The partial purification and characterisation of gibberellin 2β-hydroxylases from seeds of Pisum sativum

A wheat dominant dwarfing line with Rht12, which reduces stem cell length and affects gibberellic acid synthesis, is a 5AL terminal deletion line

Dwarfing and semi-dwarfing are important agronomic traits that have great potential for the improvement of wheat yields. Rht12, a dominant gibberellic acid (GA)-responsive dwarfing gene from the gamma-ray-induced wheat mutant Karcagi 522M7K, is located in the long arm of chromosome 5A, which is closely linked with the locus Xwmc410. Rht12 is likely an ideal gene for GA biosynthesis and deactivation research in common wheat. However, information on the Rht12 locus and sequence is lacking. In this study, Rht12 significantly shortened stem cell length and decreased GA biosynthetic components. Using bulked segregant RNA-Seq, wheat 660k single nucleotide polymorphism chip detection, and newly developed simple sequence repeat markers, Rht12 was mapped to a 11.21-Mb region at the terminal end of chromosome 5AL, and was found to be closely linked with the Xw5ac207SSR marker with a 10.73-Mb fragment deletion in all of the homologous dwarfing plants. Transcriptome analyses of the remaining 483-kb region showed significantly higher expression of the TraesCS5A01G543100 gene encoding the GA metabolic enzyme GA 2-β-dioxygenase in dwarfing plants than in high stalk plants, suggesting that Rht12 reduces plant height by activating TaGA2ox-A14. Taken together, our findings will promote cloning and functional studies of Rht12 in common wheat.

2-Oxoglutarate: linking TCA cycle function with amino acid, glucosinolate, flavonoid, alkaloid, and gibberellin biosynthesis

The tricarboxylic acid (TCA) cycle intermediate 2-oxoglutarate (2-OG) is used as an obligatory substrate in a range of oxidative reactions catalyzed by 2-OG-dependent dioxygenases. These enzymes are widespread in nature being involved in several important biochemical processes. We have recently demonstrated that tomato plants in which the TCA cycle enzyme 2-OG dehydrogenase (2-ODD) was antisense inhibited were characterized by early senescence and modified fruit ripening associated with differences in the levels of bioactive gibberellin (GA). Accordingly, there is now compelling evidence that the TCA cycle plays an important role in modulating the rate of flux from 2-OG to amino acid metabolism. Here we discuss recent advances in the biochemistry and molecular biology of 2-OG metabolism occurring in different biological systems indicating the importance of 2-OG and 2-OG dependent dioxygenases not only in glucosinolate, flavonoid and alkaloid metabolism but also in GA and amino acid metabolism. We additionally summarize recent findings regarding the impact of modification of 2-OG metabolism on biosynthetic pathways involving 2-ODDs.

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.

The SLENDER gene of pea encodes a gibberellin 2-oxidase

The amount of active gibberellin (GA) in plant tissues is determined in part by its rate of catabolism through oxidation at C-2. In pea (Pisum sativum L.) seeds, GA 2-oxidation is controlled by the SLN (SLENDER) gene, a mutation of which produces seedlings characterized by a slender or hyper-elongated phenotype. We cloned a GA 2-oxidase cDNA from immature pea seeds by screening an expression library for enzyme activity. The clone contained a full-length open reading frame encoding a protein of 327 amino acids. Lysate of bacterial cultures expressing the protein converted the C(19)-GAs, GA(1), GA(4), GA(9), and GA(20) to the corresponding 2beta-hydroxy products. GA(9) and GA(20) were also converted to GA(51) and GA(29) catabolites, respectively. The gene appeared to be one member of a small family of GA 2-oxidases in pea. Transcript was found predominantly in roots, flowers, young fruits, and testae of seeds. The corresponding transcript from sln pea contained a point mutation and did not produce active enzyme when expressed heterologously. RFLP analysis of a seedling population segregating for SLN and sln alleles showed the homozygous mutant allele co-segregating with the characteristic slender phenotype. We conclude that SLN encodes GA 2-oxidase.

