Plant Hormones

Introduction · Animal Comparisons · Growth Concepts

1. Introduction and Overview of Plant Hormones

Plant growth and development are governed by a complex integration of environmental cues, endogenous genetic programs, and chemical signals. Central to these regulatory networks are small organic compounds known as plant hormones or phytohormones.

Definition and Distinctions

What is a Phytohormone?

Plant hormones are defined as naturally occurring organic substances synthesized by specific plant cells or tissues that exert profound physiological effects at very low concentrations (typically < 1 μM).

Although conceptually analogous to animal hormones, phytohormones differ fundamentally in several physiological aspects. The table below outlines the core differences in how plants and animals utilize hormonal signalling.

🌱 Plant Hormones	🧬 Animal Hormones
Synthesized in diffuse, non-specialized tissues (e.g., meristems, young leaves)	Synthesized in highly specialized, localized endocrine glands (e.g., thyroid, pituitary)
Transport is not obligatory for activity (can act locally where produced)	Transport is strictly dependent on a dedicated vascular / circulatory system
Exert pleiotropic (multiple), overlapping effects depending on context	Exert highly specific, target-cell-restricted effects
Show concentration-dependent dual actions (e.g., stimulating growth at low levels, inhibiting at high levels)	Generally show threshold or binary regulatory activation responses

Plant Growth Concepts

Growth in plants is defined as an irreversible increase in size or volume, primarily driven by cellular turgor pressure and subsequent cell wall enlargement. While growth is easy to observe, quantifying it precisely requires careful measurement selection. Growth can be quantified through changes in fresh weight, dry weight, volume, or cell number.

Key Measurement Metrics

Dry Weight
Dry weight provides the most accurate measure of actual structural biomass accumulating over time. Because it removes the variable of water content, it is entirely independent of the transient, daily fluctuations in a plant's cellular water status.
Cell Number
While counting cells indicates division (mitosis), it can be a misleading metric for overall growth. In multicellular plant organs, rapid cell division can frequently occur without any immediate or corresponding increase in the total volume of the organ itself.

Plant Hormones

Major Classes · Biosynthetic Precursor Pathways

Major Classes and Precursor Families

Phytohormones are categorized based on their chemical structure, biosynthetic origin, and biological function. Despite their diversity, all major hormonal classes derive from three fundamental metabolic precursor pathways: Amino Acids, Isoprenoid Compounds, and Lipids.

Figure 1. Biosynthetic Origins of Major Phytohormones. Plant hormones fall into major structural categories determined by their metabolic precursors: amino acids, isoprenoids, and lipid derivatives.

CKs: Cytokinins

GAs: Gibberellins

ABA: Abscisic Acid

BRs: Brassinosteroids

SLs: Strigolactones

JAs: Jasmonates

Plant Hormones - Auxins

Auxins

The Master Growth Regulators · Discovery & Structure

2. Auxins

Auxin holds the historical distinction of being the first plant hormone ever identified. The name itself derives from the Greek word auxein, meaning "to grow" or "to increase."

Discovery and Natural Chemical Forms

Charles & Francis Darwin

1880

First demonstrated that phototropism in canary grass (Phalaris canariensis) coleoptiles was mediated by a translocatable chemical signal specifically produced at the growing tip.

Frits Went

1926

Successfully isolated this chemical substance in agar blocks using the famous Avena coleoptile curvature test, proving the chemical basis of growth regulation.

Natural Auxins
Indole-3-acetic acid (IAA) is the principal active auxin in higher plants. Other naturally occurring compounds displaying significant auxin activity include Indole-3-butyric acid (IBA) and Phenylacetic acid (PAA).
Chemical Structure
All active auxins share a conserved modular structure: they possess an unsaturated planar aromatic ring system directly linked to a carboxyl group side chain.

Figure 2. Chemical Modular Structure of Auxins. Active auxins like IAA and IBA are defined by their planar Indole ring system attached to a carboxyl side chain of varying length.

Auxin Biosynthesis Pathways

The primary sites of IAA synthesis include high-activity metabolic zones: shoot apical meristems, young expanding leaves, developing seeds, and germinating embryos.

Free vs. Conjugated Auxin

Within the cell, IAA exists in a dynamic equilibrium between two distinct forms:

Free IAA: The biologically active form capable of triggering immediate cellular responses.
Conjugated IAA: The inactive form used for storage and safe long-distance transport. IAA is reversibly bound to sugars (e.g., IAA-glucose) or amino acids/proteins (e.g., glycoproteins) to prevent premature activation.

