Plant Growth Regulators

Plant Growth Regulators

This chapter is prepared for PharmD students in accordance with the officially endorsed curriculum in Pakistan.

October 14, 2023

Chapter outline
» Introduction
» Auxins
» Gibberellins
» Cytokinins
» Abscisic acid
» Ethylene

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 substances 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

1. Auxins
» The term auxin is derived from the Greek word auxein which means to grow
» Generally compounds are considered auxins if they are able to induce cell elongation in stems and otherwise resemble indole-3-acetic acid (IAA) (Figure 1)
» Auxins usually affect other processes in addition to cell elongation of stem cells
› But cell elongation is considered the critical characteristic of all auxins

Discovery
» Auxins were the first plant hormones to be discovered
» In 1931, Dutch workers isolated two growth-promoting substances and named them ‘Auxin-A’ and ‘Auxin-B’
› Auxin-A was isolated from human urine
› Auxin-B was isolated from cereal products
› Auxin-A and Auxin-B had similar properties to indole-3-acetic acid (IAA)
» IAA is considered to be the major natural auxin of plants
› IAA is found particularly in actively growing tissues

Natural auxins
» Indole-3-acetaldehyde
» Indole-3-acetonitrile
» Indole-3-pyruvic acid (Figure 1)
› These compounds and IAA are all derived, in the plants, from tryptophan

Synthetic auxins
» Indole-3-butyric acid
» Naphthalene-1-acetic acid
» 2,4-Dichlorophenoxyacetic acid (Figure 1)

Regulation of auxins
» In plants, oxidative degradation of IAA to give a number of products is controlled by IAA oxidase
» Some substances such as the orthodiphenols (e.g., caffeic acid, chlorogenic acid, and quercetin ) inhibit the action of IAA oxidase
› In this way, orthodiphenols  stimulate the growth (Figure 2)
» Conversely, monophenols such as p-coumaric acid promote the action of IAA oxidase and so inhibit growth (Figure 2)
» IAA may also be conjugated in the plant with aspartic acid, glutamic acid, glycine, sugars, and cyclitols
› Such bound forms may represent a detoxication mechanism or are inactive storage forms of the hormone

Typical effects of auxins
» Stems and roots respond differently to high concentrations of auxins
› Cells in stems grow more in size, giving an increase in stem length
› Cells in roots grow less in size
» Auxins promote adventitious root production
» Fruit setting without pollination

Uses of auxins
» In low concentration, auxins are used to accelerate the rooting of woody and herbaceous cuttings
› For example, holly trees were formerly very difficult to propagate because they had to be raised from seeds or by grafting
› But now, naphthalene-1-acetic acid is used to develop roots from the cuttings of holly trees
› Another example is the development of roots from the cuttings of Cinchona by indole-3-butyric acid, saving some 2 to 3 years compared with growth from seeds
» In biogenetic studies, auxins are being used to induce root formation on isolated leaves
› Examples include the leaves of Nicotiana and Datura species
» Auxins used in higher concentration (usually stronger than when used for rooting) selectively destroy some species of plant and leave others more or less unaffected
› They have, therefore, a very important role as selective weed killers in horticulture and agriculture
› For example, 2,4-dichlorophenoxyacetic acid, in suitable concentration, is toxic to dicotyledon plants
› So, 2,4-dichlorophenoxyacetic is used to destroy dicotyledonous weeds from grass lawns

Effect of auxins on secondary metabolites
» There have been several reports on the effects of auxins on the formation of secondary metabolites
» For example, seedlings and young plants of Mentha piperita, when treated with derivatives of naphthalene-1-acetic acid, gave in the mature plants an increased yield (30-50%) of volatile oil, and
› This oil itself contained 4.5-9.0% more menthol than the controls
» A stimulating effect on alkaloid production was obtained with naphthalene-1-acetic acid in tissue cultures of Rauwolfia serpentina
› While an inhibitory effect was obtained with 2,4-dichlorophenoxyacetic acid
» An increased alkaloid production has been reported for submerged cultures of certain ergot strains when treated with various auxins such as IAA, naphthalene-1-acetic, 2,4-dichlorophenoxyacetic acid, and indole-3-butyric acid
» In one study, IAA was injected into poppy capsules, 1 and 2 days after flowering
› It resulted in the production of a relatively elongated capsule form
› But, a reduced alkaloid content
» Cells of Morinda citrifolia, grown in cell suspension cultures, in the presence of naphthalene-1-acetic acid were found to have a substantial anthraquinone production
› But those grown with 2,4-dichlorophenoxyacetic acid as sole auxin, do not
» IAA appears to have no beneficial effect on the production of glycosides (sennosides) in Cassia angustifolia
___

>>> What are adventitious roots?
>>> What are secondary metabolites?

