Role of Gibberellins in Dormancy and Germination

A hormone balance theory has been invoked in which ABA and GA act antagonistically to control both dormancy breakage and germination (Karssen and Lacka, 1986; Karssen, 1995 and references therein). Generally GAs are viewed as important for the promotion and maintenance of germination, while ABA controls seed developmental events including the inception of dormancy (Bewley, 1997). However, GA also appears to act as an antagonistic to ABA function during seed development (White et al., 2000). 

Role of ABA and Components of the ABA Signalling Pathway

(1) Promotion of Developmental Processes, Prevention of Precocious Germination and Induction of Dormancy During Seed Development

Whether a seed is dormant or quiescent at maturity, its quality and vigour rely heavily on processes that occurred during seed development: reserve deposition (accumulation of storage proteins and storage lipids or starch), regulation of precocious germination, and development of stress tolerance. Control of seed maturation in turn is mediated by key interactions between different hormone signalling pathways and other regulatory cues provided by the seed environment.

Types of Seed Dormancy

This review will focus on the process of primary seed dormancy. As noted above, primary dormancy is characterised by a transient inability of mature dispersed seeds to germinate under conditions that are normally conducive to germination (Grappin et al., 2000); the inception of primary dormancy occurs during seed development (reviewed in Bewley, 1997) . Dormancy can also be induced in mature, already dispersed, nondormant seeds (known as induced or secondary dormancy) by environmental conditions that are unfavourable for germination, e.g. anoxia, unsuitable temperatures or illumination (Bewley, 1997). In induced dormancy, the seed loses its sensitivity to dormancy-breaking factors (e.g. light, nitrate, etc., depending on the species) (Bewley and Black, 1994).

Primary dormancy is generally classed into two major types: embryo dormancy and coat-imposed dormancy (more accurately termed coat-enhanced dormancy) (Bewley and Black, 1994). In embryo dormancy, it is the embryo that is dormant and the embryonic axis will not elongate even if the embryo is excised from its enclosing seed tissues and placed on water. In coat-enhanced dormancy, the embryo, when isolated is capable of germination, but the intact seed is dormant; thus, it is the surrounding seed tissues that impose the block to germination. The inhibitory nature of the enclosing seed tissues may be attributed to one or a combination of the following effects: (1) interference with water uptake; (2) mechanical restraint; (3) interference with gas exchange; (4) supply of inhibitors to the embryo or promotion of the synthesis of inhibitors within the embryo and (5) prevention of the exit of inhibitors from the embryo (Bewley and Black, 1994). For many seeds, more than one of these factors operates to maintain coat-enhanced dormancy.

A variety of germination inhibitors have been identified within the seed tissues that enclose the embryo. While their presence does not necessarily imply a causal role in preventing germination, in many cases, where repeated washing (leaching) of seeds relieves dormancy, inhibitors are known to be removed (Bewley and Black, 1994). The inhibitor that has received the most attention with respect to dormancy imposition and maintenance is ABA .

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Plant growth hormones

The two most important growth hormones of plants, so far considered antagonists, also work synergistically. The activities of auxin and cytokinin, key molecules for plant growth and the formation of organs, such as leaves and buds, are in fact more closely interwoven than previously assumed. Researchers from Heidelberg, Tbingen (Gera number of) and Umea (Sweden) made this surprising discovery in a series of complex experiments using thale cress (Arabidopsis thaliana), a biological reference organism. The international team of researchers, led by Jan Lohmann, stem cell biologist at Heidelberg University, have now published their results in the scientific journal "Nature". (Nature, 24. Juni 2010).

All the above-ground parts of a plant leaves, buds, stems and seeds ultimately arise from a small tissue at the shoot tip, which contains totipotent stem cells. Since plant stem cells remain active over the entire life of the organism, plants, unlike animals, are able to grow and develop new organs over a number of decades. On the periphery of the tip, auxin triggers cells to leave the pool of stem cells, differentiate and form organs like leaves and buds. Cytokinin stimulates stem cells to divide and proliferate; it maintains the number of cells and thus the plant's growth potential.

Some of the genetic factors involved in cytokinin's effect on plant growth were already known. In the thale cress experiments, which concentrated on the growth zone at the tip of the shoot, Lohmann and his team now studied the role of auxin in the interplay of the two hormones. It turns out that auxin directly interferes with a feedback loop involving two genes activated by cytokinin ARR7 and ARR15 which limit the effect of cytokinin. Auxin suppresses these two genes, thereby boosting the effect of cytokinin.

"Auxin acts to support the pool of stem cells", explains Jan Lohmann. "When it triggers cells at the periphery of the growth zone to form organs, it still needs to ensure that enough stem cells are supplied." This keeps the number of stem cells from falling below a critical minimum, which is key for plant growth and survival. "We're gradually beginning to understand how hormonal and genetic factors are interwoven to maintain the activity of the growth zone. We now know that hormones and genes interact in multiple ways, each one affecting the other. There are no solitary factors".

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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.

http://www.plant-hormones.info

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
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