IAA biosynthesis is a complex metabolic network that occurs via two primary parallel routes:

Tryptophan-dependent pathways
Tryptophan-independent pathways:

Plant Hormones - Auxin Biosynthesis & Transport

Tryptophan-Dependent IPA Pathway (Principal Pathway)

In higher plants, the main biosynthetic route for IAA production relies on the amino acid tryptophan as a primary precursor. This process unfolds through three distinct enzymatic steps:

1. Deamination
Tryptophan is deaminated by the enzyme tryptophan transaminase to yield Indole-3-pyruvic acid (IPA).
2. Decarboxylation
IPA undergoes decarboxylation catalyzed by indole-3-pyruvic acid decarboxylase to form Indole-3-acetaldehyde (IAAld).
3. Oxidation
Finally, IAAld is oxidized by indole-3-acetaldehyde dehydrogenase to form the active hormone, Indole-3-acetic acid (IAA).

Figure 3. Tryptophan-Dependent IPA Biosynthesis Pathway. The principal metabolic route converting the amino acid tryptophan into active auxin (IAA).

✺ Tryptophan-Independent Pathway

Plants retain a secondary pathway capable of synthesizing IAA directly from indole or indole-3-glycerol phosphate without ever passing through tryptophan as an intermediate. Although functionally proven, the exact enzymatic steps of this independent pathway remain only partially characterized.

Auxin Transport Dynamics

Auxin is unique among plant hormones due to its highly regulated, directional movement. It exhibits two fundamentally distinct transport mechanisms:

Non-Polar Transport
Rapid, bidirectional, long-distance movement occurring through the phloem sieve tubes, driven passively by standard source-to-sink mass flow.
Polar Auxin Transport (PAT)
Slow, highly regulated, cell-to-cell, and strictly unidirectional movement driven by chemiosmotic gradients. In shoots, PAT moves exclusively in a basipetal direction (from the shoot apex downward toward the base).

Chemiosmotic Model of Polar Auxin Transport

Polar transport depends on strict pH differentials between the cell wall apoplast (pH ≈ 5.0) and the cytosol (pH ≈ 7.2), which are continuously maintained by plasma membrane H⁺-ATPases.

Figure 4. Chemiosmotic Model of Polar Auxin Transport. Influx is driven by a combination of passive diffusion (IAAH) and active symport (IAA⁻). Efflux is strictly managed by PIN and ABCB transporters localized at the basal membrane, determining transport directionality.

Cellular Transport Mechanisms

Influx Mechanism
In the acidic apoplast (pH ≈ 5.0), roughly half of the auxin exists in a protonated, lipophilic state (IAAH), which freely diffuses passively across the plasma membrane. The remaining anionic auxin (IAA^-) must enter via active secondary transport mediated by 2H⁺/IAA^- symporters (e.g., the AUX1/LAX protein family).
Efflux Mechanism
Inside the neutral cytosol (pH ≈ 7.2), IAA dissociates completely into the charged IAA^- anion. Because charged molecules cannot cross the lipid membrane by simple diffusion, the auxin is effectively trapped and must exit exclusively through dedicated efflux carriers.
PIN Proteins (PIN1–PIN8)
These specialized efflux carriers are asymmetrically localized to specific membrane domains (e.g., exclusively on the basal membrane in vascular parenchyma cells). This extreme structural asymmetry fundamentally determines the vector of polar transport.
ABCB Transporters
ATP-binding cassette proteins (specifically ABCB1, ABCB4, and ABCB19) serve to stabilize PIN protein localization at the membrane and actively facilitate auxin efflux.
Transport Inhibitors (Phytotropins)
Chemical compounds such as TIBA (2,3,5-triiodobenzoic acid) and NPA (1-naphthylphthalamic acid) actively inhibit polar auxin efflux by physically binding to the PIN/ABCB transporter complexes, halting cell-to-cell movement.

Plant Hormones - Auxin Flow & Physiological Effects

Auxin Flow Streams in Roots

Auxin transport in roots is highly organized and consists of two opposing streams that form an "inverted fountain" pattern, crucial for root elongation and gravity sensing:

Acropetal Stream
Moves downwards through the central root stele toward the root apex. This flow is actively mediated by basally localized PIN1 and PIN4 efflux carriers.
Basipetal Stream
Moves upward from the root cap, travelling back through the outer epidermis and cortex layers toward the elongation zone. This upward flow is mediated by apically localized PIN2. Additionally, PIN3 and PIN7 located in the columella cells direct the lateral redistribution of auxin in response to gravity.

