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Plant Hormones

How a Plant Coordinates Its Body Without Hormone Glands

Big Question: How can a plant coordinate growth, dormancy, water conservation, ripening, defence and reproduction across many organs when it has no brain and no central endocrine glands?

Quick Answer

Plants use multiple interacting signalling systems. Phytohormones are endogenous signalling molecules that can act at very low concentrations. They may be produced in many tissues, transported locally or over long distances, perceived by specific receptors and translated into changes in gene expression, ion transport, protein stability, metabolism, cell division, cell expansion or differentiation.

Important hormone groups include:

• auxins;
• cytokinins;
• gibberellins;
• abscisic acid;
• ethylene;
• jasmonates;
• salicylic acid;
• brassinosteroids;
• strigolactones.

Other peptide and small-molecule signals also coordinate plant development. The list is not a complete catalogue of every signalling compound.

signal production → transport or local action → receptor → signalling pathway → changed cell behaviour → changed plant outcome.

What You Will Learn

• Why plant hormones are not equivalent to human endocrine hormones.
• How auxin transport helps coordinate growth and organ formation.
• How cytokinins influence cell division, meristems and nutrient responses.
• How gibberellins promote growth and seed-germination programmes.
• How ABA coordinates drought responses and seed dormancy.
• How ethylene acts as a gaseous signal in ripening, senescence and stress.
• How jasmonate and salicylic-acid pathways coordinate defence.
• How brassinosteroids regulate growth and development.
• How strigolactones influence branching and below-ground interactions.
• Why hormone crosstalk matters more than memorising isolated functions.
• How receptors turn small molecules into large developmental responses.
• How reporters, mutants, grafts, chromatography and mass spectrometry reveal hormone action.

Part 1 — What Makes Something a Plant Hormone?

A phytohormone is an endogenous signalling molecule that influences physiological or developmental processes at low concentrations.

Unlike a nutrient, its main role is not to provide carbon, nitrogen or energy. Unlike a structural polymer, its main role is not to build tissue.

Its effect depends on cells being able to perceive it and respond.

The same molecule can produce different outcomes in different tissues because receptor abundance, signalling proteins, gene state and developmental context differ.

Part 2 — Plants Do Not Have Hormone Glands

Animal endocrine systems often emphasise specialised glands releasing hormones into circulation.

Plants are more distributed.

• auxin can be synthesised in young tissues and transported directionally cell to cell;
• cytokinins can be synthesised in roots and shoots and move through vascular tissues;
• ABA can be synthesised in multiple tissues and its levels rise rapidly during water stress;
• ethylene is a gas that can diffuse through tissues and air spaces;
• jasmonates can arise rapidly after wounding;
• strigolactones can be made in roots and other tissues and participate in both internal and external signalling.

The plant body is therefore both source, pathway and receiver depending on the signal.

Part 3 — Auxin: Direction Matters

The principal naturally occurring auxin in many plants is indole-3-acetic acid, or IAA.

Auxin influences cell expansion, organ initiation, vascular development, root architecture, apical dominance, tropisms and many other processes.

One of auxin’s most distinctive features is polar transport. PIN-FORMED efflux carriers can become asymmetrically positioned in cell membranes, creating directional auxin movement through tissues.

That lets a plant build spatial information from a chemical signal.

where auxin accumulates can matter as much as how much auxin exists.

Part 4 — How Auxin Changes Gene Expression

In the canonical nuclear auxin pathway, auxin promotes interaction between TIR1/AFB receptor proteins and AUX/IAA transcriptional repressors.

The repressors are ubiquitinated and degraded by the proteasome. ARF transcription factors are then released from repression and can alter expression of auxin-responsive genes.

auxin → receptor complex → repressor destruction → transcriptional response.

This is a recurring signalling strategy in plants: a hormone can work by changing the stability of a regulatory protein rather than by directly “turning on” a growth process.

Part 5 — Phototropism: Perception and Growth Happen in Different Places

Directional blue light is perceived by phototropin receptors. Signalling changes auxin distribution so that cells on one side of a young shoot can elongate differently from cells on the other side.

