The localization, metabolism and biological activity of gibberellins in maturing and germinating seeds of Pisum sativum cv. Progress No. 9

Transcriptomics and starch biosynthesis analysis in leaves and developing seeds of mung bean provide a basis for genetic engineering of starch composition and seed quality

Mung bean starch is distinguished by its exceptional high amylose content and regulation of starch biosynthesis in leaves and storage tissues, such as seeds, share considerable similarities. Genetic engineering of starch composition and content, requires detailed knowledge of starch biosynthetic gene expression and enzymatic regulation. In this study we applied detailed transcriptomic analyses to unravel the global differential gene expression patterns in mung bean leaves and in seeds during various stages of development. The objective was to identify candidate genes and regulatory mechanisms that may enable generation of desirable seed qualities through the use of genetic engineering. Notable differences in gene expression, in particular low expression of the Protein Targeting to Starch (PTST), starch synthase (SS) 3, and starch branching enzyme1 (SBE1) encoding genes in developing seeds as compared to leaves were evident. These differences were related to starch molecular structures and granule morphologies. Specifically, the starch molecular size distribution at different stages of seed development correlated with the starch biosynthesis gene expression of the SBE1, SS1, granule-bound starch synthases (GBSS) and isoamylase 1 (ISA1) encoding genes. Furthermore, putative hormonal and redox controlled regulation were observed, which may be explained by abscisic acid (ABA) and indole-3-acetic acid (IAA) induced signal transduction, and redox regulation of ferredoxins and thioredoxins, respectively. The morphology of starch granules in leaves and developing seeds were clearly distinguishable and could be correlated to differential expression of SS1. Here, we present a first comprehensive transcriptomic dataset of developing mung bean seeds, and combined these findings may enable generation of genetic engineering strategies of for example starch biosynthetic genes for increasing starch levels in seeds and constitute a valuable toolkit for improving mung bean seed quality.

Highlights in gibberellin research: A tale of the dwarf and the slender

It has been almost a century since biologically active gibberellin (GA) was isolated. Here, we give a historical overview of the early efforts in establishing the GA biosynthesis and catabolism pathway, characterizing the enzymes for GA metabolism, and elucidating their corresponding genes. We then highlight more recent studies that have identified the GA receptors and early GA signaling components (DELLA repressors and F-box activators), determined the molecular mechanism of DELLA-mediated transcription reprograming, and revealed how DELLAs integrate multiple signaling pathways to regulate plant vegetative and reproductive development in response to internal and external cues. Finally, we discuss the GA transporters and their roles in GA-mediated plant development.

Domestication has altered the ABA and gibberellin profiles in developing pea seeds

MAIN CONCLUSION

We showed that wild pea seeds contained a more diverse combination of bioactive GAs and had higher ABA content than domesticated peas. Although the role of abscisic acid (ABA) and gibberellins (GAs) interplay has been extensively studied in Arabidopsis and cereals models, comparatively little is known about the effect of domestication on the level of phytohormones in legume seeds. In legumes, as in other crops, seed dormancy has been largely or entirely removed during domestication. In this study, we have measured the endogenous levels of ABA and GAs comparatively between wild and domesticated pea seeds during their development. We have shown that wild seeds contained more ABA than domesticated ones, which could be important for preparing the seeds for the period of dormancy. ABA was catabolised particularly by an 8´-hydroxylation pathway, and dihydrophaseic acid was the main catabolite in seed coats as well as embryos. Besides, the seed coats of wild and pigmented cultivated genotypes were characterised by a broader spectrum of bioactive GAs compared to non-pigmented domesticated seeds. GAs in both seed coat and embryo were synthesized mainly by a 13-hydroxylation pathway, with GA29 being the most abundant in the seed coat and GA20 in the embryos. Measuring seed water content and water loss indicated domesticated pea seeds´ desiccation was slower than that of wild pea seeds. Altogether, we showed that pea domestication led to a change in bioactive GA composition and a lower ABA content during seed development.

