The five major classes of plant hormones

Cytokinins

Nature of Cytokinins

Cytokinins are compounds with a structure resembling adenine which promote cell division and have other similar functions to kinetin. Kinetin was the first cytokinin discovered and so named because of the compounds ability to promote cytokinesis (cell division). Though it is a natural compound, It is not made in plants, and is therefore usually considered a "synthetic" cytokinin (meaning that the hormone is synthesized somewhere other than in a plant). The most common form of naturally occurring cytokinin in plants today is called zeatin which was isolated from corn (Zea mays).

Cytokinins have been found in almost all higher plants as well as mosses, fungi, bacteria, and also in tRNA of many prokaryotes and eukaryotes. Today there are more than 200 natural and synthetic cytokinins combined. Cytokinin concentrations are highest in meristematic regions and areas of continuous growth potential such as roots, young leaves, developing fruits, and seeds (Arteca, 1996; Mauseth, 1991; Raven, 1992; Salisbury and Ross, 1992).

History of Cytokinins

In 1913, Gottlieb Haberlandt discovered that a compound found in phloem had the ability to stimulate cell division (Haberlandt, 1913). In 1941, Johannes van Overbeek discovered that the milky endosperm from coconut also had this ability. He also showed that various other plant species had compounds which stimulated cell division (van Overbeek, 1941). In 1954, Jablonski and Skoog extended the work of Haberlandt showing that vascular tissues contained compounds which promote cell division (Jablonski and Skoog, 1954). The first cytokinin was isolated from herring sperm in 1955 by Miller and his associates (Miller et al., 1955). This compound was named kinetin because of its ability to promote cytokinesis. Hall and deRopp reported that kinetin could be formed from DNA degradation products in 1955 (Hall and deRopp, 1955). The first naturally occurring cytokinin was isolated from corn in 1961 by Miller (Miller, 1961). It was later called zeatin. Almost simultaneous with Miller Letham published a report on zeatin as a factor inducing cell division and later described its chemical properties (Letham, 1963). It is Miller and Letham that are credited with the simultaneous discovery of zeatin. Since that time, many more naturally occurring cytokinins have been isolated and the compound is ubiquitous to all plant species in one form or another (Arteca, 1996; Salisbury and Ross, 1992).

Biosynthesis and Metabolism of Cytokinins

Cytokinin is generally found in higher concentrations in meristematic regions and growing tissues. They are believed to be synthesized in the roots and translocated via the xylem to shoots. Cytokinin biosynthesis happens through the biochemical modification of adenine. The process by which they are synthesized is as follows (McGaw, 1995; Salisbury and Ross, 1992):

A product of the mevalonate pathway called isopentyl pyrophosphate is isomerized.

This isomer can then react with adenosine monophosphate with the aid of an enzyme called isopentenyl AMP synthase.

The result is isopentenyl adenosine-5'-phosphate (isopentenyl AMP).

This product can then be converted to isopentenyl adenosine by removal of the phosphate by a phosphatase and further converted to isopentenyl adenine by removal of the ribose group.

Isopentenyl adenine can be converted to the three major forms of naturally occurring cytokinins.

Other pathways or slight alterations of this one probably lead to the other forms.

Degradation of cytokinins occurs largely due to the enzyme cytokinin oxidase. This enzyme removes the side chain and releases adenine. Derivitives can also be made but the pathways are more complex and poorly understood.

Cytokinin Functions

A list of some of the known physiological effects caused by cytokinins are listed below. The response will vary depending on the type of cytokinin and plant species (Davies, 1995; Mauseth, 1991; Raven, 1992; Salisbury and Ross, 1992).

•           Stimulates cell division.

•           Stimulates morphogenesis (shoot initiation/bud formation) in tissue culture.

•           Stimulates the growth of lateral buds-release of apical dominance.

•           Stimulates leaf expansion resulting from cell enlargement.

•           May enhance stomatal opening in some species.

•           Promotes the conversion of etioplasts into chloroplasts via stimulation of chlorophyll synthesis.

Auxins

Nature of Auxins

The term auxin is derived from the Greek word auxein which means to grow. Compounds are generally considered auxins if they can be characterized by their ability to induce cell elongation in stems and otherwise resemble indoleacetic 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 auxins and thus "helps" define the hormone (Arteca, 1996; Mauseth, 1991; Raven, 1992; Salisbury and Ross, 1992).

