Physiology & Biochemist
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Dr. Sumit Kumar Assistant Professor, Botany Nalanda College, Biharsharif M.Sc. Semester II MBOTCC-7: Physiology & Biochemistry Unit III: Gibberellin: Mode of action and physiological role Introduction In the 1950s the second group of hormones, the gibberellins (GAs), was characterized. The gibberellins are a large group of related compounds (more than 125 are known) that, unlike the auxins, are defined by their chemical structure rather than by their biological activity. Gibberellins are most often associated with the promotion of stem growth, and the application of gibberellin to intact plants can induce large increases in plant height. Gibberellins are weakly acidic growth hormones having gibbane ring structure which cause cell elongation of intact plants in general and increased internodal length of genetically dwarfed plants (e.g., Pea, Corn), in particular. Figure 1. Example of GA’s physiological regulation on Mendel’s tall/dwarf alleles in peas Discovery and Structure of Gibberellins Rice farmers in Asia had long known of a disease that makes the rice plants grow tall but eliminates seed production. In Japan this disease was called the ―foolish seedling,‖ or bakanae, disease. Plant pathologists investigating the disease found that the tallness of these plants was induced by a chemical secreted by a fungus that had infected the tall plants (Figure 2). This chemical was isolated from filtrates of the cultured fungus and called gibberellin after Gibberella fujikuroi, the name of the fungus. Figure 2. Illustration of foolish seedling of rice disease Not until the mid-1950s did two groups, one in Britain and the other at the U.S., succeeded in elucidating the structure of the material that they had purified from fungal culture filtrates, which they named gibberellic acid. Gibberellic acid (GA3) The structural feature that all gibberellins have in common, and that defines them as a family of molecules, is that they are derived from the ent-kaurene ring structure. ent-Kaurene The experiments with dwarf peas and dwarf corn have confirmed that the natural elongation growth of plants is regulated by gibberellins. In contrast, plants that were genetically very tall showed no further response to applied gibberellins. Because applications of gibberellins could increase the height of dwarf plants, it was natural to ask whether plants contain their own gibberellins. Shortly after the discovery of the growth effects of gibberellic acid, gibberellin-like substances were isolated from several species of plants. Gibberellin-like substance refers to a compound or an extract that has gibberellin-like biological activity, but whose chemical structure has not yet been defined. In 1958, a gibberellin (gibberellin, GA1) was conclusively identified from a higher plant. Gibberellic acid (GA1) More than 136 GAs were known by 2010 (128 of these are in plants, 7 in bacteria and 7 in fungi). We call them GA1 to GAX in the order of discovery, where X is a number, in the order of their discovery. This scheme was adopted for all gibberellins in 1968. However, the number of a gibberellin is simply a cataloging convenience, designed to prevent chaos in the naming of the gibberellins. The system implies no close chemical similarity or metabolic relationship between gibberellins with adjacent numbers. Despite the abundant number of gibberellins present in plants, genetic analyses have demonstrated that only a few are biologically active as hormones. All the others serve as precursors or represent inactivated forms. The major bioactive GAs are GA1, GA3, GA4 and GA7. Biosynthesis and Metabolism of Gibberellins Gibberellins constitute a large family of diterpene acids and are synthesized by a branch of the terpenoid pathway in plants. GAs are biosynthesized from geranylgeranyl diphosphate (GGDP), a common C20 precursor for diterpenoids. Methylerythritol phosphate pathway in the plastid provides the majority of the isoprene units to GAs in Arabidopsis seedlings, whereas there is a minor contribution from the cytosolic mevalonate pathway (Figure 3). Biosynthesis of GA involves three major steps (Figure 4): a) Production of terpenoid precursors and ent-kaurene in plastids b) Oxidation reactions on the endoplasmic reticulum form GA12 and GA53 c) Formation in the cytosol of all other gibberellins from GA12 or GA53 Figure 3. Overview of biosynthesis of terpenoids in plant cell Figure 4. Illustration of a schematic plant (left) and gibberellin (GA) biosynthesis pathway (right). CPS, ent-copalyl diphosphate synthase; KS, ent-kaurene synthase; KO, ent- kaurene oxidase; KAO, ent-kaurenoic acid oxidase. Three different classes of enzymes are required for the biosynthesis of bioactive GAs from GGDP in plants: