Gibberellins In Plants

Gibberellins In Plants: Definition & Function


 

Chemical formula C19H22O6
Molar mass 346.37 g/mol
Melting point 233 to 235 °C
Solubility in water 5 g/l (20 °C)


Gibberellins (GAs) are hormones that are produced by plants and which is responsible for developmental processes, containing stem elongation, germination, seed dormancy, flowering, flower growth, and leaf and fruit senescence. Gibberellins are one of the longest classes of plant hormone. It is believed that the (albeit unconscious) careful breeding of crop strains that were lacking in GA synthesis was one of the main drivers of the "green revolution", a revolution that is also known to save over a billion lives all around the globe.

History


The first step into the understanding of Gibberellins was the developments from the plant pathology field, with research on the bakanae, or "foolish seedling" sickness in rice. Foolish seedling illness causes a solid elongation of rice stems and leaves and ultimately causes them to collapse over. Japanese scientist Eiichi Kurosawa revealed that foolish seedling disease was caused by the fungus Gibberella fujikuroi, in early years of 1926. Later work at the University of Tokyo (notable from Yabuta, Sumiki, and Hayashi) displayed that a substance created by this fungus caused the symptoms of the foolish seedling disease and they coined this substance ‘gibberellin’.

Chemistry


As we all aware of gibberellins are diterpenoid acids that are produced by the terpenoid pathway in plastids and then altered in the endoplasmic reticulum and cytosol until they reach their biologically-active level.  All gibberellins are derived from the ent-gibberellin skeleton but are produced via ent-kaurene. The gibberellins are so-called GA1 through GAn in order of detection. Gibberellic acid, which was the first gibberellin to be fundamentally characterized, is GA3.
Gibberellins are also tetracyclic diterpene acids. There are two groups based on the presence of 19 or 20 carbons. The 19-carbon gibberellins, like gibberellic acid, have misplaced carbon 20 and, in place, holding a five-member lactone bridge that is associated between carbons 4 and 10. The 19-carbon forms are biologically active forms of gibberellins. Hydroxylation also has a great result on the biological activity of the gibberellin. Overall, the highly biologically active substances are dihydroxylated gibberellins, which have hydroxyl groups on both C- 3 and C-13. Gibberellic acid is also known as dihydroxylated gibberellin.

Bioactive of GAs


GA1, GA3, GA4, and GA7 are bioactive GAs. There are three basic structural characters among 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 functional group can be replaced for other functional groups at C-2 and/ C-3 positions. GA5 and GA6 are pure examples of bioactive GAs that do not contain a hydroxyl group on C-3β. The presence of GA1 in several plant species proposes that it is a common bioactive Gibberellin GA.

Gibberellins take part in the natural process of breaking seed dormancy and other aspects of propagation. Before the photosynthetic device develops adequately in the early stages of germination, the stored energy which reserves starch nourish the seedling. Typically in germination, the breakdown of starch to glucose in the endosperm begins soon after the seed is introduced to water. Gibberellins in the seed embryo are supposed to signal starch hydrolysis through inducing the production of the enzyme α-amylase in the aleurone cells. Gibberellin-induced synthesis of α-amylase, it is explained that gibberellins produced in the scutellum diffuse to the aleurone cells, where they excite the secretion α-amylase. α-Amylase then hydrolyzes of starch, which is rich in many seeds, into glucose that can also be used in cellular respiration to produce energy for the seed embryo. Studies of this process have shown gibberellins cause higher intensities of transcription of the gene coding for the α-amylase enzyme, to excite the production of α-amylase.

Gibberellins are a synthesis in greater mass when the plant is open to cold temperatures. They excite cell elongation, seedless fruits, breaking and budding, and seed germination. They do the last by breaking the seed’s dormancy and performing as a chemical messenger. Its hormone attaches to a receptor, and Ca2+ activates the protein calmodulin, and the complex binding to DNA, leading to producing an enzyme to exciting growth in the embryo.

Metabolism


Biosynthesis

GAs are commonly synthesized from the methylerythritol phosphate (MEP) corridor in higher plants. In this pathway, bioactive GA is created from trans-geranylgeranyl diphosphate (GGDP).In the MEP pathway, three types of enzymes are used to produce GA from GGDP: 

1) terpene synthases (TPSs)
2) cytochrome P450 monooxygenases (P450s) and
3) 2- oxoglutarate- dependent dioxygenases (2ODDs).
There are 8 stages in the methylerythritol phosphate pathway: -
1) GGDP is transformed to ent-copalyl diphosphate (ent-CPD) by ent-copalyl diphosphate synthase - 2) etn-CDP is transformed to ent-kaurene by ent-kaurene synthase - 3) ent-kaurene is transformed to ent-kaurenol by ent-kaurene oxidase (KO) - 4) ent-kaurenol is transformed to ent-kaurenal by KO -
5) ent-kaurenol is transformed to ent- kaurenoic acid by KO-
6) ent-kaurenoic acid is transformed to ent-7a-hydroxykaurenoic acid by ent-kaurene acid oxidase (KAO) 7) ent-7a-hydroxy kaurenoic acid is transformed to GA12-aldehyde by KAO - 8) GA12-aldehyde is transformed to GA12 by KAO. GA12 is treated to the bioactive GA4 by oxidations on C-20 and C-3, which is achieved by 2 soluble ODDs: GA 20-oxidase and GA 3-oxidase