History of Auxins and Pioneering Experiments

Auxins were the first plant hormones discovered. Charles Darwin was among the first scientists to dabble in plant hormone research. In his book "The Power of Movement in Plants" presented in 1880, he first describes the effects of light on movement of canary grass (Phalaris canariensis) coleoptiles. The coleoptile is a specialized leaf originating from the first node which sheaths the epicotyl in the plants seedling stage protecting it until it emerges from the ground. When unidirectional light shines on the coleoptile, it bends in the direction of the light. If the tip of the coleoptile was covered with aluminum foil, no bending would occur towards the unidirectional light. However if the tip of the coleoptile was left uncovered but the portion just below the tip was covered, exposure to unidirectional light resulted in curvature toward the light. Darwin's experiment suggested that the tip of the coleoptile was the tissue responsible for perceiving the light and producing some signal which was transported to the lower part of the coleoptile where the physiological response of bending occurred. He then cut off the tip of the coleoptile and exposed the rest of the coleoptile to unidirectional light to see if curving occurred. Curvature did not occur confirming the results of his first experiment (Darwin, 1880). 
It was in 1885 that Salkowski discovered indole-3-acetic acid (IAA) in fermentation media (Salkowski, 1885). The isolation of the same product from plant tissues would not be found in plant tissues for almost 50 years. IAA is the major auxin involved in many of the physiological processes in plants (Arteca, 1996). In 1907, Fitting studied the effect of making incisions on either the light or dark side of the plant. His results were aimed at understanding if translocation of the signal occurred on a particular side of the plant but his results were inconclusive because the signal was capable of crossing or going around the incision (Fitting, 1907). In 1913, Boysen-Jensen modified Fritting's experiment by inserting pieces of mica to block the transport of the signal and showed that transport of auxin toward the base occurs on the dark side of the plant as opposed to the side exposed to the unidirectional light (Boysen-Jensen, 1913). In 1918, Paal confirmed Boysen-Jensen's results by cutting off coleoptile tips in the dark, exposing only the tips to the light, replacing the coleoptile tips on the plant but off centered to one side or the other. Results showed that whichever side was exposed to the coleoptile, curvature occurred toward the other side (Paal, 1918). Soding was the next scientist to extend auxin research by extending on Paal's idea. He showed that if tips were cut off there was a reduction in growth but if they were cut off and then replaced growth continued to occur (Soding, 1925). 
In 1926, a graduate student from Holland by the name of Fritz Went published a report describing how he isolated a plant growth substance by placing agar blocks under coleoptile tips for a period of time then removing them and placing them on decapitated Avena stems (Went, 1926). After placement of the agar, the stems resumed growth (see below). In 1928, Went developed a method of quantifying this plant growth substance. His results suggested that the curvatures of stems were proportional to the amount of growth substance in the agar (Went, 1928). This test was called the avena curvature test.(see below) 

Much of our current knowledge of auxin was obtained from its applications. Went's work had a great influence in stimulating plant growth substance research. He is often credited with dubbing the term auxin but it was actually Kogl and Haagen-Smit who purified the compound auxentriolic acid (auxin A) from human urine in 1931 (Kogl and Haagen-Smit, 1931). Later Kogl isolated other compounds from urine which were similar in structure and function to auxin A, one of which was indole-3 acetic acid (IAA) initially discovered by Salkowski in 1985. In 1954 a committee of plant physiologists was set up to characterize the group auxins. The term comes from the Greek auxein meaning "to grow." Compounds are generally considered auxins if they are synthesized by the plant and are substances which share similar activity to IAA (the first auxin to be isolated from plants) (Arteca, 1996; Davies, 1995). 

