Notably, experiments by Sponsel and MacMillan in which substrates were injected into cotyledons of immature pea seeds provided evidence of two parallel pathways leading to hydroxylated and deoxy GAs, respectively, with hydroxylation occurring early in the pathway. It does not occur in G.
Curently Agreed Speakers:
Other inactivation mechanisms known to occur in plants are conjugation, primarily with glucose Schneider and Schliemann and the recently described epoxidation of the C,17 double bond Zhu and others The epoxides are hydrated to the 16,dihydrodiols, which have been known for some years to be endogenous GA metabolites for example, Hedden and others Several of the biosynthetic steps were of particular interest from a mechanistic standpoint. The formation of ent -kaurene from ent -copalyl diphosphate involves a complex rearrangement proposed to arise from a carbonium ion formed by heterolytic cleavage of the diphosphate evidence reviewed in MacMillan and Beale In pumpkin endosperm and the fungus G.
This latter product is further oxidised, but is not converted to GAs. Other by-products of GA biosynthesis are formed in pumpkin endosperm and G. It has been shown for the fungus that all of these by-products are produced by the highly multifunctional enzyme that converts ent -kaurenoic acid to GA 14 Rojas and others However, there is no evidence for these by-products being formed in vegetative plant tissues, which presumably possess ent -kaurenoic acid oxidases with tighter specificity.
The mechanism for the loss of C from the aldehyde is still unclear. Later Yuji Kamiya demonstrated in a cell-free system from developing pea cotyledons that C is lost from the aldehyde as CO 2 Kamiya and others This would require two oxidation steps, although no intermediate between the aldehyde and C 19 product has been identified. The GA oxidase GA20ox enzyme responsible for removing C also catalyses the oxidation of C from a methyl to the aldehyde via an alcohol. More recently, on the basis of experiments with a recombinant GA20ox from Arabidopsis , it was proposed that an initially formed free radical on C decomposes by an unknown oxidative mechanism to produce a C radical, which captures the C-4 carboxyl group Ward and others The activities were originally named ent -kaurene synthetase A and B erroneously as they do not require ATP , but the names ent -copalyl diphosphate synthase CPS and ent -kaurene synthase KS were proposed by MacMillan and have been universally adopted.
In the fungus, the two activities reside on a single polypeptide, which was purified by Fall and West in The activities were partially purified from M. Early indications that ent -kaurene synthesis occurred in plastids for example, Simcox and others were later confirmed by Aach and others , who showed conclusively that GGDP was converted to ent -kaurene in plastids from pea shoot tips and pumpkin endosperm. It is notable that despite the many demonstrations of ent -kaurene synthesis from MVA in cell-free systems, ent -kaurene was later shown to be produced mainly from pyruvate and glyceraldehyde phosphate via the methylerythritol phosphate MEP pathway in plants Fig.
Work with cell-free preparations from Marah , pumpkin, pea and Gibberella showed that the oxidative activities for the conversion of ent -kaurene to GA 12 -aldehyde were present in microsomes and were stimulated by NADPH. In the pumpkin cell-free system, GA 12 -aldehyde is oxidised to GA 12 by both microsomal and soluble enzymes reviewed in Hedden , whereas a microsomal preparation from pea cotyledons converted GA 12 -aldehyde to GA 12 and thence to GA 53 by hydroxylation Ropers and others Thus, it was demonstrated that in higher plants, the middle section of the pathway from ent -kaurene to GA 12 and GA 53 was catalysed by monooxygenases.
After the cloning of cDNAs encoding these enzymes see below , it was found that just two enzymes, ent -kaurene oxidase KO and ent -kaurenoic acid oxidase KAO , were required for GA 12 formation from ent -kaurene, with a third enzyme responsible for hydroxylation. There is still no explanation for this conundrum. After the early s, there was a hiatus in research on fungal GA biosynthesis, but the topic was reactivated by Bettina Tudzynski and her collaborators in the late s through the identification and characterisation of the biosynthetic genes, which are present as a cluster.