Figure 5. The "Inverted Fountain" of Auxin Flow in Roots. Auxin travels down the central stele (acropetal stream), is redistributed laterally in the columella, and travels back up through the outer epidermal layers (basipetal stream) to govern elongation.

Physiological Effects of Auxin

Auxin acts as the master regulator of plant development, influencing nearly every aspect of the plant's life cycle depending on local concentrations and tissue sensitivity.

Acid Growth Hypothesis
Cell Elongation
Auxin actively triggers cell elongation by activating plasma membrane H⁺-ATPases. These pumps extrude protons into the cell wall apoplast, rapidly dropping the wall pH to ∼ 4.5. This low pH environment activates expansins—specialized wall-loosening proteins that disrupt the rigid hydrogen bonds bridging cellulose microfibrils and hemicellulose. This wall relaxation permits turgor-driven cellular expansion.

Note: The fungal toxin fusicoccin induces irreversible hyper-acidification and wilting through this exact same biochemical mechanism.
Vascular Patterning
Cell Differentiation
Auxin gradients orchestrate vascular tissue development. High auxin-to-cytokinin ratios strongly promote xylem differentiation, whereas lower concentrations and altered ratios induce phloem differentiation.
Organogenesis
Root Development
While extremely high concentrations of auxin actually inhibit primary main root elongation, localized auxin accumulation is the absolute trigger for both adventitious root and lateral root initiation by stimulating targeted cell divisions deep within the pericycle layer.
Architecture Control
Apical Dominance
The actively growing shoot tip produces auxin that travels downward, suppressing the growth of lateral (axillary) buds. Auxin executes this dominance indirectly: it maintains high local levels of strigolactones (which induce the bud-repressor gene BRC1) while simultaneously suppressing bud-activating cytokinins.
Cholodny-Went Theory
Tropic Responses
Directional environmental stimuli (such as unilateral light or gravity) cause the lateral, asymmetric redistribution of auxin within an organ. This uneven hormone concentration causes differential growth rates between opposite sides of the stem or root, resulting in bending toward or away from the stimulus.

Plant Hormones - Auxin Signaling Pathway

Auxins

Molecular Signaling · The TIR1 Pathway

Auxin Signaling Pathway

Auxin perception operates through an elegant derepression mechanism heavily dependent on ubiquitin-mediated proteolysis. Rather than directly activating a transcription factor, auxin works by destroying the repressor protein that keeps the transcription factor turned off.

Figure 6. Auxin TIR1 Derepression Signaling Pathway. Auxin acts essentially as a molecular glue. By bringing the repressor (Aux/IAA) to the degradation machinery (SCF^TIR1), auxin clears the path for the ARF transcription factor to function.

Pathway Components & Mechanisms

TIR1 Receptor
Transport Inhibitor Response 1 (TIR1) is an F-box protein that serves as a soluble auxin receptor. It acts as the specific substrate-recognition subunit of the larger SCF^TIR1 E3 ubiquitin ligase complex.
Aux/IAA Proteins
These are potent transcriptional repressors. In the absence of auxin, they physically dimerize with ARFs (Auxin Response Factors) to actively block the activation of auxin-responsive genes.
The "Molecular Glue" Mechanism
Auxin acts as a structural molecular glue, physically binding within a pocket to stabilize the interaction between the Aux/IAA repressors and the SCF^TIR1 complex. Once bound, the SCF complex polyubiquitinates the Aux/IAA proteins, tagging them for rapid destruction by the 26S proteasome. This frees the ARFs to initiate gene transcription.
ABP1 (Auxin Binding Protein 1)
Distinct from the TIR1 genomic pathway, ABP1 is located in the Endoplasmic Reticulum (ER) lumen and the plasma membrane. It is primarily responsible for mediating incredibly rapid, non-genomic responses to auxin, such as immediate proton pumping and plasma membrane depolarization.

Plant Hormones - Gibberellins (GAs)

Gibberellins (GAs)

Stem Elongation · Germination · Biosynthesis

3. Gibberellins (GAs)

Gibberellins are a large family of tetracyclic diterpenoid compounds. They act as essential regulators of stem elongation, seed germination, flowering, and enzyme induction.

Discovery and Structural Classes

Eiichi Kurosawa

1926

Investigated bakanae ("foolish seedling") disease in rice, which caused stems to grow overly tall and spindly. He traced it to an infection by the fungus Gibberella fujikuroi (now Fusarium fujikuroi).

Yabuta & Sumuki

1935

Successfully isolated the active chemical compound responsible for the extreme elongation from the fungus and officially named it gibberellin.