Gibberellic acid alleviates cadmium toxicity in rice by regulating NO accumulation and cell wall fixation capacity of cadmium

Gibberellic acid (GA) has been implicated in the response of plants to cadmium (Cd) stress, but the underlying mechanism remains unclear. In the present study, our aim was to confirm the role of GA in regulating the accumulation of Cd in rice. We found that Cd stress elevated the endogenous GA level in the rice roots. Exogenous GA application not only decreased the fixation of Cd in the root cell wall through reducing the hemicelluloses content, but also decreased the expression of OsNRAMP5 (Natural Resistance-Associated Macrophage Protein 5) and OsCd1 (a major facilitator superfamily gene). Both OsNRAMP5 and OsCd1 are related to Cd absorption, therefore, less Cd was accumulated in the roots. Furthermore, GA increased the expression of OsHMA3 (Heavy Metal ATPase 3) and OsCAL1 (Cadmium accumulation in Leaf 1), which are responsible for sequestering the Cd to the vacuoles and effluxing the Cd outside the cell, respectively, as a result, less Cd was accumulated in the shoots. In contrast, more Cd was accumulated in GA deficient lines. Furthermore, GA decreased the endogenous NO levels and the activity of antioxidant enzymes, while application of a NO scavenger-cPTIO diminished the alleviatory role of GA. In summary, the GA accelerated cell wall Cd exclusion mechanism probably improved rice tolerance to Cd toxicity via regulating the accumulation of NO.

OsABF1 Represses Gibberellin Biosynthesis to Regulate Plant Height and Seed Germination in Rice (Oryza sativa L.)

Gibberellins (GAs) are diterpenoid phytohormones regulating various aspects of plant growth and development, such as internode elongation and seed germination. Although the GA biosynthesis pathways have been identified, the transcriptional regulatory network of GA homeostasis still remains elusive. Here, we report the functional characterization of a GA-inducible OsABF1 in GA biosynthesis underpinning plant height and seed germination. Overexpression of OsABF1 produced a typical GA-deficient phenotype with semi-dwarf and retarded seed germination. Meanwhile, the phenotypes could be rescued by exogenous GA₃, suggesting that OsABF1 is a key regulator of GA homeostasis. OsABF1 could directly suppress the transcription of green revolution gene SD1, thus reducing the endogenous GA level in rice. Moreover, OsABF1 interacts with and transcriptionally antagonizes to the polycomb repression complex component OsEMF2b, whose mutant showed as similar but more severe phenotype to OsABF1 overexpression lines. It is suggested that OsABF1 recruits RRC2-mediated H3K27me3 deposition on the SD1 promoter, thus epigenetically silencing SD1 to maintain the GA homeostasis for growth and seed germination. These findings shed new insight into the functions of OsABF1 and regulatory mechanism underlying GA homeostasis in rice.

A Century of Gibberellin Research

Gibberellin research has its origins in Japan in the 19th century, when a disease of rice was shown to be due to a fungal infection. The symptoms of the disease including overgrowth of the seedling and sterility were later shown to be due to secretions of the fungus Gibberella fujikuroi (now reclassified as Fusarium fujikuroi), from which the name gibberellin was derived for the active component. The profound effect of gibberellins on plant growth and development, particularly growth recovery in dwarf mutants and induction of bolting and flowering in some rosette species, prompted speculation that these fungal metabolites were endogenous plant growth regulators and this was confirmed by chemical characterisation in the late 1950s. Gibberellins are now known to be present in vascular plants, and some fungal and bacterial species. The biosynthesis of gibberellins in plants and the fungus has been largely resolved in terms of the pathways, enzymes, genes and their regulation. The proposal that gibberellins act in plants by removing growth limitation was confirmed by the demonstration that they induce the degradation of the growth-inhibiting DELLA proteins. The mechanism by which this is achieved was clarified by the identification of the gibberellin receptor from rice in 2005. Current research on gibberellin action is focussed particularly on the function of DELLA proteins as regulators of gene expression. This review traces the history of gibberellin research with emphasis on the early discoveries that enabled the more recent advances in this field.