With the exception of a 2-oxoglutarate-dependent dioxygenase that converts GA 4 to GA 7 Bhattacharya and others , the steps from ent -kaurene, including the oxidation of GA 14 to GA 4 , are catalysed by cytochrome P monooxygenases. When conversion of GA 12 -aldehyde and GA 12 to endogenous GAs was achieved with the pumpkin endosperm system, there was considerable interest in discovering the nature of the enzymes, which were found to be soluble and therefore different from the monooxygenases responsible for the earlier steps Graebe and Hedden Enzyme activity was lost after gel filtration, indicating the requirement for a small molecule cofactor.
The demonstration that activity could be restored by 2-oxoglutaric acid and stimulated by ascorbic acid established the enzymes to be 2-oxoglutarate-dependent dioxygenases ODDs Hedden and Graebe This last enzyme appears to have a restricted distribution, being so far identified in members of the Cucurbitaceae Pimenta Lange and others After the identification of the enzymes, the next step was to purify them, and this was undertaken in several laboratories, particularly in order to facilitate their cloning Griggs and others ; Kwak and others ; Lange and others b ; Smith and MacMillan In fact, cloning was enabled both by enzyme purification and the use of mutants.
The importance of GA-deficient mutants of pea and maize in establishing GAs as plant hormones has already been described. These and mutants in other species, most notably Arabidopsis , were to prove extremely valuable for studies on GA biosynthesis and in identifying transcripts and genes encoding the enzymes.
Bernard Phinney at UCLA and Ian Murfet in Hobart, Tasmania assembled a series of single gene mutants of maize and pea, respectively, for which, through a combination of substrate feeding and product identification by GC—MS, the sites of the lesions in the biosynthetic pathway were identified.
Later, the cloning of the LE cDNA allowed the amino acid substitution, causing impairment of enzyme function in the le mutant to be defined Lester and others ; Martin and others The first characterisation of a GA-biosynthetic mutation was reported for maize, in which it was demonstrated using cell-free systems from shoots that the dwarf - 5 mutant was defective in KS activity, producing ent -isokaurene rather than ent -kaurene Hedden and Phinney Also in maize, Phinney and Spray demonstrated in bioassays with dwarf - 1 that GA 1 , but none of its precursors, possessed biological activity, so confirming the structural requirements for activity, which were later substantiated when the GA receptor was identified see below.
In , Maartin Koornneef at Wageningen, The Netherlands, produced a number of GA-sensitive mutants in Arabidopsis , naming them ga1 to ga5 on the basis of epistasis Koornneef and van der Veen The ga1 , ga2 and ga3 mutants were extreme dwarfs, and were sterile with non-germinating seeds, whereas the ga4 and ga5 phenotypes were much less severe.
Redundancy for the GA3ox and GA20ox enzymes catalysing these reactions was later to explain the relatively mild phenotype, whereas the GA1 , GA 2 and GA3 genes are present as single copies. The ga1 - 3 mutant, which was produced by neutron bombardment and contained a large deletion, was utilised in the first cloning of a GA-biosynthetic gene using genomic subtraction Sun and others GA1 could then be shown by expression in E. Soon after, the Anther ear1 An1 gene of maize, predicted to encode CPS, was cloned by transposon tagging Bensen and others Both GA1 and AN1 contained chloroplast-targeting leader sequences.
Theo Lange working with Jan Graebe and Peter Hedden purified a GA20ox from pumpkin endosperm and obtained partial sequences Lange , allowing the production of peptide antibodies that were used to isolate the cDNA from an expression library Lange and others a. The identity of the clone was confirmed by functional expression in E. Expression of the genes showed different tissue specificity and was down-regulated by application of GA, confirming feedback regulation see later.
