Plant Growth Regulators, Practice Problems and FAQs

Fruits are one of the most important foods we need to include in our daily diet as they are an excellent source of minerals and vitamins. They are rich in fibres too. If you are living in a metropolitan city, then planting fruit trees at your home will be difficult. So what will we do to get fruits normally? Yes buying fruits from shops is the only way to get fruits then.

But when we buy fruits from outside, we cannot trust them completely, because for business purposes, they might spray some chemicals for the better colour, taste, ripening, and texture of the fruits. This is one reason why we say we need to wash the fruits properly, before eating them. Especially if you get fruits, in the off seasons, then it will be ripened mostly by using artificial methods. One such method is the spraying of ethephon. But this ethephon is harmful to us, since it causes headache, dizziness, fatigue, unconsciousness etc. Ethylene is also produced in the plants, which is responsible for the natural ripening of fruits.

You might have observed the ripening of fruits, when we keep it along with the ripened fruits. Right? Ethylene causes this fruit ripening. It is a gaseous hormone, responsible for the ripening of fruits, flowering, dormancy etc.

Fig: Ripening of fruits

Plants require various substances for their growth and development, such as oxygen, water, minerals, nutrients, carbon dioxide, sunlight and many more. These are considered as external factors. Plants require internal factors like chlorophyll, carotenoids etc. Plants also require some internal factors like certain organic compounds that regulate the plant growth. These organic compounds are known as plant growth regulators or phytohormones. We are going to discuss more about the plant growth regulators in this article.

Plant growth regulators

Plant growth regulators are the small, simple organic molecules, naturally produced by plants that possess several chemical compositions. These are also known as phytohormones. These are produced under appropriate conditions and can accelerate or retard the growth of plants. Plants do not have specialised glands for the production of hormones. These are produced in the buds, root tips and stems normally. Every cell in a plant can produce phytohormones. The major five plant growth regulators are as follows:

Auxin
Gibberellin
Cytokinin
Ethylene
Abscisic acid

Fig: Growth of a plant

Let's check out some of the major characteristics of plant growth regulators.

Characteristics of plant growth regulators

Plant growth regulators exhibit the following common characteristics:

Differentiation and elongation of cells.
Formation of leaves, flowers and stems.
Wilting of leaves.
Ripening of fruits.
Seed dormancy.
Cell division and cell enlargement.
Delayed senescence.

Types of plant growth regulators

There are many types of plant growth regulators and they can be classified on the basis of chemical composition and their function.

Types of plant growth regulators on the basis of chemical composition

On the basis of chemical composition, the plant growth regulators are of five types. These are as follows:

Indole compounds
Adenine derivatives
Carotenoid derivatives
Terpenes
Gases

Fig: Types of plant growth regulators based on

chemical composition

Indole compounds

Auxin or indole 3-acetic acid (IAA) is an indole derived plant growth regulator. Indole has a bicyclic structure, consisting of a benzene ring fused to a five-membered ring.

Adenine derivatives

Cytokinin is an adenine derived plant growth regulator. Distinct substitutions are attached to the N6 position of the adenine ring. The common class of cytokinins have isoprenoid side chains.

Carotenoid derivatives

Abscisic acid is a carotenoid-derived phytohormone. It is considered a plant stress hormone and therefore, plants are able to survive in dry conditions with the help of this hormone. Carotenoids are plant pigments having yellow to orange colour.

Terpenes

Gibberellic acid is a terpene-derived phytohormone. It promotes seed germination. Stimulates stem growth.

Gases

Ethylene is a gaseous phytohormone. It breaks seed dormancy and initiates germination.