The result is curvature toward light.

The critical idea is not “auxin makes plants bend.” It is:

directional perception → asymmetric signalling → differential growth → curvature.

Part 6 — Auxin and Apical Dominance

Growing shoot tips influence whether axillary buds remain suppressed or begin growing.

Auxin produced near the shoot apex moves basipetally through the stem and interacts with strigolactones, cytokinins, sugars and local bud networks.

Removing the shoot tip can change these relationships and release some buds.

“Auxin directly enters every bud and switches it off” is therefore too simple. Apical dominance is a network phenomenon.

Part 7 — Cytokinins: Growth, Meristems and Nutrient Information

Cytokinins influence cell division, shoot meristem activity, branching, leaf ageing, nutrient responses and root–shoot communication.

Their canonical receptors are histidine-kinase proteins. Signalling uses a multistep phosphorelay related to bacterial two-component systems.

cytokinin → histidine-kinase receptor → phosphorelay → response regulators → changed gene expression.

Auxin and cytokinin often interact antagonistically or cooperatively depending on tissue. Their relative activities help shape root and shoot meristems, vascular development and organ formation.

Part 8 — Gibberellins: Growth by Removing a Brake

Gibberellins, or GAs, promote processes including stem elongation, seed germination, flowering and fruit growth in many plants.

A powerful way to understand GA signalling is through DELLA proteins, which act as growth repressors.

GA binds the GID1 receptor. The GA–GID1 complex promotes recognition and destruction of DELLA repressors.

GA rises → DELLA brake removed → growth programmes become easier to execute.

This “remove a repressor” logic resembles auxin signalling even though the molecular components differ.

Part 9 — Gibberellins and the Green Revolution

Some famous semi-dwarf crop varieties carry changes affecting gibberellin production or response. Shorter stems can reduce lodging—the collapse of cereal stems under wind, rain or heavy grain heads—while allowing more resources to support grain production.

This is a powerful example of developmental biology becoming agriculture:

hormone pathway → stem architecture → lodging risk → harvest.

But dwarfing is not automatically beneficial in every environment. Architecture, water access, disease, soil fertility and farming system all matter.

Part 10 — Abscisic Acid: A Signal of Water Status and Development

Abscisic acid, or ABA, is central to drought responses and seed dormancy, among other processes.

When plant water status declines, ABA can accumulate and promote stomatal closure. In guard cells, ABA signalling activates ion-channel changes that reduce guard-cell turgor and close stomatal pores.

This reduces water loss but also restricts carbon dioxide entry.

save water now ↔ limit carbon gain now.

Part 11 — ABA Signalling: Release the Kinase

In the canonical ABA pathway, PYR/PYL/RCAR receptors bind ABA and inhibit PP2C phosphatases.

When PP2Cs are inhibited, SnRK2 protein kinases can become active and phosphorylate downstream targets, including transcription factors and ion-channel regulators.

ABA → receptor → PP2C inhibited → SnRK2 released → stress-response machinery.

Again, the signal works partly by changing which proteins are allowed to restrain other proteins.

Part 12 — ABA and Seeds: Knowing When Not to Grow

Growth is not always the correct response.

ABA contributes to seed maturation and dormancy. Gibberellins often promote germination-related processes.

The balance between ABA and GA signalling helps determine whether a seed remains protected and inactive or begins the irreversible transition into seedling growth.

This is a better model than “ABA stops growth, GA starts growth.” Their interactions depend on developmental and environmental context.

Part 13 — Ethylene: A Hormone That Is a Gas

Ethylene is a small hydrocarbon gas, C₂H₄.

Because it is gaseous, it can diffuse through intercellular spaces and surrounding air. Plants produce ethylene during many processes, including fruit ripening, senescence, flooding responses, mechanical stress and defence.

Some fruits are climacteric: ethylene production rises strongly during ripening and can stimulate further ethylene production.

one ripening fruit can change the chemical atmosphere around another fruit.

