Plant growth regulators are chemical messengers that control plant development. They include , , , , and . Each type has unique effects on growth, from to .
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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Frontiers | Auxin-Dependent Cell Elongation During the Shade Avoidance Response 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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Top images from around the web for Auxins
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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Frontiers | Auxin Control of Root Organogenesis from Callus in Tissue Culture 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 (IAA), which is synthesized in young leaves and developing seeds
Synthetic auxins, such as (IBA) and (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 , 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 (, , )
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 , 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 , 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 , 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 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 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