Types of plant growth regulators on the basis of function

On the basis of function in the plant body, plant growth regulators are of two types as follows:

Growth promoting phytohormones
Growth inhibiting phytohormones

Fig: Types of phytohormones on the basis of functions

Growth promoting phytohormones

These hormones promote growth in the plant body. These are as follows:

Auxins
Gibberellins
Cytokinins
Ethylene

GIF: Growth of plant

Functions of growth promoting phytohormones

The functions of growth promoting hormones are as follows:

Cell division
Cell elongation
Tropic growth
Organ development
Formation of reproductive structures
Flowering
Seed formation
Fruiting

Growth inhibiting hormones

These hormones inhibit growth in the plant body and are as follows:

Ethylene
Abscisic acid

Fig: Inhibition of growth

Functions of growth inhibiting hormones

The functions of growth inhibiting hormones are:

Dormancy
Stress tolerance
Abscission

Introduction to discovery of phytohormones

The discovery of all five plant growth regulators was accidental. The discovery of phytohormones started with the observations of Charles Darwin and his son Francis Darwin. Now we will discuss more about the discovery and functions of each of these phytohormones.

Auxin

The word auxin is derived from the Greek word ‘Auxein’ which means ‘to grow’. Auxins belong to a group of hormones that have indole structure. Indole has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered ring. Auxin was first isolated from human urine.

Fig: Structure of auxin

Discovery of Auxin

Presence of a growth stimulator in the tip of the stem was first discovered by Charles Darwin and his son Francis Darwin in 1880. This experiment was performed using canary grass (Phalaris canariensis). Soon after the discovery by Darwins, a series of experiments were carried out. Finally in 1926 a demonstration by F.W. Went showed that a chemical called auxin was responsible for the bending of plants. He performed this experiment using oats (Avena sativa).

Types of auxin

Auxins are naturally released by plants. They can also be produced synthetically. So there are two types of auxins as follows:

Natural auxin
Synthetic auxin

Natural auxin

These are produced naturally in plants. Examples of natural auxin are as follows:

Indole 3-acetic acid (IAA)
Indole 3-butyric acid (IBA)
4-chloro-indole acetic acid
Phenyl acetic acid

Synthetic auxin

These are synthesised artificially. Examples are as follows:

Naphthalene acetic acid (NAA)
2, 4-dichlorophenoxyacetic acid (2, 4-D)
2, 4, 5-trichlorophenoxy acetic acid (2, 4, 5-T)

Functions of auxins

The effects of auxins are classified into two as follows:

Tropic movements
Developmental effects

Fig: Functions of auxins

Tropic movements

Tropic movements are of two types as follows:

Phototropism
Gravitropism

Phototropism

It is the phenomenon of the bending of plants towards light.

GIF: Phototropism shown by plants

Gravitropism

It is the phenomenon of coordinated growth of plants in response to gravity. Gravitropism can be of two types as follows:

Positive gravitropism
Negative gravitropism

GIF: Gravitropism

Positive gravitropism

The growth of roots towards gravity is known as positive gravitropism. Here the auxin will be concentrated at the lower side of the roots. So there will be less concentration on the upper side of the roots. As a result, cell elongation takes place at the opposite side i.e., the upper side and the root tip bends downwards. In this way auxin suppresses growth in the roots.

GIF: Positive gravitropism

Negative gravitropism

The growth of stem against or opposite to gravity is known as negative gravitropism. Here the auxin will be concentrated at the lower side. Hence the cell elongation takes place on the lower side and the shoot bends upwards.

GIF: Negative gravitropism

Developmental effects

Auxins perform the following developmental effects:

Promotes growth of lateral and adventitious roots and also initiates rooting in stem cuttings in plant propagation methods.

Fig: Root initiation in stem cuttings

Auxins delay abscission.
It helps in differentiation of xylem and phloem.
It helps in cell elongation.
Auxins are used as weed killers for dicotyledons. Examples include 2, 4-dichlorophenoxy acetic acid and 2, 4, 5-trichloro phenoxy acetic acid.

GIF: Weedicide effect

It can induce a feminising effect in plants such as Opuntia stenopetala (cactus) and Cannabis sativus (marijuana).

