* DEFINITION:
Various organic compounds other than nutrients produced by plants that control and regulate germination, growth, metabolism and other physiological activities are termed plant hormones.
* INTRODUCTION:-
-> Plant hormones are biological catalysts.
-> They are also known as phytohormones and growth bio-regulators.
-> They are active in very low concentrations [ 10^-6 - 10^-5 M ] .
-> Signal molecules produced at specific locations cause altered processes in target cells at other locations.
-> They are also active in the tissue where they are produced.
-> They are produced in certain parts of plants and usually transported to other parts where they elicit specific biochemical, physiological and morphological responses.
-> Each plant hormone elicits many different effects or responses, such as stimulatory, inhibitory and overlapping responses.
-> Each plant hormone performs specific functions; however, all measurable plant responses to heredity or environment are controlled by interactions between two or more hormones.
-> Such interactions can be cooperative, antagonistic and balanced, occurring at different levels:
1) synthesis of hormone
2) hormone receptors and secondary messengers
3) ultimate action of plant hormones.
-> Plant hormones are internally secreted hormones or chemicals.
-> Synthesis of plant hormones is more diffuse and not always localised.
CLASSES :
STIMULATORY INHIBITORY
- Auxin - Ethylene
- Cytokinin - Abscisic acid
- Gibberellins - Jasmonic acid
- Brassinosteroids
* FUNCTIONS:-
-> Root and shoot formation
-> Bud formation
-> Auxin tends to move in a particular direction through plant tissues. This directional movement helps establish differences in auxin concentration.
-> Auxins occur in very small amounts in higher plants but have major effects on cell division, cell elongation, root development, and apical dominance, tropic movements, flowering and fruit development, vascular differentiation, abscission, and gene expression.
Procedure:
- Take oat (Avena) coleoptile tips.
- Remove the tips and place them on agar.
- Auxin diffuses from the coleoptile tip into the agar.
- The agar block is then placed on a decapitated coleoptile stump.
- If auxin is present, it causes curvature.
- Greater auxin activity → greater curvature.
B) Split Pea Stem Curvature Bioassay:
Procedure:
- A pea stem section is split longitudinally.
- The sections are first kept in distilled water to remove endogenous auxin.
- They are then placed in the solution suspected to contain auxin.
- After several hours, the stem segments show curvature.
-> Auxin causes:
↑ Cell elongation → ↑ Length of tissue
-> The amount of stimulation is approximately related to the logarithm of auxin concentration.
-> Etiolated (dark-grown) stem segments are generally more sensitive than green stem segments.
. Physiological effects:
1) Cell elongation:
-> Auxin promotes elongation of stem cells, which elongates cells.
-> High concentrations may inhibit growth in coleoptiles and young leaves, which are particularly sensitive.
-> Effects of auxin depend on the concentration of auxin, type of tissue, age of tissue, and plant species.
2) Root growth:
-> Roots are generally more sensitive to auxin than shoots.
Therefore:
- Very low auxin concentration → may stimulate root growth.
- Higher concentration → inhibits root growth.
3) Apical dominance:
-> The apical bud suppresses the growth of lateral buds.
-> Auxin produced in the shoot apex moves downward and inhibits lateral bud growth.
-> If the apical bud is removed, lateral buds grow; thus, branching increases.
4) Flowering and parthenocarpy:
-> Auxins can promote formation of female flowers in some plants, and also promote growth of ovary walls.
PARTHENOCARPY- Formation of fruits without fertilisation, often producing seedless fruits.
Ex: Grapes, banana, etc.
5) Abscission:
-> Abscission means the natural shedding of leaves, flowers or fruits.
-> Auxin generally delays/inhibits abscission in many young tissues.
-> However, in old or senescent tissues, auxin can sometimes promote abscission indirectly through increased ethylene production.
6) Plant growth movements:
-> Auxin plays an important role in the directional growth of plants in response to environmental stimuli.
PHOTOTROPISM - is the growth movement of a plant part in response to light.
-> Auxin moves to the shaded side; more elongation occurs there, shoot bends toward the light.
GEOTROPISM - is the growth response of a plant to gravity.
-> Effects include:
- Increased cambial cell division
- Increased secondary growth
- Promotion of vascular tissue development
- Promotion of xylem differentiation
IAA is particularly important in vascular differentiation.
8) Auxin and Gene Expression:
-> Auxin can influence gene expression.
The basic sequence is:
Auxin → receptor → regulatory factor → DNA → RNA → Protein → Response
9) Auxin and Enzyme Activity:
-> Auxin can also affect enzyme activity directly.
