Plant Growth Regulators

Plant Growth Regulators

 

  • Plant growth regulators (PGRs) are small molecules with diverse chemical compositions found in plants. 
  • They are also known as plant growth substances, plant hormones, or phytohormones in literature.

 

Types of PGRs:

 

  • PGRs can be classified into various groups based on their chemical compositions, including indole compounds (e.g., indole-3-acetic acid or IAA), adenine derivatives (e.g., kinetin), carotenoid derivatives (e.g., abscisic acid or ABA), terpenes (e.g., gibberellic acid or GA3), and gases (e.g., ethylene or C2H4).

 

Functions of PGRs:

PGRs can be broadly divided into two groups based on their functions in plants:

  • Growth-promoting PGRs: These PGRs stimulate various growth processes such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation. Examples include auxins, gibberellins, and cytokinins. 
  • Growth-inhibiting PGRs: These PGRs regulate plant responses to wounds and stresses of biotic and abiotic origin. They also inhibit certain growth activities such as dormancy and abscission. Abscisic acid is an example of a growth-inhibiting PGR. 
  • Ethylene, a gaseous PGR, can act as an inhibitor of growth activities but may also have promoting effects depending on the context.

 

Discovery of Plant Growth Regulators (PGRs):

  • The discovery of major groups of PGRs was accidental and began with observations made by Charles Darwin and his son Francis Darwin. 
  • They observed phototropism in canary grass coleoptiles, leading to the discovery of auxin, the first identified PGR.

 

 

  • The 'bakanae' disease in rice seedlings, caused by Gibberella fujikuroi fungus, led to the identification of gibberellic acid as a PGR. 
  • F. Skoog and his team discovered kinetin while studying callus proliferation in tobacco stem internodal segments. 
  • In the mid-1960s, three independent researches identified inhibitors named inhibitor-B, abscission II, and dormin, which were later found to be chemically identical and termed abscisic acid (ABA). 
  • H.H. Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas, leading to the identification of ethylene as a gaseous PGR.

 

Physiological Effects of PGRs:

  • Each category of PGRs has specific physiological effects on plant growth and development. 
  • These effects include promoting growth processes like cell division, elongation, flowering, and fruiting (e.g., auxins, gibberellins, and cytokinins), as well as regulating responses to stresses and inhibiting certain growth activities (e.g., abscisic acid and ethylene). 
  •  Understanding the physiological effects of PGRs is essential for manipulating plant growth and development in agriculture, horticulture, and biotechnology.

Auxin

  • Auxins are plant growth regulators that promote growth and development. 
  • Auxins, derived from the Greek word 'auxein' meaning 'to grow', were initially discovered in human urine. 
  • The term "auxin" refers to indole-3-acetic acid (IAA) and other natural or synthetic compounds with similar growth-regulating properties. 
  • They are primarily produced by the growing tips of stems and roots and then transported to other parts of the plant where they exert their effects.

 

Types of Auxins:

  • Natural auxins like IAA and indole butyric acid (IBA) are found in plants, while synthetic auxins include NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid). 
  • These auxins are widely used in agriculture and horticulture for various purposes.

 

Applications of Auxins:

  • Auxins are commonly used to initiate root formation in stem cuttings, facilitating plant propagation. 
  • They promote flowering in certain plants, such as pineapples, and can help prevent premature fruit and leaf drop while promoting the abscission of mature leaves and fruits. 
  • Auxins also play a role in apical dominance, where the growth of lateral buds is inhibited by the presence of a dominant apical bud. 
  • Removing the shoot tips (also known as decapitation) can release this inhibition, promoting lateral bud growth. 
  • This phenomenon is utilized in practices like tea plantation and hedge-making. 
  • Auxins induce parthenocarpy, resulting in fruit development without fertilization, as seen in tomatoes. 
  • Additionally, auxins like 2,4-D are used as herbicides to control weed growth, particularly in dicotyledonous plants, without affecting monocotyledonous plants. 
  • Auxins also play a role in controlling xylem differentiation and cell division in plants.

 

 

Gibberellin

  •  Gibberellins (GAs) are plant growth regulators (PGRs) that play a crucial role in promoting plant growth and development. 
  • There are over 100 known gibberellins found in various organisms, including fungi and higher plants, denoted as GA1, GA2, GA3, and so forth. Gibberellic acid (GA3) is one of the most extensively studied forms of gibberellins.

 

Physiological Effects:

  • Gibberellins are acidic compounds that induce a wide range of physiological responses in plants. 
  • Gibberellins induce a wide range of physiological responses in plants, including promoting elongation of the stem and other plant organs. 
  • They are used to increase the length of grape stalks, improve the shape of fruits like apples, and delay senescence, allowing fruits to remain on the tree longer for extended market periods.

