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PubMed1992

On the role of abscisic Acid and gibberellin in the regulation of growth in rice.

Submergence induces rapid elongation of rice coleoptiles (Oryza sativa L.) and of deepwater rice internodes. This adaptive feature helps rice to grow out of the water and to survive flooding. Earlier, we found that the growth response of submerged deepwater rice plants is mediated by ethylene and gibberellin (GA). Ethylene promotes growth, at least in part, by increasing the responsiveness of the internodal tissue to GA. In the present work, we examined the possibility that increased responsiveness to GA was based on a reduction in endogenous abscisic acid (ABA) levels. Submergence and treatment with ethylene led, within 3 hours, to a 75% reduction in the level of ABA in the intercalary meristem and the growing zone of deepwater rice internodes. The level of GA(1) increased fourfold during the same time period. An interaction between GA and ABA could also be shown by application of the hormones. ABA inhibited growth of submerged internodes, and GA counteracted this inhibition. Our results indicate that the growth rate of deepwater rice internodes is determined by the ratio of an endogenous growth promoter (GA) and a growth inhibitor (ABA). We also investigated whether ABA is involved in regulating the growth of rice coleoptiles. Rice seedlings were grown on solutions containing fluridone, an inhibitor of carotenoid and, indirectly, of ABA biosynthesis. Treatment with fluridone reduced the level of ABA in coleoptiles and first leaves by more than 75% and promoted coleoptile growth by more than 60%. Little or no enhancement of growth by fluridone was observed in barley, oat, or wheat. The involvement of ABA in determining the growth rate of rice coleoptiles and deepwater rice internodes may be related to the semiaquatic growth habit of this plant.

Journal Article↗

PubMed1992

Gibberellins and parthenocarpic ability in developing ovaries of seedless mandarins.

Satsuma (Citrus unshiu [Mak] Marc.) and Clementine (Citrus reticulata [Hort.] Ex. Tanaka, cv Oroval) are two species of seedless mandarins differing in their tendency to develop parthenocarpic fruits. Satsuma is a male-sterile cultivar that shows a high degree of natural parthenocarpy and a high fruit set. Seedless Clementine varieties are self-incompatible, and in the absence of cross-pollination show a very low ability to set fruit. The gibberellins (GAs) GA53, putative 17-OH-GA(53), GA(44), GA(17), GA(19), GA(20), GA(29), GA(1), 3-epi-GA(1), GA(8), GA(24), GA(9), and GA(4) have been identified from developing fruits of both species by full-scan combined gas chromatography-mass spectrometry. Using selected ion monitoring with [(2)H(2)]- and [(13)C]-labeled internal standards, the levels of GA(53), GA(44), GA(19), GA(20), GA(1), GA(8), GA(4), and GA(9) were determined in developing ovaries at anthesis and 7 days before and after anthesis, from both species. Except for GA8, levels of the 13-hydroxy-GAs were higher in Satsuma than in Clementine, and these differences were more prominent for developing young fruits. At petal fall, Satsuma had, on a nanograms per gram dry weight basis, higher levels of GA(53) (10.4x), GA(44) (13.9x), GA(19) (3.0x), GA(20) (11.2x), and GA(1) (2.0x). By contrast, levels of GA(8) were always higher in Clementine, whereas levels of GA(4) did not differ greatly. Levels of GA(9) were very low in both species. At petal fall, fruitlets of Satsuma and Clementine contained 65 and 13 picograms of GA(1), respectively. At this time, the application of 25 micrograms of paclobutrazol to fruits increased fruit abscission in both varieties. This effect was reversed by the simultaneous applications of 1 microgram of GA(3). GA(3) alone improved the set in Clementine (13x), but had little influence on Satsuma. Thus, seedless fruits of the self-incompatible Clementine mandarin may not have adequate GA levels for fruit set. Collectively, these results suggest that endogenous GA content in developing ovaries is the limiting factor controlling the parthenocarpic development of the fruits.

Journal Article↗

PubMed1998

Phytochrome regulation and differential expression of gibberellin 3beta-hydroxylase genes in germinating Arabidopsis seeds.

