Comprehensive notes on plant hormones, tropisms, photoperiodism, and defense signaling
Growth-promoting vs growth-inhibiting hormones (overview)
Plant hormones can have opposite or tissue-specific effects: the same hormone may cause different outcomes in different tissues or at different hormone concentrations.
Growth-promoting hormones: auxin, gibberellins (GA), cytokinins. They can have subtle or varied effects depending on tissue context and hormone balance.
Growth-inhibiting hormones: abscisic acid (ABA), ethylene, often associated with stress responses, dormancy, and senescence.
The balance among growth-promoting and growth-inhibiting hormones determines developmental outcomes (bud break, dormancy, flowering, senescence).
Hormone action is not highly specific to one target; rather, a tissue can respond to different hormone levels to produce different results.
Dormancy, bud break, and the ABA/GA/cytokinin balance
Bud dormancy is promoted by higher ABA (abscisic acid) and reduced cytokinin, with possible involvement of reduced GA (gibberellin) synthesis.
Higher ABA (growth-inhibiting) helps buds resist winter damage (frost, desiccation) by delaying growth when conditions are harsh.
Bud opening occurs when GA is synthesized and ABA levels drop, while cytokinin is promoted to support growth.
Mechanism example: in winter, increased ABA and reduced cytokinins lead to dormancy; in spring, GA increases and ABA decreases, promoting bud break and flowering in combination with cytokinin.
ABA (ABA, often called a stress hormone) has multiple roles beyond dormancy, including stress signaling and various protective effects.
Seed germination, dormancy, and maternal fruit signaling
In developing seeds inside fruit, the plant prevents premature germination to avoid seedling death inside the fruit by manipulating GA and ABA levels.
Early in seed development (young seed): GA (gibberellin) is produced to promote germination, ABA is low, enabling embryo activation and possible germination when conditions are right.
As seeds mature inside the fruit: ABA levels rise and GA synthesis decreases, suppressing germination inside the fruit and preserving resources for dispersal.
Toward seed dispersal (late maturation): ABA declines and GA is kept low, so the seed remains dormant until it exits the fruit and imbibes water. If external conditions are favorable after dispersal, ABA may continue to fall and GA can rise to permit germination.
If seeds germinate inside the fruit, this leads to viviparity (germination within the mother plant), a phenomenon observed in some mutants that cannot maintain ABA levels or ABA signaling.
Imbibition experiment example (mung bean lab): soaking seeds in water activates the embryo and stimulates GA production, leading to germination when ABA is not high enough to block it and when ABA is reduced after imbibition.
In drought or dry conditions, seeds may remain dormant in the soil until moisture returns, illustrating a natural drought-avoidance strategy.
General points: balance ABA/GA and, in some cases, cytokinins, determines dormancy vs germination; environmental cues (water availability, temperature, light) influence this hormonal balance.
ABA as a stress and dormancy hormone (drought response)
ABA is produced in response to drought and other stresses; it triggers stomatal closure to reduce water loss and helps the plant endure stress.
Example experiment: split-root pot where one side is kept wet and the other dry. The plant closes stomata on the whole plant due to ABA signaling produced by roots experiencing drought signals, even though some roots have ample water. This demonstrates ABA’s role as a long-distance drought signal.
Stomata are tiny openings on leaf surfaces that control gas exchange and water loss; ABA-induced stomatal closure helps prevent dehydration during drought.
Practical insight: plants lacking ABA signaling fail to close stomata efficiently under drought, leading to higher water loss and possible death.
Other growth-inhibiting hormones and fruit/leaf aging processes
Ethylene is a key aging and ripening hormone:
In climacteric fruits (e.g., bananas), ethylene promotes ripening; mature, green bananas can be kept green by packaging in oxygen-poor environments, which suppresses ethylene production and signaling.
Ethylene also promotes leaf senescence and abscission (loss of leaves) via an abscission layer formation, often accompanied by increased ethylene and ABA signaling in the aging tissues.
Mutants that cannot produce ethylene stay green longer, illustrating ethylene’s role in ripening.
Some fruit ripening may be manipulated with controlled ethylene exposure (or inhibition) in commerce. Acetylene exposure in some contexts can accelerate ripening, though it can be hazardous.
Oxygen availability interacts with ethylene production in fruits; evolving ripening strategies rely on gas exchange and hormone signaling.
Some fruits, like strawberries, do not rely primarily on ethylene for ripening; instead, other hormones and signals (e.g., auxin signaling from seeds) modulate ripening processes.
Auxin (IAA) in growth, tropisms, and plant architecture
Auxin is central to growth promotion in shoots and to differential growth leading to tropisms.
