Plant Growth Regulators

Introduction:

Plant growth regulators, also known as plant hormones, are naturally occurring or synthetic substances that influence the growth and development of plants. These substances are produced in different parts of the plant, such as the roots, leaves, stems, flowers, and fruit. Plant hormones are vital to the growth and development of plants, and they control a wide range of physiological processes, such as cell division, elongation, differentiation, and senescence. In this article, we will discuss the different types of plant growth regulators and their roles in plant growth and development.



Types of Plant Growth Regulators:

1. Auxins:

Auxin is one of the most important plant growth regulators, which plays a crucial role in the growth and development of plants. It is a type of phytohormone, a class of signaling molecules produced by plants to regulate various physiological processes. The term "auxin" is derived from the Greek word "auxein," which means "to grow."

Discovery of Auxin:

The discovery of auxin can be attributed to the pioneering work of Charles Darwin and his son Francis in the late 19th century. They observed that plants tend to grow towards the light source, which they termed as "phototropism." They also observed that the tip of the plant was responsible for this response. In the early 20th century, scientists such as Frits Went and Kenneth Thimann discovered that a chemical signal produced by the tip of the plant was responsible for phototropism. They named this signal "auxin."

Chemical Structure of Auxin:

Auxin is a simple organic compound, with the chemical formula C10H9O5. The most common form of auxin in plants is indole-3-acetic acid (IAA), which is synthesized in the shoot apical meristem and other parts of the plant, such as young leaves and developing fruits. Other forms of auxin include indole-3-butyric acid (IBA) and naphthaleneacetic acid (NAA).

Mechanism of Action of Auxin:

Auxin acts by binding to specific receptors on the cell surface, which triggers a signaling cascade that results in changes in gene expression and cell growth. The receptors are located on the plasma membrane and belong to the family of F-box proteins, which are involved in the degradation of other proteins in the cell.

One of the main effects of auxin is to stimulate cell elongation, which is critical for the growth of the plant. Auxin promotes cell elongation by activating proton pumps on the plasma membrane, which increase the acidity of the cell wall. This causes the loosening of the cell wall, which allows the cell to elongate. Auxin also promotes cell division and differentiation, by regulating the expression of genes involved in these processes.

Auxin Transport:

Auxin is synthesized in the shoot apical meristem and other parts of the plant, but it is transported throughout the plant to regulate growth and development in different parts of the plant. Auxin transport occurs through two main pathways: the polar pathway and the non-polar pathway.

In the polar pathway, auxin is transported from the shoot apex towards the roots, in a process called "basipetal transport." This involves the active transport of auxin through specialized cells called "polar auxin transporters," which are located in the vascular tissue of the plant.

In the non-polar pathway, auxin is transported laterally across the plant, in a process called "acropetal transport." This involves the diffusion of auxin through the cell wall and the intercellular spaces, which allows auxin to move from one cell to another.

Functions of Auxin:

a. Apical Dominance:

One of the main functions of auxin is to promote apical dominance, which is the tendency of the plant to grow towards the light source. This involves the inhibition of the growth of lateral buds by the shoot apical meristem, which allows the plant to direct its growth towards the light. Auxin promotes apical dominance by suppressing the growth of lateral buds, while promoting the growth of the shoot apex.

b. Tropisms:

Auxin is also involved in the regulation of various tropisms, which are plant movements in response to environmental stimuli, such as light, gravity, and touch. For example, auxin is responsible for the phototropism, which is the growth of plants towards a light source. Auxin also regulates gravitropism, which is the response of plants to gravity. In roots, auxin accumulates in the lower side of the root, which stimulates the growth of the root towards the ground. In shoots, auxin accumulates in the upper side of the stem, which inhibits the growth of the shoot towards the ground.

c. Root Development:

Auxin plays a crucial role in the development of roots. It promotes the growth of the primary root and the lateral roots. It also stimulates the differentiation of root cells into different cell types, such as epidermal cells, root hairs, and cortex cells.

d. Leaf Development:

Auxin is also involved in the regulation of leaf development. It stimulates the growth of young leaves and the differentiation of leaf cells into different cell types, such as palisade cells and spongy mesophyll cells.

e. Fruit Development:

Auxin is involved in the regulation of fruit development, particularly in the early stages of fruit growth. It stimulates the growth of the ovary, which develops into the fruit. It also promotes the differentiation of fruit cells into different cell types, such as the pericarp and the seed.