Fig: Feminising effect

The auxin produced in the apical buds inhibits the growth of lateral buds and this is called apical dominance.

Fig: Apical bud

Auxin promotes flowering. For example, foliar spray of dilute solutions of 2, 4-D and NAA causes flowering in pineapple and litchi.
Auxin is used for parthenocarpy; the production of seedless fruits.

Fig: Production of seedless fruits

Fig: Developmental effects of auxin

Deficiency of auxin

We know that auxin is produced by the apical meristem. So then the lateral buds will remain dormant. What happens, when the apex of the shoot is removed? Then the auxin will not get produced here, and this will result in the growth of lateral buds and as a result, bushy plants are formed.

Gibberellins

Gibberellin is a type of growth promoting phytohormone. More than 100 gibberellins are identified in different organisms such as fungi and higher plants. They are denoted as GA₁, GA₂, and GA₃. All GAs are acidic. They are transported throughout the plant body via vascular tissue. They are manufactured in the plastids through a terpenoid pathway. After formation, they are transferred in the ER and cytosol until they become biologically active.

Fig: Structure of gibberellin

Discovery of Gibberellins

E. Kurosawa in 1926 reported the symptoms of the ‘foolish seedling’ or bakanae disease in Japan. He reported that the symptoms appeared in seedlings on treatment with a fungus filtrate. The chemical obtained from the sterile filtrate of those tall parts was named as gibberellin, after Gibberella fujikuroi, the name of the fungus.

Fig: Rice plants

Functions of gibberellins

Gibberellins perform the following functions:

It stimulates stem elongation in dwarf plants after external application of GA₃.

Fig: Stem elongation in dwarf plants

It also promotes stem growth (bolting) in rosette plants (circular arrangement of leaves).

Fig: Bolting in rosette plants

It promotes bolting or internode elongation just prior to flowering.
The embryo produces gibberellins that trigger the synthesis of hydrolytic enzymes which promotes seed germination.
It reactivates the growth of flowering buds, i.e. they can break bud dormancy.

GIF: Breaking bud dormancy

It delays the aging process or senescence and is considered as one of the anti-aging hormones for plants.
Fruit ripening is also affected by gibberellin. They left on the plant for a longer period to extend the market period.

Fig: Functions of gibberellins

Uses of gibberellins

Gibberellin increases the length of a grape stalk and this will increase the yield of grapes.
It increases the stem length of sugarcane and thus increases the yield.
Treatment of gibberellins elongates and improves the shape of apples.

Fig: Apple untreated and treated with gibberellin

Gibberellin accelerates the malting process in the brewing industry.
Treatment of GA on juvenile conifers hastens the maturity period and this leads to early seed production.

Deficiency of gibberellin

Since gibberellin helps in the elongation of the stem, the deficiency of gibberellin will result in the formation of dwarf plants.

Cytokinins

Cytokinins promote cell division or cytokinesis and therefore, they are named as cytokinins. Along with auxin, it promotes cell division. Zeatin is a naturally occurring cytokinin. Cytokinin is transported through xylem elements. Some synthetic cytokinins act as herbicides. They are found in the regions of rapid cell division, such as root apices, young fruits, shoot buds, etc.

Fig: Cytokinins

Discovery of cytokinins

Cytokinin was discovered by F. Skoog while he was working on callus. He concluded that the naturally occurring substances having similar structure to kinetin regulate the activity of cell division in the plant. Another chemical called zeatin was identified from the maize kernel and was discovered by C.O. Miller.

Functions of cytokinins

Cytokinins perform the following functions:

It is necessary for cell division and promotes meristem growth.
It helps the plant to overcome apical dominance and promote lateral growth.

Fig: Lateral growth

It aids in producing bushy plants.
It delays senescence.
It promotes nutrient mobilisation from one part to another part of the leaf.

Fig: Nutrient mobilisation

Leaves treated with cytokinin have a developed chloroplast with extensive grana, photosynthetic enzymes and more chlorophyll.
It promotes the production of new leaves and shoots.

