6.4 Plant hormones and signaling molecules

Plant hormones are chemical messengers that regulate growth and development in plants. These signaling molecules, produced in low concentrations, have profound effects on plant physiology and morphology. They coordinate various aspects of plant life, from seed germination to senescence.

The main types of plant hormones include auxins, gibberellins, cytokinins, ethylene, and abscisic acid. Each hormone has specific roles in plant growth, such as cell elongation, stem growth, fruit ripening, and stress responses. Understanding these hormones is crucial for plant biology and agriculture.

Types of plant hormones

• Plant hormones, also known as phytohormones, are naturally occurring substances that regulate growth, development, and responses to environmental stimuli in plants
• These signaling molecules are produced in low concentrations but have profound effects on plant physiology and morphology
• Different types of plant hormones work in concert to coordinate various aspects of plant growth and development, from seed germination to senescence

Auxins

Top images from around the web for Auxins

• 

  Frontiers | A Growing Stem Inhibits Bud Outgrowth – The Overlooked Theory of Apical Dominance View original

  Is this image relevant?

• 

  Frontiers | LAZY Gene Family in Plant Gravitropism View original

  Is this image relevant?

• 

  Frontiers | Developmental Roles of AUX1/LAX Auxin Influx Carriers in Plants View original

  Is this image relevant?

• 

  Frontiers | A Growing Stem Inhibits Bud Outgrowth – The Overlooked Theory of Apical Dominance View original

  Is this image relevant?

• 

  Frontiers | LAZY Gene Family in Plant Gravitropism View original

  Is this image relevant?

1 of 3

Top images from around the web for Auxins

• 

  Frontiers | A Growing Stem Inhibits Bud Outgrowth – The Overlooked Theory of Apical Dominance View original

  Is this image relevant?

• 

  Frontiers | LAZY Gene Family in Plant Gravitropism View original

  Is this image relevant?

• 

  Frontiers | Developmental Roles of AUX1/LAX Auxin Influx Carriers in Plants View original

  Is this image relevant?

• 

  Frontiers | A Growing Stem Inhibits Bud Outgrowth – The Overlooked Theory of Apical Dominance View original

  Is this image relevant?

• 

  Frontiers | LAZY Gene Family in Plant Gravitropism View original

  Is this image relevant?

1 of 3

• Auxins, such as indole-3-acetic acid (IAA), are involved in cell elongation, apical dominance, and root formation
• They promote the differentiation of vascular tissues (xylem and phloem) and play a role in phototropism and gravitropism
• Auxins are synthesized primarily in young leaves, shoot tips, and developing seeds, and they are transported basipetally (from the shoot towards the root)
• Examples of auxin-mediated responses include the formation of adventitious roots in stem cuttings and the inhibition of lateral bud growth in apical dominance

Gibberellins

• Gibberellins (GAs) are a group of hormones that stimulate stem elongation, seed germination, and fruit development
• They promote the degradation of DELLA proteins, which are growth repressors, thereby allowing plant growth to proceed
• GAs are synthesized in young leaves, roots, and developing seeds, and they can be transported both acropetally (from the root towards the shoot) and basipetally
• Examples of gibberellin-mediated responses include the bolting of rosette plants (rapid stem elongation) and the induction of alpha-amylase production during seed germination

Cytokinins

• Cytokinins, such as zeatin and kinetin, promote cell division, shoot branching, and leaf expansion
• They delay senescence and are involved in the regulation of source-sink relationships in plants
• Cytokinins are synthesized primarily in root tips and developing seeds, and they are transported acropetally via the xylem
• Examples of cytokinin-mediated responses include the formation of shoot meristems in tissue culture and the delay of leaf senescence

Ethylene

• Ethylene is a gaseous hormone that promotes fruit ripening, leaf abscission, and senescence
• It is also involved in stress responses, such as wound healing and defense against pathogens
• Ethylene is synthesized from the amino acid methionine and is produced in response to various environmental stimuli, such as mechanical stress or pathogen attack
• Examples of ethylene-mediated responses include the ripening of climacteric fruits (bananas and tomatoes) and the abscission of leaves and flowers

