The Role of Hormones in Plants The majority of people know that hormones play a vital and important part in animal growth and development. The common layperson knows that without certain chemicals in their bodies, they would not function right. An example of a hormonal problem would be in the case of a dwarf in which a person does not produce enough growth hormone and grows only to a small height. What most people do not know is that plants also require hormones to grow and develop.
This is not my work. The average person tends to think that plants just magically grow without anything regulating them. In reality, there is a complex series of hormones that regulate all of the growth and seed development in plants. That is what this presentation is about, the role that these hormones play in the life of a plant. Before I can go into detail about plant hormones, it is important that I tell you some basic parts of plants that are affected by plant hormones.
This picture shows some of the basic parts of the plant. The parts of the plant that will be mentioned in this paper are the stem, merristems, xylem and floe m, and finally, the stomates. Merristems are located at the tips of stems, buds, and nodes. They are the permantely embryonic parts of a plant.
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All merristems are composed of undifferencated tissue, and the cells in the meristems are continuously dividing. The merristems main function in a plant is to provide the plant with new leaves, roots, and stems. Xylem and phloem are the vascular tissue of plants. Phloem is composed of live cells that are interconnected so sucrose and amino acids can pass through them from the leaves to the roots. Xylem is composed of dead cells that are hollowed out so water and minerals can move from the roots to the leaves.
Stomates are little holes in the leaves of plants. I did not write this. It is through these holes that carbon dioxide for photsynthsis enters th plant, and water evaporates out. The evaporation of water is important for a plant because of transperation. Transperation is how water is pumped up the plant. As water evaporates from the leaves, it creates a suction through the xylem which pulls water up from the roots.
The advantage of moving water this way is because this process requires no energy from the plant. Stems are the main backbone of a plant. They provide the plant its hight. They also have the most observer able affects to plant hormones (1).
There are several types of plant hormones. Of the known plant hormones, there are three known plant growth promoters.
They are Auxin, Gibberellins or GA’s, and Cytokinins. One other type of plant hormone is Ethylene, which controls ripening. There is also a plant growth inhibitor, Abscisic acid (ABA), which is very useful for plants in a drought situation (4).
All of these hormones are present in all plants. I will now go into each of the different hormones in detail and explain what they do for the plant. The paper will also tell about some of the history behind each hormone and some of the current commercial applications for plant hormones.
Auxin is the first of the growth promoting hormones. It is developed in young developing tissues such as the merristems and young leaves. It was one of the first hormones discovered, and the groundwork for this discovery was laid out by none other then Charles Darwin and his son on their research into tropism. Auxin is the main substance that controls different tropism in plans such as phototropism. In the experiments of Charles Darwin and his son Francis, this relationship of auxin to phototropism was shown.
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In their study, Charles and Francis studied why grass grew towards the light. They found out that by removing the developing tips of the shoots, the plants did not grow towards the light. Fifty years after their experiments, other scientists were able to distinguish auxin as the cause of the tropism. (2).
Auxin, in the case of phototropism, is at its highest levels in the plant at the spots that the plant needs to respond to the stimulus of light. Auxin is used by plants to kill humans, plants, and animals. In plants, auxin is transported down from the developing tissues such as merristems, to the roots through the xylem system. The main affect that auxin has on plants is cell elongation. Auxin causes plant cells to enlarge by a factor of five to ten times the original size. In order for the size of a plant cell to increase, two things must occur.
First, the plant cells must take on water. Second, the cell walls of plants must expand to accommodate the extra water (5).
How auxin triggers this effect in plants is still in debate. What is not in debate is that auxin, in conjunction with another type of plant hormones called cytokinin, causes the plant cells themselves to divide. Cytokinins are also plant growth promoter.
They are produced in the root tips and are transported up through the plant via the xylem. Cytokinins, in conjunction with auxin or gibbelelins, are the only compounds able to promote cell division (1).
The way that Cytokinins were discovered is by the effect that Auxin has on a I got this paper from school sucks. com plant cutting. Before the discovery of Cytokinins, Auxin was all that was thought to be necessary to start cell division in plants. Research showed that Auxin in its pure form could not start cell division alone.
It was then notice that the addition of some other plant material such as coconut milk caused the plant cells to divide. This fact inferred that there was something in the coconut milk other then auxin that caused the plant cells to divide. After years of research, the first cytokinin was finally isolated (8).
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Cytokinins also cause the leaves of plants to expand. This is a good thing because Cytokinin production is tied to the amount of roots there are in a plant. The amount of roots in a plant determines the needed leaf space so the more roots tips the more cytokinins that are produced to implement leaf growth (4).
Gibberellins or GA are also made in developing tissue but they are different from Auxin and Cytokinins in that they can be transported down from the leaves and up from the roots instead of just in one direction. GA is the growth hormone of the plant kingdom. It is the primary factor that controls plant height. In an experiment in Biology 100, the majority of the class studied the effects that GA had on plant height.
The class planted two types of plants. One type was a normal plant and the other type was a mutant plant that did not produce any Gibberllin. One plant of each type was treated by putting GA on the leaves. The remaining two plants had only water placed on the leaves. The results of this experiment are shown in this graph. As you can see, the normal plant that received the hormone grew the highest.
The plants that were deficient in gibberellins did not grow near as high but the one that was treated with GA did outgrow its counter part (3).
