plant growth regulation

1. Plant Hormones
Plant growth regulators

2. Factors responsible for Plant Growth
Light-- Sun
H2O, minerals, nitrogen-- Soil
Carbon dioxide-- Air
Plants hormones or phytohormones

3. Hormones
• The word “Hormone” is derived from Greek word
“horman” means “to set in motion”
Plant “hormones” are often synthesized in many tissues
simultaneously, may act on the same cells in which they are
synthesized, and often elicit opposing reactions depending on
concentration and context
Phytohormones
• Unlike animal hormones, these substances are not produced by a
particular area of the plant body, and they may be less specific in
their effects. It has therefore been suggested that they should not be
described as hormones at all.
Plant Growth Regulators:
•
They are usually synthetic and rarely natural

4. Difference between Vitamin,
Hormone and a Pheromone
• Vitamin: an organic substance which in
trace amounts is essential for normal
metabolic functioning of the body. But its
concentration is much higher than that of
a hormone
• Pheromone: A secretion or chemical
produced by an individual which causes
stimulation in other individuals of same
type.

5. A True hormone is based on Four
criteria
• 1. Organic nature
• 2. Very low conc.
• 3. Transport
• 4. Physiological response, Amplification

6. Difference between animal and
plant hormone
• Both hormones are organic. Animal
hormone is usually a protein.
• Synthesized in one place and transported
to another
• Effective in very low concentrations
• Produce physiological response
• Transport is not essential in case of plant
hormone however it certainly takes place
in case of animal hormones

7. New Definition
• Davies proposed a new definition of Plant
Hormones:
• “Plant hormones are natural compounds
with an ability to affect physiological
processes at concentrations much lower
than those where either nutrients or
vitamins would affect those processes.

8. Plant Hormones
• Chemical substances that function as
messages and regulators
• May act in very small quantities
• May act at a distance or locally
• May be “excitatory” or “inhibitory”
• Response complex
– Due to chemical itself AND
– Response of target tissue
• Response of target may change over time
• Different targets may respond to same hormone
differently

9. What is a hormone?
Plant hormones play an integral role in controlling the growth
and development of plants.
A plant hormone is generally described as an organic
compound synthesized in one part of the plant and
translocated to another part, where in low concentrations (e.g.,
<1mM and often <1uM) it elicits a physiological response
A problem with this definition is that in all cases plant
hormones are not necessarily translocated. A prime example
of such a case is the hormone ethylene which may bring about
changes in the same tissue or even the same cell in which it is
synthesized. Rather than to get caught up in semantics,
perhaps a better idea is to outline what a hormone is not.

10. Inorganic compounds such as Ca2+ and K+ cause
physiological responses and can move throughout the
plant, yet they are not synthesized by the plant and are
therefore not plant hormones.
The same is the case for synthesized growth regulatory
substances such as 2,4-D regardless of its structure being
similar to that of auxin.
The definition states that the hormone must be
translocated but mentions nothing about how or how far,
or even whether it needs to cause a response in the cells it
is translocated to. Ethylene can affect the cells in which it
is synthesized as well as cells to which it is translocated.

11. Sucrose is not a hormone even though it is
synthesized, translocated, and stimulates a response
(growth) because it is only at high concentrations that
growth will occur.
The same is the case for many other sugars as well as
amino acids, organic acids, and other metabolites
which are usually present in concentrations ranging
from >1mM up to 50 mM and in some cases even
higher than 50mM (Salisbury and Ross, 1992).

12. History of the Study of Plant Hormones
The concept of chemical messengers in plants is not a new one.
For over two millennia, people have observed that one part of the
plant may influence that of another.
Duhamel du Monceau's experiments in 1758 suggested that sap
movement controlled the growth of plants.
He showed that downward moving sap from the leaves controlled the
roots healthiness(du Monceau, 1758).
Julius von Sachs who is acknowledged as the father of plant
physiology revised du Monceau's theory by presenting evidence that
"organ-forming substances" were made by the plant and moved to
different parts of the plant where they controlled growth and
development. He also hypothesized that these "organ-forming
substances" were produced in response to environmental stimuli (von
Sachs, 1880).

