PLANT GROWTH REGULATORS / PLANT HORMONES

Introduction
▪ The growth and development of plants is regulated by a number of chemical
substances which together exert a complex interaction to meet the needs of plants.
These chemical substances are called plant hormones or plant growth regulators
▪ Five groups of plant hormones are well established;
1. Auxins [Concerned mainly with cell enlargement]
2. Gibberellins [Concerned mainly with cell enlargement]
3. Cytokinins [Cell division hormones]
4. Abscisic acid and its derivatives [Growth inhibitors]
5. Ethylene [Growth inhibitors]
▪ These substances are of wide distribution and may, in fact, occur in all higher
plants
▪ They are specific in their action
▪ They are active in very low concentrations
▪ They regulate cell enlargement, cell division, cell differentiation, organogenesis
senescence (aging) and dormancy
▪ Their action is probably sequential
▪ The essential role of these substance is illustrated by cell and tissue cultures
• Without the addition of suitable hormones, no development or cell division
occurs
▪ The effects of these very active substances on the production of secondary
metabolites, particularly with a view to producing plants containing an enhanced
proportion of active constituent, are of interest to pharmacognosists
▪ For commercial purposes, yield per hectare is an obvious criterion
• For biosynthetic studies, yield per plant or percent fresh weight may be of more
significance
 Auxins
Discovery
▪ Auxin-a (growth regulating acid) was isolated from human urine
▪ Auxin-b (growth regulating acid) was isolated from cereal products
• These substances had similar properties to indole-3-acetic acid (IAA)
• IAA is the compound now considered to be the major auxin of plants, found
particularly in actively growing tissues.
Potential precursors of auxins (IAA)
▪ Indoleacetaldehyde
▪ Indoleacetonitrile
▪ Indolepyruvic acid
• These compounds and IAA are all derived, in the plants, from tryptophan
Synthetic auxins
▪ Indole-3-butyric acid
▪ Naphthalene-1-acetic acid (NAA)
▪ 2,4-dichlorophenoxyacetic acid (2,4-D)
Typical effects of auxins
▪ Cell elongation giving an increase in stem length
▪ Inhibition of root growth

▪ Adventitious root production

▪ Fruit setting without of pollination
Uses of auxins
▪ In low concentration, to accelerate the rooting of woody and herbaceous cuttings
• NAA is used to develop roots from cuttings Examples: Holly tree, which were
formerly very difficult to propagate in this way and had to be raised from seeds or
by grafting
• Indole-3-butyric acid is used to develop roots from cuttings
Examples: Cinchona cuttings, saving some 2 to 3 years compared with growth
from seeds. Similar results have been obtained with cuttings of Carica, Coffea,
Pinus and other species
▪ In biogenetic studies, auxins are being used to induce root formation on isolated
leaves. Examples: Leaves of Nicotiana and Datura species
▪ In higher concentration, to act as a selective herbicide or weed killer. Examples:
(2,4-D), in suitable concentration, is toxic to dicotyledon plants. (2,4-D) is used to
destroy dicotyledonous weeds from grass lawns
▪ There have been several reports on the effects of auxins on the formation of
secondary metabolites in medicinal plants.

Gibberellins
Introduction
▪ This group of plant growth regulators was discovered in connection with the
‘bakanae’ (foolish seedling) disease of rice.
In this disease, the affected plants become excessively tall and are unable to
support themselves
• Through a combination of the resulting weakness and parasite damage, they
eventually die
• The causative organism of the disease is the fungus Gibberella fujikuroi
▪ It was found that extracts of the fungus could initiate the disease symptoms when
applied to healthy rice plants
▪ Later on, a crystalline sample of the active material of the fungus was isolated
and named ‘gibberellin’
Types of gibberellins
▪ GA1, GA2, GA3, etc.
▪ GA3 is commonly known as ‘gibberellic acid’ and is produced commercially by
fungal cultivation
• It is probably the best known of the series
▪ It was revealed that gibberellins also exist in higher plants
▪ By 1980, 58 gibberellins were known, of which about half were derived from the
Gibberella fungus and half from higher plants.
Synthesis and storage of gibberellins
▪ Gibberellins are synthesized in leaves, and they accumulate in relatively large
quantities in the immature seeds and fruits of some plants.
Typical effects and uses of gibberellins
▪ Bolting and flowering is induced when applied to short node plants, for example
those plants producing rosettes of leaves (Digitalis, Hyoscyamus)
▪ Dwarf varieties of many plants, when treated with the hormone, grow to the same
height as taller varieties
▪ Initiation of the synthesis of various hydrolytic and proteolytic enzymes, upon
which seed germination and seedling establishment depend
▪ The growth effect of gibberellin arises from cell elongation in the subapical
meristem region, where young internodes are developing
▪ The effects of gibberellins and auxins appear complementary, the full stimulation
of elongation by either hormone necessitating an adequate presence of the other
▪ The gibberellins have been used to treat many plants which contain useful
secondary metabolites

Cytokinins
▪ Cytokinins are cell division hormones
Types
▪ Kinetin (animal source): A scientist discovered that aged or autoclaved DNA
from herring sperm stimulated cell division
• This active degradation product was called kinetin, identified as 6-
furfurylaminopurine (6- furfuryladenine)
Zeatin (plant source): From extracts of maize embryos at the milky stage, an
active substance named zeatin was isolated
• Like most other cytokinins, it is a 6-substituted adenine derivative, 6-(4-hydroxy-
3-methylbut-2-enyl)- aminopurine
▪ The hormone complex has been detected in the cambial region of various woody
plants
Typical effects and uses
▪ Cytokinins have a more specific effect on cell division (cytokinesis)
▪ The activity of cytokinins is not only confined to cell division in a tissue; they
also regulate the pattern and frequency of organ production as well as position and
shape ▪ They have an inhibitory effect on senescence Cytokinins have been much
employed in tissue culture work, in which they are used to promote the formation
of adventitious buds and shoots from undifferentiated cells
▪ In cell cultures, they have been shown to promote the biosynthesis of berberine
(Thalictrum minus), condensed tannins (Onobrychis vicifolia) and rhodoxanthin
(Ricinus).

Abscisic acid
▪ It is a growth inhibitor substance
▪ From plants, abscisic acid [3-methyl-5-(1-hydroxy-4-oxo-2,6,6-trimethyl-2-
cyclohexane-1-yl)-cis,trans2,4-pentadienoic acid] was isolated and characterized
• It has also been isolated from the fungus Cenospora rosicola
▪ Vomifoliol: Substance related to abscisic acid, isolated from plants
• It has the same activity as abscisic acid in stomatal closure tests
▪ A number of synthetic growth inhibitors have been studied
• The first to be described was maleic hydrazide
Typical effects and uses
▪ Affect bud opening
▪ Affect seed germination
▪ Affect development of dormancy
▪ Little or no work appears to have been reported on the effects of abscisic acid on
the production of pharmacognostically interesting substances.

Ethylene
▪ It has been known for many years that ethylene induces growth responses in
plants ▪ In 1932, it was demonstrated that the ethylene evolved by stored apples
inhibited the growth of potato shoots enclosed with them
• Ethylene has a role in fruit ripening
Typical effects and uses
▪ Stimulation of the de novo synthesis and secretion of cell wall dissolving
enzymes such as cellulase during leaf abscission and fruit ripening
▪ Ethephon (Ethrel®): A compound, applied in aqueous solution in
concentrations of the order of 100-5000 ppm
• In the cell sap, at pH values above 4.0, it is broken down to ethylene and
phosphate ▪ Ethephon is now increasingly used as standard practice for enhancing
the flow of rubber latex