For example:
Auxin → increased activity of cell-wall-related enzymes → cell-wall changes → cell elongation
Auxin can affect enzymes involved in:
- Cell wall metabolism
- Protein synthesis
- Other metabolic pathways
-> One of the most important mechanisms of auxin-induced elongation involves the plasma membrane and cell wall.
hypothesis:-
Auxin → H⁺ pump → acidification of cell wall → wall loosening → water uptake → cell elongation
11) Second Messenger Mechanism:
-> Auxin may also act through second messengers.
-> A hormone is considered the first messenger.
-> After auxin binds to its receptor, intracellular signalling molecules may be activated.
Possible second messengers include:
- Ca²⁺
- Calmodulin
- Cyclic nucleotides such as cAMP.
-> Auxins are mainly synthesised in:
- Shoot apical meristems
- Young developing tissues
- Young leaves
- Developing seeds/fruits
- Other actively growing tissues
The principal natural auxin, IAA, can be synthesised from the amino acid tryptophan.
-> It involves three pathways:-
1) IPA (Indole-3-pyruvic acid ) PATHWAY - one of the most important tryptophan-dependent pathways.
-> In this pathway, L-tryptophan is first converted into indole-3-pyruvic acid by the tryptophan transaminase enzyme. This involves transferring an amino group from tryptophan (transamination reaction). The indole-3-pyruvic acid is then converted into indole-3-acetaldehyde by an enzyme, indole-3-pyruvate decarboxylase. During this reaction, the carboxyl group is removed as carbon dioxide (decarboxylation). Finally, indole-3-acetaldehyde is oxidised to indole-3-acetic acid by the enzyme indole-3-acetaldehyde dehydrogenase (oxidation).
Examples: Sterile pea seeds and cucumber seedlings.
2) TAM (Tryptamine) PATHWAY -
-> In this pathway, tryptophan undergoes decarboxylation to form tryptamine. This reaction is catalysed by the enzyme tryptophan decarboxylase. Tryptamine is then converted into indole-3-acetaldehyde by an oxidative deamination reaction. The enzyme involved is tryptamine oxidase; finally, indole-3-acetaldehyde undergoes oxidation to form indole-3-acetic acid. This reaction is carried out by indole-3-acetaldehyde dehydrogenase.
Examples: Tomato, rice and barley.
3) IAN (Indole-3-acetonitrile) PATHWAY -
-> In this pathway, tryptophan undergoes oxygenation to form indole-3-acetaldoxime. This reaction is catalysed by tryptophan monooxygenase. Then indole-3-acetaldoxime is converted into indole-3-acetonitrile by a process involving hydrolysis. The compound is then converted into the nitrile form. Finally, indole-3-acetonitrile undergoes nitrile hydrolysis to form indole-3-acetic acid.
. Mechanism of Auxin Action:
-> Auxin affects plants at different levels.
It can affect:
- Gene expression
- Protein synthesis
- Enzyme activity
- Membrane permeability
- Ion transport
- Cell wall loosening
- Cell elongation
Some auxin effects occur very rapidly, while others require changes in gene expression and protein synthesis.
. Important Applications of Synthetic Auxins:
-> Synthetic auxins are useful in agriculture and horticulture.
Uses include:
- Root initiation in cuttings
- Control of plant growth
- Fruit development
- Induction of parthenocarpy
- Prevention/delay of premature abscission
- Selective weed control
Example: 2,4-D is widely used as a selective herbicide.
- Number and position of –OH groups
- Oxidation state
- Other functional groups
Bioassay = determining the activity/presence of a hormone by observing its biological effects.
A. Dwarf maize/pea assay
- Gibberellins promote growth of dwarf plants.
- Dwarf plants have little endogenous gibberellin.
- Application of GA causes stem elongation.
- Increase in stem length is measured.
B. Barley aleurone assay
- This is an important bioassay for gibberellins.
- GA is applied to barley aleurone layers.
- GA stimulates production of Ξ±-amylase.
- Ξ±-amylase breaks down starch into sugars.
- The amount of sugar/Ξ±-amylase produced indicates GA activity.
-> Gibberellins are diterpenoid plant hormones that are synthesised mainly in young and actively growing tissues, such as shoot apices, young leaves, developing seeds and fruits. The biosynthesis of gibberellins takes place through a series of enzymatic reactions and begins with the formation of geranylgeranyl diphosphate (GGDP).
-> First, acetyl-CoA is converted into mevalonic acid, which gives rise to the isoprenoid units IPP (isopentenyl diphosphate) and DMAPP (dimethylallyl diphosphate). These units combine to form geranylgeranyl diphosphate (GGDP), which is the starting precursor for gibberellin biosynthesis.
-> GGDP is then converted into ent-copalyl diphosphate (ent-CPP) by the enzyme ent-copalyl diphosphate synthase (CPS). Ent-CPP is further converted into ent-kaurene by the enzyme ent-kaurene synthase (KS). Thus, the early part of the pathway is:
GGDP → ent-CPP → ent-kaurene
-> The ent-kaurene formed is subsequently oxidised by ent-kaurene oxidase (KO). It is converted successively into ent-kaurenol, ent-kaurenal, and finally ent-kaurenoic acid.