Biosynthesis and Metabolism of Auxin

IAA is chemically similar to the amino acid tryptophan which is generally accepted to be the molecule from which IAA is derived. Three mechanisms have been suggested to explain this conversion: 
Tryptophan is converted to indolepyruvic acid through a transamination reaction. Indolepyruvic acid is then converted to indoleacetaldehyde by a decarboxylation reaction. The final step involves oxidation of indoleacetaldehyde resulting in indoleacetic acid. 
Tryptophan undergoes decarboxylation resulting in tryptamine. Tryptamine is then oxidized and deaminated to produce indoleacetaldehyde. This molecule is further oxidized to produce indoleacetic acid. 
As recently as 1991, this 3rd mechanism has evolved. IAA can be produced via a tryptophan-independent mechanism. This mechanism is poorly understood, but has been proven using trp(-) mutants. Other experiments have shown that, in some plants, this mechanism is actually the preferred mechanism of IAA biosynthesis. 
The enzymes responsible for the biosynthesis of IAA are most active in young tissues such as shoot apical meristems and growing leaves and fruits. The same tissues are the locations where the highest concentrations of IAA are found. One way plants can control the amount of IAA present in tissues at a particular time is by controlling the biosynthesis of the hormone. Another control mechanism involves the production of conjugates which are, in simple terms, molecules which resemble the hormone but are inactive. The formation of conjugates may be a mechanism of storing and transporting the active hormone. Conjugates can be formed from IAA via hydrolase enzymes. Conjugates can be rapidly activated by environmental stimuli signaling a quick hormonal response. Degradation of auxin is the final method of controlling auxin levels. This process also has two proposed mechanisms outlined below: 
The oxidation of IAA by oxygen resulting in the loss of the carboxyl group and 3-methyleneoxindole as the major breakdown product. IAA oxidase is the enzyme which catalyzes this activity. Conjugates of IAA and synthetic auxins such as 2,4-D can not be destroyed by this activity. 
C-2 of the heterocyclic ring may be oxidized resulting in oxindole-3-acetic acid. C-3 may be oxidized in addition to C-2 resulting in dioxindole-3-acetic acid. 
The mechanisms by which biosynthesis and degradation of auxin molecules occur are important to future agricultural applications. Information regarding auxin metabolism will most likely lead to genetic and chemical manipulation of endogenous hormone levels resulting in desirable growth and differentiation of important crop species. Ultimately, the possibility exists to regulate plant growth without the use of hazardous herbicides and fertilizers (Davies, 1995; Salisbury and Ross, 1992). 

Functions of Auxin

The following are some of the responses that auxin is known to cause (Davies, 1995; Mauseth, 1991; Raven, 1992; Salisbury and Ross, 1992).

• Stimulates cell elongation 
• Stimulates cell division in the cambium and, in combination with cytokinins in tissue culture 
• Stimulates differentiation of phloem and xylem 
• Stimulates root initiation on stem cuttings and lateral root development in tissue culture 
• Mediates the tropistic response of bending in response to gravity and light 
• The auxin supply from the apical bud suppresses growth of lateral buds 
• Delays leaf senescence 
• Can inhibit or promote (via ethylene stimulation) leaf and fruit abscission 
• Can induce fruit setting and growth in some plants 
• Involved in assimilate movement toward auxin possibly by an effect on phloem transport 
• Delays fruit ripening 
• Promotes flowering in Bromeliads 
• Stimulates growth of flower parts 
• Promotes (via ethylene production) femaleness in dioecious flowers 
• Stimulates the production of ethylene at high concentrations

Ethylene

Nature of Ethylene

Ethylene, unlike the rest of the plant hormone compounds is a gaseous hormone. Like abscisic acid, it is the only member of its class. Of all the known plant growth substance, ethylene has the simplest structure. It is produced in all higher plants and is usually associated with fruit ripening and the tripple response (Arteca, 1996; Mauseth, 1991; Raven, 1992; Salisbury and Ross, 1992)

History of Discovery in Plants

Ethylene has been used in practice since the ancient Egyptians, who would gas figs in order to stimulate ripening. The ancient Chinese would burn incense in closed rooms to enhance the ripening of pears. It was in 1864, that leaks of gas from street lights showed stunting of growth, twisting of plants, and abnormal thickening of stems (the triple response)(Arteca, 1996; Salisbury and Ross, 1992). In 1901, a russian scientist named Dimitry Neljubow showed that the active component was ethylene (Neljubow, 1901). Doubt discovered that ethylene stimulated abscission in 1917 (Doubt, 1917). It wasn't until 1934 that Gane reported that plants synthesize ethylene (Gane, 1934). In 1935, Crocker proposed that ethylene was the plant hormone responsible for fruit ripening as well as inhibition of vegetative tissues (Crocker, 1935). Ethylene is now known to have many other functions as well. 

