ABA induces dormancy in seeds by blocking germination and promoting the synthesis of storage proteins. Plants adapted to temperate climates require a long period of cold temperature before seeds germinate. This mechanism protects young plants from sprouting too early during unseasonably warm weather in winter.
As the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring. Another effect of ABA is to promote the development of winter buds; it mediates the conversion of the apical meristem into a dormant bud. Low soil moisture causes an increase in ABA, which causes stomata to close, reducing water loss in winter buds.
Ethylene is associated with fruit ripening, flower wilting, and leaf fall. Ethylene is unusual because it is a volatile gas C 2 H 4. Hundreds of years ago, when gas street lamps were installed in city streets, trees that grew close to lamp posts developed twisted, thickened trunks and shed their leaves earlier than expected. These effects were caused by ethylene volatilizing from the lamps. Aging tissues especially senescing leaves and nodes of stems produce ethylene.
The best-known effect of the hormone, however, is the promotion of fruit ripening. Ethylene stimulates the conversion of starch and acids to sugars.
Some people store unripe fruit, such as avocadoes, in a sealed paper bag to accelerate ripening; the gas released by the first fruit to mature will speed up the maturation of the remaining fruit. Ethylene also triggers leaf and fruit abscission, flower fading and dropping, and promotes germination in some cereals and sprouting of bulbs and potatoes.
Ethylene is widely used in agriculture. Commercial fruit growers control the timing of fruit ripening with application of the gas. Horticulturalists inhibit leaf dropping in ornamental plants by removing ethylene from greenhouses using fans and ventilation. Recent research has discovered a number of compounds that also influence plant development. Their roles are less understood than the effects of the major hormones described so far.
Jasmonates play a major role in defense responses to herbivory. Their levels increase when a plant is wounded by a predator, resulting in an increase in toxic secondary metabolites.
They contribute to the production of volatile compounds that attract natural enemies of predators. For example, chewing of tomato plants by caterpillars leads to an increase in jasmonic acid levels, which in turn triggers the release of volatile compounds that attract predators of the pest. Oligosaccharins also play a role in plant defense against bacterial and fungal infections.
They act locally at the site of injury, and can also be transported to other tissues. Strigolactones promote seed germination in some species and inhibit lateral apical development in the absence of auxins. Strigolactones also play a role in the establishment of mycorrhizae, a mutualistic association of plant roots and fungi. Brassinosteroids are important to many developmental and physiological processes. Signals between these compounds and other hormones, notably auxin and GAs, amplifies their physiological effect.
Apical dominance, seed germination, gravitropism, and resistance to freezing are all positively influenced by hormones. Root growth and fruit dropping are inhibited by steroids. The shoot of a pea plant winds around a trellis, while a tree grows on an angle in response to strong prevailing winds. These are examples of how plants respond to touch or wind. The meristematic region of tendrils is very touch sensitive; light touch will evoke a quick coiling response.
Cells in contact with a support surface contract, whereas cells on the opposite side of the support expand. Application of jasmonic acid is sufficient to trigger tendril coiling without a mechanical stimulus. A thigmonastic response is a touch response independent of the direction of stimulus. In the Venus flytrap, two modified leaves are joined at a hinge and lined with thin fork-like tines along the outer edges.
Tiny hairs are located inside the trap. When an insect brushes against these trigger hairs, touching two or more of them in succession, the leaves close quickly, trapping the prey. Glands on the leaf surface secrete enzymes that slowly digest the insect. The released nutrients are absorbed by the leaves, which reopen for the next meal. Thigmomorphogenesis is a slow developmental change in the shape of a plant subjected to continuous mechanical stress.
When trees bend in the wind, for example, growth is usually stunted and the trunk thickens. Researchers hypothesize that mechanical strain induces growth and differentiation to strengthen the tissues. Ethylene and jasmonate are likely involved in thigmomorphogenesis. Use the menu at the left to navigate to three short movies: a Venus fly trap capturing prey, the progressive closing of sensitive plant leaflets, and the twining of tendrils.
Plants face two types of enemies: herbivores and pathogens. Herbivores both large and small use plants as food, and actively chew them. Pathogens are agents of disease.
These infectious microorganisms, such as fungi, bacteria, and nematodes, live off of the plant and damage its tissues. Plants have developed a variety of strategies to discourage or kill attackers. The first line of defense in plants is an intact and impenetrable barrier. Bark and the waxy cuticle can protect against predators.
Other adaptations against herbivory include thorns, which are modified branches, and spines, which are modified leaves. They discourage animals by causing physical damage and inducing rashes and allergic reactions. If the first line of defense is breached, the plant must resort to a different set of defense mechanisms, such as toxins and enzymes.
Secondary metabolites are compounds that are not directly derived from photosynthesis and are not necessary for respiration or plant growth and development. Many metabolites are toxic, and can even be lethal to animals that ingest them. Some metabolites are alkaloids, which discourage predators with noxious odors such as the volatile oils of mint and sage or repellent tastes like the bitterness of quinine.
