Ethylene triggers fruit ripening and other aging processes. As fruit cells age, they

Experiments on how plants turn toward light led to the discovery of a plant hormone. Hormones coordinate the activities of plant cells and tissues. Experiments carried out by Darwin and others showed that the tip of a grass seedling detects light and transmits a signal down to the growing region of the shoot.

Five major types of hormones regulate plant growth and development. By triggering signal transduction pathways, small amounts of hormones regulate plant growth and development.

Auxin stimulates the elongation of cells in young shoots. Plants produce the auxin IAA in the apical meristems at the tips of shoots. At different concentrations, auxin stimulates or inhibits the elongation of shoots and roots. It may act by weakening cell walls, allowing them to stretch when cells take up water. Auxin also stimulates the development of vascular tissues and cell division in the vascular cambium, promoting growth in stem diameter.

Cytokinins stimulate cell division. Cytokinins, produced by roots, embryos, and fruits, promote cell division. Cytokinins from roots may balance the effects of auxin from apical meristems, causing lower buds to develop into branches.

Gibberellins affect stem elongation and have numerous other effects. Gibberellins stimulate the elongation of stems and leaves and the development of fruits. Gibberellins released from embryos function in some of the early events of seed germination.

Abscisic acid inhibits many plant processes. Abscisic acid (ABA) inhibits germination. The ratio of ABA to gibberellins often determines whether a seed remains dormant or germinates. Seeds of many plants remain dormant until their ABA is inactivated or washed away. ABA also acts as a stress hormone, causing stomata to close when a plant is dehydrated.

Ethylene triggers fruit ripening and other aging processes. As fruit cells age, they give off ethylene gas, which hastens ripening. A changing ratio of auxin to ethylene, triggered mainly by shorter days, probably causes autumn color changes and the loss of leaves from deciduous trees.

Plant hormones have many agricultural uses. Auxin can delay or promote fruit drop. Auxin and gibberellins are used to produce seedless fruits. A synthetic auxin called 2,4-D kills weeds. There are questions about the safety of using such chemicals.

Tropisms orient plant growth toward or away from environmental stimuli. Plants sense and respond to environmental changes. Tropisms are growth responses that change the shape of a plant or make it grow toward or away from a stimulus. Phototropism, bending in response to light, may result from auxin moving from the light side to the dark side of a stem. A response to gravity, or gravitropism, may be caused by settling of organelles on the low sides of shoots and roots, which may trigger a change in hormone distribution. Thigmotropism, a response to touch, is responsible for coiling of tendrils and vines around objects.

Plants have internal clocks. An internal biological clock controls sleep movements and other daily cycles in plants. These cycles, called circadian rhythms, persist with periods of about 24 hours even in the absence of environmental cues, but such cues are needed to keep them synchronized with day and night.

Plants mark the seasons by measuring photoperiod. The timing of flowering is one of the seasonal responses to photoperiod, the relative lengths of night and day.

Phytochromes are light detectors that may help set the biological clock. Light-absorbing proteins called phytochromes may help plants set their biological clock and monitor photoperiod.

Defenses against herbivores and infectious microbes have evolved in plants. Plants use chemicals to defend themselves against both herbivores and pathogens. So-called avirulent plant pathogens interact with host plants in a specific way that stimulates both local and systemic defenses in the plant. Local defenses include microbe-killing chemicals and the sealing off of the infected area. Hormones trigger generalized defense responses in other organs (systemic acquired resistance).

Test your knowledge of the five major classes of plant hormones (auxins, cytokinins, gibberellins, abscisic acid, ethylene) by matching one hormone to each lettered box. (Note that some hormones will match up to more than one box.)

During winter or periods of drought, which one of the following plant hormones inhibits growth and seed germination? a. ethylene d. auxin b. abscisic acid e. cytokinin c. gibberellin

A certain short-day plant flowers only when days are less than 12 hours long. Which of the following would cause it to flower? a. a 9-hour night and 15-hour day with 1 minute of darkness after 7 hours b. an 8-hour day and 16-hour night with a flash of white light after 8 hours c. a 13-hour night and 11-hour day with 1 minute of darkness after 6 hours d. a 12-hour day and 12-hour night with a flash of red light after 6 hours e. 24 hours of continuous light

Auxin causes a shoot to bend toward light by a. causing cells to shrink on the dark side of the shoot. b. stimulating growth on the dark side of the shoot. c. causing cells to shrink on the lighted side of the shoot. d. stimulating growth on the lighted side of the shoot. e. inhibiting growth on the dark side of the shoot.

In autumn, the amount of ______ increases and the amount of ______ decreases in fruits and leaf stalks, causing a plant to drop fruit and leaves. a. ethylene . . . auxin d. auxin . . . ethylene b. gibberellin . . . abscisic acid e. gibberellin . . . auxin c. cytokinin . . . abscisic acid

Plant hormones act by affecting the activities of a. genes. d. genes, membranes, and enzymes. b. membranes. e. genes and enzymes. c. enzymes.

Buds and sprouts often form on tree stumps. Which hormone would stimulate their formation? a. auxin d. ethylene b. cytokinin e. gibberellin c. abscisic acid

A plants defense response at the site of initial infection by a pathogen will be especially strong if a. the pathogen is virulent. b. the plant makes a receptor protein that recognizes a signal molecule from the microbe. c. the pathogen is a fungus. d. the plant has an Avr gene that is the right match for one of the microbes R genes. e. the right combination of hormones travel from the infection site to other parts of the plant.

Matching. Bending of a shoot toward light 1

Matching. Growth response to touch 1

Matching. Cycle with a period of about 24 hours 1

Matching. Pigment that helps control flowering 1

Matching. Relative lengths of night and day 1

Matching. Growth response to gravity 1

Matching. Folding of plant leaves at night 1

If apples are to be stored for long periods, it is best to keep them in a place with good air circulation. Explain why. 1

Write a short paragraph explaining why a houseplant becomes bushier if you pinch off its terminal buds. 1

A plant nursery manager tells the new night security guard to stay out of a room where chrysanthemums (which are short-day plants) are about to flower. Around midnight, the guard accidentally opens the door to the chrysanthemum room and turns on the lights for a moment. How might this affect the chrysanthemums? How could the guard correct the mistake? 1

A plant biologist observed a peculiar pattern when a tropical shrub was attacked by caterpillars. After a caterpillar ate a leaf, it would skip over nearby leaves and attack a leaf some distance away. Simply removing a leaf did not trigger the same change nearby. The biologist suspected that a damaged leaf sent out a chemical that signaled other leaves. How could this hypothesis be tested? 2

Imagine the following scenario: A plant biologist has developed a synthetic chemical that mimics the effects of a plant hormone. The chemical can be sprayed on apples before harvest to prevent flaking of the natural wax that is formed on the skin. This makes the apples shinier and gives them a deeper red color. What kinds of questions do you think should be answered before farmers start using this chemical on apples? How might the scientist go about finding answers to these questions?