The same group cloned KAO from barley, where it is defined by the grd5 mutation, and then from Arabidopsis , which contains two fully redundant copies Helliwell and others a. They demonstrated that the enzymes carry out the three-step conversion of ent -kaurenoic acid to GA 12 by heterologous expression in yeast. The availability of these genes provided a means to modify GA content through ectopic expression in transgenic plants. Such studies showed that in Arabidopsis , GA biosynthesis is limited particularly by GA20ox activity Coles and others ; Fleet and others ; Huang and others The potential benefits of modifying GA metabolism in crop species were a powerful driver for such experiments, particularly with the aim of reducing GA content to control growth.
As growth inhibition by these chemicals could be reversed by application of GAs, they were thought to function as anti-gibberellins, and they were found to inhibit GA biosynthesis in the fungus Kende and others Subsequently, AMO and other quaternary ammonium-type inhibitors were shown to inhibit ent -kaurene synthesis Dennis and others Further growth retardants acting on different stages of the biosynthetic pathway have been developed, with KS, KO and GA3ox, the principal sites of action reviewed by Rademacher The simple successful strategy involved screening a cDNA expression library for clones that released 3 H from [1,2- 3 H 2 ]GA 9 Thomas and others Later, a new class of GA2ox which hydroxylates C 20 -GAs was identified in Arabidopsis by activation tagging Schomburg and others Both classes of GA2ox are ubiquitous in higher plants and have important roles in regulating GA content.
Their overexpression has proved to be a very effective method for producing dwarfism Phillips The recent cloning of GA 12 hydroxylases from rice Magome and others means that genes have now been identified for all the enzymes in the pathway. Indeed, Magome and others suggested that hydroxylation may be a form of mild deactivation because overexpression of these cDNAs caused reduced growth. This is an interesting and unexpected conclusion since in most plant species, Arabidopsis being a notable exception, the hydroxylation pathway predominates. The increasing number of plant genome sequences now available has simplified the identification of GA-biosynthetic genes.
However, the genes are often incorrectly annotated, and in only a few cases, their functions are demonstrated biochemically, being assigned on the basis of sequence homology. The focus of research on GA metabolism has now moved to its regulation by developmental and environmental factors and the determination of the underlying mechanisms. This topic has been covered in a recent review Hedden and Thomas Physiological action of GA as illustrated by comparison of the Landsberg erecta Arabidopsis plant with a GA-deficient mutant ga In the absence of a GA response stem elongation, leaf enlargement, floral development, seed set and fruit development do not occur.
The molecular mechanisms of GA action has in recent years become intensively researched, but for practical reasons, much of the early research on GA function was conducted with germinating cereal grain. Research on this topic was stimulated by the importance of the process for malt production in brewing. Margaret Radley had shown earlier that barley grain contained GA-like substances which increased during germination, prompting the suggestion that the embryo was the source of GA that stimulated amylase production in the endosperm Paleg b.
This proposal has been substantiated many times since reviewed in Bethke and others The cereal aleurone proved an ideal experimental system to study GA action, since it was dependent on an external source of GA and gave a well-defined biochemical response.
It could be easily isolated to produce a uniform population of cells and was amenable to the production of protoplasts which retain their GA response with some changes , allowing experiments on membrane properties, such as patch clamping, unencumbered by the cell wall. The role of these factors in the GA response is still not well understood. GA was shown also to promote programmed cell death of aleurone cells Bethke and others , a process that also occurs in the tapetum via a GA-regulated mechanism involving GAMYB reviewed in Plackett and others A number of lines of evidence indicated that the GA receptor in aleurone cells was present on the plasma membrane.
Furthermore, experiments with an agonist and inhibitor of heterotrimeric G proteins suggested their involvement in the response of the oat aleurone to GA Jones and others However, a membrane GA receptor has not been identified, and the discovery of a soluble, nuclear-localised GA receptor GID1 in rice Ueguchi-Tanaka and others has placed some doubt on its existence, particularly with the recent report that GID1 was the only GA receptor in rice Yano and others Indeed there is some debate as to whether plants actually contain G-protein coupled receptors Taddese and others Representation of GA perception and signal transduction.