 

Applications:

  • In the brewing industry, GA3 is used to accelerate the malting process. 
  • Spraying sugarcane crops with gibberellins increases stem length, leading to higher yields by up to 20 tonnes per acre. 
  • Gibberellins are also used to promote early seed production in juvenile conifers by hastening maturity. 
  • They induce bolting (internode elongation before flowering) in plants like beet, cabbages, and those with a rosette habit.

 

Cytokinin

  • Cytokinins are plant growth regulators that specifically affect cytokinesis, the process of cell division. 
  • They were first discovered as kinetin, a modified form of adenine, found in autoclaved herring sperm DNA. Kinetin is not naturally occurring in plants. 
  • Zeatin, another natural cytokinin, was later isolated from corn kernels and coconut milk, leading to the discovery of other naturally occurring cytokinins and synthetic compounds with cell division-promoting activity.

 

Physiological Effects:

  • Cytokinins are synthesized in regions of rapid cell division such as root apices, developing shoot buds, and young fruits. 
  • They promote various growth processes, including the production of new leaves, chloroplasts in leaves, lateral shoot growth, and the formation of adventitious shoots. 
  • Cytokinins help overcome apical dominance, where the growth of lateral buds is inhibited by the presence of a dominant apical bud. 
  • Additionally, they promote nutrient mobilization and delay leaf senescence, contributing to overall plant health and vigor.

 

Applications:

  • Cytokinins are used in agriculture and horticulture to promote plant growth, enhance crop yield, and delay senescence. 
  • They are applied to stimulate lateral shoot growth, induce adventitious shoot formation, and improve overall plant vigor. 
  • Cytokinins play a crucial role in plant development and physiological processes, making them valuable tools in crop management and plant growth regulation.

 

Ethylene

  • Ethylene is a simple gaseous plant growth regulator (PGR) synthesized in large quantities by tissues undergoing senescence and ripening fruits.

 

Physiological Effects:

  • Ethylene influences various growth processes in plants, including horizontal growth of seedlings, swelling of the axis, and apical hook formation in dicot seedlings. 
  • It promotes senescence and abscission of plant organs, especially leaves and flowers, and is highly effective in fruit ripening, increasing the respiration rate during the process.

 

Applications: 

  • Ethylene breaks seed and bud dormancy, initiating germination in peanut seeds and sprouting of potato tubers. 
  • It promotes rapid internode and petiole elongation in deep water rice plants, helping leaves and upper parts of the shoot remain above water. 
  • Ethylene also enhances root growth and root hair formation, increasing the plant's absorption surface. 
  • In agriculture, ethylene is used to initiate flowering and synchronize fruit-set in pineapples, induce flowering in mangoes, and regulate various physiological processes. 
  • The most widely used compound as a source of ethylene is ethephon, which is readily absorbed and transported within the plant, releasing ethylene slowly. 
  • Ethephon hastens fruit ripening in tomatoes and apples, accelerates abscission in flowers and fruits (thinning of cotton, cherry, walnut), and promotes female flowers in cucumbers to increase yield.

 

Abscisic acid

  • Abscisic acid (ABA) was initially discovered for its role in regulating abscission and dormancy, but it also has wide-ranging effects on plant growth and development. 
  • ABA acts as a general plant growth inhibitor and suppresses plant metabolism. 
  • It inhibits seed germination and stimulates the closure of stomata, enhancing plant tolerance to various stresses, earning it the nickname "stress hormone." 
  • ABA plays a crucial role in seed development, maturation, and dormancy, helping seeds withstand desiccation and unfavorable conditions for growth. It often acts antagonistically to gibberellins (GAs).

 

Interactions of Plant Growth Regulators (PGRs):

  • Different PGRs play complementary or antagonistic roles in various phases of plant growth, differentiation, and development. 
  • Multiple PGRs can interact to affect events such as dormancy, abscission, senescence, and apical dominance. 
  • PGRs, along with genomic control and extrinsic factors like temperature and light, regulate plant growth and development. Extrinsic factors often influence plant processes through PGRs.

 

Role of Extrinsic Factors:

  • Extrinsic factors like temperature and light play vital roles in plant growth and development by influencing PGR activity. 
  • Events such as vernalization, flowering, dormancy, seed germination, and plant movements are regulated by interactions between extrinsic factors and PGRs.

 

Applications:

  • ABA helps plants tolerate various stresses such as drought, salinity, and extreme temperatures by regulating stomatal closure and reducing water loss. 
  • ABA helps plants tolerate various stresses such as drought, salinity, and extreme temperatures by regulating stomatal closure and reducing water loss. 
  • ABA application can enhance drought resistance in plants by promoting water conservation mechanisms, such as reducing transpiration and maintaining cellular water balance. 
  • ABA plays a role in fruit ripening by regulating ethylene production and fruit softening processes. 
  • ABA can delay leaf senescence, prolonging the photosynthetic activity and lifespan of leaves.