Plant hormones and their uses in agriculture
Auxins, gibberellins, cytokinins, abscisic acid, ethylene and the newer hormones: how each was discovered, what it does, and how growers use them.
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Plant hormones are small signalling molecules, active in tiny amounts, that coordinate growth and development: which buds grow, when seeds germinate, when flowers form, when fruit ripens and falls, and how the plant responds to drought, wounding and disease. Synthetic compounds that act like them, or block them, are called plant growth regulators (PGRs), and they are used across horticulture and field cropping.
Hormones are among the most examined topics in plant physiology. Questions match hormones to discoveries, effects and uses, and ask how pairs of hormones interact.
How plant hormones differ from animal hormones
Plants have no glands. Hormones are made in many tissues, often act close to where they are made as well as far away, and their effect depends on the tissue, its stage, and the balance with other hormones. The same hormone can promote one process and inhibit another. So it is more useful to learn each hormone's characteristic effects, and the main interactions, than to expect one hormone, one job.
Auxins
Discovery. Charles and Francis Darwin noticed in 1880 that grass seedlings bend towards light only if their tips are exposed. Frits Went showed in 1928 that a chemical diffuses from the tip of oat coleoptiles and makes cells below it elongate; it was identified as indole-3-acetic acid (IAA), made mainly in shoot tips and young leaves from the amino acid tryptophan.
Effects.
- Cell elongation in stems, by loosening cell walls.
- Tropisms: in phototropism, auxin moves to the shaded side, which grows faster, so the shoot bends towards light (the Cholodny–Went model). Gravity redistributes auxin similarly in roots.
- Apical dominance: auxin from the shoot tip suppresses the growth of side buds. Cut off the tip ("pinching" or topping) and side shoots grow.
- Root initiation on cuttings.
- Fruit set and development, and delayed abscission of leaves and fruit.
- Vascular differentiation and the polarity of growth.
Uses. Synthetic auxins such as indole-3-butyric acid (IBA) and naphthaleneacetic acid (NAA) are used in rooting powders for cuttings, to prevent pre-harvest fruit drop, to thin fruit, and in tissue culture. At high doses, some synthetic auxins kill broad-leaved plants and are used as selective herbicides in cereals (the HRAC "synthetic auxin" group).
Gibberellins
Discovery. In 1926 Eiichi Kurosawa in Japan showed that rice plants infected with the fungus Gibberella fujikuroi grow abnormally tall and spindly (the "foolish seedling" or bakanae disease) because of a substance the fungus secretes. It was later found to be a family of compounds made by plants themselves; more than 130 gibberellins (GAs) are known, of which a few, such as GA1 and GA4, are active. Gibberellic acid (GA3) is the form used commercially.
Effects.
- Stem elongation, between nodes. Genetic dwarf peas and maize grow tall when given GA.
- Bolting and flowering in rosette and long-day plants, such as cabbage and lettuce.
- Seed germination: in cereal grains, GA from the embryo signals the aleurone layer to make α-amylase, which breaks starch into sugars for the seedling. This is the basis of malting in brewing.
- Breaking dormancy of some seeds and buds.
- Fruit growth, especially in seedless grapes, and parthenocarpy in some fruits.
Uses. GA sprays enlarge berries and loosen clusters of seedless grapes, delay rind ageing in citrus, increase stalk length in some crops, and help malting barley germinate evenly. Conversely, growth retardants that block gibberellin synthesis are used to shorten stems of cereals (against lodging) and of potted ornamentals.
A link to the Green Revolution. The semi-dwarfing genes of modern wheat encode altered DELLA proteins that make the plant less responsive to gibberellin; the semi-dwarf gene of modern rice reduces gibberellin synthesis. Short straw that carried heavy grain without lodging was, at root, a change in gibberellin signalling.
Cytokinins
Discovery. Folke Skoog and Carlos Miller, growing tobacco tissue in culture in the 1950s, found that a substance from degraded herring sperm DNA made the cells divide: kinetin. The first natural cytokinin, zeatin, was isolated from immature maize kernels in 1963. Cytokinins are made mainly in root tips and move up in the xylem.
Effects.
- Cell division, together with auxin.
- Shoot formation in tissue culture.
- Release of lateral buds from apical dominance (opposing auxin).
- Delay of leaf senescence: leaves treated with cytokinin stay green longer and draw nutrients to themselves.
The auxin–cytokinin balance in tissue culture is a classic exam point. On a growth medium:
| Auxin : cytokinin ratio | Result |
|---|---|
| high auxin, low cytokinin | roots form |
| roughly balanced | undifferentiated callus grows |
| low auxin, high cytokinin | shoots form |
Micropropagation of bananas, potatoes, orchids and many ornamentals relies on adjusting this ratio.
Abscisic acid (ABA)
Discovery. Found in the 1960s by groups studying cotton fruit abscission (Frederick Addicott, who named it abscisin II) and bud dormancy in sycamore (Philip Wareing's "dormin"); the two proved to be the same compound. Despite its name, ABA's role in abscission is minor; ethylene is the main abscission signal.
Effects.
- Stomatal closure under water stress. Roots in drying soil and stressed leaves make ABA, which makes guard cells lose turgor within minutes.
- Seed dormancy and maturation: ABA prevents premature germination. Mutants lacking it show vivipary, sprouting on the parent plant, as in some maize mutants.
- Stress responses to drought, salt and cold, switching on protective genes.
- Broadly antagonistic to gibberellin: ABA keeps seeds dormant, GA promotes germination.