-> The ent-kaurenoic acid then undergoes further oxidation by ent-kaurenoic acid oxidase (KAO) to form GA₁₂ (gibberellin A₁₂). GA₁₂ is an important common precursor from which many other gibberellins are synthesised.
-> Enzymes such as GA 20-oxidase (GA20ox) and GA 3-oxidase (GA3ox) further modify GA₁₂ to produce biologically active gibberellins. For example, GA₂₀ is converted to GA₁, an important active gibberellin that promotes plant growth.
-> The level of active gibberellin is also controlled by GA 2-oxidase (GA2ox), which converts active gibberellins into less active or inactive forms. In this way, plants regulate the amount of active gibberellin according to their growth requirements.
1. Promotion of plant growth
- Gibberellins promote stem and internode elongation.
- They can overcome genetic dwarfism in many plants.
- They increase the length of stems.
2. Bolting
- GA promotes rapid elongation of the stem before flowering.
- Common in rosette plants.
3. Flowering
- Gibberellins can promote flowering in some plants.
- Particularly effective in certain long-day plants.
4. Seed germination
- GA promotes seed germination.
- It stimulates the production of hydrolytic enzymes, especially Ξ±-amylase.
- These enzymes break down stored food and provide nutrients to the growing embryo.
5. Fruit growth
- GA promotes fruit growth and development.
- It can increase fruit size.
6. Parthenocarpy
- GA can induce development of seedless fruits in some plants.
- Fruit develops without normal fertilisation.
7. Breaking dormancy
- Gibberellins can help break seed and bud dormancy.
8. Effect on Membrane Permeability
- Gibberellins can increase membrane permeability.
- They influence the synthesis of membrane components.
- Increased permeability helps movement/release of substances.
- In barley aleurone cells, GA promotes secretion of hydrolytic enzymes into the surrounding medium.
9. Gibberellins and DNA
- Gibberellins may interact indirectly with DNA and gene expression.
- They can influence transcription of specific genes.
- This leads to production of specific mRNAs and proteins.
- However, the exact mechanism of direct GA-DNA interaction is not completely established.
9. Anti-Gibberellins / Growth Retardants
-> These are chemicals that reduce gibberellin action or synthesis.
Examples:
- CCC (cycocel)
- Phosphon-D
- AMO-1618
- SADH
Effects:
- Reduce stem elongation.
- Produce shorter, compact plants.
- Often act by inhibiting gibberellin biosynthesis.
10. GA-Glucosides
- Gibberellins can be stored in plants as glucosides/conjugates.
- These forms generally have low biological activity.
- They may act as storage forms.
- Under suitable conditions, they may be hydrolysed to release active gibberellins.
. Commercial Uses of Gibberellins:
1. Increase fruit size
-
GA is used in horticulture.
-
It can increase the size and quality of fruits.
2. Grapes
-
GA is sprayed on grapevines.
-
It increases the number and size of grapes in clusters.
3. Seedless fruits
-
GA₃ can be used to produce seedless fruits, such as seedless grapes.
4. Apples and pears
-
GA can improve:
-
Size
-
Colour
-
Quality
- Size
- Colour
- Quality
5. Beer industry
-
GA increases Ξ±-amylase activity in germinating barley.
-
This helps in malting and beer production.
6. Sugarcane
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GA promotes internode elongation.
-
This increases cane growth and yield.
-
It is particularly useful where sugarcane is an important crop.
- G. Haberlandt, an Austrian plant physiologist, found that vascular tissues contained a substance that could stimulate cell division.
- Discovered that coconut milk could stimulate cell division.
- It also promoted the growth of very young Datura embryos.
- Studied tobacco pith tissues.
- Found that active cell division required adenine/purine compounds along with other growth substances.
-
A substance obtained from degraded DNA was found to stimulate:
- Cell division
- Callus formation
- This substance was named kinetin.
- Kinetin was one of the first compounds recognised as a cytokinin.
- It was discovered through laboratory studies.
- Naturally occurring cytokinins were identified, including:
- Zeatin
- Zeatin riboside
- Other adenine derivatives
Most cytokinins have:
Adenine nucleus + side chain
-> The side chain attached to adenine determines the particular type of cytokinin.
A) Cell division/callus growth assay
- Cytokinins stimulate cell division and callus formation.
- In tissue culture, a suitable concentration of cytokinin can promote the formation of callus and shoots.
B) Cotyledon expansion test
-> This is a common assay for cytokinin activity.
Procedure:
- Seeds are allowed to germinate.
- Cotyledons are removed.
