🌱Plant Physiology Unit 7 – Plant Hormones: Growth and Signaling

Key Plant Hormones

• Auxins (indole-3-acetic acid, IAA) promote cell elongation, apical dominance, and root initiation
• Cytokinins (zeatin) stimulate cell division, delay senescence, and promote shoot growth
  • Adenine derivatives synthesized in roots, young leaves, and developing seeds
• Gibberellins (GA) regulate stem elongation, seed germination, and fruit development
  • Tetracyclic diterpenoid acids synthesized in young tissues and developing seeds
• Abscisic acid (ABA) mediates stress responses, seed dormancy, and stomatal closure
• Ethylene (C2H4) is a gaseous hormone that promotes fruit ripening and leaf abscission
  • Also involved in stress responses and senescence
• Brassinosteroids (BRs) promote cell elongation, vascular differentiation, and stress tolerance
• Jasmonates (JAs) mediate defense responses against herbivores and pathogens
• Salicylic acid (SA) induces systemic acquired resistance (SAR) against pathogens

Hormone Functions and Effects

• Auxins stimulate cell wall loosening and elongation through acid growth hypothesis
  • Activate plasma membrane H+-ATPases, lowering apoplastic pH
  • Induce expression of cell wall-modifying enzymes (expansins, xyloglucan endotransglucosylase/hydrolases)
• Cytokinins promote cell cycle progression by activating cyclin-dependent kinases (CDKs)
• Gibberellins stimulate α-amylase synthesis in aleurone cells during seed germination
  • Induce degradation of DELLA proteins, which are negative regulators of GA signaling
• ABA induces stomatal closure by triggering K+ and anion efflux from guard cells
  • Increases cytosolic Ca2+ concentration and activates SNF1-related protein kinases (SnRKs)
• Ethylene promotes fruit ripening by inducing expression of cell wall-degrading enzymes (polygalacturonases, cellulases)
• BRs enhance stress tolerance by upregulating antioxidant enzymes and heat shock proteins
• JAs activate defense genes encoding proteinase inhibitors, defensins, and secondary metabolites
• SA induces expression of pathogenesis-related (PR) proteins and enhances resistance to biotrophic pathogens

Biosynthesis and Regulation

• Auxin biosynthesis occurs primarily through the indole-3-pyruvic acid (IPA) pathway in young leaves and developing seeds
  • Tryptophan is converted to IPA by tryptophan aminotransferases (TAA1/TARs) and then to IAA by YUCCA flavin monooxygenases
• Cytokinin biosynthesis involves the transfer of an isoprenoid side chain to adenine nucleotides by isopentenyltransferases (IPTs)
  • CYP735A cytochrome P450 monooxygenases convert iP nucleotides to trans-zeatin nucleotides
• GA biosynthesis starts with the cyclization of geranylgeranyl diphosphate (GGDP) by ent-copalyl diphosphate synthase (CPS) and ent-kaurene synthase (KS)
  • Subsequent oxidation steps by cytochrome P450 monooxygenases (KO, KAO) and 2-oxoglutarate-dependent dioxygenases (GA20ox, GA3ox) produce bioactive GAs
• ABA is synthesized from carotenoids (zeaxanthin) through the cleavage of 9-cis-epoxycarotenoids by 9-cis-epoxycarotenoid dioxygenases (NCEDs)
• Ethylene is synthesized from S-adenosyl-L-methionine (SAM) by 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase
• BR biosynthesis involves the oxidation of campesterol by cytochrome P450 monooxygenases (DWF4, CPD, BR6ox) and other enzymes
• JA is synthesized from α-linolenic acid through the octadecanoid pathway, with key enzymes including lipoxygenases (LOXs), allene oxide synthase (AOS), and allene oxide cyclase (AOC)
• SA is synthesized from chorismate through the isochorismate pathway or the phenylalanine ammonia-lyase (PAL) pathway