Part 14 — Ethylene Receptors Work Backwards From What You Might Expect

Ethylene receptors such as ETR1 act as negative regulators when ethylene is absent. Through CTR1 and downstream components, the pathway suppresses ethylene responses.

When ethylene binds, that suppression is relieved, allowing EIN2/EIN3-associated signalling to activate ethylene-response genes.

So a receptor can control a response by keeping it off until the ligand appears.

Part 15 — Jasmonates: Wounding, Herbivory and Defence Reprogramming

Jasmonates rise rapidly after tissue damage and participate in defence against many herbivores and pathogens.

The bioactive jasmonate signal JA-Ile binds a receptor complex containing COI1. This promotes degradation of JAZ repressor proteins, releasing transcription factors that activate defence programmes.

wound → jasmonate → JAZ repressors removed → defence genes activated.

Defence has costs. Carbon and nitrogen invested in defensive compounds cannot simultaneously support maximum growth. Hormone networks help manage this growth–defence tradeoff.

Part 16 — Salicylic Acid: Defence and Systemic Information

Salicylic acid is especially important in defence against many biotrophic and hemibiotrophic pathogens and contributes to systemic acquired resistance.

The NPR1 protein is a central regulator of salicylic-acid-responsive gene expression.

Jasmonate and salicylic-acid pathways can antagonise or cooperate depending on pathogen lifestyle, timing and tissue context.

“SA fights pathogens; JA fights insects” is therefore only a first approximation.

Part 17 — Brassinosteroids: Steroid Signals in Plants

Brassinosteroids are steroid hormones that regulate cell expansion, vascular development, reproductive development, light responses and stress physiology.

They are perceived by the membrane receptor kinase BRI1, which works with co-receptors including BAK1.

Signalling alters the activity of transcription factors such as BZR1 and BES1.

This differs from auxin and GA receptors, showing that plant hormones can use fundamentally different receptor architectures while converging on growth and gene expression.

Part 18 — Strigolactones: Branching and an Underground Conversation

Strigolactones influence shoot branching, root development and nutrient responses.

They also have an extraordinary role outside the plant: roots release strigolactones into soil, where they can stimulate branching of arbuscular mycorrhizal fungal hyphae.

Parasitic plants such as Striga can exploit the same signals to detect a nearby host and trigger germination.

a hormone-like signal inside the plant can also become an ecological message outside it.

Part 19 — Hormone Crosstalk: The Outcome Belongs to the Network

Hormones rarely act in isolation.

• auxin and cytokinin interact in meristem maintenance and organ formation;
• ABA and GA often oppose one another during dormancy and germination;
• auxin, cytokinin and strigolactones interact in branching;
• ethylene interacts with auxin in roots and tropisms;
• jasmonate and salicylic-acid pathways reshape defence priorities;
• brassinosteroids interact with auxin and gibberellin during growth;
• sugar/TOR signalling feeds back into multiple hormone pathways.

A hormone concentration therefore means little without knowing the receiving tissue and its other active signals.

Explore the 2026 state-of-the-art review of hormone–metabolism crosstalk →

Part 20 — Hormones Can Move in Very Different Ways

Signal	Example movement logic
Auxin	Polar cell-to-cell transport plus vascular and local routes.
Cytokinin	Xylem/phloem and local movement depending on molecular form and source.
ABA	Local synthesis plus vascular transport; rapid redistribution during water stress.
Ethylene	Gas diffusion through tissues and atmosphere.
Jasmonates	Local synthesis and mobile wound signals with vascular/systemic components.
Strigolactones	Internal signalling plus root exudation into soil.

“Hormones travel through the bloodstream” is therefore not a useful plant analogy.

Part 21 — Hormone Concentration Is Not the Same as Hormone Response

A cell can contain a hormone but respond weakly if receptor levels are low or downstream repressors are active.

Another cell can respond strongly to a smaller hormone change if its signalling network is sensitised.

Modern biosensors have shown that hormone concentration and transcriptional response can even have different spatial and temporal patterns.

signal abundance ≠ signal interpretation.