Biosynthesis and Metabolism

Ethylene is produced in all higher plants and is produced from methionine in essentially all tissues. Production of ethylene varies with the type of tissue, the plant species, and also the stage of development. The mechanism by which ethylene is produced from methionine is a 3 step process (McKeon et al., 1995; Salisbury and Ross, 1992). 
ATP is an essential component in the synthesis of ethylene from methionine. ATP and water are added to methionine resulting in loss of the three phosphates and S-adenosyl methionine. 
1-amino-cyclopropane-1-carboxylic acid synthase (ACC-synthase) facilitates the production of ACC from SAM. 
Oxygen is then needed in order ro oxidize ACC and produce ethylene. This reaction is catalyzed by an oxidative enzyme called ethylene forming enzyme. 
The control of ethylene production has received considerable study. Study of ethylene has focused around the synthesis promoting effects of auxin, wounding, and drought as well as aspects of fruit-ripening. ACC synthase is the rate limiting step for ethylene production and it is this enzyme that is manipulated in biotechnology to delay fruit ripening in the "flavor saver" tomatoes (Klee and Lanahan, 1995). 

Functions of Ethylene

Ethylene is known to affect the following plant processes (Davies, 1995; Mauseth, 1991; Raven, 1992; Salisbury and Ross, 1992):

• Stimulates the release of dormancy. 
• Stimulates shoot and root growth and differentiation (triple response) 
• May have a role in adventitious root formation. 
• Stimulates leaf and fruit abscission. 
• Stimulates Bromiliad flower induction. 
• Induction of femaleness in dioecious flowers. 
• Stimulates flower opening. 
• Stimulates flower and leaf senescence. 
• Stimulates fruit ripening.

 Abscisic Acid

 Nature of Abscisic Acid

Abscisic acid is a single compound unlike the auxins, gibberellins, and cytokinins. It was called "abscisin 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 "dormin" because they thought it had a major role in bud dormancy. The name abscisic 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(Arteca, 1996; Mauseth, 1991; Raven, 1992; Salisbury and Ross, 1992).

History of Abscisic Acid

In 1963, abscisic acid was first identified and characterized by Frederick Addicott and his associates. They were studying compounds responsible for the abscission of fruits (cotton). Two compounds were isolated and called abscisin I and abscisin II. Abscisin II is presently called abscisic acid (ABA)(Addicot, 1963). Two other groups at about the same time discovered the same compound. One group headed by Philip Wareing was studying bud dormancy in woody plants. The other group led by Van Steveninck was studying abscission of flowers and fruits from lupine. Plant physiologists agreed to call the compound abscisic acid (Salisbury and Ross, 1992). 

Biosynthesis and Metabolism

ABA is a naturally occurring compound in plants. It is a sesquiterpenoid (15-carbon) which is partially produced via the mevalonic pathway in chloroplasts and other plastids. Because it is sythesized partially in the chloroplasts, it makes sense that biosynthesis primarily occurs in the leaves. The production of ABA is accentuated by stresses such as water loss and freezing temperatures. It is believed that biosynthesis occurs indirectly through the production of carotenoids. Carotenoids are pigments produced by the chloroplast which have 40 carbons. Breakdown of these carotenoids occurs by the following mechanism: 
Violaxanthin is a carotenoid which has forty carbons. 
It is isomerized and then split via an isomerase reaction followed by an oxidation reaction. 
One molecule of xanthonin is produced from one molecule of violaxanthonin and it is uncertain what happens to the remaining biproduct. 
The one molecule of xanthonin produced is unstable and spontaneously changed to ABA aldehyde. 
Further oxidation results in ABA. 
Activation of the molecule can occur by two methods. In the first method, an ABA-glucose ester can form by attachment of glucose to ABA. In the second method, oxidation of ABA can occur to form phaseic acid and dihyhdrophaseic acid. 
The transport of ABA can occur in both xylem and phloem tissues. It can also be translocated through paranchyma cells. The movement of abscisic acid in plants does not exhibit polarity like auxins. ABA is capable of moving both up and down the stem (Walton and Li, 1995; Salisbury and Ross). 