Other alkaloids affect herbivores by causing either excessive stimulation caffeine is one example or the lethargy associated with opioids. Some compounds become toxic after ingestion; for instance, glycol cyanide in the cassava root releases cyanide only upon ingestion by the herbivore. Mechanical wounding and predator attacks activate defense and protection mechanisms both in the damaged tissue and at sites farther from the injury location.
Some defense reactions occur within minutes: others over several hours. The infected and surrounding cells may die, thereby stopping the spread of infection. Long-distance signaling elicits a systemic response aimed at deterring the predator. As tissue is damaged, jasmonates may promote the synthesis of compounds that are toxic to predators.
Jasmonates also elicit the synthesis of volatile compounds that attract parasitoids, which are insects that spend their developing stages in or on another insect, and eventually kill their host.
The plant may activate abscission of injured tissue if it is damaged beyond repair. Plants respond to light by changes in morphology and activity. Irradiation by red light converts the photoreceptor phytochrome to its far-red light-absorbing form—Pfr. This form controls germination and flowering in response to length of day, as well as triggers photosynthesis in dormant plants or those that just emerged from the soil.
Blue-light receptors, cryptochromes, and phototropins are responsible for phototropism. Amyloplasts, which contain heavy starch granules, sense gravity. Shoots exhibit negative gravitropism, whereas roots exhibit positive gravitropism. Plant hormones—naturally occurring compounds synthesized in small amounts—can act both in the cells that produce them and in distant tissues and organs. Auxins are responsible for apical dominance, root growth, directional growth toward light, and many other growth responses.
Cytokinins stimulate cell division and counter apical dominance in shoots. Gibberellins inhibit dormancy of seeds and promote stem growth. Abscisic acid induces dormancy in seeds and buds, and protects plants from excessive water loss by promoting stomatal closure. Ethylene gas speeds up fruit ripening and dropping of leaves. Plants respond to touch by rapid movements thigmotropy and thigmonasty and slow differential growth thigmomorphogenesis.
Plants have evolved defense mechanisms against predators and pathogens. Physical barriers like bark and spines protect tender tissues.
During germination, this helps ensure that the seedling's stem will eventually find its way up out of the ground and grow towards the Sun while the roots will keep growing downward into the soil. To learn more about germination, see Seeds and Germination. Hydrotropism is plant growth response towards or away from water.
This usually happens in the roots, where the plant typically absorbs water. If a plant senses that there is water close by, the roots will grow in that direction.
A great experiment to do at home is to give any house plant a single source of light. Soon you will begin to see the plant bending towards the light source see Figure 2 , growing in that direction to get to more light for photosynthesis.
Thigmotropism is plant growth response to touch. An example of this tropism is the curling of a vine tendril around objects that it touches. This helps the plant securely position itself and keep growing, as these types of plants do not usually have a strong stem to keep themselves upright.
Scientists think that this response is to help protect the plant from insects. The Venus fly trap is another plant that responds to the external touch stimulus.
When the hairs on the inside of the leaf are triggered, usually when an insect has landed, the leaf closes shut. Insects provide nutrients to the plant. Over time, the insect is dissolved and the nutrients will be absorbed by the plant. Tropic Responses in Space Gravity has been known to be one of the major forces directing the growth of plants, including light and water. Glossary Auxin A plant hormone responsible for directing plant growth and causing the elongation of cells in stems.
Cholodny-Went theory A theory that proposes that the accumulation of auxin on the shaded part of the plant is responsible for an elongation response to light as an external stimulus. Gravitropism The movement of a plant in response to gravity. Hydrotropism The movement of a plant in response to water. Meristematic Meristematic cells or tissues in plants are those that have not differentiated. Microgravity This is when things appear to be weightless or float in space and indicates that g-forces are not zero, just very small.
Negative tropism The movement of a plant away from an external stimulus. Photosynthesis The process used by plants to change light energy into biochemical energy sugar. Phototropism The movement of a plant in response to light. Positive tropism The movement of a plant towards an external stimulus. Thigmotropism The movement of a plant in response to touch. Tropism The movement of a plant in a certain direction in response to an external stimulus.
References Controlling plant growth Retrieved August 9, This article describes various types of tropisms. Darwin, Design, and Phototropism Retrieved August 9, Geotropism Retrieved August 10, This article describes gravitropism.
Other links on this page link to descriptions of other tropisms. This article from How Stuff Works describes the mechanism by which Venus flytrap captures prey. In the Venus flytrap, two modified leaves are joined at a hinge and lined with thin fork-like tines along the outer edges. Tiny hairs are located inside the trap. When an insect brushes against these trigger hairs, touching two or more of them in succession, the leaves close quickly, trapping the prey.
Glands on the leaf surface secrete enzymes that slowly digest the insect. The released nutrients are absorbed by the leaves, which reopen for the next meal. Thigmomorphogenesis is a slow developmental change in the shape of a plant subjected to continuous mechanical stress. When trees bend in the wind, for example, growth is usually stunted and the trunk thickens.
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