13. Charles Darwin, is considered to be the scientist responsible for
beginning the modern research in plant growth substances with his
experiments on phototropism described in his book "The Power of
Movement in Plants." (Darwin, 1880).
It was in 1926 that this compound was first isolated from plants by a
graduate student in Holland named Fritz Went.
It was the first plant hormone isolated and was later termed "auxin"
(Greek auxein, "to increase") by Kogl and Haagen-Smit in 1931.
Went's pioneering work greatly influenced research on plant growth
substances and much of our current knowledge regarding auxins are
attributed to his work (Went, 1926).

14. Shortly after this time other lines of investigation led to other
plant hormones:
Gibberellins were discovered in plant pathogenesis studies;
Efforts to culture tissues led to cytokinins;
Efforts of controlling abscission and dormancy aimed to
abscisic acid;
and the effects of illuminating gas and smoke brought us to ethylene.
Other compounds which affect plant growth but are not
generally classified as hormones include
brassinosteroids, salicylates, jasmonates, and polyamines etc. etc

15. How do Plant Hormones Work?
It is known that micromolar and even smaller concentrations of
hormones are necessary in order for a response to be observed.
Because of this fact, three criteria must be true to stimulate plant
hormonal action (Salisbury and Ross,1992):
The hormone must be present in the correct quantity in the
correct location.
There must be good recognition and strong binding between
the hormone and the responding molecules.
The receptor molecule must then trigger some other metabolic
change which will trigger the amplification of the hormonal
signal.

16. There are two generally accepted mechanisms by which hormones
will act.
The first type deals with a steroid hormone.
In this type the hormone can pass through the plasma membrane into
the cytoplasm.
Here it binds with its receptor molecule to form a hormone-receptor
complex.
From this point, the complex may dissociate (If there is not tight
binding) or it may enter the nucleus and affect mRNA synthesis.
The effect of the hormone on mRNA synthesis ultimately results in
the physiological response(Arteca,1996 Moran et.al., 1994; Wolfe,
1993).

17. In the second type, a peptide hormone binds to a receptor protein on
the target cell.
The receptor protein will then undergo a conformational change
leading to a cellular cascade ultimately resulting in: modification of
enzyme activity,
altered metabolic processes,
and different phenotypes..

18. One thing plant hormones specifically control is gene
expression.
It is important to point out that the exact mechanisms by which
hormones regulate gene expression are poorly understood.
Gene expression is part of a large amplification process. This
process involves repeated transcription of DNA resulting in many
copies of mRNA (1st amplification step);
mRNA is processed and enters the cytoplasm where it is
translated many times by ribosomes into a gene product such as
an enzyme (2nd amplification step);
Enzymes are modified to become functional and capable of high
catalytic activity even at low concentrations. They catalyze the
production of many copies of an important cellular product (3rd
amplification step).

19. It is likely that gene regulation is affected by certain enzymes
after initial hormone binding.
Genes may be altered by secondary and tertiary messengers of a
cellular cascade as well.
Hormones may indirectly control gene expression through these
enzymes and messengers at a number of control sites such as:
transcription, mRNA processing, mRNA stability, translation, and
post-translation.

20. Sources and Sinks
Sites of Hormones
Synthesis
Leaf
Meristem
Mitochondria
Root
seed
Sites of Hormones
Action
Buds
Maturation zone
Choloplast
Seed

22. Growth & Development
Phytohormones – Signal Transduction Tolerance
Production of Natural Products
Plant – ON / OFF SWITCH

26. Concentration of different
hormones in plant parts
• Plant part Auxins GAs Cytokinins ABA
• Shoot tip +++ +++ +++ -
• Young leaf +++ +++ - -
• Stein Elongation ++ ++ - -
• Lateral bud + ++ + -
• Mature leaf + + - +++
• Mature stem + + - -
• Root + - - -
• Root tips ++ ++ +++ -