2. Gibberellins
» These group of plant growth regulators were 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 fungus Gibberella fugikuroi
» 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
» The gibberellins are named GA₁, GA₂, GA₃, GA_n in order of their discovery
» Gibberellic acid (Figure 3) was the first gibberellin to be structurally characterized as GA₃
» 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
» There are currently 136 GAs identified from plants, fungi, and bacteria
» It is now considered possible that these substances are present in most, if not all, 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 are induced when applied to short node plants
› For example, those plants producing rosettes of leaves (e.g., Digitalis and Hyoscyamus species)
» Dwarf varieties of many plants, when treated with gibberellins, grow to the same height as taller varieties
» Initiation of the synthesis of various hydrolytic and proteolytic enzymes
› Seed germination and seedling establishment depend upon these hydrolytic and proteolytic enzymes 
» The growth effect of gibberellin arises by cell elongation in the subapical meristem region
› In these regions young internodes develop
» The effects of gibberellins and auxins appear complementary
› The full stimulation of elongation by either hormone necessitates an adequate presence of the other
» Gibberellins appear to occur in plants in deactivated forms
› For example, β-D-glucopyranosyl esters of GA₁, GA₄, GA₈, GA₃₇, and GA₃₈ are known
› These may serve a depot function
› Additionally, the glucosyl ester of GA₃ has been prepared in several laboratories

Effect of gibberellins on secondary metabolites
» Various studies were conducted to evaluate the effect of gibberellins on the volatile oil production
› For example, several studies have been made to evaluate the effects of gibberellins treatment on Mentha piperita, and a lowering of the volatile oil content was observed
› On the other hand, gibberellins treatment increased the volatile oil content in Chenopodium species
› After bi-weekly sprays of gibberellins on the leaves of Foeniculum vulgare and Coriandrum sativum, no differences were detected in the yield of fruits and quantity and quality of volatile oil
» Considerable research work has been done and published on the effects of gibberellins on the alkaloid content of treated plants
› For example, a reduction in the yield of alkaloids was observed in Hyoscyamus niger, when treated with gibberellins
› On the other hand, gibberellins increased alkaloid yield in belladonna
› With Nicotiana tabacum, treatment again gave a generally reduced alkaloid content
» Regarding the effect of gibberellins on glycosides; the application of gibberellins to Cassia angustifolia appeared to reduce the glycosides (sennosides) content of the leaves at all concentrations used
» When the effect of gibberellins was studied on fixed oil (castor oil), no significant differences were observed regarding the quantity and quality of the fixed oil of the seed

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) (Figure 4)
» 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
» Cytokinin isolated from other sources are;
› Isopentenyladenine and dihydrozeatin
» Many more have been detected but not identified

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
› Increase the shelf life of vegetables, flowers, and fruits
» 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 viccifolia), and rhodoxanthin (Ricinus)

Effect of cytokinins on secondary metabolites
» In a study, cytokinin in field plants led to a delay in the usual seasonal fall in alkaloid content
» In another study, a favorable increase in alkaloid yield was observed in Hyoscyamus muticus treated with kinetin
» Leaves of the coffee plant after kinetin treatment developed a transient increase of up to 10% in their caffeine content
» In Cassia angustifolia, the hormone slightly increased the glycosides (sennosides) content

Abscisic acid
» It is a growth inhibitor substance
» From plants, abscisic acid was isolated and characterized
» It has also been isolated from the fungus Cenospora rosicola
» Vomifoliol: A substance related to abscisic acid, isolated from plants
› It has the same activity as abscisic acid in stomatal closure tests (causes stomata to close)
» A number of synthetic growth inhibitors have been studied
› The first to be described was maleic hydrazide

Typical effects and uses
» Prevents bud opening
» Prevents seed germination
» Induces development of dormancy

» Little or no work appears to have been reported on the effects of abscisic acid on the production of secondary metabolites

Ethylene
» In 1932, it was demonstrated that the ethylene evolved by stored apples inhibited the growth of potato shoots enclosed with them
» Ethylene is found in the gaseous form
» Ethylene has a role in fruit ripening (induces fruit ripening, e.g., green bananas)

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
» Ethylene causes leaves to drop (abscission)
» Ethylene also promotes senescence
» Plants often increase ethylene production in response to stress
» Ethylene is often found in high concentrations within cells at the end of a plant’s life
» The increased ethylene in leaf tissue in the fall is part of the reason leaves fall off trees
» 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, it is broken down to ethylene and phosphate

Effect of ethylene on secondary metabolites
» At low concentrations, ethylene has been shown to increase the glycoside (sennosides) concentration in Cassia angustifolia
» Ethephon is now increasingly used as standard practice for enhancing the flow of rubber latex
› Sprayed onto the scraped bark of the rubber tree it increases latex yields from 36 to 130%
___

>>> What is the structural formula of ethylene?
>>> Which plant growth regulator(s) can be used to keep the cut flowers fresh for a longer time?
>>> How plant growth regulators can play their role in tissue culture?
>>> How and why tissue culture technique is useful in Pharmacognosy?

Reading References

» Evans WC. Trease and Evans Pharmacognosy. Elsevier, 16th edition, 2009.

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