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HORMONE PHYSIOLOGY AND
SIGNAL TRANSDUCTION
Huseyin Tombuloglu, Phd.
GBE 304 Spring 2015
Growth and Development
Growth: irreversible increase in size (net
increase in protoplasm) brought about by
increases in cell size and number.
Development: which includes…
Morphogenesis: creation of pattern and shape
resulting in morphological and anatomical
development.
Differentiation: physiological and biochemical
specializations.
Specialization of Cells
 Coordination/Cooperation between specialized cells
for survival.
 Development involves a specific, logical sequence of
events….
 Life Cycle (highlights):
 Fertilization
 Embryo development
 Germination
 Vegetative growth
 Reproductive Growth
 Gametogenesis
 Senescence
 Plant Death
Plant Hormones
Coordinate complex steps in growth and
development.
Play key roles in signal transduction.
Integrate many different plant parts.
Coordinate “timing” of growth and
development.
Very potent at low concentrations.
Plant Hormones
 Definition of a Plant Hormone:
1.
2.
3.
4.
Naturally occurring organic substance.
Affects plant growth and development.
Operates in very low concentrations.
Actions may involve sites far removed from
hormone origin.
Hormones elicit a wide range of responses….
from growth to dormancy.
Hormones Classes
 Commonly grouped into five (5)
categories…but they have multiple,
overlapping, and interacting functions:
1.
2.
3.
4.
5.
AUXINS
GIBBERELLINS
CYTOKININS
ETHYLENE
ABSCISIC ACID
AUXINS
Auxins [indoleacetic acid (IAA)]
first plant hormone to be discovered.
early on it was believed to be the “master”
plant hormone.
studies date back to the time of Charles
Darwin (1880s).
very simple chemical structure….dramatic
influences on growth.
“growth promoting hormone”
Cell elongation is controlled by IAA
believed to cause release of H+ protons from
protoplast into cell wall.
acidic protons weaken cell wall structure
which softens the wall and allows for
expansion from turgor pressure.
Thus, auxin promotes cell elongation.
Auxin-induced cell wall expansion.
Functions of Auxins




Elongation of stems and roots.
Responsible for tropisms….phototropisms.
Enlargement of fruits and tubers.
Promotion of cell divisions in cambium
tissues.
 Vascular cambium…auxin signal in Spring
from buds.
 Secondary xylem initiation.
Auxin and root formation
Taiz and Zeiger. 2002. Plant Physiology, 3rd Ed.
Auxin and Geotropism
Taiz and Zeiger. 2002. Plant Physiology, 3rd Ed.
Functions of Auxins
 Wound healing.
 Promotes pith and cortex cells to differentiate into
sclerenchymatous xylem to cover wounds.
 Promotes rooting of plant cuttings.
 Promotes rooting of soft wood cuttings.
 Inhibits growth in areas other than root
tissues.
 Shorten stems and retards growth.
 Apical dominance…function of auxin distribution.
Functions of Auxins
 Inhibits abscission of leaves and fruit.
 Prevents leaf and fruit drop.
 Tissues send continuous auxin signal to prevent
abscission layer development.
 Auxin release and movement to zones of
abscission is halted by:
 Disease
 Injury…lighting strikes
 Natural/Seasonal development
Auxins and Leaf Abscission
In the Fall, leaves fall from the trees because
the leaf tissues STOP producing auxins.
Same mechanisms in place for fruit drops
(apples).
Auxins are in highest concentrations in the
stem apex of plants. Such high
concentrations inhibit the growth of axillary
buds….apical dominance.
Pruning, or removing the auxin signals results in elimination
of apical dominance and a flush of later meristematic growth.
Auxins function best at very low concentrations.
Auxins and Horticulture
Practical applications in agriculture and
horticulture:
Useful in propagating/rooting of plant tissues
cuttings.
As a herbicide.
Prevention of fruit drop.
Thinning of fruits….depending on the
concentration.
Auxins and Horticulture
Most often, synthetic auxins are preferred
over natural IAA.
2,4-D = 2,-4-dichlorophenoxacetic acid
Herbicide which mimics the activity of IAA.
Plant Growth Regulators are often synthetic auxins.
Remember that plant hormones are most
effective in SMALL quantities.
Low concentrations…stimulate growth.
High concentrations…herbicidal.
Auxins and Horticulture
2,4-D is very effective at controlling broadleaf
dicot plants.
It has no effect on grass monocots.
Differences in control represent metabolic
differences between the dicots and
monocots.
This fact is utilized successfully by agriculture
and horticulture professionals.
U.S. Air Force's herbicide program in Vietnam
Agent Orange
U.S. Army Huey helicopter spraying
Agent Orange over Vietnamese
agricultural land.
MOVIE SPRAY
http://upload.wikimedia.org/wikipedia/commons/transcoded/1/15/US-troops-spray-AgentOrange-from-riverboat-Vietnam.ogv/US-troops-spray-Agent-Orange-from-riverboatVietnam.ogv.360p.webm
GIBBERELLINS