2.5 Plant growth regulators

Plant growth regulators are chemical messengers that control plant development. They include auxins, gibberellins, cytokinins, ethylene, and abscisic acid. Each type has unique effects on growth, from cell elongation to fruit ripening.

These regulators interact in complex ways to coordinate plant responses to environmental cues. Understanding their mechanisms allows scientists to manipulate plant growth for agricultural and horticultural applications. Ongoing research explores new ways to optimize crop yields and stress tolerance.

Types of plant growth regulators

• Plant growth regulators are naturally occurring or synthetic substances that influence plant growth and development at low concentrations
• They act as chemical messengers that regulate various physiological processes in plants
• The five major classes of plant growth regulators are auxins, gibberellins, cytokinins, ethylene, and abscisic acid

Auxins

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Top images from around the web for Auxins

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  Frontiers | Auxin-Dependent Cell Elongation During the Shade Avoidance Response View original

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  Frontiers | Auxin Control of Root Organogenesis from Callus in Tissue Culture View original

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  Frontiers | Auxin and Its Interaction With Ethylene Control Adventitious Root Formation and ... View original

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  Frontiers | Auxin-Dependent Cell Elongation During the Shade Avoidance Response View original

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• Auxins are the first discovered plant growth regulators and are primarily involved in cell elongation and differentiation
• The most common natural auxin is indole-3-acetic acid (IAA), which is synthesized in young leaves and developing seeds
• Synthetic auxins, such as indole-3-butyric acid (IBA) and naphthaleneacetic acid (NAA), are used in agriculture and horticulture for various purposes (rooting of cuttings, weed control)

Gibberellins

• Gibberellins are a group of plant growth regulators that promote stem elongation, leaf expansion, and flowering
• They are synthesized in young leaves, roots, and developing seeds and are named GA1, GA2, etc., based on their discovery order
• The most biologically active gibberellin is GA3, also known as gibberellic acid, which is widely used in agriculture (fruit set, seed germination)

Cytokinins

• Cytokinins are plant growth regulators that stimulate cell division, delay senescence, and promote shoot formation
• They are synthesized in roots, young leaves, and developing seeds and are named based on their chemical structure (zeatin, kinetin, benzylaminopurine)
• Cytokinins are used in plant tissue culture for micropropagation and in agriculture for delaying leaf senescence and improving crop yield

Ethylene

• Ethylene is a gaseous plant growth regulator that is involved in fruit ripening, leaf abscission, and stress responses
• It is synthesized in various plant tissues, especially in ripening fruits and senescing leaves, and is produced in response to wounding and pathogen attack
• Ethylene is used in agriculture for fruit ripening (bananas, tomatoes) and degreening of citrus fruits

Abscisic acid

• Abscisic acid (ABA) is a plant growth regulator that induces dormancy, inhibits seed germination, and promotes stress tolerance
• It is synthesized in roots, leaves, and developing seeds in response to water stress, cold, and other environmental cues
• ABA is used in agriculture for inducing dormancy in buds and seeds and for improving drought tolerance in crops

Effects on plant growth and development

• Plant growth regulators have diverse effects on plant growth and development, depending on their type, concentration, and the plant species
• They act as signaling molecules that regulate gene expression and modify the activity of various enzymes and proteins involved in growth and development
• The effects of plant growth regulators are often interdependent and can be modulated by environmental factors, such as light, temperature, and nutrient availability

Auxins in cell elongation and differentiation

• Auxins stimulate cell elongation by increasing the plasticity of cell walls and promoting the uptake of water and nutrients
• They also induce the differentiation of vascular tissues (xylem and phloem) and the formation of lateral roots and adventitious roots
• Auxins are involved in apical dominance, the phenomenon where the main shoot apex inhibits the growth of lateral buds

Gibberellins in stem elongation and flowering

• Gibberellins promote stem elongation by stimulating cell division and elongation in the internodes, resulting in taller plants
• They also induce flowering in many plant species, especially in long-day plants and biennial plants that require vernalization (cold treatment)
• Gibberellins are involved in the mobilization of seed reserves during germination and in the development of male reproductive organs (stamens)

