Gibberellin

Gibberellins (GAs) are plant hormones that regulate various developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. 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 GA_n in order of discovery. Gibberellic acid, which was the first gibberellin to be structurally characterized, is GA₃.

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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.

Biological function

thumb|1. Shows a plant lacking gibberellins, and which and has an [[Plant stem|internode length of "0" as well as being a dwarf plant. 2. Shows an average plant with a moderate amount of gibberellins, and an average internode length. 3. Shows a plant with a large amount of gibberellins and so has a much longer internode length, because gibberellins promote cell division in the stem.]]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. 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). Multigene families encode the 2ODDs that catalyze the formation of GA₁₂ to bioactive GA₄. 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. During flower development, the tapetum of anthers is believed to be a primary site of GA biosynthesis.

Differences between biosynthesis in fungi and lower plants

The flower Arabidopsis and the fungus Gibberella fujikuroi possess different GA pathways and enzymes. The function of CPS and KS in plants is performed by a single enzyme in fungi (CPS/KS). In plants the Gibberellin biosynthesis genes are found randomly on multiple chromosomes, but in fungi are found on one chromosome

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Plants produce low amount of Gibberellic Acid, therefore is produced for industrial purposes by microorganisms. Industrially GA₃ can be produced by submerged fermentation, but this process presents low yield with high production costs and hence higher sale value, nevertheless other alternative process to reduce costs of its production is solid-state fermentation (SSF) that allows the use of agro-industrial residues.

Catabolism

Several mechanisms for inactivating Giberellins have been identified. 2β-hydroxylation deactivates them, and is catalyzed by GA2-oxidases (GA2oxs). Cytochrome P450 mono-oxygenase, encoded by elongated uppermost internode (eui), converts Gibberellins into 16α,17-epoxides. Rice eui mutants amass bioactive Gibberellins at high levels, which suggests cytochrome P450 mono-oxygenase is a main enzyme responsible for deactivation GA in rice. In a gamt1 and gamt2 mutant, concentrations of GA in developing seeds is increased. Levels of AtGA20ox1 and AtGA3ox1 expression are increased in a Gibberellin deficient environment, and decreased after the addition of bioactive GAs, Conversely, expression of the Gibberellin deactivation genes AtGA2ox1 and AtGA2ox2 is increased with addition of Gibberellins. Removal of IAA by removal of the apical bud, the auxin source, reduces the concentration of GA₁, and reintroduction of IAA reverses these effects to increase the concentration of GA₁. Auxin increases GA 3-oxidation and decreases GA 2-oxidation in barley. Auxin also regulates GA biosynthesis during fruit development in peas. These discoveries in different plant species suggest the auxin regulation of GA metabolism may be a universal mechanism.

Ethylene decreases the concentration of bioactive GAs.

Regulation by environmental factors

Recent evidence suggests fluctuations in GA concentration influence light-regulated seed germination, photomorphogenesis during de-etiolation, and photoperiod regulation of stem elongation and flowering.

Role in seed development

Bioactive GAs and abscisic acid (ABA) levels have an inverse relationship and regulate seed development and germination. Levels of FUS3, an Arabidopsis transcription factor, are upregulated by ABA and downregulated by Giberellins, which suggests that there is a regulation loop that establishes the balance of Gibberellins and Abscisic Acid.

In practice, this means that farmers can alter this balance to make all fruits mature a little later, at a same time, or 'glue' the fruit in the trees until the harvest day (because ABA participates in the maturation of the fruits, and many crops mature and drop a few fruits a day for several weeks, that is undesirable for markets).

Signalling mechanism

Receptor

In the early 1990s, there were several lines of evidence that suggested the existence of a GA receptor in oat seeds located at the plasma membrane. However, despite intensive research, to date, no membrane-bound GA receptor has been isolated. This, along with the discovery of a soluble receptor, GA insensitive dwarf 1 (GID1) has led many to doubt that a membrane-bound receptor exists.thumb|280x280px|GA-GID1-DELLA signal pathway: In the absence of GA, DELLA proteins bind to and inhibit transcription factors (TFs) and prefoldins (PFDs). When GA is present, GID1 triggers the degradation of DELLAs and releases the TFs and PFDs.GID1 was first identified in rice and in Arabidopsis there are three orthologs of GID1, AtGID1a, b, and c. GA binding to GID1 causes changes in GID1 structure, causing a 'lid' on GID1 to cover the GA binding pocket. The movement of this lid results in the exposure of a surface which enables the binding of GID1 to DELLA proteins.

When Gibberellins bind to the GID1 receptor, it enhances the interaction between GID1 and DELLA proteins, forming a GA-GID1-DELLA complex. In that complex it is thought that the structure of DELLA proteins experience changes, enabling their binding to F-box proteins for their degradation. F-box proteins (SLY1 in Arabidopsis or GID2 in rice) catalyse the addition of ubiquitin to their targets. It was later found that DELLAs repress a large number of other transcription factors, among which are positive regulators of auxin, brassinosteroid and ethylene signalling. DELLAs can repress transcription factors either by stopping their binding to DNA or by promoting their degradation.

Microtubules are also required for the trafficking of membrane vesicles, that is needed for the correct positioning of several hormone transporters. One of the most well characterized hormone transporters are PIN proteins, which are responsible for the movement of the hormone auxin between cells. In the absence of Gibberellins, DELLA proteins reduce the levels of microtubules and thereby inhibit membrane vesicle trafficking. This reduces the level of PIN proteins at the cell membrane, and the level of auxin in the cell. GA reverses this process and allows for PIN protein trafficking to the cell membrane to enhance the level of auxin in the cell.

References

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