Signaling Pathways and Mechanisms

• Auxin signaling is mediated by the TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX (TIR1/AFB) receptors, which are F-box proteins that form part of the SCF E3 ubiquitin ligase complex
  • Auxin binding to TIR1/AFBs promotes the degradation of Aux/IAA transcriptional repressors, allowing AUXIN RESPONSE FACTORS (ARFs) to activate target gene expression
• Cytokinin signaling involves histidine kinase receptors (AHKs), which autophosphorylate upon cytokinin binding and transfer the phosphoryl group to histidine phosphotransfer proteins (AHPs)
  • AHPs then phosphorylate type-B response regulators (RRs), which activate the transcription of cytokinin-responsive genes
• GA signaling is mediated by the GIBBERELLIN INSENSITIVE DWARF1 (GID1) receptor, which binds to bioactive GAs and promotes the degradation of DELLA proteins via the 26S proteasome
  • DELLAs are negative regulators that interact with and inhibit transcription factors involved in GA-responsive gene expression
• ABA signaling involves the PYRABACTIN RESISTANCE1/PYR1-LIKE/REGULATORY COMPONENT OF ABA RECEPTOR (PYR/PYL/RCAR) family of receptors, which bind ABA and inhibit type 2C protein phosphatases (PP2Cs)
  • Inhibition of PP2Cs allows the activation of SnRKs, which phosphorylate and activate downstream transcription factors (ABFs, AREBs)
• Ethylene signaling is mediated by the ETHYLENE RESPONSE1 (ETR1) family of receptors, which are negative regulators of the pathway
  • In the absence of ethylene, ETR1 activates CONSTITUTIVE TRIPLE RESPONSE1 (CTR1), a Raf-like kinase that inhibits the ETHYLENE INSENSITIVE2 (EIN2) protein
  • Ethylene binding inactivates ETR1 and CTR1, allowing EIN2 to stabilize EIN3/EIL1 transcription factors, which activate ethylene-responsive genes
• BR signaling involves the BRASSINOSTEROID INSENSITIVE1 (BRI1) receptor kinase, which heterodimerizes with BRI1-ASSOCIATED RECEPTOR KINASE1 (BAK1) upon BR binding
  • Activated BRI1 phosphorylates and inhibits the BRI1 KINASE INHIBITOR1 (BKI1) protein, allowing the activation of the BRI1 SUPPRESSOR1 (BSU1) phosphatase
  • BSU1 dephosphorylates and inactivates the BRASSINOSTEROID INSENSITIVE2 (BIN2) kinase, a negative regulator that phosphorylates and inhibits the BZR1/BES1 transcription factors
• JA signaling is mediated by the CORONATINE INSENSITIVE1 (COI1) receptor, an F-box protein that forms part of the SCF E3 ubiquitin ligase complex
  • In the presence of JA-Ile (the bioactive form of JA), COI1 promotes the degradation of JASMONATE ZIM-DOMAIN (JAZ) transcriptional repressors, allowing MYC transcription factors to activate JA-responsive genes
• SA signaling involves the NON-EXPRESSOR OF PR GENES1 (NPR1) protein, which acts as a master regulator of SA-mediated responses
  • SA accumulation triggers the reduction of NPR1 oligomers to monomers, which translocate to the nucleus and interact with TGA transcription factors to activate PR gene expression

Interactions and Crosstalk

• Auxin and cytokinin interact antagonistically in shoot and root development
  • Auxin promotes root initiation while cytokinin stimulates shoot growth
  • Auxin-cytokinin balance determines the shoot-root ratio and plant architecture
• Auxin and ethylene interact synergistically in root growth and development
  • Ethylene enhances auxin biosynthesis and transport, promoting lateral root formation and root hair elongation
• Gibberellins and DELLAs mediate the crosstalk between multiple hormone pathways
  • DELLAs interact with and modulate the activity of transcription factors involved in auxin, cytokinin, BR, and JA signaling
• ABA and ethylene have antagonistic effects on seed germination and early seedling growth
  • ABA promotes seed dormancy while ethylene stimulates germination
  • ABA inhibits root growth while ethylene promotes root hair elongation
• BRs and auxins interact synergistically to promote cell elongation and vascular differentiation
  • BRs enhance the expression of auxin-responsive genes and increase auxin transport
• JA and SA signaling pathways are mutually antagonistic
  • JA-mediated responses are typically associated with defense against necrotrophic pathogens and herbivores, while SA-mediated responses are associated with defense against biotrophic pathogens
  • NPR1 is a key regulator of the JA-SA crosstalk, acting as a positive regulator of SA signaling and a negative regulator of JA signaling

