PRT3402- Agricultural Biochemistry
PJJ UPM / UPMET
UNIT 9
Hormones and plant growth regulators
Introduction to Unit
In this unit hormones and plant growth regulators are discussed as biological
compounds that can have a profound effect on the metabolic activity of living
organisms. In most cases they are required only in minute amounts to trigger a
response. The types of hormones, functions and where they are produced and
secreted will be explained. Some structural differences are demonstrated and the
role of plant growth regulators or plant hormones will also be discussed.
Learning Outcomes
At the end of this unit the students will be able to:
1. Describe the types of hormones, their functions and the glands that produced
these hormones.
2. Explain the mechanism of hormones signaling and how its effect is
manifested.
3. Describe what are plant growth regulators, the types of growth regulators and
its effects on plant physiology and development.
129
PRT3402- Agricultural Biochemistry
PJJ UPM / UPMET
TOPIC 1: HORMONES, GLANDS AND CLASSES OF HORMONES
Main Points
1.1
Hormones are defined as: ‘a chemical substance produced by a cell or gland
in one part of human/animal body that send messages affecting cells in other
parts of the organism.’ It is derived from a Greek word meaning ‘set in motion
or impetus’. Hormones are produced by multicellular organisms and not in
single cell organism but can be produced by certain bacteria. In animals, they
are transported via blood circulation or blood stream and only a small, minute
amount is required to have a significant effect on cells, tissues or organs or
the whole organism.
1.2
Cells respond by producing specific receptor that binds with hormones and
the binding results in activation of signal transduction which leads to a cell
type-specific response. Endocrine hormones are released directly in
bloodstream whereas exocrine hormones are released via a duct, flows into
blood stream or by difussion from cell to cell.
1.3
Exocrine glands can be of various structural complexity – from a simple
tubular structure to a more complex compound tubularalveolar structure.
Differences in the structure of exocrine and endocrine gland.
130
PRT3402- Agricultural Biochemistry
PJJ UPM / UPMET
Secretion of hormones by endocrine glands
Various structures of exocrine glands from a simple to a more complex gland
1.4
The endocrine glands in the human body are found throughout many
organs/glands in the human body. Some hormones are only found in the male
whereas others are only found in the female. Such difference is related to the
sexuality of an individual.
131
PRT3402- Agricultural Biochemistry
PJJ UPM / UPMET
Location of endocrine gland in humans
1.5
Some of the exocrine glands in human include sweat glands, salivary glands,
mammary glands, stomach, liver, and pancreas.
1.6
Some of the hormonal effects that have been determined include:

Stimulation or inhibition of growth

Activation or inhibition of immune system

Induction or suppression of apoptosis (programmed cell death)
132
PRT3402- Agricultural Biochemistry
PJJ UPM / UPMET

Regulate metabolism

Control reproductive cycle

Preparing body for new phase of growth – puberty, menstruation,
menopause

Preparing body for mating, fleeing, fighting

Mood swings

Hunger cravings

Regulate production and release of other hormones
1.7
Hormones can be classified into several classes on the basis of the chemical
structure. Some of the classes include: Peptide hormones – chains of amino acids
i.e proteins e.g. insulin, growth hormone. These peptide hormones can have
carbohydrate side chains – glycoprotein hormones e.g. lutenizing hormormone,
follicle-stimulating hormone, and thyroid stimulating hormone
1.8
Lipid and phospholipid hormones – from linoleic acid, arachidonic acid.
Steroid hormones are derived from cholesterol & eicosanoids e.g. testosterone &
cortisol.
1.9
Monoamines from aromatic acids – phenylalanine, tyrosine, tryptophan.
Examples are thyroxine and adrenaline.
1.10
Some examples of hormone structure and their effects are given below.
Hormones
Structure
Effects
Melatonin
antioxidant, drowsiness
Adrenaline or Epinephrine
133
PRT3402- Agricultural Biochemistry
PJJ UPM / UPMET
Insulin
Intake of glucose, lipid,
glycogenesis, glycolysis
Serotonin
mood, appetite, sleep
Testosterone
libido, maleness
1.11
The signaling response of a hormone is summarized in the following steps
1. Biosynthesis of hormone in tissue
2. Storage and secretion
3. Transport to target cells
4. Recognition of hormone by cell membrane/receptor protein
5. Relay and amplification of hormonal signal via signal transduction
cell response
6. Degradation of hormone
1.12
Biosynthesis and secretion regulated by homeostatic negative feedback.
Negative feedback triggered by high hormone concentration and overproduction
‘effect’ of hormone. The secretion and inhibition of hormones can be influenced by

Other hormones (stimulating hormones)