- Cotyledons are incubated with different concentrations of cytokinin.
- Cytokinin causes expansion of cotyledons.
- Greater expansion indicates greater cytokinin activity.
C) Chlorophyll retention assay
Cytokinins can delay chlorophyll loss.
Procedure:
- Leaves are treated with different cytokinin concentrations.
- Leaves are kept under suitable conditions.
- Chlorophyll is measured after a fixed period, such as 48 hours.
- Greater retention of chlorophyll indicates greater cytokinin activity.
D) Anthocyanin production
- Cytokinins can stimulate anthocyanin synthesis.
- Therefore, an increase in pigmentation can also be used as an indication of cytokinin activity.
1. Promotion of Cell Division and Organ Formation
The most important function of cytokinins is the promotion of cell division.
Cytokinins stimulate:
Cell division → cell growth → tissue growth → organ formation.
2. Interaction with auxin
-> Cytokinins and auxins work together. The ratio between them determines the type of growth.
Example: In tissue culture, changing the cytokinin-to-auxin ratio can control whether the tissue produces:
- Roots
- Shoots
- Callus
3. Effect on Apical Dominance
The apical bud normally suppresses the growth of lateral buds. This is called apical dominance.
-> Cytokinins can reduce this effect by its effect on lateral buds, which makes them active and increasing branching. Therefore: Cytokinins promote lateral bud growth and reduce apical dominance.
On roots - Cytokinins can also promote the formation of bud primordia on root segments under suitable conditions.
4. Promotion of Seed Germination
Cytokinins can:
- Promote seed germination
- Break dormancy in some seeds
- Stimulate growth of seedlings
Example: Lettuce
-> Lettuce seeds: Germinate well in light.
* Treatment with cytokinin can improve germination even under dark conditions.
5. Expansion of Cotyledons and Leaves
Cytokinins promote the expansion of cotyledons, leaves, and leaf discs by mainly increasing cell division and cell enlargement.
Example: In excised bean/cotyledon tissues, cytokinin treatment increases cotyledon expansion.
6. Promotion of Chloroplast Development
This is an important effect, in which cytokinins promote the development of chloroplasts, particularly in tissues exposed to light.
Cytokinin treatment can cause:
- Development of chloroplasts
- Formation of grana
- Development of thylakoid membranes
- Increased chlorophyll formation
7. Anthocyanin Synthesis
-> Cytokinins can stimulate the formation of anthocyanins.
-> Anthocyanins are pigments responsible for red, blue and purple colour in many plant tissues.
8. Delay of Senescence
One of the most important functions of cytokinins is: Delay of leaf senescence.
Senescence- the natural ageing process of plant tissues.
During senescence:
- Proteins are degraded.
- Chlorophyll breaks down.
- Nucleic acids decrease.
- Lipids are degraded.
- Leaves become yellow.
- Nutrients move from old leaves to younger tissues.
9. Cytokinin and Nutrient Mobilisation
Cytokinins can cause nutrients and metabolites to move toward the tissue where cytokinin is present.
Example:
If cytokinin is applied to one part of a leaf, the nutrients move towards the cytokinin-applied region; this region remains green, metabolically active, and healthy for a longer period of time, and this phenomenon is called nutrient mobilisation.
10. Role in Flowering and Seed Development
Cytokinins may also participate in:
- Flower development
- Development of new leaves
- Development of floral buds
- Seed development
- Nutrient accumulation in developing reproductive organs
11. Increased enzyme activity
Cytokinins can increase the activity of several enzymes.
Examples include enzymes involved in:
- Nitrogen metabolism
- Nitrate reduction
- Amino acid metabolism
- Protein synthesis
- Nucleic acid metabolism
Example:
Cytokinin can increase nitrate reductase activity.
12. Cytokinin Receptor and Signal Transduction
Cytokinins act by interacting with specific receptors.
Simplified mechanism:
Cytokinin
↓
Binds to cytokinin receptor
↓
Signal transduction pathway is activated.
↓
Changes in gene expression
↓
Changes in RNA and protein synthesis
↓
Cell division, growth and development
This is why a very small amount of cytokinin can produce a significant physiological response.
13. Second Messenger
Cytokinin action involves intracellular signalling.
The binding of cytokinin to its receptor can activate a signalling cascade involving protein phosphorylation and changes in gene activity.
Cytokinins are mainly purine derivatives, and their biosynthesis occurs through the modification of adenine nucleotides. They are synthesised particularly in actively growing tissues such as root apical meristems, young leaves, developing seeds and fruits. The roots are considered an important site of cytokinin production.
The biosynthesis of cytokinins involves the addition of an isoprenoid side chain to an adenine nucleotide. The important enzyme involved in this process is isopentenyl transferase (IPT). It transfers an isopentenyl group from an isoprenoid donor, mainly dimethylallyl diphosphate (DMAPP), to an adenine nucleotide. This produces isopentenyl adenine nucleotides, which are important precursors of cytokinins.
