursors
  of sporopollenin which are polymerised
  deposited on the surface of the lamellae. The
  columns are now called the bacula.
• The distribution orientation of the bacula vary
  according to the pattern of the mature exine.
• Later, the lower ends of the bacula sideways to
  the cellulosic layer primexine combine to form
  the foot layer.

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• The foot-layer forms a sort of floor on which are
  raised the columns bacula.
• The tops of the bacula-columns may also spread
  sideways in all directions to form the tectum.
• Tips of the bacula may also simply enlarge to
  form knobs.
• To this extent the exine is formed inside the
  callose wall of the tetrads.
• The pattern of mature exine has already been laid
  down before the spores are set free.

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• With the age of the tetrads, the callose is
  gradually digested the individual spore(pollen
  grain) lies free within the anther locule.
• In the free state pollen grains sysnthesise the
  intine the inner most layer of the exine.
• The formation of the intine is by the activity of
  the dictyosomes.
• However, there are two special features of intine
  formation.

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• Firstly during the early growth of intine the
  thickening of the inner most layer of the exine
  continues, the lamellar material the
  sporopollenin precursors, which are contributed
  by the spore cytoplasm, pass through the
  developing intine.
• Secondly, certain proteinaceous plates or ribbons
  are incorporated in the intine in the vicinity of
  the germ-pores. These proteins show enzymatic
  activity.
• While the spore wall formation is going on inside
  the tetrad wall, some curious rounded bodies of
  lipid nature appear in the cytoplasm of the
  tapetalcells.

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• They are called pro-Ubisch bodies, migrate to the
  cell surface where they get coated with
  sporopollenin are released into the anther
  cavity.
• The sporopollenin-coated Ubisch bodies are
  involved in the external thickening of the exine.
• This deposition of sporopollenin from tapetum
  does not result in the appearance of a new
  structure. It simply contributes to the thickness
  of the pattern laid down in the tetrad stage.

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POLLENKITT

• The pollenkitt is an oily layer found on the
  outside of the mature pollen grains of many
  insect-pollinated species.
• Pollenkitt material is contributed by the tapetal
  cell. It comprises of lipid carotenoids.
• Carotenoids is responsible for its yellow or
  orange colour. It also contains some proteins
  responsible for its sticky nature.

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• The biological function of the pollenkitt is not
  clear
• Some of the suggestions regarding its functions
  are
• Acting as an insect-attractant.
• Protecting the pollen against the damaging
  effects of ultraviolet radiations.
• Because of the sticky nature it may be acting as
  an adherent to the insect body.
• Because of its hydrophobic nature it might even
  be associated with the dispersal of pollen.
• It may be functioning as the pollen-borne
  substances involved in sporophytic
  incompatibility.

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Sporopollenin

• Pollen grains consist of three substances
• The inside of the cell is filled with living
  cytoplasm that deteriorates rapidly during
  fossilisation.
• The inner layer of the cell wall, the intine,
  consists mainly of cellulose and pectin, this
  also degrades rapidly during fossilisation.

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• The outer cell wall, the exine, consists mainly
  of sporopollenin, an N-free polymeric substance
  belonging to the terpenes. Its chemical formula
  is C90 H130-158 024-44.
• Sporopollenin is chemically unsaturated and is
  corroded by oxygen (oxidation), but is otherwise
  resistant even to strongly alkaline substances
  and organic acids. Sporopollenin is thus one of
  the most resistant substances in the plant world.

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Diagram of Arabidopsis pollen wall structure
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REFERENCES

• Book
• The Embryology of Angiosperms
• Authors- S S Bhojwani
• S P Bhatnagar
• Publication - Vikas Publication House Pvt
  Ltd
• Year of Publication - 1974
• Edition - 1988 Edition
• Websites www.cambridgeuniversity.com
• www.google.com

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