Abscisic acid

• Abscisic acid (ABA) is a stress hormone that regulates stomatal closure, seed dormancy, and responses to drought and salinity
• It antagonizes the effects of gibberellins and promotes the accumulation of storage proteins and lipids in seeds
• ABA is synthesized in roots and mature leaves, and it can be transported both acropetally and basipetally
• Examples of ABA-mediated responses include the induction of stomatal closure during water stress and the maintenance of seed dormancy

Other signaling molecules

• In addition to the classical plant hormones, other signaling molecules, such as brassinosteroids, jasmonates, and salicylic acid, play important roles in plant growth and development
• Brassinosteroids are steroid hormones that promote cell elongation and division, and they are involved in vascular differentiation and stress responses
• Jasmonates, such as jasmonic acid and methyl jasmonate, are lipid-derived hormones that mediate defense responses against herbivores and pathogens
• Salicylic acid is a phenolic compound that induces systemic acquired resistance (SAR) against pathogens and is involved in the regulation of flowering time

Hormone synthesis and regulation

• Plant hormones are synthesized through complex biosynthetic pathways that involve multiple enzymes and intermediates
• The regulation of hormone synthesis and metabolism ensures the proper balance of hormones required for optimal plant growth and development
• Environmental factors, such as light, temperature, and nutrient availability, can influence hormone production and signaling

Biosynthetic pathways

• Each class of plant hormones has a unique biosynthetic pathway that begins with a specific precursor molecule
• For example, auxins are synthesized from the amino acid tryptophan via the indole-3-pyruvic acid (IPyA) pathway or the indole-3-acetamide (IAM) pathway
• Gibberellins are synthesized from geranylgeranyl diphosphate (GGDP) through a series of oxidation and hydroxylation reactions catalyzed by terpene synthases and cytochrome P450 monooxygenases
• Cytokinins are derived from adenine nucleotides, with the key step being the addition of an isoprenoid side chain by isopentenyltransferases (IPTs)
• Ethylene is synthesized from methionine via the Yang cycle, which involves the conversion of S-adenosylmethionine (SAM) to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase, followed by the oxidation of ACC to ethylene by ACC oxidase
• Abscisic acid is synthesized from carotenoids, specifically violaxanthin, through a series of cleavage and oxidation reactions catalyzed by zeaxanthin epoxidase, 9-cis-epoxycarotenoid dioxygenase (NCED), and abscisic aldehyde oxidase

Factors affecting hormone production

• Light is a major environmental factor that influences hormone synthesis and signaling, with different light qualities (red, far-red, blue) and photoperiods affecting the production of various hormones
• For example, red light promotes the synthesis of gibberellins and cytokinins, while far-red light inhibits their production
• Temperature also affects hormone synthesis, with low temperatures generally reducing hormone production and high temperatures increasing it
• Nutrient availability, particularly nitrogen and phosphorus, can influence hormone synthesis, with deficiencies often leading to reduced hormone production
• Biotic and abiotic stresses, such as pathogen attack, wounding, drought, and salinity, can trigger the synthesis of specific hormones, such as ethylene, jasmonates, and abscisic acid, as part of the plant's defense and adaptation mechanisms

Hormone degradation and inactivation

• The levels of active hormones in plant tissues are regulated not only by synthesis but also by degradation and inactivation
• Hormones can be degraded by specific enzymes, such as auxin oxidases and cytokinin dehydrogenases, which catalyze the oxidation or reduction of the hormone molecules
• Hormones can also be inactivated through conjugation with other molecules, such as sugars or amino acids, which renders them biologically inactive
• For example, auxins can be conjugated with glucose to form indole-3-acetyl-β-D-glucose (IAA-Glc), which is an inactive storage form of the hormone
• The balance between hormone synthesis, degradation, and inactivation determines the overall levels of active hormones in plant tissues and their effects on growth and development

Hormone transport and distribution

• Plant hormones are transported from their sites of synthesis to their target tissues through various mechanisms, ensuring the proper distribution of signaling molecules throughout the plant
• The transport of hormones can occur over long distances via the vascular system or over short distances through cell-to-cell movement
• The establishment of hormone gradients and localized concentrations plays a crucial role in regulating plant growth and development