Applications of Auxin:

Auxin has numerous applications in agriculture and horticulture. Some of the important applications are:

i. Rooting of Cuttings:

Auxin is used to promote the rooting of cuttings, which is a common method of vegetative propagation. Cuttings are treated with a solution of IBA, which stimulates the formation of roots.

ii. Fruit Development:

Auxin is used to improve fruit set in fruit crops, such as apples and pears. Spraying the flowers with a solution of NAA can increase the size and number of fruits.

iii. Weed Control:

Auxin is also used as a herbicide to control weeds. Synthetic auxins, such as 2,4-D and dicamba, are widely used as selective herbicides to control broadleaf weeds in crops such as corn, soybeans, and wheat.

iv. Plant Growth Regulation:

Auxin is also used as a plant growth regulator to control the growth and development of plants. For example, spraying young cotton plants with a solution of NAA can stimulate the development of a strong, fibrous stem.

2. Gibberellins:

Gibberellins, also known as GAs, are a group of plant growth regulators that were first discovered in Japan in the 1930s. They are involved in the regulation of various physiological processes, such as stem elongation, seed germination, leaf expansion, flower development, and fruit development. Gibberellins are present in all plant tissues, but they are particularly concentrated in the shoot apical meristem, young leaves, and developing seeds.

Types of Gibberellins:

There are over 130 different types of gibberellins, but only a few have significant biological activity in plants. The most common types are GA1, GA3, GA4, and GA7. GA3 is the most active type and is widely used in research and agriculture.

Functions of Gibberellins:

i. Stem Elongation:

Gibberellins stimulate stem elongation by promoting cell division and elongation. They also break down the cell walls by stimulating the production of enzymes that degrade the cell wall components. This allows the cells to expand and the stem to elongate.

ii. Seed Germination:

Gibberellins promote seed germination by stimulating the production of enzymes that break down the food reserves in the seed. This provides the energy and nutrients needed for the growth of the embryo.

iii. Flower Development:

Gibberellins are involved in the regulation of flower development. They stimulate the elongation of the flower stalk and the growth of the petals and sepals. They also promote the formation of the stamen and the pistil, which are the male and female reproductive organs of the flower.

iv. Fruit Development:

Gibberellins are involved in the regulation of fruit development. They promote the growth and development of the fruit by stimulating cell division and elongation. They also promote the differentiation of fruit cells into different cell types, such as the pericarp and the seed.

v. Leaf Expansion:

Gibberellins promote leaf expansion by stimulating cell elongation and division. They also promote the production of chlorophyll, which is essential for photosynthesis.

Applications of Gibberellins:

Gibberellins have numerous applications in agriculture and horticulture. Some of the important applications are:

a. Seed Germination:

Gibberellins are used to promote seed germination in crops such as rice, barley, and wheat. Seeds are treated with a solution of GA3, which stimulates the production of enzymes that break down the food reserves in the seed.

b. Fruit Development:

Gibberellins are used to improve fruit quality and size in fruit crops, such as grapes, peaches, and strawberries. Spraying the flowers with a solution of GA3 can increase the size and number of fruits.

c. Stem Elongation:

Gibberellins are used to promote stem elongation in crops such as sugarcane and bamboo. They are also used to produce seedless grapes by stimulating the elongation of the seedless fruit.

d. Malting of Barley:

Gibberellins are used in the malting of barley for the production of beer. Barley is treated with a solution of GA3, which stimulates the production of enzymes that break down the starch in the barley into sugars that can be fermented by yeast.

3. Cytokinins:

Cytokinins are plant hormones that are involved in various physiological processes, such as cell division, shoot and root growth, and the delay of senescence. They were first discovered in the 1950s by Miller and Skoog, who isolated a compound from corn kernels that promoted cell division in tobacco tissue cultures.