Ethylene
Discovery. In 1901 Dimitry Neljubow showed that ethylene in illuminating gas caused pea seedlings to show the triple response: shorter, thicker stems that grow sideways. Long before, fruit sellers had noticed that fruit ripens faster near ripe fruit, or in rooms heated with kerosene stoves. Ethylene (C2H4) is the only gaseous plant hormone, made from methionine via ACC (1-aminocyclopropane-1-carboxylic acid).
Effects.
- Ripening of climacteric fruit, whose respiration and ethylene production surge at ripening: banana, mango, tomato, apple, avocado, papaya. Non-climacteric fruits, such as citrus, grape, strawberry and pineapple, ripen without that surge and do not ripen further after harvest in the same way.
- Abscission of leaves, flowers and fruit.
- Flowering: ethylene (or compounds that release it) induces flowering in pineapple, allowing synchronised harvests.
- Senescence of flowers and leaves; sex expression towards female flowers in cucurbits.
- Responses to flooding (aerenchyma formation) and mechanical stress.
Uses. Ethylene-releasing compounds and controlled ripening rooms ripen bananas, mangoes and tomatoes evenly for market. Blocking ethylene does the opposite: ventilation, low temperature, potassium permanganate absorbers in packaging, and the receptor blocker 1-methylcyclopropene (1-MCP) extend the storage life of apples and cut flowers.
The newer hormones
- Brassinosteroids, steroid hormones first isolated from rapeseed pollen in 1979, promote cell expansion and division and help stress tolerance; mutants lacking them are dwarfs.
- Jasmonates (jasmonic acid and relatives) coordinate defence against chewing insects and responses to wounding.
- Salicylic acid, long known from willow bark, triggers defence against biotrophic pathogens and systemic acquired resistance, in which an infected leaf primes the whole plant.
- Strigolactones inhibit shoot branching, signal to mycorrhizal fungi, and, unfortunately, are what the seeds of the parasitic weeds Striga and Orobanche detect to germinate next to a host root.
- Florigen, the long-sought flowering signal, was identified in the 2000s as a protein (FLOWERING LOCUS T, FT) that moves from leaves to the shoot tip.
Making up a growth regulator solution
A nursery manual recommends dipping cuttings in a 1,000 mg/L (1,000 ppm) rooting hormone solution, and spraying grapes with 20 mg/L of GA3. The stocks available are pure powder and a 1 per cent (10,000 mg/L) liquid concentrate.
- 1,000 mg/L from powder: dissolve 1.0 g in a little solvent (many PGRs need alcohol or alkali to dissolve) and make up to 1 litre.
- 20 mg/L from the 1 per cent concentrate: C1V1 = C2V2, so V1 = 20 × 10 ⁄ 10,000 = 0.02 L (20 mL) of concentrate made up to 10 litres.
Because PGRs work at such low doses, a decimal slip of ten times can turn a promoter into an inhibitor, or a fruit-setting spray into a thinning one. Product registration and permitted uses differ between countries; follow the local label.
Key points
- Auxin (IAA): elongation, tropisms, apical dominance, rooting; discovered through coleoptile experiments (Darwin, Went).
- Gibberellins: stem elongation, bolting, germination via α-amylase, seedless grape size; discovered from the bakanae fungus of rice.
- Cytokinins: cell division, shoot formation, bud release, delayed senescence; high auxin:cytokinin gives roots, low gives shoots.
- ABA: stomatal closure, seed dormancy, stress; antagonist of gibberellin.
- Ethylene: a gas; ripening of climacteric fruit, abscission, pineapple flowering, the triple response.
- Newer hormones: brassinosteroids, jasmonates, salicylic acid, strigolactones; florigen is the FT protein.
What the exams ask
- School-leaving and entrance papers: which hormone causes a given effect (bolting, ripening, stomatal closure, apical dominance), and which is a gas.
- Degree and research-fellowship entrances: discovery experiments, biosynthesis precursors, climacteric versus non-climacteric fruit, the auxin–cytokinin ratio in tissue culture, hormone interactions, DELLA proteins and dwarfing.
- Professional examinations in horticulture and crop production: practical uses of growth regulators and retardants, ripening and storage, and preparing solutions at given concentrations.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why does one rotten apple spoil the barrel?
A ripening or damaged apple releases ethylene, which triggers ripening, and then over-ripening, in the apples around it. Cold storage and ventilation slow this.
Do plant hormones from sprays get into the food?
They are used at very low doses, many break down quickly, and plants make the same natural hormones themselves. Regulators are still subject to residue limits like other agricultural chemicals, which is why labels set doses and pre-harvest intervals.
Is there a single hormone that makes plants flower?
Flowering is controlled by day length, temperature and age, integrated in the leaves and shoot tip. The FT protein (florigen) is the mobile signal from leaves; gibberellins and other hormones also influence flowering in many species.
Sources
- Taiz, L., Zeiger, E., Møller, I. M. and Murphy, A. Plant Physiology and Development, 6th edition. Sinauer, 2015. Chapters 15–24.
- Davies, P. J. (ed.) Plant Hormones: Biosynthesis, Signal Transduction, Action!, 3rd edition. Springer, 2010.
- Peng, J. et al. "'Green revolution' genes encode mutant gibberellin response modulators." Nature 400, 256–261 (1999).
- Corbesier, L. et al. "FT protein movement contributes to long-distance signaling in floral induction of Arabidopsis." Science 316, 1030–1033 (2007).
- Kader, A. A. (ed.) Postharvest Technology of Horticultural Crops, 3rd edition. University of California, 2002.
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