For many years after the discovery of auxin, plant scientists attributed all the development and plant processes taking place in plants to auxin. In 1950s another groups of growth substances, the gibberellins, was discovered. The main effect of the hormone was tallness in plants and chemically it was recognized as gibberellic acid (GA). Later many more, about eighty gibberellins were discovered and it was found that their synthesis and level is under genetic control. Thus, gibberellins are a large group of related compounds defined not by their biological action but their chemical structure, as some of these are biologically inactive.
Gibberellins were first discovered by Japanese scientists. In japan a disease of rice plants known as the foolish seedling or bakanae had a devastating effect on the rice economy of japan. The plants effected with the disease were taller and paler (chlorotic) than the normal plants and remained flowerless and fruitless. Japanese pathologists demonstrated that the disease is cause d by a chemical produced by a fungus. Gibberella fujikuroi. They demonstrated that when the sterile filtrates of the fungus were applied to the healthy plants, these caused symptoms of the disease. In 1930s Japanese pathologists succeeded in obtaining impure crystals f the fungal growth compound responsible for the disease. It was named gibberellin A.
In mid-1950s two groups headed by Brian Cross and Frank Stodola succeeded to obtain growth compound in pure form from fungal cultures and determined its chemical nature to the gibberellic acid. At about same time Nobutaka Takahashi and Saburo Tamura isolated three gibberellins from the original gibberellin and named them gibberellin A1, gibberellin A2, gibberellin A3. Gibberellin A3 and gibberellic acid proved to be identical.
More than eighty gibberellin have now been discovered in various fungi and plants (51 from seed plants), although no single species contains more than 15 and most species have only a few. They are abbreviated GA with a subscript, such as GA1, GA2, GA3, up to Gx1 to distinguish them. All could be referred to as gibberellic acids, But GA3 is commonly referred to as gibberellic acid.
Gibberellins exist in angiosperms and gymnosperms and probably also in mosses, ferns, algae and at least two fungi, but apparently not in bacteria.
Chemical Nature of Gibberellins
Gibberellins are chemically related to a large group of naturally occurring compounds called terpenoids. All gibberellins have either 19 or 20 carbon atoms grouped in a total of four or five ring systems and all have one or more carboxyl groups. The gibberellin is synthesized from acetate units of acetyl coenzyme A. The following steps are involved a synthesis of gibberellins.
The gibberellins with 20 carbons are ent-gibberellane, whereas those with 19 carbons are ent-20 norgibberellane. The gibberellin are easily interconvertable by changes in the hydroxyl substitution. This process can be important in production of active or inactive forms and vice versa, depending on the presence of hydroxylating enzymes during the different developmental stages of the plant.
Biosynthesis & Distribution of Gibberellins
There is no evidence that gibberellin biosynthesis is confined to any particular organelle. The highest level of gibberellins is found in immature seeds, therefore, they are certainly synthesized there. However, the gibberellin contents of seed tissue differ particularly in seed coat and embryo. The gibberellins are transferred between tissue at different points of metabolic pathways. For example GA29 is formed in pea cotyledon and take part in catabolic activity only after its transfer to the seed coat.
Lower levels of gibberellins occur in vegetative tissue of plants, especially in young leaves, buds and upper stem. These tissues also appear to be in sites of gibberellin synthesis. The initial steps of gibberellin production may occur in one tissue and metabolism to active gibberellins in another. Their is some indications that gibberellins are synthesized in roots, but this has not been studied thoroughly.
Acetyl coenzyme A is reduced to melvonic acid.
Melvonic acid is phosphorylated first and then decarboxylated to an isoprenoid unit isopentenyl pyrosphosphate.
Four isoprenoid units are added successively to produce a C-10 and a C-20 pyrophosphate.
These pyrophosphates are cycled to produce karurene, the precursor of gibberellin. The methyl group of karurene is oxidized to carboxylic acid followed by contraction B ring from a 6-C to 5-C to give GA12-aldehyde, the first gibberellin formed in plants and precursor of all other gibberellins.
Certain growth retarding chemicals such as Phosphon D, Amo-1618, Cycocel, etc. That inhibit stern elongation and cause stunted growth, also inhibit gibberellin biosynthesis.
Once formed, giberellins are usually only slowly degraded, but they can be readily converted to conjugated as gibberellins that are largely inactive. The bound gibberellins exist in plant tissues as gibberellin glucose, in which glucose is connected in an ether bond to one of the OH groups or an ester bond to a carboxyl group of the gibberellins.
Conjugated gibberellins are believed to exist in the bleeding sap of maple and elm trees, developing bean seeds, and developing and germinating pea seeds.
Much work concerning gibberellin transport in plants is based on studies of the movement of externally applied or exogenous radioactive gibberellin to excised stem. Transport for most of the part is non-polar and occurs phloem according to flow patterns similar to those of carbohydrates and other organic substances. Gibberellins have been isolated from sieve elements during certain experiments, Gibberellin transport have also been reported through xylem as it moves laterally between the two vascular tissues. The actual mechanism of gibberellin transport from the site of biosynthesis to the site action is not fully known.