Fig: Functions of cytokinins

Deficiency of cytokinin

The deficiency of cytokinin in plants will result in the slow growth of shoot and some other morphological changes such as dwarfism. Formation of smaller leaves is also due to the deficiency of cytokinin. But excessive levels of cytokinin cause tumours in plants.

Fig: Tumours in plants

Ethylene

Ethylene is a volatile hormone. It has a chemical formula of C₂H₄. It has a growth promoting effect on all the stages of life of a plant. The component in gas that was causing such devastation to the trees was found to be ethylene. Ethylene is not always harmful though. In fact, it is a plant hormone that is responsible for the ripening of many fruits.

Fig: Structure of ethylene

Discovery of ethylene

H. H. Cousins showed that ripened oranges hastened the ripening of stored unripened bananas. The spoiled oranges released a volatile substance, which was later identified as ethylene.

Fig: Ripened oranges results in the ripening of bananas

Effects of ethylene

Ethylene shows its effects on different parts of the plants. The different effects of ethylene are as follows:

It breaks seed dormancy and stimulates seed germination along with gibberellin.
Ethylene promotes the horizontal growth of seedlings.

Fig: Horizontal growth of seedlings

It increases the girth of seedlings.
It promotes apical hook formation in dicot seeds.

Fig: Apical hook formation

It breaks bud dormancy.
It promotes the sprouting of potato tubers.

GIF: Sprouting of potato tubers

It promotes internode or petiole elongation in the rice plants which grow in deep-water.
It promotes root growth and also the formation of root hair to increase the area for absorption.
It promotes flowering in pineapples and mangoes and it synchronises fruit set in pineapple.
It is used to ripen the fruit.
It induces senescence and promotes abscission.
Ethephon is used for thinning in agriculture.

GIF: Senescence of a plant

Deficiency of ethylene

The ripening of fruits will get affected if there is ethylene deficiency. It may also affect the ageing of a plant. The root hair growth will get inhibited in plants if ethylene deficiency is present and hence less absorption of minerals and water will occur.

Abscisic acid

Abscisic acid is a carotenoid derivative. Carotenoids are plant pigments with yellow to orange colours. It is known as a stress hormone of plants.

Fig: Structure of abscisic acid

Discovery of Abscisic acid

In the 1960’s, different groups of scientists discovered various growth inhibitors and named them as inhibitor β, abscisin II, and dormin. Later, it was proved that all the three growth inhibitors were chemically identical and were named abscisic acid.

Fig: Abscisic acid

Physiological effects of abscisic acid

Following are the major physiological effects of abscisic acid:

It increases tolerance to stress and helps the plant to cope up with the stressful condition.
It acts as a growth inhibitor hence it is an antagonist to gibberellins.
Under unfavourable conditions, it slows down metabolic activity.
During unfavourable conditions like extreme cold, the rate of photosynthesis decreases. In such conditions, ABA decreases the metabolic activity to conserve energy. This inhibits plant growth.

It helps in seed development and maturation.
It induces seed dormancy and inhibits germination.
It stimulates the closure of stomata and helps to conserve water by minimising transpiration.

GIF: Closing of stomata

Deficiency of abscisic acid

The major deficiency symptoms of abscisic acid are the changes in cuticle permeability and pectin composition.

Morphogenesis

The generation of tissue organisation in animal and plant embryos is known as morphogenesis. Auxin : Cytokinin ratio regulates morphogenesis. This is applied in tissue culture systems. If we add only cytokinin to the nutrient agar medium of a tissue culture, then there will not be any growth. If we add intermediate auxin and cytokinin, then there will be callus formation. If we add more auxin than cytokinin, there will be the growth of roots. If we add low auxin and high cytokinin, then there will be shoot formation.