Gibberellin 2-oxidation and the SLN gene of Pisum sativum

Two cDNAs encoding gibberellin 2-oxidases were isolated from maturing pea seeds. The first, PsGA2ox1, was isolated by activity screening of a Lambda-ZAP cDNA library excised into phagemid form and expressed in Escherichia coli. The second, PsGA2ox2, was obtained initially as a PCR product using degenerate primers designed according to conserved regions of plant 2-oxoglutarate-dependent dioxygenases. E. coli heterologous expression products of PsGA2ox1 and PsGA2ox2 converted GA1 to GA8, as shown by HPLC-radiocounting, and gas chromatography-MS. PsGA2ox1 converted GA20 to GA29, but GA20 was a poor substrate for the PsGA2ox2 expression product. Furthermore, PsGA2ox1 converted GA29 to GA29-catabolite at a low level of efficiency while PsGA2ox2 did not catalyse this step. A cDNA of PsGA2ox1 isolated from plants of genotype sln contained a single base deletion which was predicted to produce a truncated protein and gibberellin 2-oxidase activity could not be demonstrated from this cDNA. A 10 bp size difference between the introns of the SLN and sln PsGA2ox1 genes was used to show co-segregation between the SLN and sln phenotypes and the size of the PCR products. PsGA2ox1 transcripts were more abundant in cotyledons than in shoots, while the reverse was the case for PsGA2ox2. The expression patterns of the genes, together with the effects of the sln mutation, indicate that PsGA2ox1 plays a major role in GA20 deactivation in both shoots and maturing seeds, while the PsGA2ox2 gene might be important for GA1 deactivation in the shoot.

Molecular cloning and functional expression of gibberellin 2- oxidases, multifunctional enzymes involved in gibberellin deactivation

A major catabolic pathway for the gibberellins (GAs) is initiated by 2beta-hydroxylation, a reaction catalyzed by 2-oxoglutarate-dependent dioxygenases. To isolate a GA 2beta-hydroxylase cDNA clone we used functional screening of a cDNA library from developing cotyledons of runner bean (Phaseolus coccineus L.) with a highly sensitive tritium-release assay for enzyme activity. The encoded protein, obtained by heterologous expression in Escherichia coli, converted GA9 to GA51 (2beta-hydroxyGA9) and GA51-catabolite, the latter produced from GA51 by further oxidation at C-2. The enzyme thus is multifunctional and is best described as a GA 2-oxidase. The recombinant enzyme also 2beta-hydroxylated other C19-GAs, although only GA9 and GA4 were converted to the corresponding catabolites. Three related cDNAs, corresponding to gene sequences present in Arabidopsis thaliana databases, also encoded functional GA 2-oxidases. Transcripts for two of the Arabidopsis genes were abundant in upper stems, flowers, and siliques, but the third transcript was not detected by Northern analysis. Transcript abundance for the two most highly expressed genes was lower in apices of the GA-deficient ga1-2 mutant of Arabidopsis than in wild-type plants and increased after treatment of the mutant with GA3. This up-regulation of GA 2-oxidase gene expression by GA contrasts GA-induced down-regulation of genes encoding the biosynthetic enzymes GA 20-oxidase and GA 3beta-hydroxylase. These mechanisms would serve to maintain the concentrations of biologically active GAs in plant tissues.

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.

DIOXYGENASES: Molecular Structure and Role in Plant Metabolism

▪ Abstract  Dioxygenases are nonheme iron-containing enzymes important in the biosynthesis of plant signaling compounds such as abscisic acid, gibberellins, and ethylene and also of secondary metabolites, notably flavonoids and alkaloids. Plant dioxygenases fall into two classes: lipoxygenases and 2-oxoacid-dependent dioxygenases. The latter catalyze hydroxylation, epoxidation, and desaturation reactions; some enzymes catalyze more than one type of reaction in successive steps in a biosynthetic pathway. This review highlights recent discoveries on both enzyme groups, particularly in relation to gibberellin biosynthesis, in vivo activity of 1-aminocyclopropane-1-carboxylate oxidase, and molecular structure/function relationships. Similarities between the roles of monooxygenases and dioxygenases are also discussed.

2-oxoglutarate-dependent dioxygenase and related enzymes: biochemical characterization

Hydroxylation reactions are catalysed by a few major subclasses of enzymes which are ubiquitously distributed in nature. Dioxygenases generally occur as soluble enzymes where they catalyse a diversity of oxygenation reactions in a large number of metabolic pathways in animals, plants and micro-organisms. This review discusses recent advances in the biochemistry and molecular biology of dioxygenases occurring in different biological systems.

