Plant hormone

Plant hormones (also known as phytohormones) are chemicals that regulate plant growth. In the UK, these are termed 'plant growth substances'. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, when moved to other locations, in other locations of the plant. Hormones also determine the formation of flowers, stems, leaves, the shedding of leaves, and the development and ripening of fruit. Plants, unlike animals, lack glands that produce and secrete hormones. Instead, each cell is capable of producing hormones. Plant hormones shape the plant, affecting seed growth, time of flowering, the sex of flowers, senescence of leaves, and fruits. They affect which tissues grow upward and which grow downward, leaf formation and stem growth, fruit development and ripening, plant longevity, and even plant death. Hormones are vital to plant growth, and, lacking them, plants would be mostly a mass of undifferentiated cells. So they are also known as growth factors or growth hormones.

The term phytohormone was coined by Thimann in 1948. The word 'phytohormone' is derived from Greek φυτόν ('phyton'), meaning 'plant' and hormone, meaning 'to set in motion'.

Types of plant hormones

Plant hormones are categorized into five main types: auxin, cytokinin, gibberellin, abscisic acid and ethylene. Each of these hormones has a distinct role in plant growth and development.

Auxin

Auxin is a plant hormone produced in the stem tip that promotes cell elongation. Auxin moves to the darker side of the plant, causing the cells there to grow larger than corresponding cells on the lighter side of the plant.

Cytokinin

Cytokinins are a class of plant growth substances that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation.

Gibberellin

Gibberellins are plant hormones that regulate growth and influence various developmental processes, including stem elongation, germination, dormancy, flowering, sex expression, enzyme induction, and leaf and fruit senescence.

Abscisic Acid

Abscisic acid (ABA) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size, and stomatal closure.

Ethylene

Ethylene is a gaseous plant hormone which regulates a wide range of biological processes. It is involved in processes such as ripening of fruits, opening of flowers, shedding of leaves, and senescence of green tissues.

See also

• Plant physiology
• Plant perception (physiology)
• Plant morphology

References

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