Fig: Regulation of morphogenesis

Practice Problems

1. Which of the following plant hormones is responsible for the parthenocarpy in tomatoes?

Auxins
Ethylene
Cytokinins
Gibberellins

Solution: The process of development of fruits without fertilisation of ovules is called parthenocarpy. This will lead to the formation of seedless fruits. The unpollinated ovaries in certain cultivars of strawberry, tomato, grape, and orange will produce the parthenocarpic fruit, if the young plants are treated with the auxin. Ethylene is a plant growth promoter as well as a plant growth inhibitor, which helps in the ripening of fruits. The plant hormone that helps in cell division in plants is cytokinin. The germination of seeds, stimulation of flowers and elongation of shoots are due to the activity of gibberellins. Hence the correct option is a.

2. Which of the following is used in the brewing industry to speed up the malting process?

NAA
GA3
Kinetin
Abscisic acid

Solution: Malting is considered as the process of steeping, germinating and drying grain, so that it gets converted into malt. The malt is then used for brewing or making whisky. GA3 is used in the brewing industry to speed up the malting or the seed germination process. One of the first gibberellins discovered was GA3. A synthetic auxin is NAA or 1-naphthalene acetic acid. Auxins are plant growth promoters that can initiate the formation of roots in stem cuttings, flowering and the natural detachment of old leaves and fruits (abscission). Kinetin belongs to the cytokinin group, which is a plant growth promoter. They can be found in the regions where rapid cell division occurs. It also helps in cytokinesis, which is the division of the cytoplasm. Hence the parent cell is divided into 2 daughter cells. Abscisic acid is a plant growth inhibitor. The development and maturation of seed along with other growth-related activities are also inhibited. It promotes the closure of stomata. Hence the correct option is b.

3. Match column I (Plant growth regulators) with column II (Functions) and find out the correct option from the following:

Column I (Plant growth regulators)	Column II (Functions)
Auxin	Promotes seed germination
Gibberellin	B. Helps to overcome apical dominance
Cytokinin	C. Promotes apical dominance

1 - A, 2 - B, 3 - C
1 - C, 2 - B, 3 - A
1 - B, 2 - A, 3 - C
1 - C, 2 - A, 3 - B

Solution: When the shoot apex inhibits the growth of lateral buds, then the plant will grow vertically, it is known as apical dominance. The plant must invest energy in growing upward in order to receive more light for photosynthesis. Auxins and cytokinins are the two plant hormones that can control apical dominance. While cytokinins are known to regulate or control apical dominance and auxin encourages it. Gibberellin aids in the germination of seeds. Hence the correct option is d.

4. What is positive gravitropism?

Answer: The growth of roots towards gravity is known as positive gravitropism. Here auxin will be concentrated at the lower side of the roots. So there will be less concentration on the upper side of the roots and as a result, cell elongation takes place at the opposite side i.e., the upper side and the root tip bends downwards. Here auxin suppresses growth in the roots.

Gif: Positive gravitropism

FAQs

1. What is respiratory climacteric?

Answer: Respiratory climacteric is the exponential change in the respiration rate of the fruits during ripening. Ethylene leads to respiratory climacteric. It is the final stage when fruit changes its color from green to yellow, red or so on. At this stage, cellular respiration increases. In some fruits, sugars are released during this time.

GIF: Change of colour in fruits

2. Do plants communicate with hormones?

Answer: Along with the plant growth and development, plants can communicate with each other through the plant hormones. This communication can be done between the same species, with related species or unrelated species and also with insects. They even serve in classical meta-organismic communication.

3. What are brassinosteroids?

Answer: A class of growth-promoting steroidal phytohormones are called brassinosteroids or BRs. It has complete control over the growth and development of a plant. It also plays a significant role in plant adaptation to abiotic and biotic stresses.

4. Which fruit has the most ethylene gas?

Answer: Fruits like apple, kiwi, banana, peaches, pears, melons, apricots, peppers, avocados etc., produce most ethylene. Hence these fruits should be stored away from other fruits and vegetables, while we keep it inside the fridge or storage.

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