Table of Contents
Plant hormones (also called phytohormones) are chemical messengers that coordinate growth, development, and responses to the environment in plants. Unlike in animals, plant hormones often act over short distances within tissues, and each hormone can influence many different processes depending on its concentration, location, and the plant’s developmental stage.
This chapter focuses on the major classes of plant hormones, selected examples of their actions, and how they interact.
General Features of Plant Hormones
Plant hormones share some common properties:
- They are usually produced in very small amounts.
- They are often synthesized in one region (e.g., young leaves, root tips) and transported to another.
- The same hormone can have different, even opposite, effects in different organs or at different concentrations.
- Responses depend on:
- hormone concentration (dose)
- sensitivity of the tissue (receptors and signaling pathways)
- interactions with other hormones (hormonal balance)
Most plant hormones are small organic molecules rather than large proteins.
Auxins
Auxins are among the first discovered plant hormones. The main natural auxin is indole-3-acetic acid (IAA).
Main Sources and Transport
- Produced mainly in:
- shoot and root apical meristems (growing tips)
- young leaves
- developing seeds
- Transport:
- can move over long distances via the phloem
- also transported cell-to-cell in a polar (directional) manner
- in shoots: mostly from tip downward (basipetal transport)
- in roots: more complex but still polarized
Selected Functions of Auxins
- Cell elongation in shoots
- Promotes elongation of cells on the shaded side of a shoot during phototropism (bending toward light).
- Apical dominance
- High auxin concentration from the shoot tip suppresses growth of lateral buds.
- Removing the tip reduces auxin level and lateral buds can grow out.
- Root formation
- Low concentrations promote lateral root initiation.
- Used in horticulture as “rooting powders” to stimulate root growth in cuttings.
- Vascular tissue differentiation
- Influences formation and patterning of xylem and phloem.
- Fruit development
- Auxin produced by seeds can stimulate growth of surrounding fruit tissue.
Synthetic auxins (e.g., 2,4-D) are used as herbicides; broad-leaf plants are more sensitive and can be selectively killed.
Gibberellins
Gibberellins (GAs) are a large family of hormones; GA₁, GA₃ and others are active forms.
Main Sources
- Young leaves
- Developing seeds
- Root and shoot tips
Selected Functions of Gibberellins
- Stem and internode elongation
- Promote cell division and elongation, especially in stems and flower stalks.
- Deficiency often leads to dwarf phenotypes.
- Seed germination
- In many seeds, gibberellins:
- are produced upon imbibition (water uptake)
- stimulate enzymes in storage tissues (e.g., endosperm) that mobilize starch and other reserves.
- Flowering and fruit growth
- Can induce flowering in some long-day or biennial plants under non-inductive conditions.
- Promote enlargement of some fruits (e.g., seedless grapes in agriculture).
Gibberellin biosynthesis or action can be inhibited to produce more compact ornamental plants.
Cytokinins
Cytokinins are adenine-derivative hormones that promote cell division (cytokinesis).
Main Sources and Transport
- Mainly synthesized in root tips.
- Transported upward in the xylem to shoots and leaves.
- Also produced in developing fruits and seeds.
Selected Functions of Cytokinins
- Promotion of cell division
- Particularly evident in tissue cultures where cytokinins, together with auxins, are required for sustained growth.
- Shoot development
- In tissue culture:
- high cytokinin : auxin ratio → favors shoot formation
- low cytokinin : auxin ratio → favors root formation
- Delay of leaf senescence
- Maintain chlorophyll and photosynthetic activity; used experimentally to keep leaves “green” longer.
- Regulation of apical dominance
- Can counteract auxin-induced apical dominance by stimulating lateral bud growth.
Abscisic Acid (ABA)
Abscisic acid (ABA) plays a central role in stress responses and developmental “brakes”.
Main Sources
- Produced in many tissues:
- mature leaves
- roots
- seeds and fruits
- Levels increase under stress (especially drought, salinity, cold).
Selected Functions of ABA
- Stomatal closure
- Under water stress, ABA produced in roots and leaves promotes closure of stomata to reduce water loss.
- Induction and maintenance of seed dormancy
- High ABA levels in seeds prevent premature germination.
- Dormancy is broken when ABA declines or is counteracted by gibberellins.
- Stress responses
- Coordinates transcriptional and metabolic changes during drought, salt stress, and cold.
ABA often acts antagonistically to growth-promoting hormones like gibberellins.
Ethylene
Ethylene (C₂H₄) is a simple gaseous hormone.
Main Sources
- Produced in many tissues:
- ripening fruits
- senescing leaves and flowers
- mechanically stressed tissues (e.g., bending, wounding)
- Can diffuse through air, allowing local “communication” between organs and even between plants in close proximity.
Selected Functions of Ethylene
- Fruit ripening
- Enhances softening, color change, and aroma production in climacteric fruits (e.g., bananas, tomatoes, apples).
- A small amount of ethylene can trigger autocatalytic production (positive feedback).
- Leaf and flower senescence and abscission
- Promotes aging and shedding of leaves, petals, and fruits.
