4.2 Plant Hormones - The Science of Plants (2023)

The five major groups of plant hormones—auxins, cytokinins, gibberellins, ethylene, and abscisic acid—are distinguished by their chemical structures and the response they elicit in the plant (see Table 4.1). For a cell to respond to a hormone, it must be able to sense the chemical. Some cells just don't have the ability to "see" the hormone and don't respond to its presence. The ability to sense a hormone depends on the cell's physiology when the hormone is present. When the hormone is sensed, its unique chemical structure causes a chain reaction, or signal transduction, that involves changes in gene expression and cell morphology. These cellular responses to hormones can result in the changes we see in the plant, such as: B. a movement towards light, a transition from vegetative growth to flowering or the closing of leaf crevices due to water stress.

Hormone perception takes place both in cells and in a tissue or organ, depending on the location of the hormone, the concentration of the hormone, the stage of development and the physiological state of the cell. Many cells within a tissue can respond in a coordinated manner, leading to plant-wide changes. Hormones are often produced in one cell and transported to other cells where they are sensed, and the response can occur far from the site of hormone synthesis. Responses to hormones are studied by applying the chemical exogenously to plant tissue - the hormone is applied to the outer (exo) part of the plant and observations are made of how the plant responds. For gaseous hormones like ethylene, this means that the hormone can be transferred from one plant to another. Plants essentially talk to each other using a variety of molecules.

Experiments in which hormones are applied exogenously to a plant show how plants respond to hormones; Much of our knowledge of the role hormones play in plant growth comes from this type of experimentation. However, when a hormone is administered exogenously, it also interacts with any hormones present in the plant. These areendogenousHormone (Endomeans internal), and the cell reacts according to the sum of all hormones in its presence. This can complicate the interpretation of responses to exogenous hormone use.

Hormones used in plant reproduction can be natural and found in many plants, or they can be synthetic or synthesized to mimic the structure and response of a natural hormone. Synthetic hormones are often used over natural versions because they are cheaper to source, can produce stronger or longer lasting reactions, and may be less prone to breakdown in the plant and during storage. Because the exogenous application of hormones plays a role in manipulating or arresting plant growth, they are used extensively as herbicides (herbicides) and can be targeted to specific plant species based on how specific species respond to different structures.

Auxin

Auxins are a group of related molecules involved in almost every aspect of the plant life cycle. Auxins stimulate growth through cell elongation, which is an integral part of plant responses to environmental change. Auxins are responsible for two types of growth responses:Phototropismus, the bending or growing of a bud towards the light, andGravitropismus, a growth change that occurs after a change in gravitational force. The chart below shows indole acetic acid (IAA, shown with pink dots), a naturally occurring auxin, moving from the sunny side to the shaded side of a bud tip. The differential accumulation of auxin on the shaded side of the bud causes these cells to increase growth and bend the bud tip towards the light. The stimulation of cell growth by auxin is also important for wound healing and callus formation after circumcision.

Auxin's ability to regulate growth can be used against weeds (plants out of place). The synthetic auxin 2,4-dichlorophenoxyacetic acid or 2,4-D is a widely used herbicide that disrupts normal growth regulation when applied to the plant, resulting in leaf fall and death. Because dicotyledonous (two-cotton) plants are more competent to respond to 2,4-D, 2,4-D can be used as a selective herbicide to kill dicot weeds in lawns and corn fields that are resistant to monocotyledonous grasses (single-cotyledonous plants). Grass is left unharmed due to its lower responsiveness, while dicots are killed.

In general, auxins are produced in the young leaves of a plant and translocated down to older tissues. This downward translocation controls apical dominance where axillary bud growth is suppressed. Removing (pinching) the tip of the stem where auxin is produced, as shown in the three mint photos below, frees the axillary buds from apical dominance and they begin to grow. This is a common horticultural practice that increases branching and flower production. Pinching is often used on plant seedlings such as basil or zinnias to achieve spherical shapes in a pot rather than large single stem plants.

One of the most important uses of auxin in plant propagation is to stimulate the growth of adventitious roots—roots that arise anywhere on the plant except the roots—in scion cuttings. The photo below shows sections of two different onesAcer ginnala(Amur Maple) Plants that have different abilities to form adventitious roots. Both cuttings were treated with auxin, but only the competent plant forms adventitious roots (left). A cutting from a plant that is unable to respond to auxin will not root (right) and will eventually die.

This form of asexual (clonal) reproduction is used by both horticultural professionals and hobbyists. Rooting competency for cuttings can be species specific or seasonal. Collecting stalks of a plant to use as seedlings can be most successful in the growing season, as with the Amur maples shown above. However, for plants like grapes, the cuttings are taken and rooted during the winter when the vines are not actively growing. The following video shows how shoots are taken from Amur maples, treated with auxin and incubated in a high humidity environment for several weeks to form adventitious roots. Exogenous auxin application is not required for adventitious rooting of all plants. Some plants can form many adventitious roots without exogenous applications because the naturally occurring endogenous auxin in the shoot is sufficient for root formation.

