Lecture 27 Auxin and Gibberellins

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Lecture 27Auxin and Gibberellins
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What are hormones?

• Form and function of multicellular organisms
  depend on efficient
• communication among cells, tissues and organs
• - Morphogenesis depends on chemical signals
  Julius von Sachs
• (1832-1897)

Hormone from Greek horman - "to set in motion"
chemical messenger that mediates intercellular
(between cells) communication interacts with
specific cellular proteins called
receptors present in cell in very low
concentrations controls plant development and
responses to stress Growth hormones
Stress hormones Auxin Salicylic
acid Gibberellins Jasmonic acid Cytokinins
Oligosaccharines Ethylene Abscisic
acid Brassinosteroids (plant steroid
hormone) Oligosaccharines
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Some concepts
1. Hormones are chemical signals that facilitate
intercellular communication. - Act at very low
concentration - Synthesized in certain tissues
and transported to target tissues 2. Level
of hormones is regulated. - By Rate of
biosynthesis, degradation, and transport 3.
Environmental signals cause changes in hormonal
balance. - Environmental cues ? change (Hormone)
? response
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The auxin concept A historical perspective
From experiments on coleoptile phototropism (see
lecture 26), Darwin concluded that a
growth stimulus is produced in coleoptile tip and
trans- mitted to growth zone.
Growth stimulus passes through gelatin but not
through water-impermeable barriers.
Coleoptile sheath/protective organ of the
youngest leaves in grasses
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The auxin concept A historical perspective
The growth stimulus is of chemical nature.
The growth-promoting substance can diffuse into a
gelatin block.
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Auxin stimulates the elongation of oat
coleoptile sections
primary leaves
coleoptile
Incubation in water for 18h
Incubation in auxin for 18h
Auxin from the greek word auxien, meaning to
increase or to grow.
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What is auxin?
Structures of three natural auxins
Indole
Peas
All plants
Auxin biosynthesis
Synthesis in meristems, young leaves, developing
fruits and seeds
Mustard and corn
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Synthetic auxins
Often used as herbicides
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The root cap is the site of gravity perception
Microsurgery experiments demonstrating that the
root cap produces an inhibitor (auxin!) that
regulates root gravitropism
Removal of half of the cap causes root to bend
toward side with half cap
Removal of root cap slight elongation growth
Auxin is redistributed laterally in the root cap
Horizontal control root with root cap showing
normal gravitropic bending
Removal of cap from horizontal root abolishes
response to gravity
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Physiological effects of Auxin
Apical dominance Lateral and adventitious
root development Flower and fruit
development Vascular differentiation
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Auxin regulates apical dominance
Apical dominance the growing apical bud
inhibits the growth of lateral (axillary) buds
Phaseolus vulgaris (bean)
Removal of apical/ terminal bud induces growth of
axillary buds
Applying IAA to cut surface prevents outgrowth of
axillary buds
Axillary buds are suppressed because of apical
dominance
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Auxin transport regulates floral bud development
Floral meristem depends on auxin being
transported to it from subapical tissue In
absence of an auxin carrier protein, meristem is
starved for auxin, disrupting normal leaf and
floral development
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Auxin promotes fruit development
Auxin is produced in pollen, endosperm, and
embryo of developing seeds.
Growth of fruit is regulated by auxin produced
by seeds, the achenes.
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GibberellinsRegulators of plant height and
Seed germination
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Effect of Gibberellins (GAs) on growth and
development
Cabbage, a long-day plant, remains a rosette
under short day, but it can be induced to bolt
and flower by applications of GAs
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Effect of Gibberellins on growth and development
GAs promote fruit set, i.e. initiation of fruit
growth after pollination ? apple
GAs promote seed germination
GAs have commercial applications
Fruit production (grapes, apples, citrus)
Increasing sugarcane yields
GA biosynthesis inhibitors
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GA1, the biologically active gibberellin,
controls stem growth
GA biosynthesis in shoot and root tip/apical
meristems
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Photoperiod control of tuber formation
Long days Short days
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GA signal transduction Cereal aleurone layer
2. GAs diffuse to aleurone layer
(Seed coat, fruit wall)
1. GAs are synthesized by embryo and released
into starchy endosperm via the scutellum
3. Aleurone layer cells are induced to synthesize
and secret aamylase and other hydrolases into
endosperm
5. Endosperm solutes are absorbed by the
scutellum and transported to growing embryo
4. Starch and other macromolecules are broken
down to small molecules