Trying to understand plant growth hormones? You may have often wondered why in a germinating seedling, roots grow in downward direction whereas shoots grow upwards. Why some flowers bloom during the day, but close at night, as if to sleep.
Why one rotten apple in a basket leads to rotting of others or what causes leaf fall? How are the processes of cell division and cell elongation controlled?
These are some of the questions, amongst many others, which do not have any simple answers, for most of these phenomena are controlled by complicated interactions among three levels of controls- genetic, hormonal and environmental.
The various genes in a species are turned on at precise times to control cell activity and various characteristics of organisms. One class of potent chemicals that coordinate growth and development in plants and animals are hormones.
They trigger cellular reactions in target cells and also determine the genes that are to be expressed at a particular stage of development. Environmental factors such as light and temperature also affect and control growth and development.
We all know that plants need light, water, oxygen and nutrition to grow and develop. All these qualify as extrinsic factors. While extrinsic factors are important, plant growth also depend on intrinsic factors too.
They can be intracellular genes or intercellular chemicals. These chemicals are called plant growth regulators. Plant growth regulators are small, simple chemicals produced naturally by plants to regulate their growth and development.
Plant growth regulators can be of a diverse chemical composition such as gases (ethylene), terpenes (gibberellic acid) or carotenoid derivates (abscisic acid). They are also referred to as plant growth substances, phytohormones or plant hormones.
Classification of Plant Growth Hormones
Based on their action, they are broadly classified as follows:
Plant Growth Promoters: They promote cell division, cell enlargement, flowering, fruiting and seed formation. Examples are auxins, gibberellins and cytokinins.
Plant Growth Inhibitors: These chemicals inhibit growth and promote dormancy and abscission in plants. An example is an abscisic acid.
Note: Ethylene can be a promoter or an inhibitor, but is largely a plant growth inhibitor.
Types of Plant Growth Hormones, their Effects and Applications
1. Auxins
Auxins were the first growth hormone to be discovered. They were discovered due to the observations of Charles Darwin and his son, Francis Darwin.
The Darwins observed that the coleoptile (protective sheath) in canary grass grows and bends towards the source of light.
This phenomenon is ‘phototropism’. In addition, their experiments showed that the coleoptile tip was the site responsible for the bending.
Finally, this led to the isolation of the first auxin by F. W. Went from the coleoptile tip of oat seedlings.
First isolated from human urine, auxin is a term applied to natural and synthetic compounds that have growth regulating properties.
Plants produce natural auxins such as Indole-3- acetic acid (IAA) and Indole butyric acid (IBA). Natural auxins are found in growing stems and roots from where they migrate to their site of action.
Naphthalene acetic acid (NAA) and 2, 4- dichlorophenoxyacetic (2, 4-D) are examples of synthetic auxins.
Effects
- Promote flowering in plants like pineapple.
- Help to initiate rooting in stem cuttings.
- Prevent dropping of fruits and leaves too early.
- Promote natural detachment (abscission) of older leaves and fruits.
- Control xylem differentiation and help in cell division.
Applications
- Used for plant propagation.
- To induce parthenocarpy i.e. the production of fruit without prior fertilization.
- 2, 4-D is widely used as an herbicide to kill dicotyledonous weeds.
- Used by gardeners to keep lawns weed-free.
Note: The growing apical bud in higher plants inhibits the growth of the lateral buds. This phenomenon is Apical Dominance. Removal of the apical bud allows the lateral buds to grow. This technique is commonly used in tea plantations and hedge-making.
2. Gibberellins
It is the component responsible for the ‘bakane’ disease of rice seedlings. The disease is caused by the fungal pathogen Gibberellafujikuroi. E. Kurosawa treated uninfected rice seedlings with sterile filtrates of the fungus and reported the appearance of disease symptoms.
Finally, the active substance causing the disease was identified as gibberellic acid. There exist more than 100 gibberellins obtained from a variety of organisms from fungi to higher plants.