Stem Elongation of Dwarf Plants
Gibberellins can overcome genetic dwarfism due to gene mutation in certain plants, for example in corn a mutant called dwarf-5 (d5) appear to be dwarf due to deficiency of gibberellins. The mutation causes a block in synthesis of gibberellin. Such plants have short internodes and attain one -fifth the size off normal plants. Application of gibberellins cause stem elongation and the plants resemble the tallest varieties of the same species. This effect has also been noted in Pisum sativum (pea), Vicia faba etc.
Promotion of Growth of Dormant Buds & Germination of Dormant Seeds
The buds of trees and shrubs growing in temperate zones usually become dormant in late summer or early fall. These become active again and grow only when exposed to low temperature during winter, or when the day length increases at the end of winter. Similarly, seeds of many species especially of wild plants also become dormant and do not grow until sufficient moisture, proper temperature (cold temperature) and adequate concentration of oxygen is supplied. In some red light is necessary for germination of seeds.
Gibberellins overcome both bud and seed dormancy in many species acting as substitute for low temperatures, long days or red lights. In seeds the gibberellins enhanced cell elongation so that the radicle can push through endosperm , seed coat or fruit wall that restricts its growth.
Bolting & Flowering in Long-Day Plants
In many plants, gibberellins also control balance between internode growth an leaf development. In these plants the internode growth is retarted and the leaves are arranged in rosette manner during short days. These plants flower only in long days and their flowers are produced at the end of a long flowering stalk called scape. This phenomenon of development of long flowering stalk is called bolting. The common examples of such plants are radish, onion etc. Treatment of these plants with gibberellins causes the plants to bolt and flower during short days. Thus, gibberellins can substitute for long-day requirement of these plants.
In addition, many long-day plants also have cold requirement (vernalization) for flowering. The application of gibberellin may overcome cold requirement in these plants.
Modification of Juvenility
Many woody plants do not flower until they reach a certain stage of maturity (juvenility). The juvenility and mature stages differ in leaf forms. When gibberellins are applied, these can regulate this juvenility in both directions, for example, in Hedera helix gibberellins revert mature plants to a juvenile state while in conifers gibberellins can induce juvenile phase to reproductive phase.
Induction of Maleness in Flower
In dioceious plants, that have flowers of a single sex, with both sexes on a single plant, e.g., in cucumber, in spinach, gibberellin application result in increasing tendency to produce male flowers.
Fruit Setting & Growth
The gibberellin applications can cause fruit set (the start of fruit growth following pollination) and growth of some fruit, where auxin may not effect. For example, fruit set has been observed in Malus sylvestris (apple) and Vitis vinifera (grapes).
Mobilization of Foods & Minerals in Seed Storage Cells
The young root and shoot systems begin to use minerals nutrients, fats, starch and proteins present in the storage cells of the seed. The young seedlings depend upon these reserves before the minerals can be absorbed from the soil and shoot system extends into light to start photosynthesis. The mineral salts are translocated through the phloem of the young root and shoot, but fats, polysaccharides and protein molecules are not transported in the seedlings. This may affect their growth. The gibberellins stimulate conversion of these polymers into sucrose and mobile amino acids so these can be carried easily. This has been noticed in cereals especially. The aleurone layers of cereal seeds are stimulated when the gibberellins are applied. These produce hydrolases, especially alpha-amylase, that digest the starch, proteins, and cell wall material present in the endosperm cell, so these can be translocated easily.
Application of Gibberellins in Agriculture
The following are commercial applications of gibberellins:
Fruit Production: A major use of gibberellin s is to increase the size of seedless grapes. Bunch size and shape also improved because of the stimulation of fruit stalk length. A mixture of benyladenine (a cytokinin) and GA4+7 is used to elongate improve the shape of apple fruit. In citrus fruits, the gibberellins delay senescence, so the fruits can be left on tree longer to extend market period.
Malting of Barley: As gibberellins applications increase amount of alpha-amylase in germination barley seed. It is used in the production of malt for the beer industry.
Increasing Sugar-Cane Yields: Gibberellin treatment can increase the yield of raw cane by 20 tons per acre and sugar yield by 2 tons per acre. This increase is a result of the stimulation of internode elongation during the winter season.
Uses in Plant Breeding: The juvenility period in conifers can be determined to a breeding programme by preventing the reproduction of desirable trees for many years. Application of GA4+7 can reduce the time to seed production considerably by causing the formation of cones on very young trees. In addition, the promotion of maleness in cucurbits and the stimulation of bolting in biennial vegetables such as Beta vulgaris (beet) and Brassica oleracea (cabbage) is brought by gibberellin application and used commercially in seed production.
Gibberellin Synthesis Inhibitors: The gibberellin synthesis inhibitors are used commercially to prevent elongation growth. In floral crops, short, stocky plants such as Lilies, Chrysanthemums and Poinsettias are desirable and restriction on elongation growth can be achieved by application of gibberellin synthesis inhibitors such as ancymidol or paclobutrazole. Restriction of extension growth in roadside shrubs can also be achieved with these inhibitors.