Cytokinins in cell division and senescence

• Cytokinins stimulate cell division in the shoot and root meristems, leading to the formation of new leaves, branches, and roots
• They also delay leaf senescence by inhibiting the breakdown of chlorophyll and proteins and by promoting the synthesis of antioxidants
• Cytokinins are involved in the regulation of source-sink relationships, nutrient mobilization, and stress responses

Ethylene in fruit ripening and leaf abscission

• Ethylene induces fruit ripening by stimulating the synthesis of enzymes involved in cell wall softening, starch degradation, and pigment accumulation
• It also promotes leaf and flower abscission by inducing the formation of the abscission layer, a specialized tissue that facilitates the separation of organs from the plant
• Ethylene is involved in the response to wounding, pathogen attack, and other stresses, leading to the activation of defense mechanisms

Abscisic acid in dormancy and stress response

• Abscisic acid induces bud and seed dormancy by inhibiting cell division and growth and by promoting the synthesis of storage proteins and lipids
• It also promotes stomatal closure and reduces water loss during drought stress by regulating the activity of ion channels and aquaporins in guard cells
• Abscisic acid is involved in the response to cold, salt, and other abiotic stresses, leading to the activation of stress-responsive genes and the accumulation of compatible solutes (proline, sugars)

Mechanisms of action

• Plant growth regulators exert their effects by binding to specific receptors and triggering a cascade of signaling events that ultimately lead to changes in gene expression and protein activity
• The mechanisms of action of plant growth regulators involve complex interactions between different signaling pathways and feedback loops that fine-tune the plant's response to environmental and developmental cues
• Recent advances in molecular biology and genetics have shed light on the molecular basis of plant growth regulator action and have opened new avenues for the manipulation of plant growth and development

Receptor binding and signal transduction

• Plant growth regulators bind to specific receptors located in the plasma membrane, cytosol, or nucleus of plant cells
• Auxin receptors include the TIR1/AFB family of F-box proteins, which are part of the SCF ubiquitin ligase complex and mediate the degradation of Aux/IAA transcriptional repressors
• Gibberellin receptors are soluble proteins called GID1, which form a complex with DELLA proteins and target them for degradation by the 26S proteasome
• Cytokinin receptors are histidine kinases (AHK2, AHK3, AHK4) that initiate a phosphorelay cascade involving histidine phosphotransfer proteins (AHPs) and response regulators (ARRs)
• Ethylene receptors are membrane-bound proteins (ETR1, ERS1, ETR2, ERS2, EIN4) that act as negative regulators of ethylene signaling in the absence of ethylene
• Abscisic acid receptors include the PYR/PYL/RCAR family of soluble proteins, which bind to and inhibit type 2C protein phosphatases (PP2Cs) in the presence of ABA

Gene expression regulation

• Plant growth regulators regulate gene expression by modulating the activity of transcription factors and other regulatory proteins
• Auxin response factors (ARFs) are transcription factors that bind to auxin response elements (AuxREs) in the promoters of auxin-responsive genes and activate or repress their expression
• Gibberellin-regulated transcription factors include the GRAS family proteins (GAI, RGA, SCR), which act as repressors of gibberellin signaling, and the bHLH family proteins (PIF3, PIF4), which mediate the gibberellin-induced expression of growth-related genes
• Cytokinin response factors (CRFs) are transcription factors that mediate the cytokinin-induced expression of cell cycle genes and other cytokinin-responsive genes
• Ethylene-responsive transcription factors (ERFs) are involved in the regulation of ethylene-responsive genes, such as those involved in fruit ripening, senescence, and stress responses
• Abscisic acid-responsive element-binding factors (ABFs) are transcription factors that bind to ABA-responsive elements (ABREs) in the promoters of ABA-responsive genes and activate their expression