Chemical Structure
All gibberellins are based on the ent-gibberellane ring skeleton, which consists of four fused isoprenoid ring systems (designated Rings A, B, C, and D).
C₂₀-GAs
These precursors retain all 20 carbon atoms from their original diterpene precursor (e.g., GA₁₂, GA₂₀).
C₁₉-GAs
These are derived from C₂₀-GAs through the oxidative loss of carbon-20 (released as CO₂). All biologically active gibberellins (e.g., GA₁, GA₃, GA₄, GA₇) belong to this C₁₉ group. Active forms require a hydroxyl group at C-3β and a carboxyl group at C-7.

Figure 7. The ent-gibberellane skeleton. Bioactive GAs (C₁₉-GAs) lose carbon-20 but require specific functional groups at C-3 and C-7 for physiological activity.

Biosynthesis and Transport

Gibberellin biosynthesis is a multi-step pathway that spans three distinct cellular compartments: the plastids, the endoplasmic reticulum (ER), and the cytosol.

Figure 8. Three-Stage Gibberellin Biosynthesis. GA synthesis begins with IPP in the plastids, moves to the ER for preliminary oxidations, and finishes in the cytosol where soluble dioxygenases produce the final bioactive hormone.

Enzymatic Regulation & Transport

Stage 1 (Plastids)
Cyclization of isoprenoid precursors (IPP → GGPP) forms the foundational tetracyclic hydrocarbon, ent-kaurene.
Stage 2 (ER)
Membrane-bound enzymes (ent-kaurene oxidase and ent-kaurenoic acid oxidase) convert ent-kaurene into GA₁₂ and GA₅₃.
Stage 3 (Cytosol)
Sequential oxidation reactions catalyzed by soluble dioxygenases:
• GA 20-oxidase: Removes C-20 to generate C₁₉-GAs (such as GA₂₀).
• GA 3-oxidase (3β-hydroxylase): Adds a crucial hydroxyl group at position C-3β to yield the bioactive GA₁ or GA₄.
• GA 2-oxidase: Inactivates bioactive GAs by adding a hydroxyl group at position C-2.

Inhibitors & Transport Dynamics

Commercial plant growth retardants (such as Phosphon D, AMO-1618, Cycocel (CCC), Ancymidol, and Paclobutrazol) deliberately stunt plant growth by blocking specific stages of this biosynthetic pathway.

Unlike auxin, GA Transport occurs primarily through non-polar movement via both the xylem and phloem, moving passively without strict directional polarity.

Physiological Effects of Gibberellins

Bolting & The Le Gene
Internodal Elongation
Exogenous GAs induce dramatic internodal lengthening in dwarf mutants and rosette plants (e.g., spinach, cabbage) through a rapid elongation process known as bolting. Mendel's famous tall/dwarf trait in peas is actually controlled by the Le gene, which naturally encodes GA 3-oxidase. Dwarf (le) mutants lack this functional enzyme, meaning they cannot convert precursor GA₂₀ into the bioactive GA₁ needed for normal stem elongation.
Endosperm Mobilization
Seed Germination
During germination, GA triggers the synthesis of hydrolytic enzymes that break down stored food reserves, fueling the embryo's emergence.

Figure 9. Gibberellin-Induced Enzyme Release. Upon imbibition, the embryo synthesizes GA, which travels to the aleurone layer to stimulate the production of hydrolytic enzymes (α-amylase). These enzymes then mobilize the starchy endosperm.

Plant Hormones - Gibberellin Signaling & Green Revolution

Gibberellin Signaling and Green Revolution Genes

Similar to auxin, gibberellin perception operates through an elegant ubiquitin-mediated proteolysis pathway where the hormone degrades a repressor to turn genes on.

GID1 Receptor
Gibberellin Insensitive Dwarf 1 (GID1) is a soluble nuclear receptor. When GA binds to GID1, it induces a critical conformational change that allows the GID1 receptor to successfully interact with DELLA proteins.
DELLA Proteins
These are potent nuclear transcriptional repressors (e.g., GAI, RGA, SLR1) that actively inhibit all GA-responsive growth processes in the plant. They are the "brakes" on plant growth.

Figure 10. Gibberellin GID1 Signaling Pathway. GA facilitates the binding between the GID1 receptor and the DELLA repressor, leading to DELLA's polyubiquitination and subsequent destruction by the 26S proteasome.

