Hormones

Categories: Cellular biology; physiology

Plant hormones are the major group of chemical messengers by which most plant activities are controlled. The five different groups of hormones regulate virtually every aspect of plant growth and development.

The majority of higher plants begin life as seeds.

When seeds germinate, the embryonic tissues begin to grow and undergo differentiation until, ultimately, the various parts of the mature plant are formed. Every aspect of this growth and development is regulated by a group of chemical messengers called hormones. These plant hormones, or phytohormones, function as plant regulators. A plant regulator is an organic compound, other than a nutrient, which in small amounts promotes, inhibits, or otherwise modifies a basic plant process. Hormones are produced in one area of the plant and transported to another area, where their effects are exerted. There are five major classes of identifiable plant hormones, and others will surely be identified in the future: auxins, cytokinins, gibberellins, ethylene, and abscisic acid.

Auxins

Although there are numerous plant responses to the auxins, one of the primary functions of this group of hormones is to cause increases in cell length by loosen-

ing cell walls and increasing the synthesis of cellwall material and protein. In order for plants to grow, cells produced at the stem or root meristems must undergo this process of elongation. No cell elongation can take place in the absence of auxin.

The cell elongation promoted by auxins results in regular growth, and it is also responsible for various tropisms. For example, phototropism causes plants

Plant Hormones and Their Functions

Hormone

Responsible For

Abscisic acid

Leaf abscission

Auxins

Cell elongation, tropisms, root growth

Brassinolides

Cell division, cell elongation

Cytokinins

Cell division, increased metabolism, chlorophyll synthesis

Ethylene

Fruit ripening, fruit abscission, dehiscence

Gibberellins

Stem elongation, breaking dormancy

Jasmonates

Seed germination, root growth, accumulation of storage and defense proteins

Salicylic acid

Pathogen defense activator

Systemin

Stimulates defense genes after tissue wounding

to grow toward a source of light. Gravitropism is a response to gravity and causes the roots to grow downward.

Besides cell elongation, auxins will initiate root growth at the base of the stem. Auxins inhibit growth of the lateral buds; as long as auxins are being transported down the stem from the apical bud, the lateral buds will not develop. This phenomenon, known as apical dominance, accounts for the fact that plants will not bush out until the apical buds are removed. While there are a number of natural and synthetic compounds that exhibit auxin activity, the major, naturally occurring auxin is a compound called indole acetic acid (IAA).

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