A dioxygenase gene (Ids2) expressed under iron deficiency conditions in the roots of Hordeum vulgare

A lambda zapII cDNA library was constructed from mRNA isolated from Fe-deficient barley roots and screened with cDNA probes made from mRNA of Fe-deficient and Fe-sufficient (control) barley roots. Seven clones were selected. Among them a clone having the putative full-length mRNA of dioxygenase as judged by northern hybridization was selected and named Ids2 (iron deficiency-specific clone 2). Using a cDNA fragment as probe, two clones from the genomic library (lambda EMBL-III) were isolated and one was sequenced. The predicted amino acid sequence of Ids2 resembled that of 2-oxoglutarate-dependent dioxygenase. Ids2 is expressed in the Fe-deficient barley roots but is not in the leaves. The expression is repressed by the availability of Fe. Ids2 was also strongly expressed under Mn deficiency and weakly under Zn deficiency or excess NaCl (0.5%). The upstream 5'-flanking region of Ids2 has a root-specific cis element of the CaMV 35S promoter and a nodule-specific element of leghemoglobin, a metal regulatory element (MRE) and several Cu regulatory elements (UAS) of yeast metallothionein (CUP1).

The partial purification and characterization of a gibberellin C-20 hydroxylase from immature Pisum sativum L. seeds

A gibberellin (GA) C-20 hydroxylase that catalyses the conversion of GA53 to GA44 was purified from developing pea embryos by ammonium-sulfate precipitation, gel filtration and anion-exchange column chromatography. The purification was about 270-fold and 15% of the enzymic activity was recovered. The relative molecular mass was 44000 by Sephadex G-200 gel filtration. The apparent Michaelis constant was 0.7 μM and the isoelectric point was 5.6-5.9. The enzymic activity was optimal at pH 7.0 2-Oxoglutarate and ascorbate were required for activity. Low concentrations of Fe(2+) stimulated the reaction, but externally added Fe(2+) was not essential, even in the most purified preparation. Catalase and bovine serum albumin also stimulated. Dithiothreitol preserved the activity during purification but was not needed during incubation. In fact, the simultaneous presence of dithiothreitol and Fe(2+) in the incubation mixture was inhibitory to the purified enzyme. The cofactor requirements are typical for those of 2-oxoglutarate-dependent dioxygenases.When the incubation time was long enough, GA53 was converted to both GA44 and GA19. The proportions of these two products remained constant throughout the purification, but this does not necessarily mean that their formations is catalysed by a single enzyme. Sodium dodecyl sulfatepolyacrylamide gel electrophoresis showed that the final preparation contained several proteins. Although the most prominent protein band was located within the range expected for the enzyme on the grounds of its molecular weight, this band did not represent the enzyme, since it separated from the GA C-20 hydroxylase activity on ultrathin-layer isoeletric focusing.

Enzymes from seeds of Phaseolus vulgaris L.: Hydroxylation of gibberellins A20 and A 1 and 2,3-dehydrogenation of gibberellin A 20

A time-course study is described relating the enzyme activities for GA20 metabolism with seed development in Phaseolus vulgaris L. Enzyme activity for the 3β-hydroxylation of GA20 to GA1, and for the 2,3-desaturation of GA20 to GA5, was confined to the cotyledons and showed maximal specific activity at 21 d after anthesis. These enzyme activities co-occurred, together with a much lower level of activity for the 2β-hydroxylation of GA20 to GA29. The observed rates of GA1, GA5 and GA29 formation from GA20 were constant under a range of incubation conditions. Enzyme activity for the conversion of GA1 to GA8 was detected only in embryos of seed from 40 d after anthesis. By deuterium-labelling and analysis of the products by gas chromatography-selected ion monitoring it was shown that 2β-hydroxylation of GA1 to GA8 and 3β-hydroxylation of GA20 to GA1 occur with retention of configuration and that the conversion of GA20 to GA5 occurs with loss of the 2β- and 3β-hydrogens. These results establish that GA1 is not formed from GA20 via GA5.