Isolation and transcript analysis of gibberellin 20-oxidase genes in pea and bean in relation to fruit development

Balancing of hormonal biosynthesis and catabolism pathways, a strategy to ameliorate the negative effects of heat stress on reproductive growth

In pea (Pisum sativum L.), moderate heat stress during early flowering/fruit set increased seed/ovule abortion, and concomitantly produced fruits with reduced ovary (pericarp) length, and fewer seeds at maturity. Plant hormonal networks coordinate seed and pericarp growth and development. To determine if these hormonal networks are modulated in response to heat stress, we analyzed the gene expression patterns and associated these patterns with precursors, and bioactive and inactive metabolites of the auxin, gibberellin (GA), abscisic acid (ABA), and ethylene biosynthesis/catabolism pathways in young developing seeds and pericarps of non-stressed and 4-day heat-stressed fruits. Our data suggest that within the developing seeds heat stress decreased bioactive GA levels reducing GA growth-related processes, and that increased ethylene levels may have promoted this inhibitory response. In contrast, heat stress increased auxin biosynthesis gene expression and auxin levels in the seeds and pericarps, and seed ABA levels, both effects can increase seed sink strength. We hypothesize that seeds with higher auxin- and ABA-induced sink strength and adequate bioactive GA levels will set and continue to grow, while the seeds with lower sink strength (low auxin, ABA, and GA levels) will become more sensitive to heat stress-induced ethylene leading to ovule/seed abortion.

Gibberellins Inhibit Nodule Senescence and Stimulate Nodule Meristem Bifurcation in Pea (Pisum sativum L.)

The development of nitrogen-fixing nodules formed during Rhizobium-legume symbiosis is strongly controlled by phytohormones. In this study, we investigated the effect of gibberellins (GAs) on senescence of pea (Pisum sativum) symbiotic nodules. Pea wild-type line SGE, as well as corresponding mutant lines SGEFix--1 (sym40), SGEFix--2 (sym33), SGEFix--3 (sym26), and SGEFix--7 (sym27), blocked at different stages of nodule development, were used in the study. An increase in expression of the GA2ox1 gene, encoding an enzyme involved in GA deactivation (GA 2-oxidase), and a decrease in the transcript abundance of the GA20ox1 gene, encoding one of the enzymes involved in GA biosynthesis (GA 20-oxidase), were observed in analyzed genotypes during nodule aging. A reduction in the amount of bioactive GA3 was demonstrated by immunolocalization in the early senescent mutant and wild-type lines during aging of symbiotic nodules. Down-regulated expression of senescence-associated genes encoding cysteine proteases 1 and 15a, thiol protease, bZIP transcription factor, 1-aminocyclopropane-1-carboxylate (ACC) synthase, ACC oxidase, and aldehyde oxidase was observed in the nodules of wild-type plants treated with exogenous GA3 relative to the untreated plants. GA3-treated plants also showed increases in nodule size and the nitrogen fixation zone, and decreases in the number of nodules and the senescence zone. Immunogold localization revealed higher levels of GA3 in the peribacteroid spaces in symbiosomes than in the matrix of infection threads. Furthermore, a decrease in GA3 label in mature and senescent symbiosomes in comparison with juvenile symbiosomes was observed. These results suggest a negative effect of GAs on the senescence of the pea symbiotic nodule and possible involvement of GAs in functioning of the mature nodule. Simultaneously, GA3 treatment led to nodule meristem bifurcation, indicating a possible role of GAs in nodule meristem functioning.

Regulation of Seed Germination and Abiotic Stresses by Gibberellins and Abscisic Acid

Overall growth and development of a plant is regulated by complex interactions among various hormones, which is critical at different developmental stages. Some of the key aspects of plant growth include seed development, germination and plant survival under unfavorable conditions. Two of the key phytohormones regulating the associated physiological processes are gibberellins (GA) and abscisic acid (ABA). GAs participate in numerous developmental processes, including, seed development and seed germination, seedling growth, root proliferation, determination of leaf size and shape, flower induction and development, pollination and fruit expansion. Despite the association with abiotic stresses, ABA is essential for normal plant growth and development. It plays a critical role in different abiotic stresses by regulating various downstream ABA-dependent stress responses. Plants maintain a balance between GA and ABA levels constantly throughout the developmental processes at different tissues and organs, including under unfavorable environmental or physiological conditions. Here, we will review the literature on how GA and ABA control different stages of plant development, with focus on seed germination and selected abiotic stresses. The possible crosstalk of ABA and GA in specific events of the above processes will also be discussed, with emphasis on downstream stress signaling components, kinases and transcription factors (TFs). The importance of several key ABA and GA signaling intermediates will be illustrated. The knowledge gained from such studies will also help to establish a solid foundation to develop future crop improvement strategies.

