Gibberellins (GAs) are plant hormones that regulate various developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. They 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 all over the world.
Chemistry All known gibberellins are diterpenoid acids synthesized by the terpenoid pathway in plastids and then modified in the endoplasmic reticulum and cytosol until they reach their biologically active form. All 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 2020, there are 136 GAs identified from plants, fungi, and bacteria. Gibberellins are tetracyclic diterpene acids. There are two classes, with either 19 or 20 carbons. The 19-carbon gibberellins are generally the biologically active forms. They have lost carbon 20 and, in place, possess a five-member lactone bridge that links carbons 4 and 10. Hydroxylation also has a great effect on its biological activity. In general, the most biologically active compounds are dihydroxylated gibberellins, with hydroxyl groups on both carbons 3 and 13. Gibberellic acid is a 19-carbon dihydroxylated gibberellin.
Bioactive GAs The bioactive Gibberellins are GA1, GA3, GA4, and GA7. There are three common structural traits between these GAs: 1) hydroxyl group on C-3β, 2) a carboxyl group on carbon 6, and 3) a lactone between carbons 4 and 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 without a hydroxyl group on C-3β. The presence of GA1 in various plant species suggests that it is a common bioactive GA.
Biological function Gibberellins are involved in the natural process of breaking dormancy and other aspects of germination. Before the photosynthetic apparatus develops sufficiently in the early stages of germination, the seed reserves of starch nourish the seedling. Usually in germination, the breakdown of starch to glucose in the endosperm begins shortly after the seed is exposed to water. Gibberellins in the seed embryo are believed to signal starch hydrolysis through inducing the synthesis of the enzyme α-amylase in the aleurone cells. In the model for gibberellin-induced production of α-amylase, it is demonstrated that gibberellins from the scutellum diffuse to the aleurone cells, where they stimulate the secretion α-amylase. α-Amylase then hydrolyses starch (abundant in many seeds), into glucose that can be used to produce energy for the seed embryo. Studies of this process have indicated gibberellins cause higher levels of transcription of the gene coding for the α-amylase enzyme, to stimulate the synthesis of α-amylase. Exposition to cold temperatures increases the production of Gibberellins. They stimulate cell elongation, breaking and budding, and seedless fruits. Gibberellins cause also seed germination by breaking the seed's dormancy and acting as a chemical messenger. Its hormone binds to a receptor, and calcium activates the protein calmodulin, and the complex binds to DNA, producing an enzyme to stimulate growth in the embryo.
Metabolism
Biosynthesis Gibberellins are usually synthesized from the methylerythritol phosphate (MEP) pathway in higher plants. In this pathway, bioactive GA is produced from trans-geranylgeranyl diphosphate (GGDP), with the participation of three classes of enzymes: terpene syntheses (TPSs), cytochrome P450 monooxygenases (P450s), and 2-oxoglutarate–dependent dioxygenases (2ODDs). The MEP pathway follows eight steps:
GGDP is converted to ent-copalyl diphosphate (ent-CDP) by ent-copalyl diphosphate synthase (CPS) ent-CDP is converted to ent-kaurene by ent-kaurene synthase (KS) ent-kaurene is converted to ent-kaurenol by ent-kaurene oxidase (KO) ent-kaurenol is converted to ent-kaurenal by KO ent-kaurenal is converted to ent-kaurenoic acid by KO ent-kaurenoic acid is converted to ent-7a-hydroxykaurenoic acid by ent-kaurenoic acid oxidase (KAO) ent-7a-hydroxykaurenoic acid is converted to GA12-aldehyde by KAO GA12-aldehyde is converted to GA12 by KAO. GA12 is processed to the bioactive GA4 by oxidations on C-20 and C-3, which is accomplished by 2 soluble ODDs: GA 20-oxidase and GA 3-oxidase. One or two genes encode the enzymes responsible for the first steps of GA biosynthesis in Arabidopsis and rice. The null alleles of the genes encoding CPS, KS, and KO result in GA-deficient Arabidopsis dwarves. Multigene families encode the 2ODDs that catalyze the formation of GA12 to bioactive GA4. AtGA3ox1 and AtGA3ox2, two of the four genes that encode GA3ox in Arabidopsis, affect vegetative development. Environmental stimuli regulate AtGA3ox1 and AtGA3ox2 activity during seed germination. In Arabidopsis, GA20ox overexpression leads to an increase in GA concentration.
Sites of biosynthesis Most bioactive Gibberellins are located in actively growing organs on plants. Both GA20ox and GA3ox genes (genes coding for GA 20-oxidase and GA 3-oxidase) and the SLENDER1 gene (a GA signal transduction gene) are found in growing organs on rice, which suggests bioactive GA synthesis occurs at their site of action in growing organs in plants. During flower development, the tapetum of anthers is believed to be a primary site of GA biosynthesis.
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