Phototropism (growth toward light):
Classic Darwin/Darwin experiments with canary grass coleoptiles showed bending toward light requires signals from the tip; removing the tip or covering it blocks bending, indicating the tip senses light and produces a signal (auxin) that moves down the stem.
The signal (auxin) distributes to the shaded side, causing cells on that side to elongate, while cells on the lit side elongate less, bending the shoot toward light.
Monochromatic light experiments reveal blue light is the trigger detected by blue light photoreceptors in the shoot tip, initiating the auxin-driven response.
Auxin distribution under directional light becomes asymmetric: more auxin on the shaded side, leading to differential growth and bending toward the light source.
When the plant is tilted or the pot is turned, the same principle applies: gravity and light influence auxin distribution, reorienting growth.
Gravitropism (growth in response to gravity):
Roots show positive gravitropism (grow toward gravity); shoots show negative gravitropism (grow away from gravity).
In roots, auxin accumulation on the lower side inhibits elongation, causing the root to curve downward toward gravity; in shoots, auxin accumulation on the lower side promotes cell elongation, causing bending upward away from gravity.
Gravity perception involves amyloplasts (statoliths) in tip cells; these starch-containing organelles settle to the bottom under gravity, guiding the redistribution of auxin by signaling pathways to the lower side, reinforcing the curvature toward gravity.
Amyloplast repositioning provides the gravity cue that directs auxin redistribution and subsequent differential growth in roots.
Thigmotropism (response to touch):
Plants respond to touch with growth changes; tendrils and climbing vines lean and wrap around supports. This involves signaling hormones such as jasmonic acid and auxin and physical growth changes on the side away from touch.
Interplay of auxin with other hormones:
Auxin can promote elongation in shoots and inhibit elongation in roots depending on tissue context and concentration.
Auxin distribution interacts with light signaling and blue light photoreceptors to guide phototropism and gravitropism.
Tropism concepts, tests, and historical context
Tropisms are directional growth responses (growth is often not reversible once it commits; though some aspects can be adjusted briefly).
Positive tropisms: growth toward a stimulus (e.g., phototropism toward light, gravitropism of roots toward gravity).
Negative tropisms: growth away from a stimulus (e.g., shoots away from gravity or toward light in certain contexts).
Hints from classic experiments:
Charles Darwin and son demonstrated tip-sensing in phototropism, establishing the role of the shoot tip in light perception and signal generation that dictates bending.
Early work led to the concept of a signal (later identified as auxin) produced at the tip and transported downwards to effect bending.
Blue light photoreceptors are involved in perceiving blue wavelengths; the response is strongest under blue light, guiding phototropism.
Gibberellins, cytokinins, and pollen/seed development interplay
Gibberellins (GA) act as growth-promoting hormones involved in seed germination and bud break; GA production promotes growth when ABA is low and can coordinate with cytokinins to stimulate growth in buds and other tissues.
Cytokinins promote cell division and shoot growth, often working with GA to promote bud opening and plant growth; cytokinins can counter ABA effects in some contexts and help with tissue differentiation.
The balance among GA, cytokinin, and ABA controls dormancy vs growth, seed germination readiness, and stress responses.
Ethylene, cutting-edge signaling, and defense interactions
Ethylene regulates fruit ripening, leaf senescence, and abscission; interplay with oxygen levels influences ethylene synthesis and signaling in tissues.
Some plants also use jasmonic acid and salicylic acid in defense signaling, with salicylic acid derivatives functioning similarly to aspirin in mammalian systems. These signals contribute to local defense responses and systemic acquired resistance (SAR).
Salicylic acid (a signal related to acetylsalicylic acid, aspirin) is produced in response to pathogen attack and can spread systemically to prime defense in uninfected tissues.
Jasmonic acid participates in local defense signaling and can interact with auxin and ethylene pathways during defense responses (including responses to insect herbivory).
Defense signaling and systemic acquired resistance (SAR)
When pathogens attack, plants can form localized hypersensitive responses (HR) that create a dead cell zone around infection to stop spread.
Salicylic acid is involved in signaling to neighboring tissues to activate defensive programs; this signal can disseminate through the plant to establish SAR, a permament state of enhanced resistance in exposed tissues.
Jasmonic acid also participates in defense signaling, often in response to insect herbivory and necrotrophic pathogens; cross-talk between salicylic acid and jasmonic acid pathways helps tailor defense responses.
Developmental photoperiodism and phytochrome signaling
Photoperiodism: developmental responses governed by the length of day and night, influencing flowering, seed germination, and leaf senescence.