🌾 The Green Revolution Dwarfing Genes (Rht)

In the mid-20th century, agronomist Norman Borlaug famously introduced highly productive semi-dwarf wheat varieties containing the mutant Rht-B1b or Rht-D1b alleles—a breakthrough that sparked the Green Revolution.

The Molecular Mechanism: These mutant genes encode altered, defective DELLA proteins that possess non-functional N-terminal domains. Because this domain is broken, these mutant DELLAs cannot successfully bind to the GA-GID1 complex.

The Agricultural Result: Since they cannot bind to the complex, they are completely immune to ubiquitin-mediated degradation. These persistent DELLA repressors permanently restrain extreme stem growth, creating short, sturdy, and robust stems that are highly resistant to wind lodging. Crucially, the plant then diverts all those saved photoassimilates directly into massive grain yield.

Plant Hormones - Cytokinins

Cytokinins

Cell Division · Organogenesis · Senescence Delay

4. Cytokinins

Cytokinins are a class of plant hormones defined by their ability to promote cytokinesis (cell division) in plant roots and shoots. They are structurally derived from adenine and are deeply involved in primarily controlling local cell proliferation, driving organogenesis, counteracting apical dominance, and dramatically delaying the aging process of leaves (senescence).

Discovery and Natural Chemical Structures

Miller & Skoog

1955

In a search for compounds that induce cell division in tobacco pith tissue, they isolated the first synthetic cytokinin, named kinetin (N⁶-furfuryladenine). Remarkably, it was discovered uniquely from autoclaved (heat-treated) herring sperm DNA. Kinetin does not occur naturally in plants.

Miller & Letham

1964

Independently isolated the first natural plant cytokinin, zeatin (trans-zeatin), extracted from the liquid endosperm of immature maize kernels (Zea mays). Zeatin remains the most abundant natural cytokinin in higher plants.

Chemical Structure
Cytokinins are fundamentally N⁶-substituted adenine derivatives. Active cytokinins exist as free bases (such as trans-zeatin, isopentenyladenine [iP], and dihydrozeatin). In the plant, they frequently exist covalently bound to sugars or phosphates as ribosides, ribotides, or glucosides for transport and storage.

Figure 11. Core Structure of Cytokinins. The hormone is an adenine base featuring an active isopentenyl (or modified) side chain attached strictly to the N⁶ position.

Biosynthesis and Transport

Biosynthetic Pathway
De novo cytokinin biosynthesis primarily begins with the transfer of an isopentenyl moiety from dimethylallyl diphosphate (DMAPP) to an adenine nucleotide (ATP, ADP, or AMP). This crucial initial step is directly catalyzed by isopentenyl transferase (IPT), the primary rate-limiting enzyme. Subsequent enzymatic cleavage of the ribose and phosphate groups from these intermediate nucleotides yields the final, active free bases.
Transport Dynamics
While cytokinins are synthesized locally in various developing tissues (seeds, fruits, young leaves), the root apical meristem is a massive site of synthesis.

• Xylem Transport: Sweeps root-derived cytokinins upward to the shoots in the transpiration stream. They primarily travel as inactive bound ribosides, acting as a profound signaling mechanism to communicate the root system's nitrogen status to the shoot.
• Phloem Transport: Carries shoot-derived cytokinins downward to the roots, helping balance systemic growth.

Physiological Effects of Cytokinins

Meristematic Activity
Cell Cycle Control
Cytokinins are fundamental regulators of the plant cell cycle, specifically serving as the biochemical "green light" for the G₁-to-S and G₂-to-M phase transitions.

G₁-to-S Transition: High cytokinin levels induce the transcription of CycD3 (Cyclin D3). This cyclin binds with a resting Cyclin-Dependent Kinase (CDK). The newly active CDK-CycD3 complex specifically phosphorylates Retinoblastoma-related (RBR) proteins. In their unphosphorylated state, RBRs trap and inhibit the E2F transcription factor. Once phosphorylated by the CDK complex, the RBR changes shape, releasing active E2F to enter the nucleus and spark S-phase (DNA replication) gene expression.

G₂-to-M Transition: Cytokinin promotes Cdc25-mediated dephosphorylation, effectively removing inhibitory phosphate groups and fully activating the CDKs required for mitosis.

Figure 12. Cytokinin Control of the Cell Cycle. By inducing CycD3, cytokinin forces the deactivation of the Retinoblastoma repressor, unleashing E2F to trigger DNA replication.