Next-generation transcriptome analysis in transgenic birch overexpressing and suppressing APETALA1 sheds lights in reproduction development and diterpenoid biosynthesis

Overexpression of BpAP1 could cause early flowering in birch. BpAP1 affected the expression of many flowering-related unigenes and diterpenoid biosynthesis in transgenic birch, and BpPI was a putative target gene of BpAP1. APETALA1 (AP1) is an MADS-box transcription factor that is involved in the flowering process in plants and has been a focus of genetic studies examining flower development. Here, we carried out transcriptome analysis of birch (Betula platyphylla Suk.), including BpAP1 overexpression lines, BpAP1 suppression lines, and non-transgenic line (NT). Compared with NT, we detected 8302 and 7813 differentially expressed unigenes in 35S::BpAP1 and 35S::BpAP1RNAi transgenic lines, respectively. Overexpression and suppression of BpAP1 in birch affected diterpenoid biosynthesis and altered expression of many flowering-related unigenes. Moreover, combining information from the RNA-seq database and the birch genome, we predicted downstream target genes of BpAP1. Among the 166 putative target genes of BpAP1, there was a positive correlation between BpAP1 and BpPI. These results provide references for further examining the relationship between BpAP1 and its target genes, and reveal that BpAP1 functions as a transcription regulator in birch.

Role of the gibberellin receptors GID1 during fruit-set in Arabidopsis

Gibberellins (GAs) play a critical role in fruit-set and fruit growth. Gibberellin is perceived by its nuclear receptors GA INSENSITIVE DWARF1s (GID1s), which then trigger degradation of downstream repressors DELLAs. To understand the role of the three GA receptor genes (GID1A, GID1B and GID1C) in Arabidopsis during fruit initiation, we have examined their temporal and spatial localization, in combination with analysis of mutant phenotypes. Distinct expression patterns are revealed for each GID1: GID1A is expressed throughout the whole pistil, while GID1B is expressed in ovules, and GID1C is expressed in valves. Functional study of gid1 mutant combinations confirms that GID1A plays a major role during fruit-set and growth, whereas GID1B and GID1C have specific roles in seed development and pod elongation, respectively. Therefore, in ovules, GA perception is mediated by GID1A and GID1B, while GID1A and GID1C are involved in GA perception in valves. To identify tissue-specific interactions between GID1s and DELLAs, we analyzed spatial expression patterns of four DELLA genes that have a role in fruit initiation (GAI, RGA, RGL1 and RGL2). Our data suggest that GID1A can interact with RGA and GAI in all tissues, whereas GID1C-RGL1 and GID1B-RGL2 interactions only occur in valves and ovules, respectively. These results uncover specific functions of each GID1-DELLA in the different GA-dependent processes that occur upon fruit-set. In addition, the distribution of GA receptors in valves along with lack of expression of GA biosynthesis genes in this tissue, strongly suggests transport of GAs from the developing seeds to promote fruit growth.

Unraveling the signal scenario of fruit set

Long-term goals to impact or modify fruit quality and yield have been the target of researchers for many years. Different approaches such as traditional breeding,mutation breeding, and transgenic approaches have revealed a regulatory network where several hormones concur in a complex way to regulate fruit set and development,and these networks are shared in some way among species with different kinds of fruits. Understanding the molecular and biochemical networks of fruit set and development could be very useful for breeders to meet the current and future challenges of agricultural problems.