Transcriptome comparison of global distinctive features between pollination and parthenocarpic fruit set reveals transcriptional phytohormone cross-talk in cucumber (Cucumis sativus L.)

Parthenocarpy is an important trait determining yield and quality of fruit crops. However, the understanding of the mechanisms underlying parthenocarpy induction is limited. Cucumber (Cucumis sativus L.) is abundant in parthenocarpic germplasm resources and is an excellent model organism for parthenocarpy studies. In this study, the transcriptome of cucumber fruits was studied using RNA sequencing (RNA-Seq). Differentially expressed genes (DEGs) of set fruits were compared against aborted fruits. Distinctive features of parthenocarpic and pollinated fruits were revealed by combining the analysis of the transcriptome together with cytomorphological and physiological analysis. Cell division and the transcription of cell division genes were found to be more active in parthenocarpic fruit. The study also indicated that parthenocarpic fruit set is a high sugar-consuming process which is achieved via enhanced carbohydrate degradation through transcription of genes that lead to the breakdown of carbohydrates. Furthermore, the evidence provided by this work supports a hypothesis that parthenocarpic fruit set is induced by mimicking the processes of pollination/fertilization at the transcriptional level, i.e. by performing the same transcriptional patterns of genes inducing pollination and gametophyte development as in pollinated fruit. Based on the RNA-Seq and ovary transient expression results, 14 genes were predicted as putative parthenocarpic genes. The transcription analysis of these candidate genes revealed auxin, cytokinin and gibberellin cross-talk at the transcriptional level during parthenocarpic fruit set.

Gibberellin biosynthesis and its regulation

The GAs (gibberellins) comprise a large group of diterpenoid carboxylic acids that are ubiquitous in higher plants, in which certain members function as endogenous growth regulators, promoting organ expansion and developmental changes. These compounds are also produced by some species of lower plants, fungi and bacteria, although, in contrast to higher plants, the function of GAs in these organisms has only recently been investigated and is still unclear. In higher plants, GAs are synthesized by the action of terpene cyclases, cytochrome P450 mono-oxygenases and 2-oxoglutarate-dependent dioxygenases localized, respectively, in plastids, the endomembrane system and the cytosol. The concentration of biologically active GAs at their sites of action is tightly regulated and is moderated by numerous developmental and environmental cues. Recent research has focused on regulatory mechanisms, acting primarily on expression of the genes that encode the dioxygenases involved in biosynthesis and deactivation. The present review discusses the current state of knowledge on GA metabolism with particular emphasis on regulation, including the complex mechanisms for the maintenance of GA homoeostasis.

Tissue-specific regulation of gibberellin biosynthesis in developing pea seeds

Previous work suggests that gibberellins (GAs) play an important role in early seed development. To more fully understand the roles of GAs throughout seed development, tissue-specific transcription profiles of GA metabolism genes and quantitative profiles of key GAs were determined in pea (Pisum sativum) seeds during the seed-filling development period (8-20 d after anthesis [DAA]). These profiles were correlated with seed photoassimilate acquisition and storage as well as morphological development. Seed coat growth (8-12 DAA) and the subsequent dramatic expansion of branched parenchyma cells were correlated with both transcript abundance of GA biosynthesis genes and the concentration of the growth effector GA, GA(1). These results suggest GA(1) involvement in determining the rate of seed coat growth and sink strength. The endosperm's PsGA20ox transcript abundance and the concentration of GA(20) increased markedly as the endosperm reached its maximum volume (12 DAA), thus providing ample GA(20) substrate for the GA 3-oxidases present in both the embryo and seed coat. Furthermore, PsGA3ox transcript profiles and trends in GA(1) levels in embryos at 10 to 16 DAA and also in embryo axes at 18 DAA suggest localized GA(1)-induced growth in these tissues. A shift from synthesis of GA(1) to that of GA(8) occurred after 18 DAA in the embryo axis, suggesting that deactivation of GA(1) to GA(8) is a likely mechanism to limit embryo axis growth and allow embryo maturation to proceed. We hypothesize that GA biosynthesis and catabolism are tightly regulated to bring about the unique developmental events that occur during seed growth, development, and maturation.