Interactions between growth regulators

• Plant growth regulators do not act in isolation but interact with each other and with other signaling pathways to coordinate plant growth and development
• Auxins and cytokinins have antagonistic effects on shoot and root development, with auxins promoting root formation and cytokinins promoting shoot formation
• Gibberellins and abscisic acid have opposite effects on seed germination and dormancy, with gibberellins promoting germination and abscisic acid inducing dormancy
• Ethylene and abscisic acid have synergistic effects on leaf senescence and abscission, with ethylene inducing the formation of the abscission layer and abscisic acid promoting the remobilization of nutrients from senescing leaves
• Auxins and ethylene have complex interactions in root development, with auxins stimulating ethylene biosynthesis and ethylene modulating auxin transport and signaling

Applications in agriculture and horticulture

• Plant growth regulators have numerous applications in agriculture and horticulture, ranging from the propagation of plants to the improvement of crop yield and quality
• The use of plant growth regulators has revolutionized modern agriculture and has enabled the production of high-quality crops under diverse environmental conditions
• However, the use of plant growth regulators also raises concerns about their potential impact on human health and the environment, and their application is subject to strict regulations and guidelines

Auxins for rooting and weed control

• Auxins are widely used for the rooting of cuttings in the propagation of ornamental plants and fruit trees
• Synthetic auxins, such as indole-3-butyric acid (IBA) and naphthaleneacetic acid (NAA), are applied as powders or solutions to the base of the cuttings to stimulate the formation of adventitious roots
• Auxins are also used as herbicides for the selective control of broadleaf weeds in cereal crops and lawns
• Synthetic auxins, such as 2,4-dichlorophenoxyacetic acid (2,4-D) and dicamba, mimic the action of natural auxins and cause uncontrolled growth and death of susceptible weeds

Gibberellins for fruit set and seed germination

• Gibberellins are used to improve fruit set and size in various fruit crops, such as grapes, citrus, and cherries
• Gibberellic acid (GA3) is applied as a spray or dip to the flowers or developing fruits to stimulate cell division and enlargement and to prevent fruit drop
• Gibberellins are also used to break seed dormancy and promote uniform germination in various crops and ornamental plants
• GA3 is applied as a soak or spray to the seeds before planting to overcome dormancy and improve germination rate and seedling vigor

Cytokinins for micropropagation and delayed senescence

• Cytokinins are used in plant tissue culture for the micropropagation of various crops and ornamental plants
• Synthetic cytokinins, such as benzylaminopurine (BAP) and kinetin, are added to the culture medium to stimulate shoot formation and multiplication from small pieces of plant tissue (explants)
• Cytokinins are also used to delay leaf senescence and prolong the shelf life of cut flowers and potted plants
• Synthetic cytokinins, such as thidiazuron (TDZ), are applied as sprays or dips to the leaves or flowers to inhibit chlorophyll degradation and maintain the aesthetic quality of the plants

Ethylene for fruit ripening and degreening

• Ethylene is used to promote fruit ripening in climacteric fruits, such as bananas, tomatoes, and avocados
• Ethylene gas is applied to the fruits in ripening rooms or chambers to stimulate the synthesis of enzymes involved in ripening and to enhance color, flavor, and texture development
• Ethylene is also used for the degreening of citrus fruits, such as oranges and lemons
• Ethylene gas is applied to the fruits in degreening rooms to break down the green pigment (chlorophyll) and reveal the underlying yellow or orange color of the rind

Abscisic acid for drought tolerance and dormancy breaking

• Abscisic acid is used to improve drought tolerance in various crops, such as wheat, maize, and soybean
• Synthetic ABA analogs, such as pyrabactin, are applied as sprays or seed treatments to induce stomatal closure and reduce water loss during drought stress
• Abscisic acid is also used to break dormancy in seeds and buds of various crops and ornamental plants
• ABA is applied as a soak or spray to the seeds or buds to overcome dormancy and promote uniform germination or bud break