Cytokinine

Like auxins, cytokinins are a group of related molecules that regulate growth and development. However, the plant response to cytokinin is very different from the response to auxin. Cytokinin comes from the word cytokinesis, which means cell division. You will learn about cytokinesis, especially mitosisChapter 13.

Cytokinins promote cell division, with one cell dividing and two new daughter cells being formed. Cytokinins are important regulators of plant growth and development.

Cytokinins have an interesting interaction with auxins in plants. In the 1950s, Skoog and Miller studied the growth ofNO. tobaccoStrains in Tissue Culture. They found that they could use specific ratios of an auxin (IAA) and a cytokinin (kinetin) to control the growth of stem tissue in culture. A high cytokinin to auxin ratio led to the formation of sprouts, a higher auxin level led to the formation of roots and equal amounts of each produced callus growth, which is the undifferentiated growth of plant cells. The transformative discovery of Skoog and Miller formed the basis of "MS" plant medium, which remains popular for tissue culture plant propagation.

Watch this video to learn more about plant propagation on synthetic media with exogenous hormones in tissue culture.

Gibberin

Gibberellins or gibberellic acid (GA) are a group of more than 100 molecules that are primary regulators of stem elongation and seed germination. They were discovered while researching the cause of "silly seedling disease" in rice. The disease, characterized by tall plants with few grains, is caused by infection withGibberella fujikora, a parasitic fungus that produces GA in the rice shoots, leading to increased stem elongation.

EmChapter 9.2, on seed physiology, you will learn that some seeds are dormant and will not germinate even if the right environment is in place. Seed dormancy, which has many causes and evolutionary advantages, always has the common trait of preventing seed germination until the time, season, or seed physiology is right. Planting a dormant seed or a dead seed produces the same result: no germination. For plant growers, the dormant period can be confusing, leading to the question, "Are my seeds dead or dormant?" Any condition prevents the germination and reproduction of the plant. Seed treatment with GA is a common method to break dormancy and facilitate germination. GA treatment ofYellow gentian(Bitterroot), for example, increases germination from 0% (no germination) to over 80% when treated with 100 parts per million (ppm) GA (see germination chart at left). If a multiplierG. luteaknew nothing about seed dormancy, they may have assumed their bitter roots were dead. For most plants, GA is the endogenous hormone that triggers seed germination.

abscisic acid

While GA facilitates seed germination, abscisic acid (ABA) inhibits it. Abscisic acid is a unique molecule that regulates germination and a plant's response to reduced water availability during water stress. ABA levels increase when there is less water available to the plant, causing several responses including stomata closure. The closing of the stomata slows transpiration (also called evapotranspiration), the movement of water in the plant from the root, through the stem, to the leaf, and out through the stomata to the atmosphere. Read more about stomata and water movement inChapter 11, Plants and Water.

Reducing water content is one of the final steps in seed maturation and is important to seed longevity by keeping metabolism to a minimum, which is the dormant nature of mature seeds. Increasing endogenous ABA levels in seeds prepares them to survive lower water levels, is important for seed maturation, and prevents early germination (voviparity). Seeds with low ABA levels during seed development may germinate prematurely. Low ABA levels can result from a genetic mutation or environmental causes. Viviparity is not uncommon in some fruits and can occur during fruit storage in supermarkets.

ethylene

Ethylene is known as the gaseous hormone of maturation. It also regulates seedling growth and root hair formation, and can lead to epinasty — the bending of branches downwards.

Many plants are sensitive to the effects of ethylene on fruit ripening. Iconic examples are the tomato and the banana. These fruits are climacteric - they continue to ripen after harvest. The perception of ethylene by the cells that make up the fruit triggers the ripening process and the production of more ethylene. As the ethylene concentration increases, the rate of ripening also increases. Harvesting unripe or immature fruit allows for long-distance transportation because the fruit is firmer and less prone to damage in transit. The unripe fruit can then be treated with ethylene from an ethylene generator (right) to accelerate ripening.

In other fruit crops, the introduction or production of ethylene should be avoided to prevent overripening and spoilage. To prevent the formation of ethylene during fruit storage, ethylene is removed from the air by an air filtration system. A reduction in ethylene concentration means slower ripening and less spoilage.

The senescence process is also triggered by the production of ethylene and is important in the cut flower industry. Keeping cut flowers away from gases with ethylene-like activity will help keep floral arrangements looking fresh. Reducing the effect of ethylene extends the shelf life of many cut flowers as well as fruit storage.

The five major groups of plant hormones control many aspects of plant growth and development and have important applications in plant reproduction. But many other molecules are also essential for the plant's response to its environment. These very different signaling molecules modulate plant physiology through complex interactions. A cell's response to many different hormones is a sum of its genetic makeup, physiology, and environment.