Despite extensive studies on the roles of phytochrome in photostimulated seed germination, the mechanisms downstream of the photoreceptor that promote germination are largely unknown. Previous studies have indicated that light-induced germination of Arabidopsis seeds is mediated by the hormone gibberellin (GA). Using RNA gel blot analyses, we studied the regulation of two Arabidopsis genes, GA4 and GA4H (for GA4 homolog), both of which encode GA 3beta-hydroxylases that catalyze the final biosynthetic step to produce bioactive GAs. The newly isolated GA4H gene was expressed predominantly during seed germination. We show that expression of both GA4 and GA4H genes in imbibed seeds was induced within 1 hr after a brief red (R) light treatment. In the phytochrome B-deficient phyB-1 mutant, GA4H expression was not induced by R light, but GA4 expression still was, indicating that R light-induced GA4 and GA4H expression is mediated by different phytochromes. In contrast to the GA4 gene, the GA4H gene was not regulated by the feedback inhibition mechanism in germinating seeds. Our data demonstrate that expression of GA 3beta-hydroxylase genes is elevated by R light, which may result in an increase in biosynthesis of active GAs to promote seed germination. Furthermore, our results suggest that each GA 3beta-hydroxylase gene plays a unique physiological role during light-induced seed germination.

Amino Acid Sequence↗

PubMed1998

Heterotrimeric G proteins are implicated in gibberellin induction of a-amylase gene expression in wild oat aleurone

The role of heterotrimeric G proteins in gibberellin (GA) induction of a-amylase gene expression was examined in wild oat aleurone protoplasts. Mas7, a cationic amphiphilic tetradecapeptide that stimulates GDP/GTP exchange by heterotrimeric G proteins, specifically induced alpha-amylase gene expression and enzyme secretion in a very similar manner to GA1. In addition, Mas7 stimulated expression of an alpha-Amy2/54:GUS promoter and reporter construct in transformed protoplasts. Both Mas7 and GA1 induction of alpha-amylase mRNA were insensitive to pertussis toxin. Hydrolysis-resistant nucleotides were introduced into aleurone protoplasts during transfection with reporter gene constructs. GDP-beta-S, which inhibits GDP/GTP exchange by heterotrimeric G proteins, completely prevented GA1 induction of alpha-Amy2/54:GUS expression, whereas GTP-gamma-S, which activates heterotrimeric G proteins, stimulated expression very slightly. Novel cDNA sequences from Galpha and Gbeta subunits were cloned from wild oat aleurone cells. By using RNA gel blot analysis, we found that the transcripts were expressed at a low level. Heterotrimeric G proteins have been implicated in several events during plant growth and development, and these data suggest that they may be involved in GA regulation of alpha-amylase gene expression in aleurone.

Journal Article↗

PubMed1993

Derivative Alleles of the Arabidopsis Gibberellin-Insensitive (gai) Mutation Confer a Wild-Type Phenotype.

The gai mutation of Arabidopsis confers a dwarf phenotype resembling that of mutants defective in gibberellin (GA) biosynthesis. However, gai mutant plants differ from GA biosynthesis mutants because they fail to respond to exogenous GAs and accumulate endogenous GA species to higher (rather than lower) levels than found in wild-type controls. The gai mutation, therefore, identifies a gene that modulates the response of plant cells to GA. We have mapped gai with respect to visible and restriction fragment length polymorphism (RFLP) markers from chromosome 1. To observe the phenotype exhibited by individuals potentially lacking wild-type (GAI) function, we have also isolated novel irradiation-induced derivative alleles of gai. When homozygous, these alleles confer a revertant phenotype that is indistinguishable from the wild type. gai is a semidominant mutation that exerts its effects either because it is a gain-of-function mutation or because it is a loss-of-function or reduced-function mutation. The genetic and physiological properties of the derivative alleles are considered with reference to these alternative modes of dominance of gai. Because these alleles are potential deletion or rearrangement mutations, together with the closely linked RFLP markers identified in the linkage mapping experiments, they provide useful resources for the isolation of the gai locus via a map-based cloning approach.

Journal Article↗

PubMed1997

Sugar Repression of a Gibberellin-Dependent Signaling Pathway in Barley Embryos.