Synthetic plant growth regulators

• Synthetic plant growth regulators are man-made compounds that mimic the action of natural plant growth regulators or have novel activities not found in nature
• The development of synthetic plant growth regulators has expanded the range of applications and has enabled the fine-tuning of plant growth and development for specific purposes
• However, the use of synthetic plant growth regulators also raises concerns about their potential impact on human health and the environment, and their registration and use are subject to strict regulations and testing

Development and classification

• Synthetic plant growth regulators are developed by modifying the chemical structure of natural plant growth regulators or by screening large numbers of compounds for their effects on plant growth and development
• The development of synthetic plant growth regulators involves extensive testing for their efficacy, specificity, and safety, and their registration and use are regulated by national and international agencies (EPA, EU, FAO)
• Synthetic plant growth regulators are classified based on their chemical structure and mode of action, and they include various classes of compounds, such as phenoxyacetic acids, benzoic acids, pyridines, and others

Advantages and limitations vs natural regulators

• Synthetic plant growth regulators have several advantages over natural plant growth regulators, such as higher stability, longer shelf life, and more specific activities
• They can be applied at lower concentrations and have more predictable effects on plant growth and development, which facilitates their use in agriculture and horticulture
• However, synthetic plant growth regulators also have limitations, such as higher cost, potential off-target effects, and environmental risks
• They may have unintended effects on non-target organisms, such as pollinators and soil microbes, and their residues may accumulate in the environment and pose risks to human and animal health

Environmental and health concerns

• The use of synthetic plant growth regulators raises concerns about their potential impact on the environment and human health
• Some synthetic plant growth regulators, such as 2,4-D and dicamba, have been associated with the development of herbicide-resistant weeds and the contamination of water sources and food products
• Other synthetic plant growth regulators, such as daminozide and chlormequat chloride, have been banned or restricted due to their potential carcinogenic and teratogenic effects on humans and animals
• The environmental and health risks of synthetic plant growth regulators are assessed through extensive toxicological and ecological studies, and their use is regulated by national and international agencies to minimize the potential risks

Research and future prospects

• The research on plant growth regulators has made significant advances in recent years, thanks to the development of new technologies and approaches in molecular biology, genetics, and biochemistry
• The future prospects of plant growth regulator research include the elucidation of the molecular basis of their action, the genetic engineering of their biosynthesis and signaling pathways, and the development of novel applications in crop improvement and stress management
• The integration of plant growth regulator research with other disciplines, such as genomics, proteomics, and metabolomics, will provide a more comprehensive understanding of plant growth and development and will enable the rational design of new plant growth regulators and the optimization of their use in agriculture and horticulture

Molecular basis of growth regulator action

• The molecular basis of plant growth regulator action involves the identification and characterization of the receptors, signaling components, and target genes that mediate their effects on plant growth and development
• The use of genetic and biochemical approaches, such as mutant screens, protein-protein interaction assays, and chromatin immunoprecipitation, has enabled the dissection of the signaling pathways and the identification of the key regulators of plant growth regulator responses
• The integration of omics approaches, such as transcriptomics, proteomics, and metabolomics, has provided a more comprehensive view of the molecular networks that underlie plant growth regulator action and has revealed novel targets for the manipulation of plant growth and development

Genetic engineering of growth regulator biosynthesis and signaling

• The genetic engineering of plant growth regulator biosynthesis and signaling pathways offers new opportunities for the modulation of plant growth and development and the improvement of crop yield and quality
• The use of transgenic approaches, such as overexpression, silencing, and genome editing, has enabled the manipulation of plant growth regulator levels and responses in a tissue- and stage-specific manner
• The engineering of plant growth regulator biosynthesis pathways has been used to increase the production of valuable secondary metabolites, such as fragrances, flavors, and pharmaceuticals, in plant cell cultures and whole plants
• The engineering of plant growth regulator signaling pathways has been used to enhance the resistance to abiotic and biotic stresses, such as drought, salinity, and pathogens, and to improve the yield and quality of crops

Novel applications in crop improvement and stress management

• The research on plant growth regulators has opened new avenues for the improvement of crop yiel