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, , GIBBERELLINS, ITS DISCOVERY AND, PHYSIOLOGICAL EFFECTS

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INTRODUCTION, , Gibberellins (GAs) are plant hormones that regulate various, developmental, processes, including, stem, elongation,, germination, dormancy, flowering, flower development, and, leaf and fruit senescence. GAs are one of the longest-known, classes of plant hormone. It is thought that the selective, breeding (albeit unconscious) of crop strains that were deficient, in GA synthesis was one of the key drivers of the -green, revolution" in the 1960s, a revolution that is credited to have, saved over a billion lives worldwide.

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DISCOVERY, , , , , The discovery of gibberellins is quite fascinating and dates, back to about the same period when auxins were, discovered, but it was only after 1950 that they came into, prominence. A young Japanese scientist Kurosawa had, been trying to find out why the rice seedlings infected by, the fungus Gibberella fujikuroi (asexual stage Fusarium, monoliforme) grew taller and turned very thin and pale., These are the symptoms of ‘Backanae disease’ (meaning, foolish) which is known to Japanese for over a century. In, 1926, he succeeded in obtaining a filtered extract of this, fungus which could cause symptoms of the Backanae, disease in healthy rice seedlings. In 1935, Yabuta isolated, the active substance which was quite heat stable and gave, it the name gibberellin.

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, , Gibberellins are tetracyclic diterpene acids. There are, two classes based on the presence of either 19 or 20, carbons. The 19-carbon gibberellins, such as gibberellic, acid, have lost carbon 20 and, in place, possess a fivemember lactone bridge that links carbons 4 and 10. The, 19-carbon forms are, in general, the biologically active, forms of gibberellins. Hydroxylatio also has a great, effect on the biological activity of the gibberellin. In, general, the most biologically active compounds are, dihydroxylated gibberellins, which possess hydroxyl, groups on both carbon 3 and carbon 13. Gibberellic acid, is a dihydroxylated gibberellin.

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ANTIGIBBERELINS, , , A large number of chemical substances are, known to inhibit the biosynthesis and activity of, gibberellins in plants. These chemicals are, known as antigibberellins., , Examples are, 1) AMO-1618, 2) CCC-β, 3) Phosphon-D, 4) B-995, 

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TRANSPORT, , , , , , , Unlike the auxin, transport of gibberellins is nonpolar., It moves from one part to the another in the, phloem, similar to the transport of carbohydrate, and other organic substances., , GA is also translocated in the xylem, due to the, lateral movement between two vascular bundles.

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PHYSIOLOGICAL EFFECTS, , , , , , , , , , , , , Stem elongation, due to cell division and cell, elongation induced by GA., Dwarfism reversal, Bolting and flowering: very well discussed in your, practical classes., Parthenocarpy: eg. Tomato, apples, cucumbers, Seed germination: eg. Lactuca sativa, barley etc. can, germinate even in complete dark., Breaking of dormancy, Cell division in cambium, Cold treatment substitution, Synthesis of hydrolytic enzymes like α-amylase,, protease in aleurone cells of barley.

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COMMERCIAL APPLICATIONS, , , Seed dormancy must be broken for seeds to germinate, and this, can be done by using gibberellins., , , , Gibberellins are used in the production of seedless fruit such as, seedless grapes, which are normally sprayed with gibberellin to, increase the size of each grape., , , , Induce parthenocarpy (seedless fruits)., , , , Affects positively on fruit setting., , , , Gibberellins are sometimes used to increase the amount of a amylase in germinating barley (Hordeum vulgare) which is used, in production of malt for brewing industry.