These isopentenyl adenine compounds can undergo further modifications, including hydroxylation, to form zeatin-type cytokinins. The cytokinin nucleotides may subsequently be converted into ribosides and free bases. The free-base forms are generally the biologically active forms of cytokinins.
The exact mechanism is complex, but cytokinins mainly affect:
- Nucleic acid synthesis
- Protein synthesis
- Enzyme activity
- Gene expression
- Cell division
-> Abscisic acid (ABA) is a major plant growth regulator/hormone. It is mainly known as a growth-inhibiting and stress hormone.
-> Main functions of ABA
- Inhibits seed germination.
- Promotes seed and bud dormancy.
- Inhibits seedling and shoot growth.
- Causes stomatal closure during water stress.
- Helps plants tolerate drought and water deficiency.
- Promotes leaf senescence and abscission in many situations.
- Regulates several metabolic and developmental processes.
-> ABA is widely distributed in plants and has been found in leaves, roots, seeds, fruits, buds, flowers, developing and dormant tissues.
A bioassay is a biological test used to detect or measure the activity/concentration of a plant hormone by observing its effect on a living plant tissue.
For ABA, bioassays are mainly based on its growth-inhibiting effect.
Important ABA bioassays
1. Seed germination inhibition test
- Seeds are placed on solutions containing different concentrations of ABA.
- A control is maintained without ABA.
- Germination percentage is recorded after a fixed time.
- Higher ABA concentration → greater inhibition of germination.
- Thus, the degree of inhibition indicates ABA activity.
2. Seedling growth inhibition test
- Young seedlings are grown in solutions containing ABA.
- Their growth/elongation is compared with untreated seedlings.
- ABA reduces seedling growth.
- The amount of growth inhibition is used as an indication of ABA activity.
3. Stomatal closure bioassay
- Epidermal strips containing guard cells are treated with ABA.
- ABA causes stomatal closure.
- The degree of stomatal closure can be measured under a microscope.
- Greater closure indicates greater ABA activity.
- ABA is a 15-carbon (C₁₅) sesquiterpenoid compound.
- It has a relatively small molecular structure.
- ABA contains an asymmetric carbon atom, so it can exist in different optical forms.
- Naturally occurring ABA is mainly the S-(+)-form.
ABA can be synthesised through two major pathways:
1. Direct pathway
ABA can be synthesised from mevalonic acid/isoprenoid precursors.
Simplified:
Mevalonic acid → isoprenoid compounds → farnesyl pyrophosphate → ABA-related compounds
2. Indirect pathway
Carotenoids such as xanthophylls can be broken down to form ABA precursors.
Simplified:
Carotenoids → xanthophylls → oxidative cleavage → C₁₅ aldehyde (xanthoxin) → ABA
The aldehyde intermediate is oxidised to form abscisic acid.
1. Inhibition of Seed Germination
-> The most important effect of ABA is that it prevents or delays seed germination.
How?
- ABA maintains seeds in a dormant state.
- It inhibits enzymes required for germination.
- It suppresses metabolic activity needed for embryo growth.
- Removal or reduction of ABA helps the seed begin germination.
Example
In cereal grains, ABA can inhibit the production/action of enzymes such as Ξ±-amylase, which are important for mobilising stored food.
Therefore:
High ABA → dormancy/inhibition of germination
Low ABA → germination becomes easier
2. Inhibition of Seedling Growth
ABA can inhibit the growth of young seedlings.
- It reduces elongation growth.
- It can inhibit growth of shoots and coleoptiles.
- The effect depends on the concentration of ABA and the plant tissue.
- Higher concentrations generally produce stronger inhibition.
3. Inhibition of Bud Growth and Promotion of Dormancy
ABA plays an important role in bud dormancy, especially in woody plants.
Effects
- ABA concentration is often high in dormant buds.
- It inhibits the growth of lateral buds.
- When dormancy-breaking conditions occur, ABA levels may decrease.
- Reduction in ABA allows the bud to resume growth.
Therefore:
High ABA → bud dormancy
Low ABA → bud growth
This is especially important for plants that experience winter or unfavourable environmental conditions.
4. Stomatal Closure
This is one of the most important functions of ABA.
During drought or water shortage:
Water stress → ABA increases → ABA reaches guard cells → stomata close → transpiration decreases → water is conserved
Why is this useful?
It prevents excessive water loss and helps the plant survive drought.
5. ABA and Ion Movement During Stomatal Closure
ABA affects several ions in guard cells.
Important ions include:
- K⁺
- H⁺
- Cl⁻
- Organic anions such as malate
ABA modifies ion transport across the guard-cell membrane.