Types of Cytokinins:

There are various types of cytokinins, but the most common types are isopentenyladenine (IPA), zeatin, and kinetin. Zeatin is the most active and abundant cytokinin in plants.

Functions of Cytokinins:

i. Cell Division:

Cytokinins promote cell division by stimulating the production of proteins that are involved in the cell cycle. They also activate the expression of genes that are involved in the control of cell division.

ii. Shoot and Root Growth:

Cytokinins promote shoot and root growth by stimulating the production of new cells in the shoot and root apical meristem. They also promote the differentiation of cells into different cell types, such as xylem and phloem.

iii. Delay of Senescence:

Cytokinins delay the senescence of leaves and other plant organs by promoting the synthesis of proteins that are involved in stress response and defense mechanisms. They also promote the production of antioxidants that protect cells from damage by reactive oxygen species.

iv. Chloroplast Development:

Cytokinins are involved in the regulation of chloroplast development. They promote the differentiation of proplastids into chloroplasts by activating the expression of genes that are involved in chloroplast biogenesis.

Applications of Cytokinins:

Cytokinins have numerous applications in agriculture and horticulture. Some of the important applications are:

a. Seed Germination:

Cytokinins are used to promote seed germination in crops such as soybeans, peas, and beans. Seeds are treated with a solution of cytokinins, which stimulate the production of proteins that are involved in the cell cycle and the differentiation of cells.

b. Tissue Culture:

Cytokinins are used in tissue culture to promote the growth and differentiation of plant cells into whole plants. They are often used in combination with auxins to promote the growth of shoots and roots.

c. Delay of Senescence:

Cytokinins are used to delay the senescence of cut flowers and potted plants. They are often used in floral preservatives to extend the vase life of cut flowers.

d. Fruit Development:

Cytokinins are used to improve fruit quality and size in fruit crops, such as apples and grapes. They are often used in combination with gibberellins to promote the growth and development of the fruit.

4. Abscisic Acid:

Abscisic acid (ABA) is a plant hormone that plays an important role in plant growth and development. It was first discovered in the 1960s, and since then, much research has been conducted on its functions and applications.

Functions of Abscisic Acid:

i. Water Regulation:

One of the most important functions of ABA is the regulation of water uptake and loss in plants. ABA is involved in the closure of stomata, which reduces water loss through transpiration. This mechanism helps plants to survive during drought conditions.

ii. Seed Dormancy:

ABA is involved in the regulation of seed dormancy. It inhibits the germination of seeds by preventing the synthesis of enzymes that are involved in the breakdown of stored nutrients in the seed. This mechanism helps the seed to remain dormant until the environmental conditions are favorable for germination.

iii. Stress Response:

ABA is involved in the stress response of plants. It helps plants to withstand various environmental stresses, such as drought, salinity, and extreme temperatures. ABA regulates the expression of stress-responsive genes, which helps plants to adapt to their surroundings.

iv. Leaf Senescence:

ABA is involved in the regulation of leaf senescence. It helps plants to conserve resources by promoting the degradation of chloroplasts and other cellular components during the senescence process.

Applications of Abscisic Acid:

a. Drought Tolerance:

ABA is used to increase the drought tolerance of crops. It can be applied as a foliar spray or seed treatment to help plants survive in drought conditions.

b. Seed Preservation:

ABA is used to preserve the viability of seeds during storage. It can be applied as a coating on seeds to prevent their germination and maintain their viability during long-term storage.

c. Fruit Ripening:

ABA is involved in the regulation of fruit ripening. It can be used to delay or promote fruit ripening, depending on the concentration and timing of application.

d. Stress Response:

ABA is used to improve the stress response of plants. It can be applied to plants before exposure to stress to improve their ability to withstand stress conditions.

5. Ethylene:

Ethylene is a gaseous plant hormone that is involved in various physiological processes, such as fruit ripening, senescence, and abscission. It was first discovered in the 20th century as the "aging hormone," as it was found to be involved in the regulation of the senescence process in plants.