A breakthrough in GA signalling was achieved with the cloning from Arabidopsis of the genes responsible for GA-insensitivity. On the basis of genetic evidence, they proposed that GAI, which had the characteristics of a transcriptional co-activator, is a growth repressor, that the repression is relieved by GA signalling, and that the gai mutant form is resistant to GA, that is, gai is a gain-of-function mutation. Tai-ping Sun and colleagues at Duke University, USA, substantiated this scenario when they characterised a loss of function mutation that partially rescued the semi-dwarf phenotype of the GA-deficient mutant ga1 - 3 Silverstone and others Furthermore, a mutant form of RGA with the same deletion as in gai was resistant to GA-induced degradation.
Thus, the N-terminal region is required for degradation in the presence of GA, but not for growth repression. Strikingly, missense mutations in their N-terminus produce dwarfism gain of function , while loss of function mutations result in an overgrowth slender phenotype. On the basis of domain analysis and mutagenesis experiments, Ueguchi-Tanaka and others proposed a molecular model, later confirmed by the X-ray crystal structure of GID1 Shimada and others and of an Arabidopsis ortholog AtGID1a Murase and others , whereby binding of GA GA 4 was the most effective GA in a pocket allowed the flexible N-terminal strand of GID1 to associate with the top of the pocket, acting as a lid.
Rice contains a single GID1 receptor, whereas Arabidopsis has three paralogs Nakajima and others , with considerable redundancy such that loss of a single paralog has no effect on the phenotype, while the two double knockouts produce different phenotypes and loss of all three receptors results in a very extreme GA-insensitive dwarf Griffiths and others This redundancy may explain why GID1 was discovered in rice rather than Arabidopsis , in which mutant screens for the receptor were unsuccessful.
It is known that they regulate gene expression with as many genes activated as suppressed Zentella and others They do not contain a recognisable DNA-binding domain, but act in association with transcription factors. A number of DELLA partners are components of signalling pathways for other hormone classes, as, for example, the transcription factors BZR1, involved in brassinosteroid signalling Gallego-Bartolome and others , and JAZ in jasmonate signalling Hou and others , indicating the high degree of cross-talk between GA signalling and these pathways.
The early application experiments indicated that GA 3 was mobile in plants and the first studies to investigate GA transport, such as that by Kato in which he measured movement through pea stems between agar blocks, established that GA transport, unlike that of auxin, was non-polar, with equal movement in acropetal and basipetal directions.
To enable detection, Kato used very high amounts of GA, but subsequent experiments by several groups with radiolabelled GAs at physiological concentrations confirmed the non-polar nature of GA transport in shoot tissue sections, although there was evidence for polar, basipetal movement from root tips reviewed in Jacobs and Jacobs The rate of movement was much less than for polar auxin transport. Based on the observed inhibition of [ 3 H]GA 1 movement through oat coleoptiles by sodium azide, Drake and Carr concluded that GA transport is symplastic, occurring via plasmodesmata.
Kramer estimated that the decay length of GAs in the apoplast and xylem would be measured in micrometers, with hydroxylated GAs surviving slightly longer in this environment. They suggested also that accumulation of GA 1 in the cytosol would disrupt the membrane pH gradient and stressed the importance of metabolism to more polar metabolites that could be stored in the vacuole.
Indeed, Musgrave and others had suggested earlier that accumulation of [ 3 H]GA 1 in barley aleurones was associated with metabolism to more polar products. What is the physiological relevance of GA transport? On the basis of the co-location of genes encoding GA-biosynthetic enzymes and signalling components, Kaneko and others concluded that GAs are synthesised at their site of action in shoot apices and stamens of rice. However, some organs are dependent on an external source of GAs, notable examples being the cereal aleurone, which receives GA from the embryo scutellum Lenton and others , and petals, which are dependent on the anthers as their GA source Weiss and Halevy Long distance transport of GAs from leaves has been implicated in floral initiation at the shoot apex in a number of species King , and in the promotion of elongation and secondary growth of the stem Dayan and others ; Garcia-Martinez and Rappaport Rescue of GA-deficient mutants in grafting experiments has also demonstrated long distance movement of GAs, and while grafting between wild-type and mutant maize seedlings implied movement of bioactive GA Katsumi and others , experiments with pea and potato indicated that the precursor GA 20 rather than GA 1 was the mobile form reviewed in Ross and others Recently, grafting experiments with Arabidopsis mutants provided clear evidence that GA 12 is the main mobile form in this species in both the xylem and phloem Regnault and others However, as shoots would normally be autonomous for GA, the physiological relevance of these observations needs clarification.