Long-distance transport

• Long-distance transport of hormones occurs primarily through the vascular system, which includes the xylem and phloem
• Xylem transport is driven by transpiration and is responsible for the movement of water and nutrients from the roots to the shoots
• Phloem transport is driven by the pressure-flow mechanism and is responsible for the movement of sugars and other organic compounds from source to sink tissues
• Auxins, cytokinins, and abscisic acid are transported long-distance via the xylem, while gibberellins and ethylene are transported via the phloem
• The direction of hormone transport in the vascular system can be acropetal (from the root towards the shoot) or basipetal (from the shoot towards the root), depending on the hormone and the plant's developmental stage

Short-distance transport

• Short-distance transport of hormones occurs through cell-to-cell movement, which can be symplastic (through plasmodesmata) or apoplastic (through the cell wall and intercellular spaces)
• Auxins are transported cell-to-cell through a combination of passive diffusion and active transport mediated by influx and efflux carriers, such as AUX1 and PIN proteins
• Cytokinins and gibberellins are thought to move symplastically through plasmodesmata, while abscisic acid can move both symplastically and apoplastically
• Short-distance transport is important for establishing local hormone gradients and regulating cell-specific responses to hormonal signals

Hormone gradients and localization

• The establishment of hormone gradients and localized concentrations is crucial for regulating plant growth and development
• Auxin gradients, for example, are essential for the formation of vascular tissues, the establishment of apical dominance, and the response to tropic stimuli (phototropism and gravitropism)
• The polar transport of auxin, mediated by the asymmetric distribution of influx and efflux carriers, creates auxin maxima and minima in specific tissues, which then trigger downstream developmental responses
• Cytokinin gradients are important for the regulation of shoot branching, with high cytokinin levels promoting bud outgrowth and low levels inhibiting it
• Abscisic acid gradients are involved in the regulation of stomatal aperture, with high ABA levels in guard cells promoting stomatal closure and low levels promoting stomatal opening
• The localization of hormone synthesis and transport, combined with the spatial distribution of hormone receptors and signaling components, allows plants to fine-tune their growth and development in response to environmental cues

Hormone receptors and signaling pathways

• Plant hormones exert their effects on growth and development by binding to specific receptors and triggering downstream signaling cascades
• Hormone receptors are diverse in their structure and cellular localization, and they mediate the perception and transduction of hormonal signals
• The activation of hormone receptors leads to changes in gene expression, protein activity, and cellular metabolism, ultimately resulting in the physiological and morphological responses associated with each hormone

Receptor types and structures

• Plant hormone receptors can be broadly classified into three main types: receptor kinases, nuclear receptors, and soluble receptors
• Receptor kinases are transmembrane proteins that consist of an extracellular ligand-binding domain, a single transmembrane domain, and an intracellular kinase domain
• Examples of receptor kinases include the brassinosteroid receptor BRI1 and the ethylene receptors ETR1 and ERS1
• Nuclear receptors are intracellular proteins that bind to hormone ligands and directly regulate gene expression by acting as transcription factors
• The gibberellin receptor GID1 and the abscisic acid receptors PYR/PYL/RCAR are examples of nuclear receptors
• Soluble receptors are cytoplasmic or membrane-associated proteins that bind to hormone ligands and mediate downstream signaling through protein-protein interactions or enzymatic activities
• The auxin receptor TIR1, which is part of the SCF ubiquitin ligase complex, is an example of a soluble receptor

Signal transduction mechanisms

• Upon binding to their receptors, plant hormones trigger a series of signal transduction events that relay the hormonal message to downstream effectors
• Receptor kinases typically initiate signaling through autophosphorylation and the phosphorylation of downstream targets, such as kinases, phosphatases, and transcription factors
• For example, the binding of brassinosteroids to BRI1 leads to its autophosphorylation and the phosphorylation of the kinase BAK1, which then activates a cascade of phosphorylation events that culminate in the regulation of gene expression
• Nuclear receptors, upon binding to their ligands, undergo conformational changes that allow them to interact with specific DNA sequences and regulate the transcription of target genes
• The gibberellin receptor GID1, when bound to gibberellins, interacts with DELLA proteins and targets them for degradation by the 26S proteasome, thereby relieving their repressive effects on growth and development
• Soluble receptors often mediate signaling through the regulation of protein stability or enzymatic activity
• The auxin receptor TIR1, when bound to auxin, promotes the ubiquitination and degradation of Aux/IAA transcriptional repressors, allowing auxin-responsive genes to be expressed