Developmental and embryo axis regulation of gibberellin biosynthesis during germination and young seedling growth of pea

The expression patterns of five genes (PsGA20ox1, PsGA20ox2, PsGA3ox1, PsGA2ox1, and PsGA2ox2) encoding five regulatory gibberellin (GA) biosynthesis enzymes (two GA 20-oxidases, a GA 3beta-hydroxylase, and two GA 2beta-hydroxylases) were examined to gain insight into how these genes coordinate GA biosynthesis during germination and early postgermination stages of the large-seeded dicotyledonous plant pea (Pisum sativum). At the time the developing embryo fills the seed coat, high mRNA levels of PsGA20ox2 (primarily responsible for conversion of C20-GAs to GA(20)), PsGA2ox1 (primarily responsible for conversion of GA(20) to GA(29)), and PsGA2ox2 (primarily responsible for conversion of GA(1) to GA(8)) were detected in the seeds, along with high GA(20) and GA(29) levels, the enzymatic products of these genes. Embryo maturation was accompanied by a large reduction in PsGA20ox2 and PsGA2ox1 mRNA and lower GA(20) and GA(29) levels. However, PsGA2ox2 transcripts remained high. Following seed imbibition, GA(20) levels in the cotyledons decreased, while PsGA3ox1 mRNA and GA(1) levels increased, implying that GA(20) was being used for de novo synthesis of GA(1). The presence of the embryo axis was required for stimulation of cotyledonary GA(1) synthesis at the mRNA and enzyme activity levels. As the embryo axis doubled in size, PsGA20ox1 and PsGA3ox1 transcripts increased, both GA(1) and GA(8) were detectable, PsGA2ox2 transcripts decreased, and PsGA2ox1 transcripts remained low. Cotyledonary-, root-, and shoot-specific expression of these GA biosynthesis genes and the resultant endogenous GA profiles support a key role for de novo GA biosynthesis in each organ during germination and early seedling growth of pea.

Parthenocarpic fruit development in tomato

Parthenocarpic fruit development is a very attractive trait for growers and consumers. In tomato, three main sources of facultative parthenocarpy, pat, pat-2, pat-3/pat-4, are known to have potential applications in agriculture. The parthenocarpic fruit development in these lines is triggered by a deregulation of the hormonal balance in some specific tissues. Auxins and gibberellins are considered as the key elements in parthenocarpic fruit development of those lines. An increased level of these hormones in the ovary can substitute for pollination and trigger fruit development. This has opened up genetic engineering approaches for parthenocarpy that have given promising results, both in quality and quantity of seedless fruit production.

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.

Genetic analysis of growth-regulator-induced parthenocarpy in Arabidopsis

In Arabidopsis, seedless silique development or parthenocarpy can be induced by the application of various plant growth regulators (PGRs) to unfertilized pistils. Ecotype-specific responses were observed in the Arabidopsis ecotypes Columbia and Landsberg relative to the type of PGR and level applied. The parthenocarpic response was greatest in ecotype Landsberg, and comparisons of fruit growth and morphology were studied primarily in this ecotype. Gibberellic acid application (10 micromol pistil(-1)) caused development similar to that in pollinated pistils, while benzyladenine (1 micromol pistil(-1)) and naphthylacetic acid (10 micromol pistil(-1)) treatment produced shorter siliques. Naphthylacetic acid primarily modified mesocarp cell expansion. Arabidopsis mutants were employed to examine potential dependencies on gibberellin biosynthesis (ga1-3, ga4-1, and ga5-1) and perception (spy-4 and gai) during parthenocarpic silique development. Emasculated spy-4 pistils were neither obviously parthenocarpic nor deficient in PGR perception. By contrast, emasculated gai mutants did not produce parthenocarpic siliques following gibberellic acid application, but silique development occurred following pollination or application of auxin and cytokinin. Pollinated gai siliques had decreased cell numbers and morphologically resembled auxin-induced parthenocarpic siliques. This shows that a number of independent and possibly redundant pathways can direct hormone-induced parthenocarpy, and that endogenous gibberellins play a role in regulating cell expansion and promoting cell division in carpels.

