Gibberellin

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 (albeit unconscious) breeding of crop strains that were deficient in GA synthesis was one of the key drivers of the 'green revolution' in the 1960's, a revolution that is credited to have saved over a billion lives worldwide. 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 (albeit unconscious) breeding of crop strains that were deficient in GA synthesis was one of the key drivers of the 'green revolution' in the 1960's, a revolution that is credited to have saved over a billion lives worldwide. The first inroads into the understanding of GAs were developments from the plant pathology field, with studies on the bakanae, or 'foolish seedling' disease in rice. Foolish seedling disease causes a strong elongation of rice stems and leaves and eventually causes them to topple over. In 1926, Japanese scientist Eiichi Kurosawa identified that foolish seedling disease was caused by the fungus Gibberella fujikuroi. Later work at the University of Tokyo (notable from Yabuta, Sumiki and Hayashi) showed that a substance produced by this fungus triggered the symptoms of foolish seedling disease and they named this substance 'gibberellin'. The increased communication between Japan and the west following World War II enhanced the interest in gibberellin in the United Kingdom (UK) and the United States (US). Workers at Imperial Chemical Industries in the UK and the Department of Agriculture in the US both independently isolated gibberellic acid (with the Americans originally referring to the chemical as 'gibberellin-X', before adopting the British name and the chemical is known as gibberellin A3 or GA3 in Japan) Knowledge of gibberellins spread around the world as the potential for its use on various commercially important plants became more obvious. For example, research that started at the University of California, Davis in the mid-1960s led to its commercial use on Thompson seedless table grapes throughout California by 1962. A known gibberellin biosynthesis inhibitor is paclobutrazol (PBZ), which in turn inhibits growth and induces early fruitset as well as seedset. A chronic food shortage was feared during the rapid climb in world population in the 1960s. This was averted with the development of a high-yielding variety of rice. This variety of semi-dwarf rice is called IR8, and it has a short height because of a mutation in the sd1 gene. Sd1 encodes GA20ox, so a mutant sd1 is expected to exhibit a short height that is consistent with GA deficiency. All known gibberellins are diterpenoid acids that are synthesized by the terpenoid pathway in plastids and then modified in the endoplasmic reticulum and cytosol until they reach their biologically-active form. All gibberellins are derived via the ent-gibberellane skeleton, but are synthesised via ent-kaurene. The gibberellins are named GA1 through GAn in order of discovery. Gibberellic acid, which was the first gibberellin to be structurally characterized, is GA3. As of 2003, there were 126 GAs identified from plants, fungi, and bacteria. 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 five-member lactone bridge that links carbons 4 and 10. The 19-carbon forms are, in general, the biologically active forms of gibberellins. Hydroxylation 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. The bioactive GAs are GA1, GA3, GA4, and GA7. There are three common structural traits between these GAs: 1) a hydroxyl group on C-3β, 2) a carboxyl group on C-6, and 3) a lactone between C-4 and C-10. The 3β-hydroxyl group can be exchanged for other functional groups at C-2 and/or C-3 positions. GA5 and GA6 are examples of bioactive GAs that do not have a hydroxyl group on C-3β. The presence of GA1 in various plant species suggests that it is a common bioactive GA.