The identification of GA-like substances in phloem and xylem exudates Hoad and Bowen ; Reid and others is consistent with GAs being transported by both these routes. However, as discussed above, while phloem transport of GAs would be predicted, transport in the xylem is not consistent with the ion trap model based on passive diffusion of the neutral molecules through membranes. Furthermore, on the basis of scanning colorimetry and electron spin resonance experiments with artificial phospholipid membranes, Pauls and others concluded that GA 4 and GA 7 associate with the membrane surface, but do not penetrate.
Transport of GAs would therefore appear to require trans-membrane transporters, particularly efflux transporters, which would also fit with the apparent high structural specificity of the GAs that are transported Regnault and others The recent report that GA-fluorescein conjugates accumulated in the endodermis of Arabidopsis roots is evidence also of cellular specificity Shani and others GA transporters are now being identified, although they lack specificity and are capable of transporting other hormones as well as unrelated molecules Chiba and others ; Saito and others It is anticipated that further GA transporters will be found in the near future.
The availability of genome sequences for numerous organisms has prompted interest in the evolution of GA production and signalling.
The lycophyte Selaginella moellendorffii , but not the bryophyte Physcomitrella patens , contains functional GA-biosynthesis and signalling pathways, indicating that they evolved in vascular plants Hirano and others ; Vandenbussche and others ; Yasumura and others In Selaginella , GA signalling regulates sporulation, but not growth, and it is suggested that the pathway evolved to regulate GAMYB, which is involved in reproductive development even in less advanced plants such as Physcomitrella Aya and others The development of a role for GA in growth responses in higher plants may have occurred through modifications to DELLA that extended the range of transcription factors with which it can interact.
Gibberellin production has evolved in some fungal and bacterial species and, at least in fungi, this seems to have occurred independently of that in plants Hedden and others On the other hand, GA production is present in a number of distantly related species Kawaide and Sassa ; Rademacher and Graebe , and may have been passed between fungal species by gene transfer.
Some bacteria also produce GAs, the nitrogen-fixing endophyte Bradyrhizobium japonica , for example, is capable of producing GA 9 Mendez and others , although there is as yet no indication of function. Since the first experiments in the late s, the chemistry, biochemistry and genetics of GA biosynthesis have been resolved to a considerable extent. Nevertheless, a few unsolved questions remain. For example, an alternative GA oxidase that converts the lactone form of the C alcohol to the aldehyde Ward and others is likely to make a major contribution to GA biosynthesis, but has not been characterised.
Furthermore, the precise mechanism by which C is lost is still unresolved. Although GA hydroxylases have been identified in rice as cytochrome Ps, other enzymes with this activity must be present, as mutants lacking both GA13ox paralogs are not completely deficient in hydroxy GAs Magome and others The regulation of GA biosynthesis by developmental and environmental factors is an area of considerable current interest, and the recent progress in understanding the molecular mechanism for GA homeostasis at the transcriptional level is an important advance.
However, work suggesting that GA feedback regulation may also operate at the level of protein stability Lee and Zeevaart needs to be followed up. Research on GA signalling is focussed on identifying the transcription factors with which DELLA proteins associate to activate or suppress gene expression, as well as their gene targets. By enabling microtubule assembly and orientation in the cytosol, GA promotes the transverse orientation of microfibrils, producing the anisotropic cell growth characteristic of GA action Shibaoka Mapping precisely the sites of GA biosynthesis and action in plants is an essential prerequisite for understanding how GA signalling is regulated.