Environmental Influences

• Light regulates hormone biosynthesis, transport, and signaling
  • Red light promotes auxin biosynthesis and transport, while blue light enhances auxin signaling and cytokinin biosynthesis
  • Shade avoidance response is mediated by changes in auxin, GA, and BR levels
• Temperature affects hormone metabolism and signaling
  • Cold stress induces ABA accumulation and inhibits GA biosynthesis, promoting cold acclimation and dormancy
  • Heat stress stimulates ethylene production and enhances BR signaling, promoting thermotolerance
• Drought stress triggers ABA accumulation, which induces stomatal closure and activates stress-responsive genes
  • ABA interacts with JA and ethylene signaling to fine-tune drought responses
• Nutrient availability modulates hormone signaling and plant growth
  • Nitrogen deficiency promotes root growth through changes in auxin and cytokinin signaling
  • Phosphate starvation enhances auxin sensitivity and inhibits GA signaling, promoting root growth and phosphate acquisition
• Biotic stress (pathogen infection, herbivory) elicits complex hormonal responses
  • Biotrophic pathogens typically induce SA-mediated defenses, while necrotrophic pathogens and herbivores induce JA/ethylene-mediated defenses
  • Hormonal crosstalk fine-tunes the balance between growth and defense in response to biotic stress

Practical Applications

• Synthetic auxins (2,4-D, NAA) are used as herbicides and rooting agents in plant propagation
• Cytokinins (kinetin, BA) are used to promote shoot proliferation in plant tissue culture and delay leaf senescence in cut flowers
• GA inhibitors (paclobutrazol, uniconazole) are used to control plant height in horticulture and agriculture
• Ethylene releasers (ethephon) are used to promote fruit ripening and uniform crop maturation
• BR analogs (brassinolide) are used to improve crop stress tolerance and yield
• Methyl jasmonate is used to enhance the production of secondary metabolites in medicinal plants
• SA and its analogs (BTH, INA) are used to induce systemic acquired resistance in crops against pathogens
• ABA analogs and antagonists are being developed to modulate plant stress responses and improve water use efficiency

Recent Discoveries and Future Directions

• New hormone receptors and signaling components are being identified through genetic and biochemical approaches
  • The discovery of the PYR/PYL/RCAR family of ABA receptors has revolutionized our understanding of ABA signaling
  • The identification of the FERONIA receptor kinase as a modulator of auxin and ethylene signaling has revealed new aspects of hormonal crosstalk
• Hormone transport and spatial distribution are being studied using advanced imaging techniques and mathematical modeling
  • Fluorescent biosensors and reporters are being used to visualize hormone dynamics in vivo
  • Computational models are being developed to predict hormone gradients and signaling outcomes
• Epigenetic regulation of hormone signaling is an emerging area of research
  • Chromatin modifications and noncoding RNAs are being investigated as potential regulators of hormone-responsive gene expression
• Hormone-based strategies for improving crop stress tolerance and yield are being developed
  • Genome editing tools (CRISPR/Cas) are being used to manipulate hormone biosynthesis and signaling genes in crops
  • Synthetic biology approaches are being explored to engineer novel hormone signaling pathways and optimize plant growth and development
• Hormonal regulation of plant-microbe interactions is an active area of research
  • The role of hormones in shaping the plant microbiome and modulating plant-microbe symbioses is being investigated
  • Microbial production of plant hormones and their impact on plant growth and development are being explored
• Integration of hormone signaling with other signaling pathways (e.g., light, circadian clock, sugar) is being investigated to understand the complex regulation of plant growth and development
  • Systems biology approaches are being used to unravel the crosstalk and feedback loops between different signaling pathways
  • Mathematical modeling and network analysis are being employed to predict the emergent properties of hormone signaling networks