Plant Hormones: Definition, Types, and Examples

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Plant Hormones Definition

Plant hormones are molecules that allow plants to communicate, coordinate, and grow their numerous cells. Plants, like mammals, rely on chemical signals to control the expression of DNA and the cell’s functions.

Plant hormones are naturally occurring chemicals that regulate a variety of elements of plant growth. Everything from the distance between nodes on branches to the planned death, or senescence, found in many annual plants is controlled by them.

Plant hormones are divided into five categories, each controlling different elements of plant growth. There are a few more plant hormones that have only lately been discovered. Keep in mind that these are broad categories, and different species may have evolved unique applications for different hormones.

Types of Plant Hormones

i. Abscisic Acid

Because plant hormones are significantly engaged in the dormancy process, abscisic acid was formerly known as dormin. These plant hormones are now known to have two major roles in plants.

To begin with, they govern the seed development process. This aids in the development of the embryo into a full-fledged seedling. Second, these plant hormones are important in the plant’s reaction to changes in temperature and water loss.

As the temperature rises, more water vaporises from the stoma, creating small holes in the leaves. Abscisic acid is generated and released into the leaves when the temperature reaches a point where significant water loss begins.

The stoma closes as a result, and the water is trapped within the leaves. Plants would be unable to control their water content without these hormones. Vascular plants have an essential and vital function in this regard.

ii. Auxins

Auxins are a type of plant hormone that regulates a variety of elements of plant development. They usually have an impact on cell enlargement and elongation. Phototropism and geotropism, respectively, enable the plant to respond to sunlight and gravity.

Auxins are crucial for forming the apical meristem and determining the plant’s growth direction in many plants. Auxins are generally distributed in a manner in which they are concentrated in the shoots and less concentrated in the roots for this reason.

Depending on the species, the concentration of these plant hormones also guides development in different regions of the plant. Auxins are involved in the beginning and end of the fruiting process in fruit plants.

Auxins have a role in the control and storage of carbohydrates in the roots of potatoes and carrots. Several synthetic auxin compounds have been produced in the lab and can be used as plant hormones.

Plant growth regulators are what they’re called. Auxins synthesised in this way have a wide range of commercial applications. By triggering new growth, they can start the rooting process. They’re also used to destroy weeds.

Many plants’ growth cycles can be disrupted by synthetic auxins, resulting in their death. Synthetic auxins can also be used to develop new plants from tissues or inhibit undesirable branches from growing on attractive trees.

Auxins have a wide range of applications due to the fact that they were one of the first plant hormones to be identified.

iii. Cytokinins

A category of plant hormones known as cytokinins interacts directly with auxins. They control cell differentiation and several aspects of cell metabolism in this way. The cytokinins interact with the DNA of the plant, causing it to express or conceal different proteins.

This allows the plant to generate distinct tissues for different functions by directing cell differentiation inside the plant. The cytokinins, unlike the auxins, are most concentrated in the roots and become less concentrated as they approach the shoots.

This auxin balancing allows the plant to grow in both directions and form and retain an axis. Auxins alone generate roots, cytokinins alone produce buds, and when they’re together, they cause undifferentiated development. While numerous tests have been conducted with these plant hormones, there are few commercial uses.

iv. Ethylene

Ethylene is a single chemical, unlike the other plant hormones. It takes the form of a gas at room temperature, and it helps plants to communicate quickly between cells and with other plants. Ethylene gas was discovered to start the ripening process in fruits in the early 1900s.

Ethylene is commonly detected in plants after they have been damaged. Ethylene is released when a stem is twisted, damaged, or fractured.

Ethylene is a gas that swiftly diffuses through the plant’s fluids and can move into the air. This hormone is used by plants to convey harm to other plants, causing them to ripen their fruit or build herbivore defences.

Ethylene was discovered in the 1960s, and it has since been used in a variety of commercial applications. Ethylene may be blasted over a crop as a gas, causing the entire crop to ripen at once. This enables commercial farmers to harvest a whole crop at once.

Ethylene causes fruits, nuts, and vegetables to reach the end of their growth cycle and separate from the stem. This makes collecting a breeze. Plant hormones are also utilised to control the sex expression of specific plants, allowing producers to manage their harvest.

v. Gibberellins

Gibberellins, like auxins, are plant hormones that regulate growth. They regulate cell division and overall plant development to a higher extent than auxins. Gibberellins cannot be synthesised or used by dwarf plants due to a genetic flaw.

Extra gibberellins will cause a dwarf plant to expand to its full size. Gibberellins are also involved in the activation of a variety of enzymes. Gibberellins are used by some plants as sexual hormones, assisting in the production of male and female flowers.

Gibberellin plant hormones, like auxins, influence the senescence of plant components. Gibberellins are also crucial in bringing dormant seeds to life. Gibberellins activate enzymes like amylases in the seed, which break down starches into glucose and supply energy to the embryo.

Other enzymes are activated by the plant hormones, which give amino acids and lipids for the embryo’s growth. These plant hormones have a variety of applications in commercial agriculture. If administered at the proper time, gibberellins can enhance the size of grapes and other fruits.

Gibberellins naturally encourage seed germination, and synthetic gibberellins can help seeds germinate as well. This can aid in the sprouting and viability of all seedlings.

Gibberellins can also be used by commercial farmers to encourage male or female blooms, allowing them to selectively breed a variety of plants. Gibberellins are frequently obtained from bacteria that are cultivated to produce gibberellins for commercial use.

Other Plant Hormones

In recent years, four additional fundamental plant hormone families have been identified. Brassinolides, like oestrogen and testosterone, are steroid hormones. These plant hormones appear to have a role in cell division, but their exact mechanism of action is unknown.

Another recently found hormone, salicylic acid, works similarly to ethylene and helps plants to interact with one another. Like an immune system hormone, these plant hormones respond to invaders and assault.

A related plant hormone is represented by the jasmonates. Finally, systemin is a type of plant defence hormone that helps plants activate their defence genes when a part of their system has been damaged.

Plant Hormones Citations
  • The role of plant hormones during grafting. J Plant Res . 2018 Jan;131(1):49-58.
  • Role of plant hormones in plant defence responses. Plant Mol Biol . 2009 Mar;69(4):473-88.
  • Strigolactones: Plant Hormones with Promising Features. Angew Chem Int Ed Engl . 2019 Sep 9;58(37):12778-12786.
  • Melatonin and its relationship to plant hormones. Ann Bot . 2018 Feb 12;121(2):195-207.
  • How Plant Hormones Mediate Salt Stress Responses. Trends Plant Sci . 2020 Nov;25(11):1117-1130.
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