Part 22 — Hormone Metabolism Is Part of Signalling

Plants regulate hormones by controlling:

• biosynthesis;
• activation from inactive precursors;
• conjugation;
• degradation;
• transport;
• sequestration;
• receptor abundance;
• downstream signalling components.

A hormone pathway is therefore not merely “make molecule → response.” Turnover and sensitivity are equally important.

Part 23 — A Plant Response Is Usually Multi-Layered

Consider drought:

1. soil water availability falls;
2. root and leaf water status changes;
3. ABA biosynthesis and transport change;
4. guard-cell ABA signalling alters ion channels;
5. stomata close;
6. carbon dioxide entry falls;
7. photosynthesis slows;
8. sugar status changes;
9. growth programmes are rebalanced;
10. root architecture and gene expression may change;
11. other hormones interact with the ABA response.

“ABA closes stomata” is correct at one resolution. The full plant response is a network of water, carbon, ions, genes and interacting signals.

Part 24 — Ripening Is Not Just Ethylene

In climacteric fruits, ethylene is a major driver of coordinated ripening changes such as softening, pigment changes, aroma production and altered sugar/acid balance.

But ripening also depends on developmental state, transcription factors, other hormones and metabolism.

Non-climacteric fruits do not use the same strong ethylene-autocatalytic programme.

So placing every fruit into one “ethylene makes fruit ripe” rule creates immediate errors.

Part 25 — How a Signal Becomes a Different Body Shape

Hormones do not sculpt leaves and roots directly.

They change cell behaviour. Those local behaviours accumulate into organ shape.

• change cell division rate;
• change cell expansion;
• change wall properties;
• change differentiation;
• change transporter position;
• change gene expression;
• change ion flux;
• change metabolism.

Development emerges when thousands or millions of cells execute these local responses in spatial patterns.

How Scientists Discover Hormone Pathways

• Classic bioassays: measure bending, elongation, germination or other responses to extracts or purified compounds.
• Genetic mutants: reveal plants that cannot make, perceive or respond to a hormone.
• Grafting: separates local production from long-distance signalling.
• Radiotracers: follow movement of labelled hormones.
• LC–MS/MS: quantifies extremely small concentrations of hormones and metabolites.
• Fluorescent reporters: show where hormone responses occur in living tissues.
• Biosensors: estimate hormone concentration or signalling dynamics at cellular resolution.
• Protein interaction assays: reveal receptor–ligand and repressor interactions.
• Structural biology: resolves how hormones fit receptor pockets and alter protein complexes.
• Single-cell and spatial omics: show how neighbouring cells differ in signalling state.

No single method proves the whole network. Confidence comes from converging evidence.

Observation vs Inference

A student sprays an auxin-like compound on one group of cuttings and observes more roots.

• Observation: treated cuttings produced more roots under those conditions.
• Inference: auxin signalling may have promoted adventitious-root initiation.
• Not proven: natural auxin concentration in untreated plants was necessarily limiting.
• Better test: use concentration series, untreated controls, receptor or transport mutants, hormone measurements and repeated experiments.

A hormone application experiment shows what an added signal can do. It does not automatically reveal how the intact plant normally regulates the pathway.

Common Misconceptions and Better Models

Misconception	Better model
Auxin is the growth hormone.	Auxin regulates many developmental processes, and growth outcomes depend on tissue and interacting signals.
ABA is the stress hormone.	ABA has major drought and dormancy roles but also participates in normal development and interacts with many pathways.
Ethylene only ripens fruit.	Ethylene also affects senescence, flooding, mechanical stress, defence and development.
Each hormone has one job.	Hormone networks are pleiotropic and context-dependent.
Plants have endocrine glands like animals.	Hormone production and perception are distributed across tissues.
More hormone means stronger response.	Responses can be non-linear and depend on receptors, metabolism, transport and other signals.
Hormones are nutrients.	They are signalling molecules, usually effective at low concentrations.
Hormones command passive cells.	Cells integrate hormones with metabolic, mechanical and environmental information.
A hormone molecule must travel far to matter.	Many hormone effects are highly local; long-distance coordination uses multiple routes.
Plant hormone pathways are finished science.	Receptors are well characterised for many hormones, but network dynamics and context remain active research areas.

