What are Plant Hormones?

Study Notes

Plant hormones are chemicals produced in many parts of the plant that promote and regulate functions in other parts of the plant.
Hormones are chemical messengers that tell different parts of the plant to do different things, depending on the type of hormone.
Functions regulated by plant hormones include promotion and inhibition of growth, protection from pathogens, repair from damage, ripening of fruit, and start or delay of senescence.

Hormones are synthesized in many parts of the plant, including roots, stems, and leaves.
Hormones are then transported to the part of the plant that needs a specific hormone through vascular tissues, such as phloem and xylem.

There are five types of hormones in plants: auxins, cytokinins, gibberellins (GAs), ethylene, and abscisic acid (ABA).
Each type of hormone has a specific function, with several secondary functions that help the plant in its life.

Three plant hormones fall under the category of growth promotion: auxins, cytokinins, and gibberellins.
Auxins are responsible for cell elongation in response to phototropism and gravitropism, and are produced in the stems, buds, and root tips of the plant.
Cytokinins are responsible for cell division and are most abundant in areas of the plant that produce cells, such as seed embryo, fruit, and root and shoot tips.
Gibberellins are responsible for stem, fruit, and seed growth, and are produced in the roots of plants.

The two main hormones synthesized by plants and used for growth inhibition are ethylene and abscisic acid.
Ethylene is responsible for aging in plants, ripening fruit, and is released as a gas.
Abscisic acid is responsible for dormancy of different organs within the plant, and causes leaves to drop in response to environmental factors such as cold, heat, dehydration, or nutrient deficiency.

Plant hormones are used for protection from pathogens, and many hormones act as the plant's immune system.
Plant hormones are used by humans as medicine, such as salicylic acid, which is a powerful anti-inflammatory used in painkillers.