Physiology of Seed Plants

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Physiology of Seed Plants
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Regulating Growth and Development The Plant
Hormones

• Auxins
• Cytokinins
• Ethylene
• Abscisic Acid
• Gibberellins

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Hormones

• Chemical signals that help both plants and
  animals regulate and coordinate metabolism,
  growth, and differentiation.
• Phytohormones- plant hormones

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Three basic elements of Hormones

1. Synthesis of the hormone in one part of the
   organism
2. Transport of the hormone to another part (target
   tissue)
3. Induction of chemical response

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Phytohormones

• Produced in tissues or glands
• Very active in small quantities
• Pineapple Ananas comosus for example are only 6
  micrograms of indoleacetic acid (IAA) a common
  plant hormone per kg of plant material.
  (analogous to a needle in 20 metric tons)
• Can stimulate or inhibit depends on chemical
  structure and how it is read by the target tissue

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Five classes of plant hormonesThe Classic Five

• Auxins- today
• Cytokinins- today
• Ethylene- thursday
• Abscisic acid- thursday
• Gibberellins- thursday

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Auxins

• Charles Darwin and Francis Darwin- The Power of
  Movement in Plants 1881

In response to light an influence that
causes bending is transmitted from the tip to
area below the tip
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The Principle naturally occuring Auxin-
Indoleacetic Acid

• In plants variety of pathways to produce
• tryptophan usually precursor
• All tissue produces IAA but typically
• found in shoot apical meristems, young
• leaves and developing fruit and seeds.
• Mutants lacking either auxin or cytokinin
• Have yet to be found- mutations eliminating
• Them are lethal

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Auxin synthesis- the site of auxin synthesis
along the margin of a young leaf Site
corresponds to the location of cells that will
differentiate into a hydothode (gland like
structure) GIS reporter gene detects
auxin synthesis
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Auxin transport- experimental demo of polar auxin
transport in stems represented here by a segment
of hypocotyl from a seedling
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In the root, nonpolar transport of IAA takes
place in the phloem of the vascular cylinder
whereas the polar transport occurs in the
epidermis and cortical parenchyma cells
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Schematic model for polar auxin transport
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• Arrowhead- vascular regeneration
• Basipetal polar movement of auxin from
• Above the arrow and then around the wound

IAA induced xylem regeneration around a wound
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• - When the apical bud is cut off a plant, the
  development of axillary buds in lateral branches
  is observed.
• - If the apical bud is replaced by cotton
  impregnated with auxin, no axillary bud
  development is observed.
• - So the auxin replaces the apical bud. - It can
  be deduced that this hormone is produced in the
  apical part of the plant.

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• The inferior part of the plant including roots (
  or root cap) is cut off and the plant is
• put in a medium containing auxin or free of it.
• Without auxin, adventitious roots can developed.
  This is the principle of cuttings.
• However, with auxin, root development is much
  better.
•  

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Auxin promotes fruit development

• Auxin is involved with the formation of fruit
• Parthenocarpic fuit- by treating a female flower
  parts (carpels) of certain species with auxin it
  is possible to produce a fruit (without
  fertilization- a virgin fruit) i.e. seedless
  tomatoes, cucumbers and eggplants.
• Developing seed is a source of auxin

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Auxin and fruit development- Normal strawberry
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Strawberry with all seeds removed
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Strawberry with horizontal band of seeds removed
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Other characteristics of Auxin

• Auxin provides chemical signals that communicate
  information over long distances
• Promotes the formation of lateral and
  adventitious roots
• Synthetic auxin (2,4 dichlorophenoxyacetic acid)
  are used to kill weeds (broad leaf)
• Mechanism unk.

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Cytokinins

• In 1941 Johannes van Overbeek found that coconut
  milk (liquid endosperm) contained potent growth
  factors
• Factors greatly accelerated the development of
  plant embryos and promoted the growth of isolated
  tissue and cells in vitro (test tube)

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Discovery had two affects

• It gave impetus to studies of isolated plant
  tissues
• Launched the search for another major group of
  growth regulators

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Basic medium used for tissue culture of plant
cells

• Contained sugar
• Vitamins
• Various salts
• Grown in this culture, growth slowed or stopped
• Thus some growth stimulus declined and the
  addition of IAA had no affects
• Adding coconut milk encouraged the cells to
  divide and growth to resume

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Growth factor from DNA

• Isolation of growth factor from DNA identifying
  its chemical nature called kinetin and the group
  of regulators called cytokinins because its
  involvement with cytokinesis

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Kinetin

• Resembles purine- adenine
• Probably does not occur naturally in plants
• Has relatively simple structure
• Chemist able to synthesize a number of related
  compounds
• Zeatin- most active naturally occurring cytokinin
  (maize)

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Cytokinins

• Found in active dividing structures, seeds,
  fruits, leaves and root tips
• Found also in SVP horsetail, fern
• Central to tissue culture methods and extremely
  important in biotech.
• Tx of lateral buds causes growth even in the
  presence of auxin thus modifying apical growth.