The Skoog-Miller Classic
Tissue Morphogenesis (Organogenesis)
In a hallmark experiment defining modern plant tissue culture, Skoog and Miller proved that the precise ratio of auxin to cytokinin rigorously dictates organ differentiation in undifferentiated plant callus:
• High Auxin : Low Cytokinin powerfully promotes root initiation (rhizogenesis).
• Low Auxin : High Cytokinin forces shoot initiation (caulogenesis).
• Intermediate Ratio maintains the rapid proliferation of an undifferentiated callus mass.
Antagonism
Counteracting Apical Dominance
While auxin flowing downward from the shoot apex suppresses lateral axillary buds, cytokinins flowing upward from the roots directly stimulate those same buds. Direct application of cytokinin to a dormant axillary bud will break its dormancy, proving cytokinins are the natural antagonists to auxin in shaping plant architecture.
Nutrient Sinks
The Richmond-Lang Effect
The Richmond-Lang Effect elegantly proves cytokinins act as potent anti-aging hormones. Direct application to detached, yellowing leaves abruptly prevents chlorophyll degradation and protein breakdown. Cytokinins achieve this by essentially converting the treated tissues into aggressive metabolic sinks, overriding standard physiological decline by actively pulling amino acids and nutrients from surrounding untreated areas.

🦠 Agrobacterium tumefaciens & Crown Gall Tumors

Nature's primary genetic engineer, the soil bacterium Agrobacterium tumefaciens, exploits plant hormone pathways to build its own habitat. It infects wounded plants, transferring a specific piece of DNA (the T-DNA from its Ti Plasmid) permanently into the plant's own nuclear genome.

This T-DNA contains specific, active genes (such as iaaM, iaaH, and ipt) that force the plant cells to autonomously synthesize massive quantities of both auxin and cytokinin. Because the infected cell is producing intermediate ratios of both hormones simultaneously, it perfectly mimics the Skoog-Miller callus conditions—producing massive, undifferentiated, and completely uncontrolled tumorous tissue growth known as a "crown gall."

Two-Component Multistep Phosphorelay Signaling Pathway

Unlike Auxin and Gibberellin, which operate via protein degradation, Cytokinin signal transduction is an evolutionary remnant inherited from bacteria. It utilizes a highly sophisticated multistep phosphorelay mechanism, passing a phosphate group like a baton from the membrane receptor to the nucleus.

Figure 13. Cytokinin Multistep Phosphorelay. The signal is transmitted via the rapid transfer of high-energy phosphate groups. It moves from Histidine to Aspartate internally within the AHK receptor, then to a Histidine on the AHP shuttle, and finally to an Aspartate on the ARR response regulators in the nucleus.

Pathway Components

Receptors (AHKs)
Arabidopsis Histidine Kinases (e.g., AHK2, AHK3, AHK4/CRE1) act as the primary structural sensors. Positioned at the plasma membrane and ER, they contain an extracellular/lumenal CHASE domain that directly binds cytokinin molecules, triggering crucial internal autophosphorylation.
Transmitters (AHPs)
Arabidopsis Histidine Phosphotransfer proteins (AHP1–AHP5) act as highly mobile molecular shuttles. They receive the activated phosphate group and physically shuttle it from the membrane-bound receptors through the cytosol and deep into the nucleus.
Response Regulators (ARRs)
The final targets residing in the nucleus that firmly dictate the plant's genomic response:
• Type-B ARRs: Contain a receiver domain paired with a DNA-binding output domain. They act strictly as transcriptional activators that turn on cytokinin-responsive genes, driving cell division.
• Type-A ARRs: Lack DNA-binding domains entirely. They are rapidly induced by Type-B ARRs specifically to act as negative feedback inhibitors. They soak up the phosphates from AHP shuttles, successfully dampening the cytokinin signal and preventing fatal overactivation.

Plant Hormones - ABA & Ethylene

Abscisic Acid (ABA)

Stress Responses · Stomatal Closure · Seed Dormancy

5. Abscisic Acid (ABA)

Abscisic acid is a 15-carbon sesquiterpene hormone that functions primarily as an inhibitory and stress-response signal. It plays central roles in combating water stress, inducing stomatal closure, and regulating seed maturation and deep dormancy.

Discovery and Biosynthesis

Frederick Addicott

1963

Working on abscission (leaf/fruit drop), he isolated a potent compound from cotton fruits and named it abscisin II.

Philip Wareing

1960s

Working on bud dormancy, he isolated a compound from sycamore leaves he called dormin. Both groups later realized they had discovered the identical compound: Abscisic Acid.

Biosynthetic Pathway
ABA is synthesized via the carotenoid pathway, spanning both the plastids and the cytosol. The primary rate-limiting step is the cleavage of a 40-carbon carotenoid down into a 15-carbon precursor.