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.

Plant regeneration from pea protoplasts via somatic embyogenesis

Plant regeneration via somatic embryogenesis was obtained from pea protoplasts. Strong auxins (picloram or 2.4-D) and increased osmolarity of the medium were necessary for embryo induction. Relatively high amounts of embryogenic calli could be obtained in 2 genotypes. After a period on hormone-free medium, a second induction of somatic embryos was possible. Further development of somatic embryos was accomplished on GA3 - containing medium.

Gibberellins in developing fruits of Pisum sativum cv. Alaska: Studies on their role in pod growth and seed development

Gibberellins A1, A8, A20 and A29 were identified by capillary gas chromatography-mass spectrometry in the pods and seeds from 5-d-old pollinated ovaries of pea (Pisum sativum cv. Alaska). These gibberellins were also identified in 4-d-old non-developing, parthenocarpic and pollinated ovaries. The level of gibberellin A1 within these ovary types was correlated with pod size. Gibberellin A1, applied to emasculated ovaries cultured in vitro, was three to five times more active than gibberellin A20. Using pollinated ovary explants cultured in vitro, the effects of inhibitors of gibberellin biosynthesis on pod growth and seed development were examined. The inhibitors retarded pod growth during the first 7 d after anthesis, and this inhibition was reversed by simultaneous application of gibberellin A3. In contrast, the inhibitors, when supplied to 4-d-old pollinated ovaries for 16 d, had little effect on seed fresh weight although they reduced the levels of endogenous gibberellins A20 and A29 in the enlarging seeds to almost zero. Paclobutrazol, which was one of the inhibitors used, is xylem-mobile and it efficiently reduced the level of seed gibberellins without being taken up into the seed. In intact fruits the pod may therefore be a source of precursors for gibberellin biosynthesis in the seed. Overall, the results indicate that gibberellin A1, present in parthenocarpic and pollinated fruits early in development, regulates pod growth. In contrast the high levels of gibberellins A20 and A29, which accumulate during seed enlargement, appear to be unnecessary for normal seed development or for subsequent germination.

Metabolism of gibberellins in a cell-free system from immature seeds of Phaseolus vulgaris L

The biosynthetic steps from gibberellin A12-aldehyde (GA12-aldehyde) to C19-GAs were studied by means of a cell-free system from the embryos of immature Phaseolus vulgaris seeds. Stable-isotope-labeled GAs were used as substrates and the products were identified by gas chromatography-mass spectrometry. Gibberellin A12-aldehyde was converted to GA4 via non-hydroxylated intermediates and to GA1 via 13-hydroxylated intermediates. 13-Hydroxylation took place at the beginning of the pathway by the conversion of GA12-aldehyde to GA53-aldehyde. The conversion of GA20 to GA5 and GA6 was also shown but no 2β-hydroxylating activity was found. Endogenous GAs from embryos and testas of 17-dold seeds were re-examined by gas chromatography-selected ion monitoring using stable-isotopelabeled GAs as internal standards. Gibberellins A9, A12, A15, A19, A23, A24, and A53 were identified for the first time in P. vulgaris, in addition to GA1, GA4, GA5, GA6, GA8, GA17, GA20, GA29, GA37, GA38 and GA44, which were previously known to occur in this species. The levels of all GAs, except the 2β-hydroxylated ones, were greater in the embryos than in the testas. Conversely, the contents of GA8 and GA29, both 2β-hydroxylated, were much higher in the testas than in the embryos.

