Gibberellin
Template:Short description Template:Overly detailed Gibberellins (GAs) are plant hormones that regulate various developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence.<ref name="Hedden-2015" /> 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,<ref name="Spielmeyer-2002">Template:Cite journal</ref> a revolution that is credited to have saved over a billion lives worldwide.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
ChemistryEdit
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.<ref name="Campbell-2002">Template:Cite book</ref> 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.<ref name="Sponsel-2010">Template:Citation</ref> Gibberellic acid, which was the first gibberellin to be structurally characterized, is GA3.<ref name="Hedden-2020">Template:Cite journal</ref>
Template:As of,<ref name="Sponsel-2010" /> there are 136 GAs identified from plants, fungi, and bacteria.<ref name="Hedden-2015" /><ref name="Hedden-2020" /><ref name="Sponsel-2010" />
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.<ref name="AccessScience">Template:Cite journal</ref>
Bioactive GAsEdit
The bioactive Gibberellins are GA1, GA3, GA4, and GA7.<ref name="Yamaguchi-2008">Template:Cite journal</ref> 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.<ref name="Yamaguchi-2008" />
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- Gibberellic acid.svg
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- Ent-Gibberellane.svg
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- Ent-Kauren.svg
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The 3β-hydroxyl group can be exchanged for other functional groups at C-2 and/or C-3 positions.<ref name="Yamaguchi-2008" /> GA5 and GA6 are examples of bioactive GAs without a hydroxyl group on C-3β.<ref name="Yamaguchi-2008" /> The presence of GA1 in various plant species suggests that it is a common bioactive GA.<ref>Template:Cite journal</ref>
Biological functionEdit
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.<ref>Template:Cite journal</ref> 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.<ref name="Campbell-2002" /> 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.<ref name="AccessScience" />
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.
MetabolismEdit
BiosynthesisEdit
Gibberellins are usually synthesized from the methylerythritol phosphate (MEP) pathway in higher plants.<ref name="Hedden-2012">Template:Cite journal</ref> 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).<ref name="Hedden-2012" /><ref name="Yamaguchi-2008" /> The MEP pathway follows eight steps:<ref name="Yamaguchi-2008" />
- 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.<ref name="Yamaguchi-2008" /> The null alleles of the genes encoding CPS, KS, and KO result in GA-deficient Arabidopsis dwarves.<ref>Template:Cite journal</ref> Multigene families encode the 2ODDs that catalyze the formation of GA12 to bioactive GA4.<ref name="Yamaguchi-2008" />
AtGA3ox1 and AtGA3ox2, two of the four genes that encode GA3ox in Arabidopsis, affect vegetative development.<ref>Template:Cite journal</ref> Environmental stimuli regulate AtGA3ox1 and AtGA3ox2 activity during seed germination.<ref name="Yamaguchi-1998">Template:Cite journal</ref><ref name="Yamauchi-2004">Template:Cite journal</ref> In Arabidopsis, GA20ox overexpression leads to an increase in GA concentration.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Sites of biosynthesisEdit
Most bioactive Gibberellins are located in actively growing organs on plants.<ref name="Hedden-2012" /> 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.<ref name="Kaneko-2003">Template:Cite journal</ref> During flower development, the tapetum of anthers is believed to be a primary site of GA biosynthesis.<ref name="Kaneko-2003" /><ref>Template:Cite journal</ref>
Differences between biosynthesis in fungi and lower plantsEdit
The flower Arabidopsis and the fungus Gibberella fujikuroi possess different GA pathways and enzymes.<ref name="Yamaguchi-2008" /> P450s in fungi perform functions analogous to the functions of KAOs in plants.<ref>Template:Cite journal</ref> The function of CPS and KS in plants is performed by a single enzyme in fungi (CPS/KS).<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> In plants the Gibberellin biosynthesis genes are found randomly on multiple chromosomes, but in fungi are found on one chromosome .<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref>
CatabolismEdit
Several mechanisms for inactivating Giberellins have been identified. 2β-hydroxylation deactivates them, and is catalyzed by GA2-oxidases (GA2oxs).<ref name="Hedden-2012" /> Some GA2oxs use 19-carbon Gibberellins as substrates, while other use C20-GAs.<ref name="Thomas-1999">Template:Cite journal</ref><ref>Template:Cite journal</ref> Cytochrome P450 mono-oxygenase, encoded by elongated uppermost internode (eui), converts Gibberellins into 16α,17-epoxides.<ref name="Zhu-2006">Template:Cite journal</ref> 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.<ref name="Zhu-2006" /> The Gamt1 and gamt2 genes encode enzymes that methylate the C-6 carboxyl group of GAs.<ref name="Varbanova-2007">Template:Cite journal</ref> In a gamt1 and gamt2 mutant, concentrations of GA in developing seeds is increased.<ref name="Varbanova-2007" />
