Gibberellins are classified on the basis of structure as well as function. The gibberellins are named GA1….GAn in order of discovery. Gibberellins are a large group of related compounds defined not by their biological functions but by their structures. Gibberellic acid, which was the first gibberellin to be structurally characterised, is GA3. There are currently 136 GAs identified from plants, fungi and bacteria. The gibberellins are carried by the xylem and phloem.
Biosynthesis and metabolism
Gibberellins are diterpenes synthesized from acetyl CoA via the mevalonic acid pathway. They all have either 19 or 20 carbon units grouped into either four or five ring systems. The fifth ring is a lactone. Gibberellins are believed to be synthesized in young tissues of the shoot and also the developing seed. It is uncertain whether young root tissues also produce gibberellins. 3-acetyl CoA molecules are oxidized by 2 NADPH molecules to produce 3 CoA molecules as a side product and mevalonic acid. Mevalonic acid is then phosphorylated by ATP and decarboxylated to form isopentyl pyrophosphate. Four of these molecules form geranylgeranylpyrophosphate, which serves as the donor for all GA carbon atoms. This compound is then converted to copalyl pyrophosphate which has a two ring systems, copalyl pyrophosphate is then converted to kaurene which has a four ring systems. Subsequent oxidations reveal kaurenol (alcohol form), kaurenal (aldehyde form) and kaurenoic acid respectively. Kaurenoic acid is converted to the aldehyde form of GA12 by decarboxylation. GA12 is the first true gibberellin ring system with 20 carbons. From the aldehyde form of GA12 arise both 20 and 19 carbon gibberellins but there are many mechanisms by which these other compounds arise. Certain commercial chemicals which are used to stunt growth do so in part because they block the synthesis of gibberellins. Some of these chemicals are Phosphon D, Amo-1618, Cycocel (CCC), ancymidol and paclobutrazol. During active growth, the plant will metabolize most gibberellins by hydroxylation to inactive conjugates quickly with the exception of GA3.
Role of gibberellins
• The gibberellins are especially abundant in seeds and young shoots where they control stem elongation by stimulating both cell division and elongation (auxin stimulates only cell elongation).
• They stimulate bolting or flowering in response to long days.
• They break seed dormancy in some plants, which require stratification of light to induce germination.
• They stimulate enzyme production (α-amylase) in germinating cereal grains for mobilization of seed reserves.
• They induce maleness in dioecious flowers (sex expression).
• They can cause parthenocarpic (seedless) fruit development.
• They can delay senescence in leaves and citrus fruits.
Gibberellins are synthesized primarily in the apical tissues and young leaves. It is uncertain whether root tissues also produce gibberellins. Gibberellins highest levels are found in immature seeds and developing fruits. The condensation of four isoprenoids subunits synthesizes gibberellins. Isopentenyl pyrophosphate (IPP) is the basic biological isoprene unit. 20 carbons geranylgeranyl pyrophosphate (GGPP) is produced by IPPs condense. GGPP acts as a biosynthetic precursor for gibberellins. Gibberellins biosynthetic pathway can be divided into three stages. Each completes in a different cellular compartment. Stage 1 Geranylgeranyl pyrophosphate (20-carbon linear molecules) is converted to ent-kaurene in plastids. Stage 2 Kaurene is transported from the plastid to the endoplasmic reticulum (ER). In the ER, ent-kaurene is oxidized to GA12, the first gibberellins in the biosynthetic pathway in all plants and the precursor of all other gibberellins. The hydroxylation of carbon-13 of GA12 gives GA53. Stage 3 In the cytosol, G12 or G53, each of which has 20 carbon atoms is converted to other gibberellins by sequential oxidation of carbon 20, followed by the loss of this carbon in the form of CO2 to give 19-carbon gibberellins. Synthesis of gibberellins is blocked by certain commercial chemicals. Phosphon D, AMO-1618, Cycocel (CCC), ancymidol and paclobutrazol are some of these chemicals. The first stage of gibberellins biosynthesis is inhibited by these chemicals. Gibberellins are translocated via phloem and xylem. The gibberellins that are synthesized in the shoot are transported to the rest of the plant via the phloem. The synthesis of gibberellins in roots is supported by the presence of gibberellins in root exudates and root extracts. The gibberellins that are synthesized in the root are transported to the shoot via the xylem. The gibberellins movements in plants do not exhibit polarity like auxins. Gibberellins are capable of moving both up and down of the stem.
