Plant Hormones Jessica Harwood Douglas Wilkin, Ph.D.
Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required)
To access a customizable version of this book, as well as other interactive content, visit www.ck12.org
AUTHORS Jessica Harwood Douglas Wilkin, Ph.D. EDITOR Douglas Wilkin, Ph.D.
CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-source, collaborative, and web-based compilation model, CK-12 pioneers and promotes the creation and distribution of high-quality, adaptive online textbooks that can be mixed, modified and printed (i.e., the FlexBook® textbooks). Copyright © 2015 CK-12 Foundation, www.ck12.org The names “CK-12” and “CK12” and associated logos and the terms “FlexBook®” and “FlexBook Platform®” (collectively “CK-12 Marks”) are trademarks and service marks of CK-12 Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution-Non-Commercial 3.0 Unported (CC BY-NC 3.0) License (http://creativecommons.org/ licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/about/ terms-of-use. Printed: November 10, 2015
CONTRIBUTORS Doris Kraus, Ph.D. Niamh Gray-Wilson Jean Brainard, Ph.D. Sarah Johnson Jane Willan Corliss Karasov
www.ck12.org
C HAPTER • • • •
Chapter 1. Plant Hormones
1
Plant Hormones
Define hormone. Define abscission and apical dominance. List the major types of plant hormones. Describe the main functions of plant hormones.
How do fruits know when to fall? Fruits, such as these peaches, do not leave their trees until it’s the right time. The seeds are mature, and the fruit is ripe. But what tells the fruits it’s time to drop? The signal that is sent through the tree is a type of hormone. Hormones also send signals through your body. Plant Hormones
Plants may not move, but that does not mean they don’t respond to their environment. Plants can sense gravity, light, touch, and seasonal changes. For example, you might have noticed how a house plant bends toward a bright window. Plants can sense and then grow toward the source of light. Scientists say that plants are able to respond to "stimuli," or something—usually in the environment—that results in a response. For instance, light is the stimulus, and the plant moving toward the light is the "response." Hormones are special chemical messengers molecules that help organisms, including plants, respond to stimuli in their environment. In order for plants to respond to the environment, their cells must be able to communicate with other cells. Hormones send messages between the cells. Animals, like humans, also have hormones, such as testosterone or estrogen, to carry messages from cell to cell. In both plants and animals, hormones travel from cell to cell in response to a stimulus; they also activate a specific response. Types of Plant Hormones
Five different types of plant hormones are involved in the main responses of plants, and they each have different functions ( Table 1.1).
1
www.ck12.org
TABLE 1.1: Plant Hormones Hormone Ethylene Gibberellins Cytokinins Abscisic Acid Auxins
Function Fruit ripening and abscission Break the dormancy of seeds and buds; promote growth Promote cell division; prevent senescence Close the stomata; maintain dormancy Involved in tropisms and apical dominance
Ethylene
The hormone ethylene has two functions. It (1) helps ripen fruit and (2) is involved in the process of abscission, the dropping of leaves, fruits, and flowers. When a flower is done blooming or a fruit is ripe and ready to be eaten, ethylene causes the petals or fruit to fall from a plant ( Figure 1.1 and Figure 1.2). Ethylene is an unusual plant hormone because it is a gas. That means it can move through the air, and a ripening apple can cause another apple to ripen, or even over-ripen. That’s why one rotten apple spoils the whole barrel! Some farmers spray their green peppers with ethylene gas to cause them to ripen faster and become red peppers. You can try to see how ethylene works by putting a ripe apple or banana with another unripe fruit in a closed container or paper bag. What do you think will happen to the unripe fruit?
FIGURE 1.1 The hormone ethylene is signaling these tomatoes to ripen.