Functions of Abscisic Acid

The following are some of the phyysiological responses known to be associated with abscisic acid (Davies, 1995; Mauseth, 1991; Raven, 1992; Salisbury and Ross, 1992).

• Stimulates the closure of stomata (water stress brings about an increase in ABA synthesis). 
• Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots. 
• Induces seeds to synthesize storage proteins. 
• Inhibits the affect of gibberellins on stimulating de novo synthesis of a-amylase. 
• Has some effect on induction and maintanance of dormancy. 
• Induces gene transcription especially for proteinase inhibitors in response to wounding which may explain an apparent role in pathogen defense. 

Gibberellins

The Nature of Gibberellins

Unlike the classification of auxins which are classified on the basis of function, gibberellins are classified on the basis of structure as well as function. All gibberellins are derived from the ent-gibberellane skeleton. The structure of this skeleton derivative along with the structure of a few of the active gibberellins are shown above. The gibberellins are named GA₁....GA_n in order of discovery. Gibberellic acid, which was the first gibberellin to be structurally characterised , is GA₃. There are currently 136 GAs identified from plants, fungi and bacteria.  GA's are widespread and so far ubiquitous in both flowering (angiosperms) and non-flowering (gymnosperms) plants as well as ferns.

Gibberellin Biosynthesis and Metabolism

Gibberellins are diterpenes synthesized from acetyl CoA via the mevalonic acid pathway. They all have either 19 or 20 carbon units grouped into either four or five ring systems. The fifth ring is a lactone ring as shown in the structures above attached to ring A. Gibberellins are believed to be synthesized in young tissues of the shoot and also the developing seed. It is uncertain whether young root tissues also produce gibberellins. There is also some evidence that leaves may be the source of some biosynthesis (Sponsel, 1995; Salisbury and Ross). The pathway by which gibberellins are formed is outlined below and illustrated in figure1. 
3 acetyl CoA molecules are oxidized by 2 NADPH molecules to produce 3 CoA molecules as a side product and mevalonic acid. 
Mevalonic acid is then Phosphorylated by ATP and decarboxylated to form isopentyl pyrophosphate. 
4 of these molecules form geranylgeranyl pyrophosphate which serves as the donor for all GA carbon atoms. 
This compound is then converted to copalylpyrophosphate which has 2 ring systems 
Copalylpyrophosphate is then converted to kaurene which has 4 ring systems 
Subsequent oxidations reveal kaurenol (alcohol form), kaurenal (aldehyde form), and kaurenoic acid respectively. 
Kaurenoic acid is converted to the aldehyde form of GA12 by decarboxylation. GA12 is the 1st true gibberellane ring system with 20 carbons. 
From the aldehyde form of GA12 arise both 20 and 19 carbon gibberellins but there are many mechanisms by which these other compounds arise. 
Certain commercial chemicals which are used to stunt growth do so in part because they block the synthesis of gibberellins. Some of these chemicals are Phosphon D, Amo-1618, Cycocel (CCC), ancymidol, and paclobutrazol. During active growth, the plant will metabolize most gibberellins by hydroxylation to inactive conjugates quickly with the exception of GA3. GA3 is degraded much slower which helps to explain why the symptoms initially associated with the hormone in the disease bakanae are present. Inactive conjugates might be stored or translocated via the phloem and xylem before their release (activation) at the proper time and in the proper tissue (Arteca, 1996; Sponsel, 1995). 

Functions of Gibberellins

Active gibberellins show many physiological effects, each depending on the type of gibberellin present as well as the species of plant. Some of the physiological processes stimulated by gibberellins are outlined below (Davies, 1995; Mauseth, 1991; Raven, 1992; Salisbury and Ross, 1992).

• Stimulate stem elongation by stimulating cell division and elongation. 
• Stimulates bolting/flowering in response to long days. 
• Breaks seed dormancy in some plants which require stratification or light to induce germination. 
• Stimulates enzyme production (a-amylase) in germinating cereal grains for mobilization of seed reserves. 
• Induces maleness in dioecious flowers (sex expression). 
• Can cause parthenocarpic (seedless) fruit development. 
• Can delay senescence in leaves and citrus fruits.