Distribution and prediction of catalytic domains in 2-oxoglutarate dependent dioxygenases

BACKGROUND

The 2-oxoglutarate dependent superfamily is a diverse group of non-haem dioxygenases, and is present in prokaryotes, eukaryotes, and archaea. The enzymes differ in substrate preference and reaction chemistry, a factor that precludes their classification by homology studies and electronic annotation schemes alone. In this work, I propose and explore the rationale of using substrates to classify structurally similar alpha-ketoglutarate dependent enzymes.

FINDINGS

Differential catalysis in phylogenetic clades of 2-OG dependent enzymes, is determined by the interactions of a subset of active-site amino acids. Identifying these with existing computational methods is challenging and not feasible for all proteins. A clustering protocol based on validated mechanisms of catalysis of known molecules, in tandem with group specific hidden markov model profiles is able to differentiate and sequester these enzymes. Access to this repository is by a web server that compares user defined unknown sequences to these pre-defined profiles and outputs a list of predicted catalytic domains. The server is free and is accessible at the following URL (http://comp-biol.theacms.in/H2OGpred.html).

CONCLUSIONS

The proposed stratification is a novel attempt at classifying and predicting 2-oxoglutarate dependent function. In addition, the server will provide researchers with a tool to compare their data to a comprehensive list of HMM profiles of catalytic domains. This work, will aid efforts by investigators to screen and characterize putative 2-OG dependent sequences. The profile database will be updated at regular intervals.

Engineering gibberellin metabolism in Solanum nigrum L. by ectopic expression of gibberellin oxidase genes

Gibberellins (GAs) control many aspects of plant development, including seed germination, shoot growth, flower induction and growth and fruit expansion. Leaf explants of Solanum nigrum (Black Nightshade; Solanaceae) were used for Agrobacterium-mediated delivery of GA-biosynthetic genes to determine the influence of their encoded enzymes on the production of bioactive GAs and plant stature in this species. Constructs were prepared containing the neomycin phosphotransferase (nptII) gene for kanamycin resistance as a selectable marker, and the GA-biosynthetic genes, their expression under the control of the CaMV 35S promoter. The GA-biosynthetic genes comprised AtGA20ox1, isolated from Arabidopsis thaliana, the product from which catalyses the formation of C(19)-GAs, and MmGA3ox1 and MmGA3ox2, isolated from Marah macrocarpus, which encode functionally different GA 3-oxidases that convert C(19)-GAs to biologically active forms. Increase in stature was observed in plants transformed with AtGA20ox1, MmGA3ox2 and MmGA3ox1 + MmGA3ox2, their presence and expression being confirmed by PCR and RT-PCR, respectively, accompanied by an increase in GA(1) content. Interestingly, MmGA3ox1 alone did not induce a sustained increase in plant height, probably because of only a marginal increase in bioactive GA(1) content in the transformed plants. The results are discussed in the context of regulating plant stature, since this strategy would decrease the use of chemicals to promote plant growth.

Gibberellin A1 metabolism contributes to the control of photoperiod-mediated tuberization in potato

Some potato species require a short-day (SD) photoperiod for tuberization, a process that is negatively affected by gibberellins (GAs). Here we report the isolation of StGA3ox2, a gene encoding a GA 3-oxidase, whose expression is increased in the aerial parts and is repressed in the stolons after transfer of photoperiod-dependent potato plants to SD conditions. Over-expression of StGA3ox2 under control of constitutive or leaf-specific promoters results in taller plants which, in contrast to StGA20ox1 over-expressers previously reported, tuberize earlier under SD conditions than the controls. By contrast, StGA3ox2 tuber-specific over-expression results in non-elongated plants with slightly delayed tuber induction. Together, our experiments support that StGA3ox2 expression and gibberellin metabolism significantly contribute to the tuberization time in strictly photoperiod-dependent potato plants.

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.