Gibberellins
Gibberellins are hormones that cause the plant to grow. When gibberellins are applied to plants by scientists, the stems grow longer. Some gardeners or horticulture scientists add gibberellins to increase the growth of plants. 2
www.ck12.org
Chapter 1. Plant Hormones
FIGURE 1.2 The hormone ethylene causes flower petals to fall from a plant, a process known as abscission.
Dwarf plants (small plants), on the other hand, have low levels of gibberellins ( Figure 1.3). Another function of gibberellins is to stop dormancy (resting time) of seeds and buds. Gibberellins signal that it’s time for a seed to germinate (sprout) or for a bud to open.
FIGURE 1.3 Dwarf plants like this bonsai tree often have unusually low concentrations of gibberellins.
Cytokinins
Cytokinins are hormones that cause plant cells to divide. Cytokinins were discovered from attempts to grow plant tissue in artificial environments ( Figure 1.4). Cytokinins prevent the process of aging (senescence). So florists sometimes apply cytokinins to cut flowers, so they do not get old and die. 3
www.ck12.org
FIGURE 1.4 Cytokinins promote cell division and are necessary for growing plants in tissue culture. A small piece of a plant is placed in sterile conditions to regenerate a new plant.
Abscisic Acid
Abscisic acid is misnamed because it was once believed to play a role in abscission (the dropping of leaves, fruits, and flowers), but we now know abscission is caused by ethylene. The actual role of abscisic acid is to close the stomata, the tiny openings in leaves that allow substances to enter and leave, and to maintain dormancy. When a plant is stressed due to lack of water, abscisic acid tells the stomata to close. This prevents water loss through the stomata. When the environment is not good for a seed to germinate, abscisic acid signals for the dormancy period of the seed to continue. Abscisic acid also tells the buds of plants to stay in the dormancy stage. When conditions improve, the levels of abscisic acid drop and the levels of gibberellins increase, signaling that is time to break dormancy ( Figure 1.5).
Auxins
Auxins are hormones that play a role in plant growth. Auxins produced at the tip of the plant are involved in apical dominance, when the main central stem grows more strongly than other stems and branches. When the tip of the plant is removed, the auxins are no longer present, and the side branches begin to grow. This is why pruning a plant by cutting off the main branches helps produce a fuller plant with more branches. You actually need to cut branches off of a plant for it to grow more branches! Auxins are also involved in tropisms, responses to stimuli in the environment
Summary
• Plant hormones are chemical signals that control different processes in plants. 4
www.ck12.org
Chapter 1. Plant Hormones
FIGURE 1.5 A decrease in levels of abscisic acid allows these buds to break dormancy and put out leaves.
• Plant hormones include ethylene, gibberellins, cytokinins, absciscic acid, and auxins. Explore More
Use the resource below to answer the questions that follow. • Plant Hormones at http://www.youtube.com/watch?v=ZbRiKlIYa-k (5:39) 1. 2. 3. 4.
What hormones stimulate plant growth? What hormones inhibit plant growth? Why is it important for a plant to be able to both stimulate and inhibit growth? Explain your answer. Do you think hormones are more important to plants or mammals? Explain your thinking.
Review
1. 2. 3. 4. 5.
What are three stimuli that plants respond to? List the roles of gibberellins. What hormone is involved in fruit ripening? What happens to an unripe fruit when placed in a paper bag with a ripe fruit? What is apical dominance?
References 1. Todd Petit. The hormone ethylene is signaling these tomatoes to ripen . CC BY 2.0 2. Lynn Wohlers. The hormone ethylene causes flower petals to fall from a plant, a process known as abscission . CC BY 2.0 3. Sheila Thomson. Dwarf plants like this bonsai tree often have unusually low concentrations of gibberellins . CC BY 2.0 4. Image copyright Zenphotography, 2014. Cytokinins promote cell division and are necessary for growing plant s in tissue culture . Used under license from Shutterstock.com 5
www.ck12.org 5. nvainio. =A decrease in levels of abscisic acid allows these buds to break dormancy and put out leaves . CC BY 2.0
6