Figure 14. ABA Biosynthesis Pathway. 9-cis-epoxycarotenoid dioxygenase (NCED) specifically catalyzes the cleavage of 9-cis-neoxanthin to yield xanthoxin. This cleavage is the critical rate-limiting step governing ABA production during stress.

Physiological Effects and Stomatal Closure

Under severe drought conditions, ABA synthesized in the roots is rapidly mobilized to the leaves, where it massively accumulates in guard cells. This accumulation triggers an immediate and powerful signaling cascade designed to slam the stomata shut and halt transpirational water loss.

Figure 15. ABA-Induced Stomatal Closure Mechanism. The intricate ion-channel choreography orchestrated by ABA in guard cells. The ultimate goal is dumping osmotically active solutes, forcing water out, collapsing cell turgor, and physically closing the pore.

LEA Proteins & Vivipary
Seed Maturation
ABA actively promotes the synthesis of dense storage proteins and Late Embryogenesis Abundant (LEA) proteins. These specialized proteins coat delicate cellular structures, conferring extreme desiccation (drying) tolerance to the maturing seed.

Conversely, low ABA levels, or genetic mutations in ABA synthesis/signaling pathways, directly cause vivipary—the premature and precocious germination of seeds while they are still physically attached to the parent plant (commonly seen in vp maize mutants or naturally in mangrove species).
Hormone Balance Theory
Dormancy Maintenance
The transition of a seed from deep dormancy into active germination is not controlled by one hormone, but rather by the delicate fluctuating ratio of ABA to GA. ABA heavily suppresses germination to ensure seeds only sprout when environmental conditions are universally favorable.

Ethylene

Fruit Ripening · Triple Response · Organ Abscission

6. Ethylene

Ethylene (H₂C=CH₂) holds the unique distinction of being the only gaseous plant hormone. Moving freely through diffusion, it broadly regulates fruit ripening, organ abscission, the famous "triple response" in seedlings, and general stress defense.

Biosynthesis and Inhibitors

The Yang Cycle
Ethylene is biochemically derived from the essential amino acid methionine through a cyclical pathway known as the Yang Cycle.

Figure 16. Ethylene Biosynthesis Pathway. The conversion of AdoMet to ACC by ACS is the principal rate-limiting step. Both major enzymatic steps can be chemically blocked for commercial or experimental purposes.

Physiological Effects

Dark-Grown Adaptations
The Triple Response
When dark-grown seedlings encounter an obstacle (like a rock in the soil), physical stress induces ethylene production, triggering a specific three-part morphological adaptation designed to push past the obstruction:
1. Reduced stem elongation (stops pushing uselessly upward).
2. Increased radial stem thickening (builds physical pushing strength).
3. Horizontal growth habit / Loss of normal gravitropism (steers around the obstacle).
Climacteric Spike
Fruit Ripening
In specific fruits known as climacteric fruits (e.g., apples, bananas, tomatoes), a sharp spike in ethylene strictly triggers autocatalytic (self-amplifying) ethylene production alongside a massive respiratory burst, rapidly accelerating ripening. Conversely, non-climacteric fruits (e.g., citrus, grapes) ripen steadily and do not exhibit this distinct respiratory or ethylene spike.
Auxin Sensitization
Organ Abscission
Ethylene actively induces the production of cell-wall dissolving enzymes (like cellulase and polygalacturonase) directly in the abscission zone of leaves and fruits. Normally, a steady downward stream of auxin keeps this tissue insensitive to ethylene. However, as leaves age, dropping auxin levels strongly sensitize the abscission zone to ethylene's destructive signals.
Flooding Response
Epinasty
Commonly triggered by severe flooding and anaerobic soil stress, epinasty is the aggressive downward bending of leaves. It occurs because ethylene forces the upper (adaxial) side of the petiole to grow significantly faster than the lower (abaxial) side.

Ethylene Receptor Signaling Pathway

Similar to auxin and gibberellin, ethylene signaling utilizes a negative regulation mechanism. The actual ethylene receptors naturally keep the signaling pathway turned off. Ethylene's job is to shut down the receptors, which releases the brakes and turns the genes on.

Figure 17. Ethylene Receptor Signaling Pathway. Ethylene physically deactivates the receptors and the CTR1 kinase. This stops the phosphorylation of EIN2, allowing its C-terminal end to be cleaved and transported into the nucleus to stabilize the necessary transcription factors.