Hormonal regulation of tomato gibberellin 20-oxidase1 expressed in Arabidopsis

Gibberellin 20-oxidases, enzymes of gibberellin (GA) biosynthesis, play an important role in (GA) homeostasis. To investigate the regulation of tomato SlGA20ox1 expression, a genomic clone was isolated, its promoter transcriptionally fused to the GUS reporter gene, and the construct used to transform Arabidopsis. Expression was found in diverse vegetative (leaves and roots) and reproductive (flowers) organs. GUS staining was also localized in the columella of secondary roots. GA negative feed-back regulation of SlGA20ox1:GUS was shown to be active both in tomato and in transformed Arabidopsis. Auxin (indol-3-acetic acid, 2,4-dichlorophenoxyacetic acid and naphtaleneacetic acid), triiodobenzoic acid (an inhibitor of auxin transport) and benzyladenine (a cytokinin) treatment induced SlGA20ox1:GUS expression associated with increased auxin content and/or signaling, detected using DR5:GUS expression as a marker. Interestingly, SlGA20ox:GUS expression was induced by auxin and root excision in the hypocotyl, an organ not showing GUS staining in control seedlings. In etiolated seedlings, SlGA20ox1:GUS expression occurred in the elongating hypocotyl region of etiolated seedlings and was down-regulated upon transfer to light associated with decrease of growth rate elongation. Our results show that feed-back, auxin and light regulation of SlGA20ox1 expression depends on DNA elements contained within the first 834bp of the 5' upstream promoter region. Putative DNA regulatory sequences involved in negative feed-back regulation and auxin response were identified in that promoter.

Characterization of gibberellin 20-oxidases in the citrus hybrid Carrizo citrange

CcGA20ox1 is a gene encoding a GA 20-oxidase, a gibberellin (GA) biosynthetic enzyme, previously isolated from the citrus hybrid Carrizo citrange (Citrus sinensis (L.) Osbeck x Poncirus trifoliata (L.) Raf.). Southern blot analysis of genomic DNA of Carrizo citrange with CcGA20ox1 suggested the presence in the hybrid of another gene encoding another GA 20-oxidase. A cDNA clone from this new gene (CcGA20ox2) was isolated using RNA from the other parent C. sinensis. CcGA20ox2 encoded a protein of 372 amino acids that showed 67.1% identity with CcGA20ox1, and its expression product catalyzed the in vitro conversion of GA12 to GA9, confirming that it corresponds to another active GA20ox. Amplification of genomic DNA and isolation of genomic clones of CcGA20ox1 and CcGA20ox2 revealed that the parental sources of these genes in the hybrid were P. trifoliata and C. sinensis, respectively. The sequences of CcGA20ox1 and CcGA20ox2 showed that both genes contained two introns, which are also conserved in GA20ox genes of other species like Arabidopsis thaliana L., Pisum sativum L. and Solanum lycopersicum L. Determination of transcript levels in the Carrizo citrange hybrid by quantitative real-time polymerase chain reaction showed that CcGA20ox1 was expressed mainly in internodes, leaves and seeds, and CcGA20ox2 in flower buds and flowers at anthesis, with the genes having similar transcript levels in young developing fruits.

Fertilization-dependent auxin response in ovules triggers fruit development through the modulation of gibberellin metabolism in Arabidopsis

Fruit development is usually triggered by ovule fertilization, and it requires coordination between seed development and the growth and differentiation of the ovary to host the seeds. Hormones are known to synchronize these two processes, but the role of each hormone, and the mechanism by which they interact, are still unknown. Here we show that auxin and gibberellins (GAs) act in a hierarchical scheme. The synthetic reporter construct DR5:GFP showed that fertilization triggered an increase in auxin response in the ovules, which could be mimicked by blocking polar auxin transport. As the application of GAs did not affect auxin response, the most likely sequence of events after fertilization involves auxin-mediated activation of GA synthesis. We have confirmed this, and have shown that GA biosynthesis upon fertilization is localized specifically in the fertilized ovules. Furthermore, auxin treatment caused changes in the expression of GA biosynthetic genes similar to those triggered by fertilization, and also restricted to the ovules. Finally, GA signaling was activated in ovules and valves, as shown by the rapid downregulation of the fusion protein RGA-GFP after pollination and auxin treatment. Taken together, this evidence suggests a model in which fertilization would trigger an auxin-mediated promotion of GA synthesis specifically in the ovule. The GAs synthesized in the ovules would be then transported to the valves to promote GA signaling and thus coordinate growth of the silique.