Gibberellins in dark- and red-light-grown shoots of dwarf and tall cultivars of Pisum sativum: The quantification, metabolism and biological activity of gibberellins in Progress no. 9 and Alaska

The stem growth in darkness or in continuous red light of two pea cultivars, Alaska (Le Le, tall) and Progress No. 9 (le le, dwarf), was measured for 13 d. The lengths of the first three internodes in dark-grown seedlings of the two cultivars were similar, substantiating previous literature reports that Progress No. 9 has a tall phenotype in the dark. The biological activity of gibberellin A20 (GA20), which is normally inactive in le le geno-types, was compared in darkness and in red light. Alaska seedlings, regardless of growing conditions, responded to GA20. Dark-grown seedlings of Progress No. 9 also responded to GA20, although red-light-grown seedlings did not. Gibberellin A1 was active in both cultivars, in both darkness and red light. The metabolism of [(13)C(3)H]GA20 has also been studied. In dark-grown shoots of Alaska and Progress No. 9 [(13)C(3)H]GA20 is converted to [(13)C(3)H]GA1, [(13)C(3)H]GA8, [(13)C]GA29, its 2α-epimer, and [(13)C(3)H]GA29-catabolite. [(13)C(3)H] Gibberellin A1 was a minor product which appeared to be rapidly turned over, so that in some feeds only its metabolite, [(13)C(3)H]GA8, was detected. However results do indicate that the tall growth habit of Progress No. 9 in the dark, and its ability to respond to GA20 in the dark may be related to its capacity to 3β-hydroxylate GA20 to give GA1. In red light the overall metabolism of [(13)C(3)H]GA20 was reduced in both cultivars. There is some evidence that 3β-hydroxylation of [(13)C(3)H]GA20 can occur in red light-grown Alaska seedlings, but no 3β-hydroxylated metabolites of [(13)C(3)H]GA20 were observed in red light-grown Progress. Thus the dwarf habit of Progress No. 9 in red light and its inability to respond to GA20 may be related, as in other dwarf genotypes, to its inability to 3β-hydroxylate GA20 to GA1. However identification and quantification of native GAs in both cultivars showed that red-light-grown Progress does contain native GA1. Thus the inability of red light-grown Progress No. 9 seedlings to respond to, and to 3β-hydroxylate, applied GA20 may be due to an effect of red light on uptake and compartmentation of GAs.

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

The gibberellin (GA) 2β-hydroxylases in mature and immature seeds of Pisum sativum have been partially purified and characterised. The enzymes are unstable when stored below pH 7.0 or in the absence of a thiol reagent. The optimum assay pH is between 7.4 and 7.8 and activity is dependent upon the presence of α-ketoglutarate, Fe(2+) and ascorbate. The 2β-hydroxylase activities for GA1, GA4, GA9 and GA20 are chromatographically inseparable and correspond to a protein of Mr 44000. The rate of GA 2β-hydroxylation varies according to substrate and some evidence indicates that the 2β-hydroxylase activities for GA1 and GA4 and for GA9 and GA20 may reside in different proteins. During pea seed maturation, the specific activity of the enzyme(s) increases dramatically and reaches a maximum at a time when endogenous GA9, GA20, GA29 and GA51 are also at their greatest concentration. This correlation is not the result of substrate induction of enzyme activity. Since the GA 2β-hydroxylases operate at maximal rate at low substrate concentrations they are incapable of rapidly 2β-hydroxylating excessive quantities of (exogenously applied) GA1 or GA20. On the basis of the kinetic parameters of the GA 2β-hydroxylase activities, a generalised model is discussed for the control of the steady-state levels of bioactive hormone under normal physiological conditions.