HomeostasisEdit
Feedback and feedforward regulation maintains the levels of bioactive Gibberellins in plants.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Levels of AtGA20ox1 and AtGA3ox1 expression are increased in a Gibberellin deficient environment, and decreased after the addition of bioactive GAs,<ref name="Yamaguchi-1998" /><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Conversely, expression of the Gibberellin deactivation genes AtGA2ox1 and AtGA2ox2 is increased with addition of Gibberellins.<ref name="Thomas-1999" />
RegulationEdit
Regulation by other hormonesEdit
The auxin indole-3-acetic acid (IAA) regulates concentration of GA1 in elongating internodes in peas.<ref name="Ross-2000">Template:Cite journal</ref> 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.<ref name="Ross-2000" /> This has also been observed in tobacco plants.<ref>Template:Cite journal</ref> Auxin increases GA 3-oxidation and decreases GA 2-oxidation in barley.<ref>Template:Cite journal</ref> Auxin also regulates GA biosynthesis during fruit development in peas.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
Regulation by environmental factorsEdit
Recent evidence suggests fluctuations in GA concentration influence light-regulated seed germination, photomorphogenesis during de-etiolation, and photoperiod regulation of stem elongation and flowering.<ref name="Yamaguchi-2008" /> Microarray analysis showed about one fourth cold-responsive genes are related to GA-regulated genes, which suggests GA influences response to cold temperatures.<ref name="Yamauchi-2004" /> Plants reduce growth rate when exposed to stress. A relationship between GA levels and amount of stress experienced has been suggested in barley.<ref>Template:Cite journal</ref>
Role in seed developmentEdit
Bioactive GAs and abscisic acid (ABA) levels have an inverse relationship and regulate seed development and germination.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
In the 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 mechanismEdit
ReceptorEdit
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.<ref name="Hedden-2015">Template:Cite journal</ref>
GID1 was first identified in rice<ref name="Ueguchi-Tanaka-2007">Template:Cite journal</ref> and in Arabidopsis there are three orthologs of GID1, AtGID1a, b, and c.<ref name="Hedden-2015" /> GID1s have a high affinity for bioactive GAs.<ref name="Ueguchi-Tanaka-2007" /> GA binds to a specific binding pocket on GID1; the C3-hydroxyl on GA makes contact with tyrosine-31 in the GID1 binding pocket.<ref name="Murase-2008">Template:Cite journal</ref><ref name="Shimada-2008">Template:Cite journal</ref> 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.<ref name="Murase-2008" /><ref name="Shimada-2008" />
DELLA proteins: Repression of a repressorEdit
DELLA proteins (such as SLR1 in rice or GAI and RGA in Arabidopsis) are repressors of plant development, characterized by the presence of a DELLA motif (aspartate-glutamate-leucine-leucine-alanine or D-E-L-L-A in the single letter amino acid code).<ref name="Davière-2013" />
DELLAs inhibit seed germination, seed growth, flowering and GA reverses these effects.<ref>Template:Cite journal</ref> 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.<ref name="Lechner-2006">Template:Cite journal</ref><ref name="Davière-2013">Template:Cite journal</ref><ref>Template:Cite journal</ref> F-box proteins (SLY1 in Arabidopsis or GID2 in rice) catalyse the addition of ubiquitin to their targets.<ref name="Lechner-2006" /> Adding ubiquitin to DELLA proteins promotes their degradation via the 26S-proteosome.<ref name="Davière-2013" /> This releases cells from DELLAs repressive effects.
Targets of DELLA proteinsEdit
Transcription factorsEdit
The first targets of DELLA proteins identified were Phytochrome Interacting Factors (PIFs). PIFs are transcription factors that negatively regulate light signalling and are strong promoters of elongation growth. In the presence of GA, DELLAs are degraded and this then allows PIFs to promote elongation.<ref name="Zheng-2016">Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> DELLAs can repress transcription factors either by stopping their binding to DNA or by promoting their degradation.<ref name="Zheng-2016" />
Prefoldins and microtubule assemblyEdit
In addition to repressing transcription factors, DELLAs also bind to prefoldins (PFDs). PFDs are molecular chaperones (they assist in the folding of other proteins) that work in the cytosol, but when DELLAs bind to them are restricted to the nucleus. An important function of PFDs is to assist in the folding of β-tubulin, a vital component of the cytoskeleton in the form of microtubules. As such, in the absence of Gibberellins (high level of DELLA proteins), PFDs reduce its activity, leading to a lower cellular pool of β-tubulin. When GA is present the DELLAs are degraded, PFDs can move to the cytosol and assist in the folding of β-tubulin. As such, GA allows for re-organisation of the cytoskeleton, and the elongation of cells.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref>
ReferencesEdit
External linksEdit
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