Download Plant Hormones: Responding to External Signals

WEB TUTORIAL 30.3
Plant Hormones:
Responding to External Signals
Text Sections
Section 30.5 Responding to External Signals
Introduction
Plant hormones, like animal hormones, are organic molecules that are active in low
concentrations at target sites removed from the site where they are produced. This
tutorial uses a plant hormone from the auxin family, indole acetic acid (IAA), to
illustrate the principles of hormone structure, transport, and activity.
Learning Objectives
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Understand what hormones do.
Know the characteristics of hormones.
Understand how hormones can produce different responses in different tissues.
Narration
Hormone Characteristics
Plants, like animals, produce hormones. A hormone is a molecule that regulates
developmental processes, such as the transition to flowering, at extremely low concentrations. There are several categories of plant hormones. This example uses the
auxin hormone indole acetic acid (IAA) to illustrate some properties of plant hormones. IAA, like other hormones, is a small organic molecule with a relatively
simple chemical structure. Hormones are produced in specific areas of the plant
and transported to their sites of action, where they elicit a particular response.
One function of IAA is to stimulate cell elongation in the shoot. IAA transport is
polar, or unidirectional: IAA always travels downward. In this example, IAA is
being produced in the developing tip of a grass leaf. The IAA travels downward a
short distance to its site of action and accumulates on the side of the leaf opposite
the sunlight. The cells in that region then elongate, due to the increased concentration of IAA on one side of the leaf. The higher concentration of IAA causes the leaf
to bend toward the light, a response known as phototropism.
Hormone Transport and Activity
Each plant hormone has a characteristic mechanism and pathway of transport.
Hormones may have different functions in different tissues, however. IAA, for
example, causes cell elongation (phototropism) in areas of high concentration in
the shoot, but it inhibits cell elongation in areas of high concentration in the root.
As we've seen, IAA is produced in the shoot tip and transported downward. When
IAA reaches the root tip, it spreads outward toward the epidermal cells of the root.
IAA regulates the downward growth of roots, a response called gravitropism. The
lack of evidence that IAA is produced at the root tip suggests that the IAA is transported from the shoot tip. It has a pronounced effect at a location far from its site
of production.
When roots are in their normal orientation, IAA is distributed in an inverted
umbrella pattern. If the root orientation changes—for example, if a potted plant is
placed on its side—an area of cells in the root senses the change in the direction of
gravity. IAA accumulates on the lower surface of the root, inhibiting the normal
elongation of the cells. As the cells on the upper surface elongate more than those
on the lower surface, the tip of the root bends downward, and the root once again
begins growing in its normal orientation.
You should now be able to…
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Explain how IAA produces a phototropic response.
Explain how IAA produces a gravitropic response.