A Text Map of the Network

ENVIRONMENT + DEVELOPMENT + METABOLISM
              ↓
      hormone biosynthesis
              ↓
 transport / diffusion / local action
              ↓
           RECEPTOR
              ↓
 protein stability / phosphorylation /
 ion channels / transcription factors
              ↓
         CELL RESPONSE
              ↓
 division / expansion / differentiation /
 stomatal movement / defence / dormancy
              ↓
        WHOLE-PLANT OUTCOME
              ↑
       metabolic feedback

No single hormone acts alone.

Primary Science / PSLE Bridge

Primary learners do not need nine signalling pathways.

Use three transferable ideas:

• plants can sense changes in their environment;
• one part of a plant can affect how another part grows or responds;
• chemical signals help coordinate these responses even though the plant has no brain.

Then choose one concrete example—shoot bending toward light, fruit ripening or stomatal closure—and build the mechanism only as far as the learner needs.

Go Beyond Primary Science

Simple idea	Higher-resolution model
Auxin changes growth	TIR1/AFB perception, AUX/IAA degradation, ARF transcription, polar transport and tissue mechanics.
ABA closes stomata	PYL receptors, PP2C inhibition, SnRK2 activation, ion-channel regulation and guard-cell water loss.
GA promotes growth	GID1 perception removes DELLA repressors and reconfigures transcriptional networks.
Ethylene ripens fruit	Negative-regulator receptors feed through CTR1/EIN2/EIN3 and autocatalytic production in climacteric fruits.
Plants defend themselves	JA, SA, ethylene and other pathways prioritise responses according to attacker and tissue context.
Hormones interact	Cross-regulation occurs at biosynthesis, transport, receptors, protein stability, transcription and metabolism.

Deep Science Window — Several Plant Hormone Receptors Work by Destroying Repressors

Auxin, gibberellin and jasmonate pathways use different receptor complexes but share a striking systems logic: hormone perception promotes removal of proteins that suppress the response.

• auxin → AUX/IAA repressors degraded;
• GA → DELLA repressors degraded;
• jasmonate → JAZ repressors degraded.

That architecture allows rapid switching because the cell already contains transcriptional machinery that can act once the brake is removed.

Deep Science Window — Spatial Information Can Be Built From Transport

Auxin is especially important because directional membrane placement of transport proteins can create local maxima, minima and flux patterns.

A chemical concentration field can therefore encode position. Cells read not just “how much hormone?” but “where am I inside the pattern?”.

This helps explain how new leaves, roots and vascular strands arise at specific places rather than everywhere.

Deep Science Window — A Plant Is Not a Hierarchy With One Boss

No single hormone is the plant’s master controller. No single organ acts like a brain issuing all commands.

Coordination emerges from many interacting local and long-distance loops involving hormones, peptides, electrical signals, calcium, reactive oxygen species, sugars, hydraulic changes and gene networks.

The existing A Plant Has No Brain | So How Does It Sense and Respond? page owns that broader sensing-and-response architecture. This manual owns the hormone layer inside it.

Deep Science Window — Hormone Networks Can Be Reprogrammed by Metabolic State

Plants cannot commit to growth without checking energy and carbon availability.

Sugar signalling and TOR kinase activity interact with auxin, ABA, ethylene and strigolactone pathways. Hormones also change primary metabolism.

This means development is constrained by what the plant can actually afford.

signal says “grow” + metabolism says “no resources” = the network must resolve the conflict.

Real, Simplified or Wrong?

REAL	Plants produce endogenous hormone signals and possess specific receptors and signalling pathways.
REAL	Ethylene is a gaseous plant hormone.
REAL	Auxin, cytokinin, GA, ABA and other hormones interact extensively.
USEFUL SIMPLIFICATION	“Auxin promotes growth” can introduce the topic if immediately bounded by tissue and context.
WRONG	Every hormone has one exclusive job.
WRONG	A plant has a central hormone gland controlling the rest of the body.
OPEN RESEARCH	How entire hormone, metabolic, mechanical and environmental networks are integrated quantitatively across whole plants.