terpene synthases (TPSs) cytochrome P450 monooxygenases (P450s) 2-oxoglutarate–dependent dioxygenases (2ODDs) The specific steps in the modification of GA12 vary from species to species, and between organs of the same species. Although active gibberellins are free, a variety of gibberellin glycosides are formed by a covalent linkage between gibberellin and a sugar. These gibberellin conjugates are particularly prevalent in some seeds. The conjugating sugar is usually glucose, and it may be attached to the gibberellin via a carboxyl group forming a gibberellin glycoside, or via a hydroxyl group forming a gibberellin glycosyl ether. When gibberellins are applied to a plant, a certain proportion usually becomes glycosylated. Glycosylation therefore represents another form of inactivation. GA1 is the biologically active gibberellin controlling stem growth. Endogenous GA1 levels are correlated with tallness in plants. The highest levels of gibberellins are found in immature seeds and developing fruits. However, because the gibberellin level normally decreases to zero in mature seeds, there is no evidence that seedlings obtain any active gibberellins from their seeds. The apical bud and young leaves appear to be the principal sites of gibberellin synthesis. The gibberellins that are synthesized in the shoot can be transported to the rest of the plant via the phloem. Intermediates of gibberellin biosynthesis may also be translocated in the phloem. The transport of GAs is not polar. Indeed, the initial steps of gibberellin biosynthesis may occur in one tissue and metabolism to active gibberellins in another. The tapetum of anthers is one of the major sites of bioactive GA synthesis during flower development. Gibberellins also have been identified in root exudates and root extracts, suggesting that roots can also synthesize gibberellins and transport them to the shoot via the xylem. Endogenous gibberellin regulates its own metabolism by either switching on or inhibiting the transcription of the genes that encode enzymes of gibberellin biosynthesis and degradation (feedback and feed-forward regulation, respectively). Environmental factors such as photoperiod (e.g., leading to bolting and potato tuberization) and temperature (vernalization), also modulate gibberellin biosynthesis. Although we often discuss the action of hormones as if they act singly, the net growth and development of the plant are the results of many combined signals. In addition, hormones can influence each other’s biosynthesis so that the effects produced by one hormone may in fact be mediated by others. It is now evident that gibberellin can induce auxin biosynthesis and vice versa. Both bioactive GAs and auxin positively regulate stem elongation. In pea, IAA is essential for the maintenance of GA1 levels in elongating internodes. Decapitation dramatically reduces the level of endogenous GA1 in elongating internodes, and application of IAA to the decapitated plants completely reverses these effects (Figure 5). Figure 5. Decapitation reduces, and IAA (auxin) restores, endogenous GA1 content in pea plants. Physiological Roles of Gibberellin on Plant Growth and Development Though they were originally discovered as the cause of a disease of rice that stimulated internode elongation, endogenous gibberellins influence a wide variety of developmental processes. In addition to stem elongation, gibberellins control various aspects of seed germination, including the loss of dormancy and the mobilization of endosperm reserves. In reproductive development, gibberellin can affect the transition from the juvenile to the mature stage, as well as floral initiation, sex determination, and fruit set. Some of the major physiological effects of gibberellins are as follows: (A) Elongation of intact stems Many plants respond to application of GA by a marked increase in stem length; the effect is primarily of internode elongation. Applied gibberellin promotes internodal elongation in a wide range of species (Figure 6). However, the most dramatic stimulations are seen in dwarf and rosette species, as well as members of the grass family. Exogenous GA3 causes such extreme stem elongation in dwarf plants that they resemble the tallest varieties of the same species (Figure 7). Accompanying this effect are a decrease in stem thickness, a decrease in leaf size, and a pale green color of the leaves. It appears that dwarfness of such varieties is due to internal deficiency of gibberellins. Figure 6. Illustration of internodal elongation in dwarf varieties after treatment with GA Figure 7. Dwarf pea plants without (left) or with GA (right) treatment (5 μg) for one week. GA also promotes internodal elongation in members of the grass family. The target of gibberellin action is the intercalary meristem—a meristem near