The sensitivity of physicochemical methods for analysing GA concentrations, GC—MS and more recently liquid chromatography—mass spectrometry has improved enormously, but is still not sufficient for measuring the concentrations of GAs and precursors at the cellular level. The development of in situ methods for identifying the cells that produce, accumulate and respond to bioactive GAs is an important objective as is the further characterisation of GA transporters. This review is dedicated to Professor Jan Graebe, a pioneer in the study of gibberellin biosynthesis, on the occasion of his 85th birthday.
Skip to main content Skip to sections. Advertisement Hide. Download PDF. A Century of Gibberellin Research. Open Access. First Online: 13 October Introduction The origins of gibberellin research can be traced to the late 19th century in Japan with the demonstration that a disease of rice that produced symptoms of excessive seedling elongation and infertility, among others, was the result of fungal infection Hori The Biosynthetic Pathways Following the structural determination of gibberellic acid GA 3 , experiments to determine its biosynthetic origin in G. The main pathways for GA biosynthesis in G.
In contrast, hydroxylation commonly occurs before loss of C in plants and is the last step in GA 3 biosynthesis in G. Work on GA biosynthesis in G. A cell-free system from G. These steps were subsequently confirmed in a number of other cell-free and intact systems reviewed in Hedden and others The next intermediate, GA 12 -aldehyde, the first with the ent -gibberellane carbon skeleton, was shown to be formed from MVA in the pumpkin cell-free system, which also produced GA 12 Graebe and others Refeeding these products established the pathway shown in Fig.
Open image in new window. Investigations into the physiological responses of higher plants to GA were advanced even before the active compounds had been isolated and structurally characterised.
The early work was reviewed by Stowe and Yamaki , who listed the numerous effects of GAs on plant development. Some of these are illustrated in Fig. Promotion of elongation in young still growing stems is one of the most obvious effects of GA and it occurs without a change in the number of nodes. Internode growth is promoted through enhanced cell elongation, shown later to be due to relaxation of the cell wall rather than increased cell turgor Cosgrove and Sovonick-Dunford However, GAs also promote cell division in some circumstances, notably in the induction of bolting in rosette species Sachs Stowe and Yamaki noted that GA promotes leaf expansion, but inhibits root growth, from which they concluded that GA changes the root—shoot ratio.
It is now known that GA action is essential for root elongation, but high GA concentrations are inhibitory and in most cases roots contain close to saturating GA levels Tanimoto Another notable action of GA is the promotion of seed germination: of particular note was the observation that GA substituted for the light requirement for germination of photoblastic seeds, whereas it reversed the light inhibition of stem elongation.
These contrasting effects could be later explained by the opposite responses of GA metabolism to red light in these tissues reviewed by Kamiya and Garcia-Martinez The effect of GAs on flowering is complex and can be promotive, inhibitory or neutral depending on the species Pharis and King ; Zeevaart Some long-day plants growing under non-inductive conditions can be induced to bolt and flower by GA application, while others will bolt without flowering.
The ability of GA to substitute for long-days prompted speculation that it was the long sought-after leaf-derived signal, florigen, and, although this is now generally recognised to be flowering locus T FT or related peptides, there is no doubt that GA can act as a mobile inductive signal, if not the major one King Until the discovery of GAs, elongation growth was thought to be regulated exclusively by auxin, and many of the early experiments tested the hypothesis that GA acted by stimulating auxin levels reviewed in Paleg However, the reverse scenario is now known to occur with auxin promoting stem elongation by increasing GA biosynthesis Ross and others , although in a recent report, it was shown that GA is required for auxin transport Willige and others The mechanism by which GAs promote growth, summarised in Fig.
The basic concept that GAs act by suppressing a growth inhibitor was proposed from studies with GA-insensitive mutants Harberd and others The characteristics of such mutants had been known for many years.