ABA → ion transport changes → K⁺/anions leave guard cells → water leaves → guard cells lose turgor → stomata close
ABA can also affect the H⁺/K⁺ exchange system and ion channels involved in stomatal movement.
6. ABA and Leaf Senescence π
ABA is associated with senescence, the natural ageing process of plant tissues.
Effects
- Promotes loss of chlorophyll.
- Reduces photosynthetic activity.
- Promotes changes in RNA and protein metabolism.
- Can accelerate the ageing of detached leaves.
- May promote abscission in some plants.
Important point
ABA was initially associated with abscission, but its role is much broader than simply causing abscission.
7. ABA and Abscission
Abscission = natural separation/falling of plant organs such as leaves, flowers or fruits.
ABA can promote abscission in some plant species.
However, ABA is not considered the only hormone responsible for abscission.
8. Role of ABA in Drought Resistance π΅
ABA is extremely important when plants experience water stress.
Major roles
1. Stomatal closure
ABA closes stomata and therefore:
Transpiration ↓ → water loss ↓
2. ABA concentration increases
When plants experience water stress: Water stress → ABA concentration ↑
When water becomes available again: Water stress relieved → ABA concentration ↓
3. Helps plants survive drought
Plants capable of producing an appropriate ABA response can reduce water loss and improve their ability to tolerate drought.
However, drought resistance is not controlled by ABA alone. Other physiological and genetic mechanisms are also involved.
9. ABA and Chloroplasts
Chloroplasts are important sites associated with ABA metabolism.
According to the mechanism shown in the figure:
Drought → ABA released from chloroplasts → stomatal closure
During continued stress:
- ABA synthesis may increase.
- ABA may accumulate.
- Further ABA synthesis can eventually be reduced by feedback/end-product inhibition.
After rewatering:
- ABA production decreases.
- Chloroplast-related ABA metabolism returns toward normal.
- Stomata can reopen.
10. ABA and Ion Transport
ABA influences the movement of ions across membranes.
It can affect:
- K⁺ channels
- H⁺ transport
- Cl⁻ movement
- Anion channels
- ATPase activity
This is particularly important in guard cells.
Result:
ABA → ion movement → water movement → change in guard-cell turgor → stomatal closure
11. ABA and Lipid Metabolism
ABA also affects lipid metabolism.
It can:
- Increase the proportion of saturated fatty acids.
- Decrease some unsaturated fatty acids.
- Change membrane properties.
- Therefore influence membrane permeability.
So, ABA can modify cell membranes both through ion transport and changes in membrane lipids.
12. ABA and Protein Synthesis
ABA can regulate protein synthesis in different ways.
ABA may:
- Inhibit synthesis of some proteins.
- Affect the formation of specific enzymes.
- Change gene expression.
- Affect mRNA production or stability.
For example, during seed germination, ABA can inhibit the synthesis of enzymes required for mobilisation of stored food.
Important exception
ABA does not always inhibit all protein synthesis.
Some proteins called storage proteins may actually be synthesised or accumulated in the presence of ABA, particularly during seed development.
13. ABA and mRNA
ABA can decrease the amount of certain mRNAs.
This can happen through:
- Reduced RNA polymerase activity
- Increased ribonuclease activity
- Increased degradation of existing mRNA
Result
ABA → mRNA changes → protein synthesis changes → growth/development changes
14. Role of Calcium as a Second Messenger
ABA can act through Ca²⁺ (calcium ions) as a secondary messenger.
Simple mechanism
ABA → Ca²⁺ concentration changes → activation of signalling proteins → ion channels change → physiological response
Calcium-dependent processes are particularly important in stomatal closure.
15. ABA and Gene Expression
ABA can influence gene expression by affecting:
- Transcription
- mRNA formation
- mRNA stability
- Protein synthesis
Therefore, ABA can produce both:
Rapid effects
Occur within minutes.
Example:
- Stomatal closure
- Ion movement
Slow effects
Require changes in gene expression/protein synthesis.
Example:
- Changes in growth
- Seed dormancy
- Stress-related proteins
ABA affects:
- Membrane permeability
- Ion transport
- Ca²⁺ signalling
- Gene expression
- mRNA
- Protein synthesis
- Lipid metabolism
-> Ethylene (C₂H₄) is a simple, gaseous plant growth regulator (phytohormone). Unlike most plant hormones, ethylene is a gas and can easily diffuse through plant tissues and the surrounding air.
-> It is produced naturally by plants and regulates several important processes such as:
- Fruit ripening
- Leaf and fruit senescence
- Leaf and fruit abscission
- Seed germination
- Root growth
- Flowering in some plants
- Response to mechanical stress
- Response to flooding and other environmental stresses
Ethylene is often called the "ripening hormone" because of its major role in the ripening of many climacteric fruits.