One or two genes encrypt the enzymes which are responsible for the first steps of gibberellins biosynthesis in Arabidopsis and rice.  Multigene families convert the 2ODDs that catalyze the production of GA12 to bioactive GA4.
AtGA3ox1 and AtGA3ox2, 2 of the 4 genes that convert GA3ox in Arabidopsis, affect vegetative development. Environmental stimuli control AtGA3ox1 and AtGA3ox2 activity in seed germination. In Arabidopsis, GA20ox overexpression will lead to an increase in gibberellins concentration. 

Sites of biosynthesis


Most bioactive gibberellins are situated in actively growing organs on plants. Each GA3ox and GA20ox genes (genes coding for GA 3-oxidase and GA 20-oxidase) and the SLENDER1 gene (a GA sign transduction gene) are seen in growing organs on rice, which proposes bioactive gibberellins synthesis happens at their site of action in growing organs of plants. Throughout flower development, the tapetum of anthers is supposed to be a primary site of gibberellins biosynthesis.

Differences between biosynthesis in fungi and lower plants:


A plant Arabidopsis and a fungus “Gibberellafujikuroi” have different gibberellins pathways and enzymes. P450s in fungi do functions analogous as compared to functions of KAOs in plants. The role of CPS and KS in plants is done by a single enzyme, CPS/KS. In fungi, the gibberellins biosynthesis genes are present on one chromosome, but in plants, they are found casually on multiple chromosomes. Plants yield a low amount of GA3, hence the GA3 is made for industrial uses by microorganisms. For industrial use, the gibberellic acid can be manufactured by submerged fermentation, but this process presents low production with high production costs and therefore higher sale value, however, another alternative process to decrease costs of the GA3 making is Solid-State Fermentation (SSF) that lets the use of agro-industrial residues. 

Catabolism


Numerous mechanisms for inactivating gibberellins have been recognized. 2β-hydroxylation disables GA and is catalyzed by GA2-oxidases (GA2oxs). Certain GA2oxs use C19-GAs as substrates, and others GA2oxs use C20-GAs. Cytochrome P450 mono-oxygenase, determined by elongated highest internode (eui), transforms GAs into 16α, 17-epoxides. Rice eui mutants amass bioactive gibberellins at high levels, which suggests cytochrome P450 mono-oxygenase is a key enzyme is responsible for deactivation gibberellins in rice. The Gamt1 and gamt2 genes convert enzymes that methylate the C-6 carboxyl group of gibberellins. In a gamt1 and gamt2 mutant, amount of gibberellins is developing seeds is increased.

Homeostasis


Feedback and feedforward parameter keeps the levels of bioactive gibberellins in plants. Levels of AtGA20ox1 and AtGA3ox1 expression are improved in a gibberellins deficient environment and reduced after the addition of bioactive gibberellins. Equally, the expression of AtGA2ox1 and AtGA2ox2, gibberellins deactivation genes, is improved with the addition of gibberellins.

Regulation by other hormones


The auxin indole-3-acetic acid (IAA) controls the concentration of GA1 in getting longer internodes in peas. Removal of IAA by removal of the apical bud, the auxin source, decreases the amount of GA1, and reintroduction of IAA reverses these effects to raise the amount of GA1. This process has also been observed in tobacco plants. Auxin surges GA 3-oxidation and drops GA 2-oxidation in barley. Auxin also controls gibberellins biosynthesis during fruit growth in peas. These detections in different plant species explain the auxin regulation of gibberellins metabolism may be a common mechanism.
Ethylene reduces the concentration of bioactive GAs.
Regulation by environmental factors
According to recent studies, variations in gibberellins concentration impact light-regulated germination, photomorphogenesis through de-etiolation, and photoperiod parameter of flowering and stem elongation. Microarray observation displayed about one-fourth cold-responsive genes is linked to gibberellins regulated genes, which suggests gibberellins impacts response to cold temperatures. Plants decrease growth rate when open to stress. A connection between gibberellins levels and concentration of stress experienced has been proposed in barley.


Role in seed development


Bioactive gibberellins and abscisic acid levels have an opposite relationship and regulate seed germination and development. Levels of FUS3, an Arabidopsis transcription aspect, are unregulated by ABA and down-regulated by gibberellin, which proposes that there is a regulation loop that creates the balance of gibberellin and ABA.

Applications of Gibberellins:


  • 1. Elongation of genetically dwarf plants

  • 2. Bolting and flowering in long-day plants

  • 3. Substitution of cold treatment

  • 4. The breaking of dormancy

  • 5. Parenthocarpy

  • 6. The hormone tested with leaf and root culture of digitalis exhibited higher production of digoxin.

  • 7. Spraying juvenile conifers with gibberellins accelerates the maturity period, this leads to early seed production

  • 8. Spraying sugarcane crop with gibberellins will increase the length of the stem.