Plant Hormones

Hormones are chemicals released from some cells that travel to other, target cells to stimulate or inhibit gene activity. Hormones affect growth (increase in number, size, and volume of cells) and development (emergence of specialized, morphologically different body parts).

At least five classes of hormones control the selective gene expression that underlies cell differentiation.

More than 80 forms of gibberellins have been isolated from plants, as well as from fungi. In nature, gibberellin helps seeds and buds break dormancy, makes stems lengthen, and influences flowering. It is often applied by growers to enhance stem length, and control ripening of fruit such as apples.

Auxins promote stem and coleoptile elongation and play a role in responses to gravity and light. Indoleacetic acid (IAA) is the most common auxin in nature. Synthetic auxins are used as herbicides to kill weeds.

This animation (Audio - Important) describes the function of auxin.

Cytokinins stimulate cell division, promote leaf expansion, and retard leaf aging. They are most abundant in root and shoot meristems and in maturing fruits. In mature plants, they are produced in roots and transported to shoots. They are used to artificially extend the shelf life of cut flowers and delay leaf death.

Abscisic acid (ABA) promotes seed and bud dormancy, causes the suspension of growth, and also causes stomatal closure. It also plays a role in drought response. It is used to induce dormancy in plants to be shipped.

Ethylene induces aging responses and stimulates the ripening of fruit. Unlike other plant hormones, ethylene is a gas. It is used commercially to ripen fruits for market.

Other unidentified hormones can inhibit lateral growth (apical dominance) and promote flowering.

REVIEW: The signaling chemicals produced by one group of cells that affect distant target cells are called
Answer hormones.