Plant hormones also known as plant growth regulators (PGRs) and phytohormones) are chemicals that regulate a plant's growth. According to a standard animal definitions, hormones are signal molecules produced at specific locations, that occur in very low concentrations and cause altered processes in target cells at other locations. The word hormone is derived from Greek and means 'set in motion'. They are naturally produced within plants though very similar chemicals are produced by fungi and bacteria that can effect plant growth. The concentration of hormones required for plant responses are very low. Plant hormones affect gene expression and transcription levels, cellular division and growth.
The five major classes are:
1. Abscisic acid:- Abscisic acid also called ABA, was discovered and researched under two different names before its chemical properties were fully known,it was called dormin and abscicin 2. Once it was determined that the two latter named compounds were the same, it was named abscisic acid. The name "abscisic acid" was given because it was found in high concentrations in newly abscissed or freshly fallen leaves.
In plants that are water stressed, ABA plays a role in closing the stomata. Soon after plants are water stressed and the roots are deficient in water, a signal moves up to the leaves causing the formation on ABA precursors, these precursors moves to the roots which release ABA that is trans-located to the foliage through the vascular system, which regulates the potassium or sodium uptake within the guard cells then loses turgidity, closing the stomata.
2. Auxins:- Auxins are compounds that positively influence cell enlargement, bud formation and root initiation.They also promote the production of other hormones and in conjuction with cytokinins, they control the growth of stems, roots, flowers and fruits. Auxins were the first class of growth regulators discovered. They affect cell elongation by altering cell wall plasticity. Auxins decrease in light and increase where its dark. They stimulate cambium cells to divide and in stems cause secondary xylem to differentiate. Auxins act to inhibit the growth of buds lower down the stems, affecting a process called apical dominance and also promote lateral and adventitious root development and growth. Auxins especially NAA and IBA, are also commonly applied to stimulate root growth when taking cuttings of plants. The most common auxin found in plants is indoleacitic acid or IAA.
The five major classes are:
1. Abscisic acid:- Abscisic acid also called ABA, was discovered and researched under two different names before its chemical properties were fully known,it was called dormin and abscicin 2. Once it was determined that the two latter named compounds were the same, it was named abscisic acid. The name "abscisic acid" was given because it was found in high concentrations in newly abscissed or freshly fallen leaves.
In plants that are water stressed, ABA plays a role in closing the stomata. Soon after plants are water stressed and the roots are deficient in water, a signal moves up to the leaves causing the formation on ABA precursors, these precursors moves to the roots which release ABA that is trans-located to the foliage through the vascular system, which regulates the potassium or sodium uptake within the guard cells then loses turgidity, closing the stomata.
2. Auxins:- Auxins are compounds that positively influence cell enlargement, bud formation and root initiation.They also promote the production of other hormones and in conjuction with cytokinins, they control the growth of stems, roots, flowers and fruits. Auxins were the first class of growth regulators discovered. They affect cell elongation by altering cell wall plasticity. Auxins decrease in light and increase where its dark. They stimulate cambium cells to divide and in stems cause secondary xylem to differentiate. Auxins act to inhibit the growth of buds lower down the stems, affecting a process called apical dominance and also promote lateral and adventitious root development and growth. Auxins especially NAA and IBA, are also commonly applied to stimulate root growth when taking cuttings of plants. The most common auxin found in plants is indoleacitic acid or IAA.3. Cytokinins:- Cytokinins are CKs are a group of chemicals that influence cell division and shoot formation. They were called Kinins in the past when the first cytokinins were isolated from yeast cells. They also help delay senescence or the aging of tissues are responsible for mediating auxin transport throughout the plant and affect internodel length and leaf growth. Cytokinins counter the apical dominance induced by auxins, they in
conjunction with ethylene promote abscission of leaves, flower parts and fruits.
4. Ethylene:- Ethylene is a gas that forms from the breakdown of methionine, which is in all cells. Ethylene has very limited solubility in water, and does not accumulate within the cell but diffuses out of the cell and escapes out of the plant. Its effectiveness as a plant hormone is dependent on its rate of escaping into the atmosphere. Ethylene is produced at a faster rate in rapidly growing and dividing cells especially in darkness. Ethylene affects cell growth and cell shape. Ethylene affects fruit ripening normally when the seeds are mature, ethylene production increases and builds-up within the fruit resulting in a climacteric event just before seed dispersal. The nuclear protein ETHYLENE INSENSITIVE 2 (EIN2) is regulated by ethylene production and in turn regulates other hormones including ABA and stress hormones.
5. Gibberellins:- Gibberellins or GAs include a large range of chemicals that are produced naturally within plants and by fungi. They were first discovered when japanese researches noticed a chemical produced by a fungus called Gibberella fujikuroi that produced abnormal growth in rice plants. They play a major role in seed germination, affecting enzyme production that mobilizes food production new cells need for growth. This is done by modulating chromosomal transcription. Gibberellins also reverse the inhibition of shoot growth and dormancy induced by ABA.
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