They are all acidic and are denoted as follows – GA1, GA2, GA3 etc. GA3 (Gibberellic acid) is the most noteworthy since it was the first to be discovered and is the most studied.
Effects
- Increase the axis length in plants such as grape stalks.
- Delay senescence (i.e. ageing) in fruits. As a result, their market period is extended.
- Help fruits like apples to elongate and improve their shape.
Applications
- The brewing industry uses GA3 to speed the malting process.
- Spraying gibberellins increase sugarcane yield by lengthening the stem.
- Used to hasten the maturity period in young conifers and promote early seed production.
- Help to promote bolting (i.e. sudden growth of a plant just before flowering) in cabbages and beet.
3. Cytokinins
F. Skoog and his co-workers observed a mass of cells called ‘callus’ in tobacco plants. These cells proliferated only when the nutrient medium contained auxins along with yeast extract or extracts of vascular tissue. Skoog and Miller later identified the active substance responsible for proliferation and called it kinetin.
Cytokinins were discovered as kinetin. Kinetin does not occur naturally but scientists later discovered several natural (example – zeatin) and synthetic cytokinins. Natural cytokinins exist in root apices and developing shoot buds – areas where rapid cell division takes place.
Effects
- Help in the formation of new leaves and chloroplast.
- Promote lateral shoot growth and adventitious shoot formation.
- Help overcome apical dominance.
- Promote nutrient mobilisation which in turn helps delay leaf senescence.
4. Abscisic Acid
Three independent researchers reported the purification and characterisation of three different inhibitors – Inhibitor B, Abscission II and Dormin.
Later, it was found that all three inhibitors were chemically identical and were, therefore, together were given the name abscisic acid. Abscisic acid mostly acts as an antagonist to Gibberellic acid.
Effects
- Regulate abscission and dormancy.
- Inhibit plant growth, metabolism and seed germination.
- Stimulates closure of stomata in the epidermis.
- It increases the tolerance of plants to different kinds of stress and
- Is, therefore, called ‘stress hormone’.
- Important for seed development and maturation.
- It induces dormancy in seeds and helps them withstand desiccation and other unfavourable growth factors.
5. Ethylene
A group of cousins showed that a gaseous substance released from ripe oranges hastens the ripening of unripe oranges.
Consequently, they found that the substance was ethylene – a simple gaseous plant growth regulator. Ripening fruits and tissues undergoing senescence produce ethylene in large amounts.
Effects
Affects horizontal growth of seedlings and swelling of the axis in dicot seedlings.
Promotes abscission and senescence, especially of leaves and flowers.
Enhances respiration rate during ripening of fruits. This phenomenon is ‘respiratory climactic’.
Increases root growth and root hair formation, therefore helping plants to increase their absorption surface area.
Application
Ethylene regulates many physiological processes and is, therefore, widely used in agriculture. The most commonly used source of ethylene is Ethephon. Plants can easily absorb and transport an aqueous solution of ethephon and release ethylene slowly.
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Used to break seed and bud dormancy and initiate germination in peanut seeds.
To promote sprouting of potato tubers.
Used to boost rapid petiole elongation in deep water rice plants.
To initiate flowering and synchronising fruit-set in pineapples.
To induce flowering in mango.
Ethephon hastens fruit ripening in apples and tomatoes and increases yield by promoting female flowering in cucumbers. It also accelerates abscission in cherry, walnut and cotton.
In summary, plant growth regulators can be of a diverse chemical composition such as gases, terpenes, or carotenoid derivates.
They are small but simple chemicals produced naturally by plants to regulate their growth and development. Plant growth regulators are indispensable in our study of plant growth and development.
One or the other plant growth regulator influences every phase of growth or development in plants. These roles could be individualistic or synergistic; promoting or inhibiting. Additionally, more than one regulator can act on any given life event in a plant.
Along with genes and extrinsic factors, plant growth regulators play critical roles in plant growth and development. Factors like temperature and light affect plant growth events (vernalisation) via plant growth regulators.
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