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• The cytokinin/auxin ratio regulates the
  production of roots and shoots in tissue cultures
• Undifferentiated plant cell has two courses open
  to it
• It can enlarge, divide, enlarge and divide again
  (undifferentiate) or
• without undergoing cell division, it can elongate
  and differentiate

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In tobacco stem tissue

• Applicatin of IAA causes rapid cell expansion-
  giant cells are formed
• Kinetin alone has little or no effect
• IAA Kinetin results in rapid cell division, so
  that large numbers of relatively small,
  undifferentiated cells are formed.
• High IAA, callus tissue- a growth of undiff.
  Cells in tissue culture frequently gives rise to
  roots.

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Callus development- effects of increasing IAA
at various kinetin
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• - At the top, the apical dominance have been
  annulled by cutting the apical bud.- At the
  centre, the terminal bud and the root cap have
  been cut. No futher development of axillary buds
  on the explant can be observed. So the roots are
  necessary for the development of buds.
• - on the other hand (at the bottom), if one
  identical explant is introduiced in a medium
  containing cytokinins, the development of
  axillary buds occurs. So the cytokinins replace
  the roots and it can be deduced that these
  hormones are produced in the roots.
• It is observed also that there is less root
  development in the presence of cytokinins.

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Summary

• Auxins
• - They are produced in the apical part of the
  plant.- They prevent the development of the
  axillary buds. - They favour the rhizogenesis
  ...(development of roots).
• Cytokinins
• - They are produced in the roots.- They prevent
  the development of roots.- They favour the
  development of the axillary buds.

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• With large concentrations of cytokinins and low
  concentrations of auxins, the development of
  axillary or adventitious buds can be obtain and
  in this way the plants are multiplied.
• With large concentrations of auxins and low
  concentrations of cytokinins or no cytokinins,
  the rooting of the shoots can be arrived at.
• - With equal concentrations of the two hormones,
  a callus is obtain. The callus is the result of
  the anarchic proliferation of cells which are
  more or less differentiated but which cannot
  organized them and form tissues and distinct
  organs.

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Cytokinins delay leaf senescence

• Yellowing (loss of chlorophyll) of leaf can be
  delayed with the addition of cytokinins
• Xanthium strumarium Leaves turned yellow in about
  ten days in plain water
• Add 10mg of kitenin help retain green

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Additional chemicals used by plants

• Brassinosteroids- naturally occuring polyhydroxyl
  steroids (tissue growth)
• Salicylic acid- phenolic compound similar
  structure to aspirin implicated in defense
  responses
• Jasmonates- class of compounds known as oxylipins
  plant growth regulation and defense
• Polyamines- strongly basic molecules found in all
  organisms (bacteria, fungi, animal, plants) are
  essental for growth and development and affects
  cell division
• Systemin- a polypeptide functions as a long
  distance signal to activate chemical defenses
  against herbivores.
• Nitric oxide (NO) serves as a signal in hormonal
  defense responses.

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Ethylene

• Plays a role in fruit ripening
• Promotes abscission (shedding of leaves, flowers
  and fruit)
• Triggers enzymes that promote fruit loosening
  from trees
• Auxin prevents abscission (prevents preharvest)
• High concentration does the opposite

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Abscisic Acid (ABA)

• First discovered as dormin and abscisin
• Dormin ash and potatoes
• In other plants abscisin
• Identical compounds now called Abscisic acid

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Abscisic Acis

• Levels increase during early seed development
• Stimulates the productio of seed storage protein
• Prevents premature seed germination
• Decline in ABA leads to germination

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Gibberellin

• Found in immature seeds- highest concentration
• Stimulates cell division and cell elongation
• Application to dwarf mutants cause them to grow
  tall
• Plays a role in breaking seed dormancy and
  germination