Results and Discussion
In , Margaret Radley showed that the Japanese dwarf wheat cultivar Norin and related dwarf lines did not respond to applied GA, unlike tall lines, and that they accumulated much higher levels of GA-like substances than the tall cultivars Radley This proposal proved correct, although the link between GA action and metabolism was not as direct as Radley may have envisaged.
Norin is the source of the Reduced height Rht genes that were introduced by Norman Borlaug into high yielding wheat varieties in the Green Revolution to stabilise the stem and increase harvest index Hedden The two homoeologous semi-dwarfing genes present in Norin, Rht1 renamed RhtB1b to indicate its genome location and allele and Rht2 RhtD1b , are still used widely in modern wheat cultivars. Similar results had been obtained for the GA-insensitive dwarf - 8 mutant of maize Fujioka and others and GA - insensitive gai Talon and others a , an Arabidopsis deletion mutant obtained by Koornneef and others In contrast, Potts and others reported that slender pea mutants containing the la cry s gene combination grew independently of GA status and possessed abnormally low levels of GA-like substances.
Similarly, slender , an overgrowth mutant of barley with a constitutive GA response was shown to contain lower levels of C 19 -GAs, but elevated C 20 -GA levels relative to its wild type Croker and others The demonstration by Cowling and others that the transcript level for GA4 , which encodes a GA3ox enzyme AtGA3ox1 , was similarly regulated by GA signalling extended the number of genes under feedback control. Subsequently, it was reported that some GA2ox genes are up-regulated by GA Thomas and others , whereas the GID1 receptor genes are down-regulated Griffiths and others , indicating the existence of a complex system of homeostatic regulation in GA signalling.
Acknowledgments This review is dedicated to Professor Jan Graebe, a pioneer in the study of gibberellin biosynthesis, on the occasion of his 85th birthday. Planta — CrossRef Google Scholar. Phytochemistry — CrossRef Google Scholar. Part XIV. Metabolic pathways from ent -kaurenoic acid to fungal gibberellins in mutant Ba of Gibberella fujikuroi. Molecular and physiological characterization. Part I. Chem Ind Lond Google Scholar. Gibberellins are diterpenoids derived from four isoprenoid units forming a system of four rings. It is distinguished between gibberellins of 19 and such of 20 C-atoms.
Both enumeration and the illustration below show, how the single structures differ. Young, growing meristematic cauline tissue, apical root cells, young fruits, as well as unripe or germinating seeds are all rich in gibberellins. In sun flowers is the gibberellin content highest in the young leaves and in the uppermost internodes while it decreases continuously towards the basal leaves and internodes. Gibberellin content and growth velocity of a tissue are correlated. Despite the shoot are high concentrations measured, too, in the root tips.
The idea that they are produced in the roots seemed logical though indications exist that they are only converted in the roots, i. The new product is then transported back via the xylem into the shoot. Both in xylem- and phloem exuded matter have gibberellins been detected showing how the distribution mechanism for hormones works in this type of plant.
Moreover exist reliable indications that symplastic transport exists in the tissues themselves. Biological activities. Especially impressive is the demonstration of the gibberellin GA 3 onto mutants of Phaseolus vulgaris characterized by dwarfism due to a genetic defect. The result points to a defect in the synthesis of GA 3 as the cause of dwarfism. Similar results have been obtained with dwarfism mutants of other cultured species. Gibberellins promote especially elongation and not growth by cell division.
Gibberellins | Laboratory of growth regulators
They stimulate the germination of pollen and the growth of pollen tubes. They induce the development of parthenocarpous fruits like apples, pumpkin, and egg-plants twice the size of normal fruit. In a number of plant species is flower formation controlled by extern factors, especially by light short-day plants and long-day plants or low temperatures vernalization. After addition of GA 3 , though, do they flower even without the otherwise necessary extern signals. This does not mean that gibberellins take actively part in flower formation.
It seems more as if flower formation in the mentioned species were dependent on a previous elongation of the stem axis, and that gibberellins stimulate mainly this step.