Evidence Boundaries

• Hormone application ≠ natural function. Exogenous doses can create responses outside normal physiological ranges.
• Reporter signal ≠ hormone concentration. Many reporters measure downstream response rather than molecule abundance.
• Hormone abundance ≠ biological effect. Receptor and tissue state matter.
• One mutant ≠ one-function gene. Developmental compensation and pleiotropy can complicate interpretation.
• Auxin gradient ≠ full explanation of tropism. Photoreceptors, transporters, mechanics and tissue competence are also required.
• Ethylene ≠ universal ripening switch. Fruit types differ.
• ABA ≠ only drought. It also regulates seeds and other developmental processes.
• Defence hormones ≠ simple enemies. JA and SA can antagonise or cooperate depending on context.
• Phytohormone list ≠ complete signalling system. Peptides, calcium, ROS, electrical and hydraulic signals also matter.

Explore Elsewhere

• Wikipedia — Plant Hormone → broad overview of hormone groups.
• Wikipedia — Auxin → transport, signalling and development.
• Wikipedia — Abscisic Acid → drought and seed responses.
• Wikipedia — Ethylene as a Plant Hormone → gaseous signalling and ripening.
• National Geographic — Photosynthesis → connect hormonal growth decisions back to carbon supply.

Checkpoint Questions

1. What is a phytohormone?
2. Why is the human endocrine-gland analogy limited?
3. What makes auxin transport spatially informative?
4. How does the TIR1/AFB pathway alter transcription?
5. Why does phototropism require both perception and differential growth?
6. What are some cytokinin functions?
7. How does gibberellin signalling remove DELLA repression?
8. Why can ABA close stomata?
9. How does the ABA receptor–PP2C–SnRK2 pathway work?
10. Why can ABA and GA oppose one another in seeds?
11. What makes ethylene unusual among hormones?
12. What happens to JAZ proteins after jasmonate perception?
13. What is one role of salicylic acid?
14. How are brassinosteroids perceived?
15. Why are strigolactones both developmental and ecological signals?
16. What does hormone crosstalk mean?
17. Why is hormone concentration not identical to hormone response?
18. What experiment could distinguish local hormone action from a mobile signal?

Can You Explain WHY?

• Why can two tissues respond differently to the same hormone concentration?
• Why is destroying a repressor an effective signalling mechanism?
• Why does a plant need both growth-promoting and growth-restraining signals?
• Why can pruning change branching without adding any hormone from outside?
• Why might drought alter growth even in tissues that are not visibly wilted?
• Why can a molecule released by roots function both as an internal hormone and an ecological signal?
• Why is “plants have hormones” true while “plants have an endocrine system like ours” is misleading?

Manual Summary

Plant hormones are distributed chemical signals embedded inside larger networks. They are synthesised, transported, perceived, degraded and interpreted in tissue-specific ways. Their pathways change protein stability, phosphorylation, ion transport, transcription and metabolism. The resulting cellular changes accumulate into growth, dormancy, stomatal movement, ripening, branching and defence.

not one hormone → one job, but many signals → one changing plant.

Where to Go Next

• eduKate Learning Manual: Plant Growth & Meristems
• eduKate Learning Manual: Water Transport
• eduKate Learning Manual: Phloem Transport
• A Plant Has No Brain | So How Does It Sense and Respond?
• eduKate Learning Manual: Seed Dormancy

Research Sources and Further Reading

• Plant Physiology (2026) — Bidirectional Crosstalk Between Plant Hormone Signaling and Metabolism
• Journal of Experimental Botany — Understanding Phototropism: From Darwin to Today
• Journal of Experimental Botany — Auxin Signaling: A Big Question to Be Addressed by Small Molecules
• Journal of Experimental Botany — Spatial Regulation of Plant Hormone Action
• Plant Physiology — Hormonal Impact on Photosynthesis and Photoprotection
• Plant Physiology — Mechanisms of Cross Talk Between Gibberellin and Other Hormones
• Wikipedia — Plant Hormone

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