Functions of Ethylene:

i. Fruit Ripening:

Ethylene is involved in the regulation of fruit ripening. It promotes the breakdown of cell walls, the conversion of starches to sugars, and the production of aroma compounds that give fruits their characteristic flavor and scent.

ii. Senescence:

Ethylene is involved in the regulation of senescence in plants. It promotes the breakdown of cellular components and the recycling of nutrients during the aging process. It also promotes the formation of abscission zones, which are areas of cell separation that lead to the shedding of leaves, flowers, and fruits.

iii. Stress Response:

Ethylene is involved in the stress response of plants. It helps plants to adapt to various environmental stresses, such as flooding, drought, and extreme temperatures. Ethylene regulates the expression of stress-responsive genes, which help plants to survive in stressful conditions.

iv. Growth and Development:

Ethylene is involved in the regulation of various aspects of plant growth and development, such as seed germination, root growth, and flowering. It promotes the elongation of roots and stems and the inhibition of lateral growth.

Applications of Ethylene:

a. Fruit Ripening:

Ethylene is used to promote the ripening of fruits, such as bananas, tomatoes, and avocados. It can be applied to fruits in a gaseous form to promote the production of aroma compounds and the softening of the fruit.

b. Flowering:

Ethylene is used to promote the flowering of some plant species. It can be applied to plants as a gas or spray to induce flowering, especially in tropical and subtropical plants.

c. Seed Germination:

Ethylene is used to promote the germination of some seeds, such as lettuce and onion. It can be applied to seeds as a gas or spray to promote their germination.

d. Stress Response:

Ethylene is used to improve the stress response of plants. It can be applied to plants before exposure to stress to improve their ability to withstand stress conditions.

Roles of Plant Growth Regulators:

i. Cell Division and Elongation:

Plant growth regulators play a vital role in regulating cell division and elongation, which are essential for plant growth and development. Cytokinins promote cell division, while auxins promote cell elongation. These two hormones work together to promote the growth of roots, shoots, and leaves.

ii. Lateral Root Formation:

Plant growth regulators also promote the formation of lateral roots, which are important for nutrient uptake and anchoring the plant in the soil. Cytokinins promote the formation of lateral roots, while auxins promote their elongation.

iii. Senescence:

Plant growth regulators also play a role in delaying leaf senescence, which allows plants to retain their leaves for a longer period and continue to photosynthesize. Cytokinins and gibberellins are involved in delaying senescence, while abscisic acid promotes senescence.

iv. Fruit Development:

Plant growth regulators are also involved in the development of fruits, which are important for seed dispersal and human consumption. Auxins, cytokinins, and gibberellins all promote fruit growth, while ethylene is involved in the ripening of fruits.

v. Environmental Stress Responses:

Plant growth regulators also play a role in the responses of plants to environmental stresses, such as drought, salt, and cold. Abscisic acid promotes drought tolerance by regulating stomatal closure and other physiological responses. Ethylene is involved in the response to stress, such as cold stress and mechanical stress.

Applications of Plant Growth Regulators:

Plant growth regulators have many practical applications in agriculture and horticulture. Some of the common applications are:

a. Fruit Production:

Plant growth regulators are commonly used to increase fruit production by promoting fruit growth and ripening. For example, the application of gibberellins can increase the size of grapes, while the application of ethylene can promote the ripening of bananas.

b. Weed Control:

Plant growth regulators can also be used as herbicides to control weeds. The synthetic auxin 2,4-D is commonly used to control broadleaf weeds in crops.

c. Delaying Senescence:

Plant growth regulators can be used to delay senescence in flowers, which can extend the vase life of cut flowers. Synthetic cytokinins are commonly used for this purpose.

d. Rooting of Cuttings:

Plant growth regulators can also be used to promote the rooting of cuttings, which is an important technique for propagating plants. Synthetic auxins, such as indole-3-butyric acid, are commonly used for this purpose.

Conclusion:

In conclusion, plant growth regulators play a crucial role in the growth and development of plants. They control a wide range of physiological processes, such as cell division, elongation, differentiation, and senescence. Plant growth regulators have many practical applications in agriculture and horticulture, including fruit production, weed control, delaying senescence in flowers, and promoting the rooting of cuttings. Understanding the roles and applications of plant growth regulators is essential for sustainable agriculture and horticulture.