The term ‘phytohormone’ was coined by Dutch biologist FW Went and American microbiologist KV Thimann in 1937.
Where are Plant Hormones Produced
Classification of Plant Hormones with Their Functions
Other Growth Hormones in Plants
1) Auxins
‘Auxin’ is derived from the Greek term ‘Auxein,’ meaning to grow. They were the first group of plant growth hormones discovered. Auxin is mainly found in the tips of shoots and roots of plants, although they migrate to other parts for further action. Indole-3-acetic acid (IAA) is a naturally produced auxin found in plants. Whereas 2,4-dichloro-phenoxy acetic acid (2,4-D), naphthalene acetic acid (NAA), and indole butyric acid (IBA) are some synthetically prepared auxins.
Functions
Promoting cell growth and elongationPromoting apical dominance, that is, the apical buds grow, suppressing the lateral budsInducing parthenocarpy (producing fruit without prior fertilization)Preventing premature fall of leaves, fruits, and flowers (abscission)Initiating rooting in stem cuttings and graftingUsing as a herbicide in agriculture (example: 2,4-D)Helping in cell division and differentiationPromoting phototropism (bending towards light based on their concentration)
2) Gibberellins
They were first isolated from the fungus Gibberella fujikuroi, which promotes abnormal growth in rice plants. Later they were found to be produced by plants, controlling their growth.
Functions
Causing stem elongation by promoting cell division and lengthening the stem internodesTriggering germination in dormant seeds, thus initiating plant growthDelaying senescenceProducing a seedless variety of grapes and tomatoes by parthenocarpyInducing maleness in cannabisBreaking seed dormancy
3) Cytokinins
They are naturally synthesized in plants where rapid cell division occurs, such as root apices and shoot buds. They are produced in the roots of plants that migrate to the stems and leaves for their function. Zeatin and isopentenyl adenine are some naturally occurring cytokinins. In contrast, kinetin and benzyl adenine are synthetically prepared.
Functions
Promoting lateral and adventitious shoot growth and developmentSuppressing apical dominance induced by auxinPromoting cell division, precisely the step of cytokinesis
4) Ethylene
It is a gaseous plant inhibitor in ripened fruits and tissues undergoing senescence. Ethylene helps plants to survive stress and is also widely used in agriculture.
Functions
Promoting fruit ripeningInducing senescencePromoting abscission of leaves, flowers, and fruitsHelping in the rapid elongation of petioles and internodesBreaking seed dormancyControlling epinastyPromoting femaleness in monoecious plants
5) Abscisic Acid
It is another hormone that helps plants to cope with stress. Lack of moisture induces stomata closing of stomata thus preventing water loss by transpiration. Abscisic acid is another plant growth inhibitor apart from ethylene.
Functions
Inducing seed dormancyControlling the closing of stomataInhibiting plant growth and development
Brassinosteroids are the only steroid-based plant hormone. Brassinosteroids control cell elongation, division, gravitropism, stress resistance, and xylem differentiation and inhibit root growth and leaf abscission. Jasmonates (JAs) are lipid-based hormones that help plants respond to stress and defend themselves from the attack of herbivores and pathogens. The most active JA in plants is jasmonic acid. Salicylic acid (SA) is a phenol-based plant hormone initially isolated from an extract of white willow bark. SA protects plants against pathogens and responds to abiotic stress, such as drought, extreme temperatures, heavy metals, and osmotic stress. They are also found to influence seed germination, cell growth, respiration, stomatal closure, gene expression, and response to biotic and abiotic stress.