Internode length in Pisum sativum L. The kinetics of growth and [(3)H]gibberellin A 20 metabolism in genotype na Le

The relationship between shoot growth and [(3)H]gibberellin A20 (GA20) metabolism was investigated in the GA-deficient genotype of peas, na Le. [17-(13)C, (3)H2]gibberellin A20 was applied to the shoot apex and its metabolic fate examined by gas chromatographic-mass spectrometric analysis of extracts of the shoot and root tissues. As reported before, [(13)C, (3)H2]GA1, [(13)C, (3)H2]GA8 and [(13)C, (3)H2]GA29 constituted the major metabolites of [(13)C, (3)H2]GA20 present in the shoot. None of these GAs showed any dilution by endogenous (12)C-material. [(13)C, (3)H2]GA29-catabolite was also a prominent metabolite in the shoot tissue but showed pronounced isotope dilution probably due to carry-over of endogenous [(12)C]GA29-catabolite from the mature seed. In marked contrast to the shoot tissue, the two major metabolites present in the roots were identified as [(13)C, (3)H2]GA8-catabolite and [(13)C, (3)H2]GA29-catabolite. Both of these compounds showed strong dilution by endogenous (12)C-material. Only low levels of [(13)C, (3)H2]GA1, [(13)C, (3)H2]GA8, [(13)C, (3)H2]GA20 and [(13)C, (3)H2]GA29 accumulated in the roots. It is suggested that compartmentation of GA-catabolism may occur in the root tissue in an analogous manner to that shown in the testa of developing seeds. Changes in the levels of [1β,3α-(3)H2]GA20 metabolites over 10 d following application of the substrate to the shoot apex of genotype na Le confirmed the accumulation of [(3)H]GA-catabolites in the root tissues. No evidence was obtained for catabolic loss of [(3)H]GA20 by complete oxidation or conversion to a methanol-inextractable form. The results indicate that the root system may play an important role in the regulation of biologically active GA levels in the developing shoot of Na genotypes of peas.

Gibberellins in immature seeds and dark-grown shoots of Pisum sativum : Gibberellins identified in the tall cultivar Alaska in comparison with those in the dwarf Progress No. 9

Gibberellins (GAs) A17, A19, A20, A29, A44, 2βOH-GA44 (tentative) and GA29-catabolite were identified in 21-day-old seeds of Pisum sativum cv. Alaska (tall). These GAs are qualitatively similar to those in the dwarf cultivar Progress No. 9 with the exception of GA19 which does not accumulate in Progress seeds. There was no evidence for the presence of 3-hydroxylated GAs in 21 day-old Alaska seeds. Dark-grown shoots of the cultivar Alaska contein GA1, GA8, GA20, GA29, GA8-catabolite and GA29-catabolite. Dark-grown shoots of the cultivar Progress No.9 contain GA8, GA20, GA29 and GA29-catabolite, and the presence of GA1 was strongly indicated. Quantitation using GAs labelled with stable isotope showed the level of GA1 in dark-grown shoots of the two cultivars to be almost identical, whilst the levels of GA20, GA29 and GA29-catabolite were significantly lower in Alaska than in Progress No. 9. The levels of these GAs in dark-grown shoots were 10(2)- to 10(3)-fold less than the levels in developing seeds. The 2-epimer of GA29 is present in dark-grown-shoot extracts of both cultivars and is not thought to be an artefact.

Conversion of gibberellin A20 to gibberellins A 1 and A 5 in a cell-free system from Phaseolus vulgaris

The soluble fraction of a cell-free system from immature seeds of Phaseolus vulgaris L. converts gibberellin A20 (GA20) to GA1 and GA5. It does however not metabolize GA1 and GA29 to GA5, showing that in this system GA20 is converted directly to GA5. The steps from GA20 to GA1 (3-hydroxylation) and from GA20 to GA5 (Δ(2) double-bond formation) require oxygen, Fe(2+) and α-ketoglutarate, and are stimulated by ascorbate. The enzymes catalyzing these conversions bate. The enzymes catalyzing these conversions have properties similar to those of GA oxidases found in Cucurbita maxima and Pisum sativum.