Important characteristics
- Ethylene is a gaseous hormone.
- Its chemical formula is C₂H₄.
- It is a very simple hydrocarbon.
- It is synthesised from the amino acid methionine.
- Its biosynthesis involves SAM and ACC.
- Ethylene production is generally increased during:
- Fruit ripening
- Senescence
- Wounding
- Mechanical stress
- Pathogen attack
- Flooding
- Ethylene can act at very low concentrations.
- It affects plants by changing gene expression and protein activity.
-> The first important observation was made by the Russian scientist Dimitry Neljubow while studying the growth of pea seedlings.
-> He observed that pea seedlings growing in the laboratory near illuminating gas showed abnormal growth compared with normal seedlings. The seedlings showed:
- Reduced elongation of the stem
- Increased thickness of the stem
- Horizontal growth instead of normal vertical growth
This characteristic response was later called the triple response.
Neljubow concluded that a component of illuminating gas was identified as ethylene.
2. Work of Henry S. F. Denny (1917)
Henry Denny investigated the effect of illuminating gas on fruits and demonstrated that ethylene was responsible for promoting fruit ripening.
His work established the important role of ethylene in the ripening process.
3. Later recognition as a natural plant hormone
Further research showed that plants themselves naturally produce ethylene. This established that ethylene was not merely an external gas but an endogenous plant hormone involved in growth and development.
Ethylene is the simplest alkene and consists of two carbon atoms and four hydrogen atoms.
Molecular formula: C₂H₄
Structural formula
H H \ / C = C / \ H H
The two carbon atoms are connected by a double covalent bond.
Important structural features
- Molecular formula: C₂H₄
- Molecular weight: approximately 28.05 g/mol
- It is an unsaturated hydrocarbon.
- It contains a C=C double bond.
- It is a colourless gas.
- It is relatively small and can diffuse rapidly through plant tissues.
Why is its structure important?
Because ethylene is a small, non-polar molecule, it can readily move through cell membranes and reach its receptors. Therefore, unlike many hormones that require transport systems, ethylene can diffuse from its site of production.
A bioassay is a method of detecting or measuring the biological activity of a plant hormone by observing a characteristic response produced in a living plant or plant tissue.
The classical bioassay for ethylene is based on the triple response of etiolated pea seedlings.
A. Pea Seedling Triple-Response Bioassay
Principle
Young pea seedlings grown in darkness (etiolated seedlings) show a characteristic response when exposed to ethylene. This response consists of three changes:
- Inhibition of stem elongation
- Increase in stem thickness
- Horizontal growth of the stem
These three effects together are called the triple response.
Procedure
Young pea seedlings are grown in darkness and placed in a closed container. They are then exposed to different concentrations of ethylene for a fixed period. A control group is kept without ethylene.
After exposure, the seedlings are examined for changes in their growth, particularly:
- Length of the stem
- Thickness of the stem
- Direction/angle of growth
The intensity of these changes can be compared with the control to estimate the biological activity of ethylene.
Ethylene is synthesised in plants from the amino acid methionine. The biosynthesis occurs through two major intermediates, S-adenosylmethionine (SAM) and 1-aminocyclopropane-1-carboxylic acid (ACC). The pathway can be written as Methionine → SAM → ACC → Ethylene.
1. Conversion of Methionine into SAM
The biosynthesis begins with methionine, which reacts with ATP to form S-adenosylmethionine (SAM). This reaction is catalysed by the enzyme SAM synthetase.
Methionine + ATP → SAM
SAM acts as the immediate precursor for ACC formation.
2. Conversion of SAM into ACC
SAM is then converted into 1-aminocyclopropane-1-carboxylic acid (ACC) by the enzyme ACC synthase (ACS). This is an important regulatory and rate-limiting step in ethylene biosynthesis. During processes such as fruit ripening, wounding, and stress, ACC synthase activity may increase, resulting in increased ethylene production.
SAM → ACC
During this reaction, 5′-methylthioadenosine (MTA) is also produced. MTA can be recycled through the methionine salvage cycle, allowing the plant to regenerate methionine.
3. Conversion of ACC into Ethylene
In the final step, ACC is converted into ethylene (C₂H₄) by the enzyme ACC oxidase (ACO). This reaction requires oxygen (O₂) and other cofactors such as Fe²⁺ and ascorbate.
ACC + O₂ → Ethylene + CO₂ + HCN + other products
The hydrogen cyanide (HCN) produced as a by-product is detoxified by the plant, mainly through conversion into Ξ²-cyanoalanine.
Ethylene has many effects on plant growth and development.
A. Fruit Ripening
This is one of the most important functions of ethylene.