This simple experiment showed that GA plays a role in determining plant height. Another role that Gibberellins play is that it is used by plants to break dormancy (9).
Many plants stop their activities during the winter to help protect themselves during the cold months.
Gibberllin is the hormone that is used to break the plant out of dormancy. The way gibberellins were discovered was by the affects of a fungal rice disease called bakanae (8).
Bakanae is Japanese for foolish seedling. Rice affected by this disease grew 50% taller then their counter parts. The problem with having the tall rice is that the rice plants did not produce as much grain as they would normally because more of their energy was used in stem development. This fungus was shown through different experiments to produce a substance that made the plants grow tall.
It was named gibberllin after the fungus that it was found in (8).
Ethylene is unique in that it is a gas. It belongs in the hydrocarbon family with such chemicals as propane and methane and like those two, Ethylene is flammable (6).
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A fruit warehouse where bananas were being treated with Ethylene actually exploded because the concentration of the gas became too high (7).
Because Ethylene is a gas, its mode of transportation is diffusion through the plant tissues.
It can also move from a plant that is producing ethylene to another plant that is not causing effects of ethylene to appear in both plants. Ethylene is produces by stressed parts of a plant. Its effect on plants is the ripening of fruit and the dropping of leaves. The only plant growth inhibitor is abscisic acid.
This substance is produced in mature leaves and is transported throughout the plant through the phloem. In a plant, Abscisic acid or ABA for short counter acts the effects of gibberellins (1).
In most plants, the ratio of GA to ABA is what’s important in determining the plant’s height. The amounts by themselves do not matter. The main use of abscisic acid is it is one of a plants main defense in a drought situation.
In a drought, the normal transpiration of a plant can become lethal. Without adequate soil Still is not mine moisture, water will evaporate from the leaves without being replaced from the ground. This action can severely dehydrate the plant if left unchecked. ABA can stop that. When there is a lack of water in a plant, the leaves produces a massive amount of ABA. The abscisic in the leaves interacts with the stomates, causing them to close.
That stops plant transpiration and helps slow the drying of the plants. Concurrently in a drought, ABA promotes root growth and stops plant shoots from growing (2).
These measures help the plant by first increasing the area in which the plant can get water and also by reducing the amount of water that the plant needs to survive. It takes a while for the effects of ABA wear off which creates a lag in the recovery of a plant when the plant finally gets water.
This is why plants that are even stressed lightly have severe reduction in their growth and yield. There are many commercial applications of plant hormones. Auxin, in the form of 2-4 D, is a powerful herbicide (7).
It works by giving the plant and overdose of a powerful form of Auxin. This type of Auxin is much more powerful then the regular type produces in plants. It is also more persistent which means it stays in plants longer then plant based Auxin.
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The overdose of Auxin affects the plant making it grow using extremely rapid cell elongation. The rate that this happens becomes too fast for the plant to maintain itself and the plant dies. Auxin, along with the other hormones, also has a non lethal use is the promotion of plants from stem cuttings. Using different plant hormones on plant tissues will cause plant tissues, and even individual cells to develop into complete plants.
A combination of auxin and cytokinin are used to start the formation of root tissue. Then later on in the growth of the culture, gibberellins are applied to promote specialized growth such as stems. This use of plant hormones is very important for plant breeders who are trying to clone a plant (10).
One thing that a plant breeder must remember is that more is not always better.
The response to plant hormones will increase up to a point but then it will decline to normal or below normal levels (7).
This is why hormones need to be used sparingly; too much of a hormone might harm or even kill a plant. Ethylene is also another popularly used plant hormone. This substance is used to make fruits ripe such as bananas in a warehouse. It is also used out in the field to make fruits ripe such as vine ripened tomatoes.
Using ethylene on the tomatoes causes them to ripen at the same time, which is very beneficial for harvesting. In addition, because it makes leaves drop, ethylene can be used as a defoliant. The danger in using ethylene is that it is explosive. Another problem that arises from ethylene is it’s naturally mixed with propane so sometimes it can be inadvertently applied on accident from un vented gas heaters in greenhouses (6).
Early on this fact led to the discovery of the powers of ethylene but, because it takes only a little ethylene to do a lot, the effect on plants inadvertently applied can be very devastating (8).
The other plant hormones do not have any major commercial use today.
Gibberellins in other that plant cuttings do not produce any favorable affects in the majority of plants. In making a plant taller, the plant uses up a lot of the energy that could go for crop yield. GA does have a favorable effect of grapes by making bigger vines but the cost of GA in not worth the increase in yield. Abscisic acid has no beneficial affects to plants.
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It could conceivable be used as a herbicide but the cost of production make it not a very wise choice when there are cheaper herbicides such as roundup around Plant hormones are an important part of plant metabolisms. They regulate many aspects of a plant’s life cycle including growth, fruit ripening, and breaking dormancy. Plant hormones affect virtually all parts of a plant including stems, merristems, leaves, roots, and the ripening fruit. They also help protect plants from disease, drought, and other circumstances, which may stress a plant. Although research is limited in how plant hormones work, there are still many important uses for plant hormones such as plant cloning, weed control, and ripening. With further research into plant hormones, scientist will be able to better understand how a plant functions.
It is truly a field in biology that has room for expansion.