Increasing evidence shows that sugars can act as signals affecting plant metabolism and development. Some of the effects of sugars on plant growth and development suggest an interaction of sugar signals with hormonal regulation. We investigated the effects of sugars on the induction of [alpha]-amylase by gibberellic acid in barley embryos and aleurone layers. Our results show that sugar and hormonal signaling interact in the regulation of gibberellic acid-induced gene expression in barley grains. The induction of [alpha]-amylase by gibberellic acid in the aleurone layer is unaffected by the presence of sugars, but repression by carbohydrates is effective in the embryo. [alpha]-Amylase expression in the embryo is localized to the scutellar epithelium and is hormone and sugar modulated. The effects of glucose are independent from the effects of sugars on gibberellin biosynthesis. They are not due to an osmotic effect, they are independent of abscisic acid, and only hexokinase-phosphorylatable glucose analogs are able to trigger gene repression. Overall, the results suggest the existence of an interaction between the hormonal and metabolic regulation of [alpha]-amylase genes in barley grains.

Journal Article↗

PubMed2004

Evidence of a cross-talk regulation of a GA 20-oxidase (FsGA20ox1) by gibberellins and ethylene during the breaking of dormancy in Fagus sylvatica seeds.

Gibberellin 20-oxidase (GA 20-oxidase) is an enzyme that catalyses the last three steps in the synthesis of active GAs and is a potential control point in the regulation of GA biosynthesis. Reverse transcriptase-polymerase chain reaction with degenerated oligonucleotides conserved among GA 20-oxidases was used to isolate a cDNA clone for this enzyme in Fagus sylvatica L. seeds. This clone contains all the features and exhibits homology to GA 20 oxidases from several plant species. Expression of this clone, named FsGA20ox1, as a fusion protein expressed in Escherichia coli confirmed that it was able to metabolize [(14)C]GA(12) to [(14)C]GA(9) and [(14)C]GA(53) to [(14)C]GA(20). Analysis of FsGA20ox1 transcript levels showed similar low expression during stratification at 4 degrees C and in the presence of gibberellic acid or ethephon (compound that releases ethylene in solution), treatments proved to be efficient in breaking the dormancy of beech seeds. However, there was a drastic increase of FsGA20ox1 transcript levels in the presence of paclobutrazol (PCB), a well-known GAs biosynthesis inhibitor, or of 2-aminoxyacetic acid (AOA), an inhibitor of ethylene biosynthesis. Furthermore, the effect of AOA was reversed by the addition of GA(3) and that of PCB by ethephon. This indicates that the gene product is subjected to down-regulation by GA and ethylene, and further suggests a cross-talk gene regulation by these two hormones during the transition from seed dormancy to germination.

Journal Article↗

PubMed2005

Modification of gibberellin biosynthesis in the grafted apple scion allows control of tree height independent of the rootstock.

The availability of short stature apple scions that required minimal applications of chemical growth retardants and could be used with a range of rootstocks would be of considerable benefit to fruit growers. We have suppressed the expression of a gene encoding the gibberellin (GA) biosynthetic enzyme GA 20-oxidase to reduce the levels of bioactive GAs in a scion variety, resulting in significant reductions in stem height. Application of GA3 reversed the effect. The scion remained dwarfed after grafting on to normally invigorating rootstocks, whilst control plants of the same cultivar displayed the expected vigour when grafted on to these rootstocks. This approach could be applicable to any perennial crop variety, allowing dwarf trees to be obtained on any available rootstock or on their own roots without the need for chemical growth retardant application. In effect, seedlings that are well suited to local conditions (drought, salinity) could be employed as tree rootstocks, as could existing rootstocks valued for characters other than vigour control, such as pest and disease resistance.

Journal Article↗

PubMed1997

GIBBERELLIN BIOSYNTHESIS: Enzymes, Genes and Their Regulation.

The recent impressive progress in research on gibberellin (GA) biosynthesis has resulted primarily from cloning of genes encoding biosynthetic enzymes and studies with GA-deficient and GA-insensitive mutants. Highlights include the cloning of ent-copalyl diphosphate synthase and ent-kaurene synthase (formally ent-kaurene synthases A and B) and the demonstration that the former is targeted to the plastid; the finding that the Dwarf-3 gene of maize encodes a cytochrome P450, although of unknown function; and the cloning of GA 20-oxidase and 3beta-hydroxylase genes. The availability of cDNA and genomic clones for these enzymes is enabling the mechanisms by which GA concentrations are regulated by environmental and endogenous factors to be studied at the molecular level. For example, it has been shown that transcript levels for GA 20-oxidase and 3beta-hydroxylase are subject to feedback regulation by GA action and, in the case of the GA 20-oxidase, are regulated by light. Also discussed is other new information, particularly from mutants, that has added to our understanding of the biosynthetic pathway, the enzymes, and their regulation and tissue localization.