• Stem Elongation Intermodal elongation in genetically dwarf plant species is promoted by exogenous gibberellins. Dwarf mutant restoring a normal, tall phenotype is an application of exogenous gibberellins. Intermodal elongation by targeting the intercalary meristem and causing increase in both cell elongation and cell division is promoted by gibberellins. Elongation of stems in rosette plants (such as spinach, cabbage) after application of gibberellins also supports its role in stem elongation. The effect of GAs is very dramatic on stem elongation. GA1 is the biologically active gibberellins which controls stems elongation. GA20 is the precursor of GA1 in higher plants. Enzyme GA 3-oxidase (functions as a 3β-hydroxylase) catalyzes the conversion of GA1 to GA20, by hydroxylation of carbon 3.
• Seed Germination By activating vegetative growth of the embryo, gibberellin causes seeds germination and mobilizing stored food reserves of the endosperm by promoting the synthesis of a variety of hydrolytic enzymes that are involved in the solubilization of endosperm reserves. One α-amylase principal hydrolytic enzyme which is synthesized and secreted by the scutellum and the aleurone layer hydrolyzes starch chain. Synthesis and release of hydrolytic enzymes is the sole function of the aleurone layer of the seeds of graminaceous monocots (i.e. barley, wheat, rice, rye and oats). Gibberellins act primarily by inducing the expression of the gene for α-amylase.
• Flowering and sex determination A role of gibberellins in flower induction in reproductively competent plants has been established in long day and biennial species. In these plants, flowering in non-inductive conditions can be achieved by the application of gibberellins. Rosette plants (such as spinach, cabbage) generally do not flower in the rosette form. These plants show extensive internode elongation just before flowering, a phenomenon known as bolting. Environmental signal normally triggers the bolting, either photoperiod or a combination of low temperature and photoperiod. Rosette plants can be induced to bolt by an exogenous application of gibberellin. Bolting involves gibberellins-dependent cell division and elongation and has been directly correlated with increased gibberellins in the shoot apex. In many plants, gibberellins can substitute for the long day or cold requirement for flowering. It also plays a role in floral sex determination. Application of gibberellins promotes the formation of staminate flowers in dicots such as cucumber and spinach. On the other hand, it suppresses stamen development in maize.
• Fruit Set can be caused by application of gibberellins. For example, in apple (Malus sylvestris), stimulation of fruit set by gibberellins has been observed.
Gibberellin Signaling Pathway
Throughout the life cycle of the plant, gibberellins regulate various developmental processes, from seed germination through leaf expansion, stem elongation, flower induction and seed development. The identification of positive and negative signaling components leads by the studies of gibberellin signal transduction. The most extensively characterized among these are the DELLA proteins, a class of nuclear proteins acting as transcriptional regulators and supressors of gibberellins signaling. The molecular mechanism by which DELLA proteins suppress gibberellin responses is not yet clear. The Arabidopsis (Arabidopsis thaliana) genome contains five DELLA genes whereas in the rice (Oryza sativa) genome, only one family member has been identified. The gibberellins-signaling pathway involves the soluble gibberellin receptor termed GA insensitive dwarf1 (GID1). Gibberellin binding to GID1 triggers its interaction with the DELLA proteins. This interaction stimulates binding of the DELLA proteins to an ubiquitin ligase leading to polyubiquitination and degradation of the DELLA protein by the 26S proteosome. While this relatively simple GA-signaling cascade involves two major players, a receptors and a DELLA protein, other studies have identified additional factors that affect gibberellin responses.