Ions and nutrients concentration in plasma
134
PRT3402- Agricultural Biochemistry
PJJ UPM / UPMET

Neurons and mental activity

Environmental effects – light, temperature
TOPIC 2; PLANT GROWTH HORMONES, PLANT GROWTH REGULATORS AND
PHYTOHORMONES
2.1
In the United Kingdom the term plant growth substances are more
commonly used. Phytohormones is another term used to describe plant
hormones. The compounds can be synthetic or man-made thus the term plant
growth regulators or PGR may be used. Plant hormones are produced by cells
rather than glands as in animals or human. Their effects can be manifested even
in extremely low amounts. Although they produce a very profound effect on plant
growth, however, they are not considered as nutrients. PGR are routinely used in
plant propagation from cuttings, grafting, micropropagation and tissue culture
laboratories where they are used to propagate plant growth of explants.
2.2
There are several major classes of phytohormones as shown in the table
below.
Hormones
Effects
Abscisic acid , ABA
Inhibitory, bud growth, seed and dormancy, seed
germination, closing of stomata in response to water
stress
Auxins
Cell enlargement, bud formation, root initiation.
Control growth of stems, roots, fruits and convert stem
into flowers.
Cytokinins, CKs
Celll division, shoot formation, delaying senescence,
auxin transport, with etylene promote abscission of
leaves, flower parts and fruits
Ethylene, ET
Cell growth and cell shape, stem diameter and height,
regulates ABA and tress hormones
Gibberelins, GAs
Seed germination, bolting of rossette-formation,
internodal length, reverse inhibition of shoot growth
and dormancy of ABA
2.3
Other classes of phytohormones include and their effects on plants are
listed below.
 Brassinosteroids (BAs)– cell elongation, gravitropism, stress resistance,
xylem differentiation.
135
PRT3402- Agricultural Biochemistry
PJJ UPM / UPMET
 Salicylic acid (SA) – activates genes producing chemicals in defense against
plant pathogen.
 Jasmonates – defence proteins, seed germination, protein storage and root
growth.
 Peptide hormones – cell-tocell signaling, cell division and expansion, pollen
self-incompatibilty.
 Polyamines – plant growth, mitosis and meiosis.
 Nitric oxide (NO) – signal in hormonal and defense responses.
 Striglactones – inhibition of shoot branching.
 Karrikins – stimulate germination of seeds.
2.4
Some chemical structurs of the more important PGRs are shown below. They
can be simple compounds or complex molecules.
2.5
The lack of certain plant growth hormones can have a very profound effect
on the growth development of plants as indicated below.
136
PRT3402- Agricultural Biochemistry
PJJ UPM / UPMET
Lack of auxin hormone can cause
Effect of too much gibberellin
abnormal growth (plant on the right)
(plant on the right)
2.6
Another example on the effect of gibberellins is shown below where the effect
can be countered by using other chemicals as inhibitors.
137
PRT3402- Agricultural Biochemistry
PJJ UPM / UPMET
In the picture above the action of the plant hormone gibberellin (GA) on
developmental processes is demonstrated:
Treatment of pumpkin seedlings with GA, one of the classical plant hormones,
results in an increase of e.g. hypocotyl length (see second plant from left). On the
other hand application of LAB 150978, an inhibitor of GA-biosynthesis, results in
dwarf plants (third plant from left). However, such dwarfs can be restored by
additional application of GAs (fourth plant from left).
2.7 Plant hormones can be grouped by the functions carried out the hormones.
These groupings are:
 Growth hormones – released under long term good growth conditions &
synthesized in shoot and root. – Auxins & Cytokinins
 Stress hormones – made by plants under stress conditions, re-distribute and
releasing stored resources. – GAs, ET and BAs
 Shock/Synchronizer hormones – general metabolic inhibitors/senescence
stimulators and general metabolic stimulators/senescence blockers. – ABAs,
SAs
2.8
In fish and other marine animals, hormones play a similar role regulating
various biological processes such as molting of the exoskeleton, growth,
reproduction, and development. While the endocrine organs of fish may differ from
those in humans, their hormones are similar in structure, similarly synthesized and
have many of the same mechanisms of action. Many of the same building blocks in
the biosynthesis of these hormones are shared between species and conserved in
human hormones thus similar hormonal effects are seen in fishes and marine
animals.
138
PRT3402- Agricultural Biochemistry
PJJ UPM / UPMET
139
PRT3402- Agricultural Biochemistry
2.9
PJJ UPM / UPMET
As shown in the diagram above, the sex determination in fishes is very much
the function and regulation of hormones. Both environmental factors and inheritance
of genes interplay to determine the outcome of sexuality in fishes.
140