Journal Article↗

PubMed2000

GROWTH RETARDANTS: Effects on Gibberellin Biosynthesis and Other Metabolic Pathways.

Plant growth retardants are applied in agronomic and horticultural crops to reduce unwanted longitudinal shoot growth without lowering plant productivity. Most growth retardants act by inhibiting gibberellin (GA) biosynthesis. To date, four different types of such inhibitors are known: (a) Onium compounds, such as chlormequat chloride, mepiquat chloride, chlorphonium, and AMO-1618, which block the cyclases copalyl-diphosphate synthase and ent-kaurene synthase involved in the early steps of GA metabolism. (b) Compounds with an N-containing heterocycle, e.g. ancymidol, flurprimidol, tetcyclacis, paclobutrazol, uniconazole-P, and inabenfide. These retardants block cytochrome P450-dependent monooxygenases, thereby inhibiting oxidation of ent-kaurene into ent-kaurenoic acid. (c) Structural mimics of 2-oxoglutaric acid, which is the co-substrate of dioxygenases that catalyze late steps of GA formation. Acylcyclohexanediones, e.g. prohexadione-Ca and trinexapac-ethyl and daminozide, block particularly 3ss-hydroxylation, thereby inhibiting the formation of highly active GAs from inactive precursors, and (d) 16,17-Dihydro-GA5 and related structures act most likely by mimicking the GA precursor substrate of the same dioxygenases. Enzymes, similar to the ones involved in GA biosynthesis, are also of importance in the formation of abscisic acid, ethylene, sterols, flavonoids, and other plant constituents. Changes in the levels of these compounds found after treatment with growth retardants can mostly be explained by side activities on such enzymes.

Journal Article↗

PubMed2005

Purification and kinetic studies of recombinant gibberellin dioxygenases.

BACKGROUND: The 2-oxoglutarate-dependent dioxygenases (2ODDs) of gibberellin (GA) biosynthesis have a key role in the metabolism of a major plant hormone. The activity of recombinant GA 2ODDs from many species has been characterised in detail, however little information relates to enzyme purification. Native GA 2ODDs displayed lability during purification. RESULTS: Two GA 2ODDs were expressed in Escherichia coli and purified to homogeneity. The GA 2-oxidase from Pisum sativum L., PsGA2OX1, was expressed as a glutathione s-transferase (GST) fusion. It was purified in the three steps of affinity chromatography, GST removal and gel filtration. Highly pure PsGA2OX1 was obtained at a yield of 0.3 mg/g of cells. It displayed a Km of 0.024 microM and a Vmax of 4.4 pkat/mg toward [1beta,2beta,3beta-3H3]GA20. The GA 3-oxidase from Arabidopsis thaliana, AtGA3OX4, was expressed as a poly(His)-tagged thioredoxin fusion. It was purified by Immobilised Metal Affinity Chromatography followed by gel filtration. Cleavage of the fusion took place between the two purification steps. Highly pure AtGA3OX4 was obtained at a yield of 0.01 mg/g of cells. It displayed a Km of 0.82 microM and Vmax of 52,500 pkat/mg toward [1beta,2beta,3beta-3H3]GA20. CONCLUSION: Fusion tags were required to stabilise and solubilise PsGA2OX1 and AtGA3OX4 during E. coli expression. The successful purification of milligram quantities of PsGA2OX1 enables mechanistic and structural studies not previously possible on GA 2ODDs. A moderate yield of pure AtGA3OX4 requires the further optimisation of the latter stages of the enzyme purification schedule. PsGA2OX1's action in planta as deduced from the effect of the null mutation sln on GA levels in seeds is in agreement with the kinetic parameters of the recombinant enzyme.

Arabidopsis↗

PubMed1963