- Responses to mechanical stress (thigmomorphogenesis)
- High ethylene can cause shorter, thicker stems, helping plants withstand wind or obstacles.
- Triple response in seedlings
- In etiolated seedlings: inhibited elongation, thickened stem, and horizontal growth to help circumvent physical barriers.
Ethylene production and perception are important targets in post-harvest storage and transport of fruits.
Brassinosteroids
Brassinosteroids are steroid-like plant hormones structurally related to animal steroid hormones but with distinct receptors and signaling pathways.
Main Sources
- Widespread in plant tissues:
- pollen
- young seeds
- growing shoots
Selected Functions of Brassinosteroids
- Cell expansion and division
- Promote elongation and expansion, particularly in young tissues.
- Vascular differentiation
- Involved in proper development of xylem and phloem.
- Photomorphogenesis
- Influence seedling development in light and dark conditions.
- Stress tolerance
- Can enhance resistance to some environmental stresses.
Mutants defective in brassinosteroid synthesis or signaling are often severely dwarfed with altered leaf morphology.
Jasmonates and Salicylates
Jasmonates (e.g., jasmonic acid, JA) and salicylic acid (SA) are key signaling molecules in plant defense.
Jasmonates
- Sources and signaling
- Produced in response to wounding and herbivore attack.
- Can move locally and systemically to signal damage.
- Selected functions
- Induce defense gene expression (e.g., proteinase inhibitors that reduce digestibility to herbivores).
- Regulate aspects of growth, fertility, and senescence.
- Mediate some plant–plant interactions and responses to insect-derived signals.
Salicylic Acid
- Sources and signaling
- Synthesized in infected tissues.
- Can be transported and converted into volatile derivatives (e.g., methyl salicylate) that spread through air.
- Selected functions
- Essential for systemic acquired resistance (SAR) against pathogens.
- Induces expression of pathogenesis-related (PR) proteins.
- In some cases, contributes to local cell death around infection sites (hypersensitive response).
Jasmonate and salicylate pathways often interact antagonistically, fine-tuning defense strategies against different types of attackers (e.g., chewing insects vs. biotrophic pathogens).
Strigolactones
Strigolactones are carotenoid-derived hormones with dual roles in development and belowground interactions.
Main Sources
- Mainly produced in roots.
- Can be exuded into the soil and transported upward in the plant.
Selected Functions
- Regulation of shoot branching
- Inhibit outgrowth of axillary buds; work together with auxin and cytokinins to control branch architecture.
- Rhizosphere signaling
- Released from roots to:
- stimulate germination of certain parasitic plants (e.g., Striga)
- attract arbuscular mycorrhizal fungi and promote symbiosis.
Balancing strigolactone levels is important for optimizing shoot architecture and symbiotic associations while limiting parasitic plant infection.
Peptide and Other Hormone-Like Signals
Beyond classical hormones, plants use many peptide signals and small molecules acting in a hormone-like manner.
Peptide Hormones
- Short polypeptides secreted by cells, perceived by receptor kinases at the cell surface.
- Involved in:
- meristem maintenance (e.g., CLAVATA peptides)
- regulation of stomatal density
- coordination of nutrient status between root and shoot.
Other Signaling Molecules
- Nitric oxide (NO) and reactive oxygen species (ROS) participate in stress and developmental signaling.
- Polyamines, melatonin, and others are proposed to have hormone-like roles, though their status as “classic” hormones is still debated.
These signals extend the range and specificity of plant internal communication.
Hormone Interactions (Crosstalk)
Plant responses nearly always depend on combinations of hormones rather than a single one acting alone. Important patterns include:
- Synergistic interactions
- Auxins and cytokinins together promote strong cell division and organ formation.
- Auxins and gibberellins can both enhance stem elongation.
- Antagonistic interactions
- ABA vs. gibberellins in seed dormancy and germination.
- Auxins vs. cytokinins and strigolactones in shaping shoot branching.
- JA vs. SA in defense signaling against different types of attackers.
- Context dependence
- The same hormone can promote growth in one tissue while inhibiting it in another, depending on receptor types, downstream signaling components, and external conditions.
Understanding these interactions is essential for interpreting complex plant phenomena such as tropisms, stress acclimation, and developmental patterning.
Practical Applications of Plant Hormones
Plant hormones are widely used in agriculture and horticulture. Examples include:
- Auxins
- Rooting powders for cuttings.
- Control of fruit set and prevention of premature fruit drop.
- Gibberellins
- Increasing fruit size (e.g., in seedless grapes).
- Overcoming seed or bud dormancy in some crops.
- Cytokinins
- Micropropagation (tissue culture) for mass plant multiplication.
- Delaying senescence in cut flowers and leafy vegetables.
- Ethylene and ethylene inhibitors
- Controlled fruit ripening during storage and transport.
- Inhibitors (e.g., 1-MCP) to delay ripening and senescence.
- Growth retardants (gibberellin inhibitors)
- Producing compact ornamental plants less prone to lodging.
By manipulating hormone levels or responses, humans can modify plant growth, yield, quality, and stress tolerance in targeted ways.