Plant hormones are specialized chemical substances produced by plants. They are the main internal factors controlling growth and development. Hormones are produced in one part of a plant and transported to others, where they are effective in very small amounts. Depending on the target tissue, a given hormone may have different effects. Plant hormones play an integral role in controlling the growth and development of plants. A plant hormone is generally described as an organic compound synthesized in one part of the plant and translocate d to another part, where in low concentrations elicits a physiological response.

There are five generally recognized classes of plant hormones; some of the classes are represented by only one compound, others by several different compounds. They are all organic compounds, they may resemble molecules which turn up elsewhere in plant structure or function, but they are not directly involved as nutrients or metabolites. Hormone Source ActionAuxins apical meristem (only moves down), embryo of seed, young leaves o Control of cell elongation o apical dominance (prevents lateral buds) o prevents abscission o continued growth of fruit o cell division in vascular and cork cambium -- formation of lateral roots from pericycle -- formation of adventitious roots from cuttings Gibberellins Roots and young leaves o Cell (stem) elongation (works in stems and leaves, but not roots) o breaking seed / bud dormancy o stimulating fruit set Cytokinins roots, embryos, fruits actively growing o Promote cell division -- signal axillary / lateral bud growth -- prevent leaf abscission o chloroplast development o breaking dormancy in some seeds o enhance flowering o promote fruit development Abscisic Acid leaves, stems, green fruit o Reduces cell division (helps maintain dormancy of seeds and buds) o prepare plants for winter 1. decreasing cell division 2.

developing protective scales 3. deposition of waterproofing substances o closes stomata Ethylene tissues of ripening fruit, nodes of stems, senescent leaves, flowers Growth inhibitor o fruit ripening o leaf abscission o initiation of flowering o apical hook of some dicots AUXINS: Nature of Auxins Compounds are generally considered auxin's if they can be characterized by their ability to induce cell elongation in stems and otherwise resemble indole acetic acid (the first auxin isolated) in physiological activity. Auxins usually affect other processes in addition to cell elongation of stem cells but this characteristic is considered critical of all auxin's and therefore defines the hormone. Functions: 1.

Stimulates cell elongation 2. Stimulates cell division in the cambium and, in combination with in tissue culture 3. Stimulates differentiation of phloem and xylem 4. Stimulates root initiation on stem cuttings and lateral root development in tissue culture 5. Mediates the response of bending in response to gravity and light 6. The auxin supply from the apical bud suppresses growth of lateral buds 7.

Delays leaf senescence 8. Can inhibit or promote (via ethylene stimulation) leaf and fruit abscission 9. Can induce fruit setting and growth in some plants 10. Involved in assimilate movement toward auxin possibly by an effect on phloem transport 11. Delays fruit ripening 12.

Promotes flowering in Bromeliad's 13. Stimulates growth of flower parts 14. Promotes (via ethylene production) femaleness in dioecious flowers 15. Stimulates the production of ethylene at high concentrations Gibberellins: The Nature of Gibberellins Unlike the classification of auxin's which are classified on the basis of function, are classified on the basis of structure as well as function. All are derived from the ent- skeleton. All are acidic compounds and are therefore also called acids (GA) with a different subscript to distinguish between them.

GA's are widespread and so far present in both flowering (angiosperms) and non-flowering (gymnosperms) plants as well as ferns. They have also been isolated from lower plants such as mosses and algae, at least two fungal species and most recently from two bacterial species. There have been over 90 GA's isolated, all of which are most likely not essential to the plant. Instead, these forms are probably inactive precursors or breakdown products of active. Function: Active show many physiological effects, each depending on the type of present as well as the species of plant. Some of the physiological processes stimulated by are: 1.

Stimulate stem elongation by stimulating cell division and elongation. 2. Stimulates bolting / flowering in response to long days. 3. Breaks seed dormancy in some plants which require stratification or light to induce germination. 4.

Stimulates enzyme production (a-amylase) in germinating cereal grains for mobilization of seed reserves. 5. Induces maleness in dioecious flowers (sex expression). 6. Can cause (seedless) fruit development.

7. Can delay senescence in leaves and citrus fruits. Cytokinins: Nature of Cytokinins Cytokinins are compounds with a structure resembling adenine which promote cell division and have other similar functions to kine tin. Kinetic was the first discovered and so named because of the compounds ability to promote cytokinesis. Though it is a natural compound, It is not made in plants, and is therefore usually considered a 'synthetic' (meaning that the hormone is synthesized somewhere other than in a plant). The most common form of naturally occurring in plants today is called which was isolated from corn.

