11: Plant Hormones

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Learning Objectives

Explain the defining characteristics of a hormone.
Name the five major plant hormones.

A plant’s sensory response to external stimuli relies on chemical messengers (hormones). Plant hormones affect all aspects of plant life, from flowering to fruit setting and maturation (Figure \(\PageIndex{a}\), and from phototropism to leaf fall. Just as in animals, hormones are signaling molecules which are present in very small amounts, transported throughout the plant body, and only elicit in responses in cells which have the appropriate hormone receptors. In plants, hormones travel large throughout the body such as through the vascular tissue (xylem and phloem) or cell-to-cell via plasmodesmata. Potentially every cell in a plant can produce plant hormones. In contrast, many animal hormones are produced only in specific glands. Plants do not have specialized hormone-producing glands.

Plant hormones are a group of unrelated chemical substances that affect plant growth, development, and other physiological processes. Five major plant hormones are traditionally described: auxins (particularly IAA), cytokinins, gibberellins (GAs), abscisic acid (ABA), and ethylene. Auxins, cytokinins, and gibberellins are groups of plant hormones whereas abscisic acid and ethylene are single compounds. Additional signaling molecules, such as jasmonates and salicylic acid are key in defense against pathogens and herbivores.

A bunch of tightly clustered, ripening bananas. The bananas in the middle of the cluster are yellow and ripe. — Figure \(\PageIndex{1}\): Ripening of fruits such as bananas is mediated by plant hormones. Image by Forest Starr and Kim Starr (CC-BY).

11.1: Auxin
The most important auxin produced by plants is indole-3-acetic acid (IAA). It undergoes both polar (unidirectional) and nonpolar transport. Key functions include mediating tropisms, growth and development, apical dominance, flowering and fruit development, and preventing abscission.
11.2: Cytokinins
Cytokinins stimulate cell division during germination, support leaf formation, and balance the effects of auxin.
11.3: Gibberellins
Gibberellins stimulate shoot elongation, seed germination, and fruit and flower maturation.
11.4: Abscisic Acid
The plant hormone abscisic acid (ABA) is the major player in mediating the adaptation of the plant to stress. It maintains dormancy in seeds and buds, and induces stomatal closing when water is low.
11.5: Ethylene
Ethylene is a gaseous plant hormone important in fruit ripening, flower wilting, and leaf and fruit abscission.
11.6: Summary Table
11.7: Other Signaling Molecules
In addition to the five major categories of plant hormones, a variety of signaling molecules coordinate plant function. These include brassinosteroids, systemin, jasmonates, salicylic acid, oligosaccharins, strigolactones, and florigen.
11.8: Chapter Summary

Attributions

Curated and authored by Melissa Ha from the following sources:

30.6 Plant Sensory Systems and Responses from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
Plant Hormones and Sensory Systems by Biology 1520 Introduction to Organismal Biology (licensed CC BY-NC-SA)

Thumbnail image: A normal barley seedling (right) and a mutant deficient in the hormone gibberellic acid. The mutant produces less of the active hormone, and does not grow as rapidly. Image and description (modified) by CSIRO (CC-BY).