Phytochrome is a plant pigment that exists in two interconvertible forms: Pr (red-absorbing) and Pfr (far-red absorbing). The two forms can be represented as:
Under long days (more red light and PFR), long-day plants promote flowering and seed germination via PFR signaling; under long nights (or short days), conversion back to PR supports flowering in short-day plants.
In practical terms:
Long-day plants flower when the night is short (or day length is long), with PFR promoting flowering.
Short-day plants flower when the night is long (or day length is short), as PR promotes flowering.
Some plants are day-neutral, showing little to no flowering response to day length, maintaining year-round growth in regions with stable photoperiods.
Blue light photoreceptors in shoot tips trigger signaling that leads to DNA/enzyme changes and hormone production (notably auxin), driving phototropism and possibly influencing photoperiodic responses.
Non-directional rapid movements (nastic/gust movements) and plant behavior
Nastic movements (gnostic movements) are rapid, non-directional movements that are reversible (not growth-dependent):
Example: Mimosa (mimosa pudica) leaf closure when touched; leaves reopen after minutes, independent of direction of touch.
Venus flytrap captures prey with rapid leaf closure; the trap may then digest prey to obtain nitrogen in nutrient-poor soils.
These responses are not tropisms and do not involve directional growth but are rapid, reversible movements driven by physiological signaling.
Vegetation-level physiology and real-world relevance
The plant hormone network is an intricate, interconnected system in which auxin sits at the center, influencing both growth and defense interactions with ABA, GA, cytokinins, ethylene, jasmonic acid, and salicylic acid.
Tropisms (phototropism, gravitropism, thigmotropism) demonstrate the dynamic, directional growth responses that help plants find resources (light, gravity, touch-based supports) and adjust orientation.
Photoperiodism and phytochrome signaling explain seasonal flowering and seed germination patterns, enabling plants to synchronize life cycles with seasonal cues.
ABA-mediated drought responses, stomatal regulation, and seed dormancy strategies illustrate how plants prioritize survival under adverse conditions.
Ethylene and other defense signaling molecules (salicylic acid, jasmonic acid) coordinate responses to biotic stress, including systemic signaling that enhances resistance across the plant.
Lab and field relevance: understanding hormone balance and signaling aids in agriculture, horticulture, crop improvement, postharvest handling (fruit ripening), and stress management.
Key recap questions and ideas to review
Which hormones are growth-promoting vs growth-inhibiting, and how does their balance affect dormancy, germination, and bud break?
How does ABA mediate drought responses and stomatal closure, and what happens in ABA-deficient plants?
What roles do ethylene play in fruit ripening, leaf senescence, and abscission, and how do oxygen levels influence ethylene signaling?
How does auxin drive phototropism and gravitropism, and why does it promote elongation in shoots but inhibit elongation in roots?
What are amyloplasts, statoliths, and how do they contribute to gravity sensing and auxin redistribution?
What are the major defense signaling molecules in plants, and how do systemic acquired resistance (SAR) and local defense responses work?
What is photoperiodism, and how do phytochromes (PR and PFR) regulate flowering and seed germination in long-day vs short-day plants?
What distinguishes tropisms from nastic movements, and what are some classic examples (phototropism vs thigmotropism vs gravitropism vs Mimosa and Venus flytrap)?
How can researchers leverage knowledge of hormone signaling to improve crop resilience and postharvest quality?
Notes on terminology and common spellings used in this material
Abbreviations and terms used here align with standard biology terminology, including ABA (abscisic acid), GA (gibberellins), IAA (indole-3-acetic acid, a common form of auxin), PR/PFR (phytochrome forms), SAR (systemic acquired resistance), and HR (hypersensitive response).
Some spellings in the source transcript are informal or misspelled (e.g., epsisic acid = ABA; upcycic acid = ABA; abxyzic acid; oxin = auxin). The notes above standardize to the canonical terms: abscisic acid (ABA), auxin (IAA), gibberellin (GA), ethylene, salicylic acid, jasmonic acid, cytokinin, and so on.
Labs and demonstrations you might encounter
Split-root ABA experiment to illustrate drought signaling and stomatal closure.
Mutants unable to produce ABA or ethylene to show consequences on dormancy and ripening.
Mimosa pudica and Venus flytrap demonstrations to illustrate non-tropic, rapid responses (gnastic and developmental movements).
Historical phototropism experiments (Darwin and colleagues) demonstrating tip-sensing and auxin distribution.
Blue light photoreceptor signaling and monocromic light experiments to illustrate blue-light-driven phototropism and the downstream auxin response.
End of section notes