Pathway Components

Receptors
These are dimeric membrane proteins (ETR1, ETR2, ERS1, ERS2, EIN4) heavily localized on the Endoplasmic Reticulum (ER) membrane. They critically contain a copper cofactor (Cu⁺) that serves as the binding site for ethylene gas.
CTR1 (Constitutive Triple Response 1)
A Raf-like Serine/Threonine protein kinase. When active (no ethylene), it continuously phosphorylates EIN2, actively blocking signal propagation.
EIN2 & EIN3
When ethylene incapacitates CTR1, EIN2 remains unphosphorylated. This unphosphorylated state allows a special protease to cleave the C-terminus fragment (EIN2-C). This fragment physically travels into the nucleus to stabilize the EIN3/EIL1 transcription factors, firmly turning on ethylene-responsive genes.

Plant Hormones - Other Phytohormones & Summary

Other Phytohormones

BRs · Strigolactones · Jasmonates · Summary

7. Other Important Classes of Phytohormones

Beyond the "classical five" hormones, plants rely on several other highly specialized lipid and sterol-derived signaling molecules to govern specialized development, fungal symbiosis, and intense defense mechanisms.

Brassinosteroids (BRs) Steroid Hormones Growth
Structure: Polyhydroxylated steroid hormones (with >70 identified forms; brassinolide is the most active). They are biochemically derived from plant sterols such as campesterol.
Perception: Uniquely among plant hormones, BRs are perceived directly on the extracellular surface by the plasma membrane receptor kinase BRI1 and its essential co-receptor BAK1.
Physiological Effects: BRs strongly promote general cell elongation, tracheary element (xylem) differentiation, pollen tube growth, and seed germination. Plant mutants lacking BR synthesis or perception exhibit severe, distinct dwarfism.
Strigolactones (SLs) Root Exudates Architecture
Structure: Carotenoid-derived terpenoid lactones that are primarily synthesized in the root system.
External Effects (Rhizosphere): SLs are actively exudated into the soil to stimulate hyphal branching in symbiotic arbuscular mycorrhizal fungi. However, parasitic weeds (like Striga and Orobanche) have evolved to detect these exudates and use them as a chemical signal to trigger their own seed germination.
Internal Effects: Endogenously, SLs act as powerful inhibitors of axillary bud outgrowth. They work downstream of auxin as the final messengers enforcing apical dominance.
Jasmonates (JAs) Lipid-Derived Defense
Structure: Oxylipins derived from the lipid α-linolenic acid via the octadecanoid pathway. Common active forms include jasmonic acid (JA), volatile methyl jasmonate (MeJA), and the highly active conjugate JA-isoleucine (JA-Ile).
Perception: Similar to Auxin and GA signaling, JA perception involves protein degradation. The COI1 F-box protein recruits JAZ repressors for destruction by the 26S proteasome in direct response to the binding of JA-Ile.
Physiological Effects: JAs are the key master mediators of plant defense against chewing herbivores (e.g., caterpillars) and necrotrophic pathogens. They trigger the massive systemic production of proteinase inhibitors (which ruin the herbivore's digestion) and toxic secondary metabolites.

Summary Comparison Matrix

A comprehensive high-level review of the primary plant hormones, their metabolic origins, dominant transport routes, and defining physiological roles.

Hormone Class	Primary Precursor	Primary Transport Mode	Key Physiological Roles
Auxins	Tryptophan / Indole	Polar (cell-to-cell) & Phloem	Cell elongation, apical dominance, adventitious root formation
Gibberellins (GAs)	IPP / GGPP (Isoprenoid)	Non-polar (Phloem / Xylem)	Internodal stem elongation, seed germination, flowering
Cytokinins (CKs)	DMAPP + AMP (Isoprenoid)	Xylem (upward) / Phloem (downward)	Cell division (cytokinesis), morphogenesis, senescence delay
Abscisic Acid (ABA)	Carotenoids / Neoxanthin	Xylem, Phloem, Parenchyma	Stomatal closure, seed dormancy, deep stress tolerance
Ethylene	Methionine / ACC	Gaseous Diffusion	Fruit ripening, triple response, organ abscission
Brassinosteroids	Campesterol (Sterol)	Local / Short-distance diffusion	Stem elongation, vascular (xylem) differentiation
Strigolactones	Carotenoids / Carlactone	Xylem (Acropetal / upward)	Axillary bud inhibition, arbuscular mycorrhizal symbiosis
Jasmonates	α-Linolenic Acid (Lipid)	Vascular / Systemic signaling	Defense against chewing herbivores & necrotrophic pathogens

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Plant Hormones and Growth Regulators

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