Crosstalk between signaling pathways

• Plant hormone signaling pathways do not operate in isolation but rather interact with each other in a complex network of crosstalk and feedback regulation
• Crosstalk between signaling pathways allows plants to fine-tune their responses to multiple hormonal and environmental cues and ensures the coordination of growth and development
• Auxin and cytokinin signaling pathways, for example, interact antagonistically in the regulation of shoot branching, with auxin inhibiting bud outgrowth and cytokinin promoting it
• Gibberellin and abscisic acid signaling pathways interact antagonistically in the regulation of seed germination, with gibberellins promoting germination and abscisic acid inhibiting it
• Ethylene and jasmonate signaling pathways interact synergistically in the regulation of defense responses against necrotrophic pathogens
• The integration of multiple hormone signaling pathways allows plants to respond to complex environmental challenges and optimize their growth and development under changing conditions

Physiological effects of plant hormones

• Plant hormones regulate a wide range of physiological processes throughout the plant life cycle, from seed germination to senescence
• The effects of hormones on plant growth and development are mediated through changes in cell division, cell elongation, and cell differentiation, as well as through the regulation of metabolic pathways and stress responses
• The specific physiological effects of each hormone depend on its concentration, distribution, and interaction with other hormones and environmental factors

Growth and development

• Auxins promote cell elongation and division, particularly in shoots, and are involved in the formation of vascular tissues, lateral roots, and fruit development
• Gibberellins stimulate stem elongation, leaf expansion, and seed germination, and they are involved in the regulation of flowering and fruit set
• Cytokinins promote cell division and shoot branching, delay leaf senescence, and are involved in the regulation of root growth and vascular development
• Ethylene inhibits cell elongation and promotes fruit ripening, leaf abscission, and senescence, and it is involved in the regulation of root hair formation and nodulation in legumes
• Abscisic acid inhibits seed germination and promotes seed dormancy, regulates stomatal closure and water balance, and is involved in the regulation of bud dormancy and leaf senescence

Stress responses and adaptation

• Plant hormones play crucial roles in mediating plant responses to biotic and abiotic stresses, such as drought, salinity, extreme temperatures, and pathogen attack
• Abscisic acid is a key regulator of plant responses to water stress, promoting stomatal closure, root growth, and the accumulation of osmoprotectants, such as proline and sugars
• Ethylene and jasmonates are involved in the regulation of plant defense responses against necrotrophic pathogens and herbivores, inducing the production of antimicrobial compounds and the reinforcement of cell walls
• Salicylic acid is a central player in the induction of systemic acquired resistance (SAR) against biotrophic pathogens, promoting the expression of defense-related genes and the accumulation of pathogenesis-related (PR) proteins
• Cytokinins and brassinosteroids are involved in the regulation of plant responses to abiotic stresses, such as cold, heat, and salinity, through the modulation of antioxidant systems and the expression of stress-responsive genes

Senescence and abscission

• Plant hormones are involved in the regulation of senescence and abscission, which are programmed processes of cell death and organ separation, respectively
• Ethylene is a key promoter of leaf and flower senescence, inducing the degradation of chlorophyll, proteins, and other cellular components, and promoting the remobilization of nutrients to other parts of the plant
• Abscisic acid also promotes leaf senescence, particularly under stress conditions, by inducing the expression of senescence-associated genes (SAGs) and the degradation of photosynthetic proteins
• Auxins and cytokinins, on the other hand, delay leaf senescence by maintaining chlorophyll content and photosynthetic activity, and by inhibiting the expression of SAGs
• Ethylene and abscisic acid are also involved in the regulation of leaf and fruit abscission, promoting the formation of the abscission zone and the separation of organs from the plant

Fruit ripening and seed germination

• Plant hormones play critical roles in the regulation of fruit ripening and seed germination, two processes that are essential for the dispersal and establishment of the next generation
• Ethylene is the primary hormone involved in the ripening of climacteric fruits, such as tomatoes, bananas, and apples, inducing changes in color, texture, flavor, and aroma
• Auxins, gibberellins, and abscisic acid are also involve