Cytokinins have been found in almost all higher plants as well as mosses, fungi, bacteria, and also in t RNA of many prokaryotes and eukaryotes. Today there are more than 200 natural and synthetic combined. Function: A list of some of the known physiological effects caused by are listed below. The response will vary depending on the type of and plant species.

1. Stimulates cell division. 2. Stimulates morphogenesis (shoot initiation / bud formation) in tissue culture. 3. Stimulates the growth of lateral buds-release of apical dominance.

4. Stimulates leaf expansion resulting from cell enlargement. 5. May enhance stomatal opening in some species. 6. Promotes the conversion of into chloroplasts via stimulation of chlorophyll synthesis.

Abscisic Acid: Nature of Abscisic Acid Abscisic acid is a single compound unlike the auxin's, , and. It was called 'abscising II' originally because it was thought to play a major role in abscission of fruits. At about the same time another group was calling it 'dorm in' because they thought it had a major role in bud dormancy. The name acid (ABA) was coined by a compromise between the two groups. Though ABA generally is thought to play mostly inhibitory roles, it has many promoting functions as well. Function: The following are some of the physiological responses known to be associated with acid.

1. Stimulates the closure of stomata (water stress brings about an increase in ABA synthesis). 2. Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots. 3. Induces seeds to synthesize storage proteins.

4. Inhibits the affect of on stimulating synthesis of a-amylase. 5. Has some effect on induction and maintenance of dormancy. 6. Induces gene transcription especially for proteinase inhibitors in response to wounding which may explain an apparent role in pathogen defense.

Ethylene: Nature of Ethylene Ethylene, unlike the rest of the plant hormone compounds is a gaseous hormone. Like acid, it is the only member of its class. Of all the known plant growth substances, ethylene has the simplest structure. It is produced in all higher plants and is usually associated with fruit ripening and the triple response. Function: Ethylene is known to affect the following plant processes: 1. Stimulates the release of dormancy.

2. Stimulates shoot and root growth and differentiation (triple response) 3. May have a role in adventitious root formation. 4. Stimulates leaf and fruit abscission. 5.

Stimulates Bromeliad flower induction. 6. Induction of femaleness in dioecious flowers. 7. Stimulates flower opening. 8.

Stimulates flower and leaf senescence. 9. Stimulates fruit ripening. How Do Plant Hormones Work? It is known that micro molar and even smaller concentrations of hormones are necessary in order for a response to be observed. Because of this fact, three criteria must be true to stimulate plant hormonal action. 1.

The hormone must be present in the correct quantity in the correct location. 2. There must be good recognition and strong binding between the hormone and the responding molecules. 3. The receptor molecule must then trigger some other metabolic change which will trigger the amplification of the hormonal signal. There are two mechanisms by which hormones will act.

The first type deals with a steroid hormone. In this type the hormone can pass through the plasma membrane into the cytoplasm. Here it binds with its receptor molecule to form a hormone-receptor complex. From this point, the complex may dissociate or it may enter the nucleus and affect m RNA synthesis. The effect of the hormone on m RNA synthesis ultimately results in the physiological response. In the second type, a peptide hormone binds to a receptor protein on the target cell.

The receptor protein will then undergo a conformational change leading to a cellular cascade ultimately resulting in modification of enzyme activity, altered metabolic processes, and different phenotypes. One thing plant hormones specifically control is gene expression. The exact mechanisms by which hormones regulate gene expression are poorly understood. Gene expression is part of a large amplification process. This process involves repeated transcription of DNA resulting in many copies of m RNA (1 st amplification step); m RNA is processed and enters the cytoplasm where it is translated many times by ribosomes into a gene product such as an enzyme (2 nd amplification step); enzymes are modified to become functional and capable of high catalytic activity even at low concentrations. They catalyze the production of many copies of an important cellular product (3 rd amplification step).

It is likely that gene regulation is affected by certain enzymes after initial hormone binding. Genes may be altered by secondary and tertiary messengers of a cellular cascade as well. Hormones may indirectly control gene expression through these enzymes and messengers at a number of control sites such as transcription, m RNA processing, m RNA stability, translation, and post-translation.