Ethylene promotes ripening in climacteric fruits, such as:
- Banana
- Apple
- Tomato
- Mango
- Avocado
During ripening, ethylene promotes:
1. Softening
Ethylene stimulates enzymes that modify the cell wall and middle lamella.
Examples include:
- Pectin-degrading enzymes
- Polygalacturonase
- Pectin methylesterase
This results in:
Cell wall modification → fruit softening
2. Colour change
Ethylene promotes changes in pigments.
For example:
Chlorophyll degradation → loss of green colour
and synthesis/unmasking of other pigments results in the characteristic colour of ripe fruits.
3. Conversion of starch into sugars
Ethylene-associated ripening promotes metabolic changes that increase soluble sugars and alter flavour.
Therefore:
Starch → soluble sugars → sweeter fruit
4. Aroma development
Ethylene promotes biochemical pathways responsible for the production of many volatile compounds.
These contribute to the characteristic aroma and flavour of ripe fruits.
B. Senescence
Senescence is the final stage of development in plant organs before their death or removal.
Ethylene promotes senescence in:
- Leaves
- Flowers
- Fruits
It can stimulate:
- Chlorophyll degradation
- Protein degradation
- Membrane changes
- Nutrient remobilisation
- Expression of senescence-associated genes
Therefore:
Ethylene → senescence-related gene expression → degradation processes → senescence
C. Abscission
Ethylene plays an important role in the falling of leaves, flowers and fruits.
At the abscission zone, ethylene promotes the expression/activity of cell-wall-modifying enzymes.
These weaken the cells in the abscission zone.
Ethylene ↓ Cell-wall-modifying enzymes ↓ Cell separation ↓ Organ detachment ↓ Abscission
Ethylene often works together with changes in auxin sensitivity/transport during abscission.
D. Triple Response
Ethylene causes a characteristic response in young seedlings.
The three components are:
1. Inhibition of elongation
Ethylene reduces excessive longitudinal growth.
2. Radial thickening
The stem becomes thicker.
3. Horizontal growth
The seedling grows more horizontally rather than straight upward.
Hence:
Triple response = short + thick + horizontal
This response can help seedlings growing through soil to avoid obstacles and protect the shoot apex.
E. Apical Hook Formation
Ethylene promotes and maintains the apical hook in dark-grown seedlings.
The apical hook protects the delicate shoot apical meristem as the seedling pushes through soil.
F. Seed Germination
Ethylene can promote germination in some plant species.
It may:
- Reduce certain dormancy constraints
- Promote embryo growth
- Interact with other hormones such as ABA and gibberellins
Its effect depends on:
- Plant species
- Seed condition
- Ethylene concentration
- Environmental conditions
G. Root Growth
Ethylene affects root development.
At appropriate levels, it can:
- Modify root elongation
- Increase root diameter
- Promote formation of root hairs
- Alter root architecture
High ethylene levels can strongly inhibit root elongation.
H. Root Hair Formation
Ethylene can promote root hair initiation and development.
This can increase the effective surface area of roots for absorption.
I. Flowering
Ethylene has different effects on flowering depending on plant species.
Important example: Pineapple
Ethylene can promote flower induction in pineapple.
Therefore, ethylene has applications in the synchronisation of flowering in certain crops.
J. Sex Expression
Ethylene can influence sex expression in some plants.
For example, in cucurbits, ethylene generally promotes the development of female flowers.
Thus:
More ethylene → greater tendency toward female flower formation
This is an important agricultural application of ethylene-related regulation.
K. Response to Mechanical Stress
When plants experience:
- Touch
- Wind
- Bending
- Wounding
- Mechanical pressure
ethylene production can increase.
It can cause changes such as:
- Reduced elongation
- Increased thickening
- Altered growth direction
This helps plants adapt to mechanical conditions.
L. Response to Flooding
Ethylene is particularly important during waterlogging and flooding.
Because ethylene is a gas, it can accumulate inside submerged tissues when its diffusion out of the plant is restricted.
In some plants, ethylene promotes formation of aerenchyma, which creates air spaces in tissues.
These spaces help transport gases through submerged parts of the plant.
M. Stress and Defence
Ethylene participates in responses to:
- Wounding
- Pathogen attack
- Salt stress
- Drought
- Flooding
- Mechanical stress
It interacts with other signalling molecules and hormones, especially:
- Jasmonates
- Salicylic acid
- Auxin
- Abscisic acid
Thus, ethylene is an important component of the plant's stress-response network.
Plant hormones are the chemical messengers of plants, controlling and coordinating growth, development, reproduction, and responses to the environment. From auxin, gibberellin, cytokinin, ethylene, and ABA to other signalling molecules, each hormone has a unique role, while their interactions ensure that plants grow, adapt, and survive efficiently. Understanding these natural regulators helps us appreciate the fascinating ways plants respond to and interact with their surroundings. πΏ
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