Popular Posts

Tuesday, August 28, 2012

Introduction to Gibberlins


                                                                                   GIBBERLINS
Introduction

Gibberellic Acid (GA) is a natural hormone in plant that promotes and regulates growth and development in plants. It belongs to the family of hormone known as Gibberellins. Scientists are exploring the benefits it can offer in enhancing and maximizing fruit and vegetable yield. There is also evidence that it helps in the seed germination process in some plants. Gibberellins (GAs) are a large family of tetracyclic, diterpenoid compounds, some of which function as endogenous plant growth regulators. Through phenotypic analyses of mutants with reduced GA production, it has been revealed that bioactive GAs play an essential role in many aspects of plant growth and development, such as stem elongation, flower and fruit development, and seed germination. Despite considerable effort, it has not yet been determined precisely where the bioactive GAs are synthesized in plants, or which cells/tissues are targeted by the bioactive GAs to initiate GA-mediated biologic actions. GAs is primarily synthesized at the site of their action. There is some evidence for the presence of GAs in xylem and phloem exudates, indicating a long-distance transport of GAs through these tissues.




                                                            

                                                                         Gibberllins


The nature of gibberlins

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 structures of this skeleton derivative along with the structure of a few of the active gibberellins are shown above. The gibberellins are named GA1....GAn in order of discovery. Gibberellic acid, which was the first gibberellins to be structurally characterized, is GA3. There are currently 136 GAs identified from plants, fungi and bacteria.  The most widely available compound is GA3, or gibberellic acid, which is a fungal product. The most important GA in plants is GA1, which is the GA primarily responsible for stem elongation.


Gibberellin Biosynthesis and Metabolism

 GAs are synthesized from mevalonic acid in young tissues of the shoot (exact location uncertain) and developing seed. GAs are probably transported in the phloem and xylem. 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 .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.





Functions of Gibberellins

Active gibberellins show many physiological effects, each depending on the type of gibberellins present as well as the species of plant. Some of the physiological processes stimulated by gibberellins are outlined below 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. 

Physiological effect of gibberellins:
Germination:
This hormone also plays the central role in breaking seed dormancy in plant species that require their seeds to be exposed to light or their seed coats to be cracked before germination. During the germination process, seeds initially take water, which triggers the germ of the plant to begin growing. The seed begins to produce hormones and enzymes that stimulate the plant to grow. Among these hormones are the gibberellins, which regulate the synthesis of proteins and the growth of stem tissues. Introducing additional gibberellins in the form of gibberellic acid can greatly accelerate seed germination. A standard solution is typically used for most seeds and is effective for both breaking the dormancy of seeds and accelerating their sprouting and initial growth. Gibberellins are responsible for promoting growth in the embryo of a seed. Gibberellin released by the embryo travels towards the aleurone layer, its target tissue situated in the endosperm region of the seed (alongside the embryo). Gibberellin acts as the inducer, as its presence allows the enzyme induction of amylase, which can break down starch INTO a sugar to be used in the embryo. Sugar is used in the plant to synthesize proteins and break out of dormancy.

.

Increased Growth:

Gibberellins are a group of plant hormones that influence growth and development. Gibberellic acid occurs naturally in the plant and is used commercially to manipulate plant growth. Stem growth - GA1 causes hyperelongation of stems by stimulating both cell division and cell elongation. The hormone gibberellic acid is used by plants to induce growth .Growth is regarded as the elongation of plant cells, which is most profound at the tips of stem shoots. As days lengthen, gibberellic acid acts to induce flowering, as in the case of plants in the mustard family like radish, cauliflower and broccoli. Thus, the new seedling begins growth and breaks out of the seed. Gibberellins initiate this process in Summer, when the external environment exhibits favorable conditions for plant growth. This produces tall, as opposed to dwarf, plants. Bolting in long day plants - GAs cause stem elongation in response to long days.

Flowering:
If Gibberellic acid is applied to young plants, the result is often premature flowering of the plant. According to the California Rare Fruit Growers, “formation of male flowers is generally promoted by concentrations of 10 to 200 ppm.; female flowers by concentrations of 200 to 300 ppm.” These dosage amounts should be taken into consideration when using Gibberellic acid for premature flowering.
Increased Yield from Fruit Plants:
In fruit plants that have not been properly pollinated, Gibberellic acid is used to increase the fruit yield from these plants. Often, the resulting fruit is seedless.  Gibberellic acid is applied to plants to increase overall plant growth, and it is often used on grapes. Gibberellic acid not only increases the number of grapes per bunch but also the average size of each grape.
Frost Protection:
Gibberellic acid, if sprayed on plants in full-bloom, can counteract the deadly effects of frost when the weather gets cold.
Commercial Use of gibberlin:
Commercially, gibberellins are produced by fungal cultures, and it is the purified natural products that are applied to plants. Generally, gibberellic acid (GA3) is used, because this is the only gibberellin obtainable in commercial quantities, although an expensive mixture of GA4 and GA7 is now commercially available for specific purposes. Dwarf  landscape plants and potted flowering plants are treated with a hormone that inhibits the synthesis of gibberellic acids. The absence of gibberellic acid results in smaller, more compact plants.The most popularly used gibberlins are written as follows:

Seed Treatment:
Seeds are protected by a thick seed coat that must be permeated before germination can take place. Seeds that are difficult to germinate are often treated with giberellic acid to increase germination rates. Gibberellic acid is frequently used as a substance to stimulate germination in seeds that have become dormant. This is one of the most common and widespread uses of Gibberellic acid on plants.

Sex Expression

Presence of gibberellic acid is needed for male flower parts, the anther with its stamen, pollen and filament, to be formed in the developing flower bud. This is the case for plants with perfect flowers, those that have both male and female parts in the same blossom, as well as for plants that form single-sexed male flowers.

Secondary Effects

The presence of gibberellic acid, depending on specific plant species, can have secondary effects. One is the prevention of seeds in fruits, or the fleshy swelling of the ovary without forming seeds, such as in modern bananas. An abundance of this hormone can retard the dropping of dying leaves or the ripe fruits of citrus, a process called senescence.

Enhanced production of seedless grapes

 Bigger, more uniform bunches with larger fruit are produced. Among other effects, the gibberellin causes lengthening of the peduncle (stalk) attaching each grape to the cluster, thus permitting larger grapes to form. Virtually all the grapes that go to market are now treated with gibberellin.
The production of hybrid cucumber seed:
 Most high-producing cucumbers are F1 hybrids. GA sprays induce the production of male flowers on cucumber plants that normally produce only female flowers. The seed from neighboring all-female plants of a different strain is then exclusively hybrid.

Increased malt production:
 The addition of GA to germinating barley during beer production enhances a-amylase production so that more malt is produced more quickly. As the malt is the raw material for fermentation, a greater production of beer is made possible by this technique.

Increased sugarcane yield:
 GA promotes the elongation of sugarcane stalks with no change in the sugar concentration, so that the net yield of sugar is increased.

                                                                                 Mukesh RamjaliNepali
                                                                              MSc.Ag first sem
                                                                          Article unpublished


No comments:

Post a Comment