Gibberellin regulation of fruit set and growth in tomato

The role of gibberellins (GAs) in tomato (Solanum lycopersicum) fruit development was investigated. Two different inhibitors of GA biosynthesis (LAB 198999 and paclobutrazol) decreased fruit growth and fruit set, an effect reversed by GA(3) application. LAB 198999 reduced GA(1) and GA(8) content, but increased that of their precursors GA(53), GA(44), GA(19), and GA(20) in pollinated fruits. This supports the hypothesis that GA(1) is the active GA for tomato fruit growth. Unpollinated ovaries developed parthenocarpically in response to GA(3) > GA(1) = GA(4) > GA(20), but not to GA(19), suggesting that GA 20-oxidase activity was limiting in unpollinated ovaries. This was confirmed by analyzing the effect of pollination on transcript levels of SlCPS, SlGA20ox1, -2, and -3, and SlGA3ox1 and -2, encoding enzymes of GA biosynthesis. Pollination increased transcript content of SlGA20ox1, -2, and -3, and SlCPS, but not of SlGA3ox1 and -2. To investigate whether pollination also altered GA inactivation, full-length cDNA clones of genes encoding enzymes catalyzing GA 2-oxidases (SlGA2ox1, -2, -3, -4, and -5) were isolated and characterized. Transcript levels of these genes did not decrease early after pollination (5-d-old fruits), but transcript content reduction of all of them, mainly of SlGA2ox2, was found later (from 10 d after anthesis). We conclude that pollination mediates fruit set by activating GA biosynthesis mainly through up-regulation of GA20ox. Finally, the phylogenetic reconstruction of the GA2ox family clearly showed the existence of three gene subfamilies, and the phylogenetic position of SlGA2ox1, -2, -3, -4, and -5 was established.

Tomato fruit set driven by pollination or by the parthenocarpic fruit allele are mediated by transcriptionally regulated gibberellin biosynthesis

We investigated the role of gibberellins (GAs) in the phenotype of parthenocarpic fruit (pat), a recessive mutation conferring parthenocarpy in tomato (Solanum lycopersicum L.). Novel phenotypes that parallel those reported in plants repeatedly treated with gibberellic acid or having a GA-constitutive response indicate that the pat mutant probably expresses high levels of GA. The retained sensitivity to the GA-biosynthesis inhibitor paclobutrazol reveals that this condition is dependent on GA biosynthesis. Expression analysis of genes encoding key enzymes involved in GA biosynthesis shows that in normal tomato ovaries, the GA20ox1 transcript is in low copy number before anthesis and only pollination and fertilization increase its transcription levels and, thus, GA biosynthesis. In the unpollinated ovaries of the pat mutant, this mechanism is de-regulated and GA20ox1 is constitutively expressed, indicating that a high GA concentration could play a part in the parthenocarpic phenotype. The levels of endogenous GAs measured in the floral organs of the pat mutant support such a hypothesis. Collectively, the data indicate that transcriptional regulation of GA20ox1 mediates pollination-induced fruit set in tomato and that parthenocarpy in pat results from the mis-regulation of this mechanism. As genes involved in the control of GA synthesis (LeT6, LeT12 and LeCUC2) and response (SPY) are also altered in the pat ovary, it is suggested that the pat mutation affects a regulatory gene located upstream of the control of fruit set exerted by GAs.

Regulation of gibberellin 20-oxidase1 expression in spinach by photoperiod

The multifunctional gibberellin (GA) 20-oxidase [GA(53), 2-oxoglutarate:oxygen oxidoreductase (20-oxidizing), EC 1.14.11] has been extensively investigated in various species at the genetic and molecular levels, but not at the protein level. Here, we report on expression of GA20ox1 protein in spinach (Spinacia oleracea L.) in response to photoperiod. Polyclonal antibodies were raised against recombinant SoGA20ox1 in a chicken. These antibodies immuno-inhibited the enzymatic activity of the recombinant SoGA20ox1 and immuno-precipitated SoGA20ox1 (43 kDa) isolated from spinach shoot tips. Northern and western analyses showed that the levels of SoGA20ox1 transcript and protein increased in the blades, petioles, young leaves, and tips in response to long-day (LD) conditions. The transcript and protein levels of the SoGA20ox1 gene were up-regulated in the petioles and tips in a time-dependent manner. The estimated number of SoGA20ox1 protein molecules per cell was approximately 13-fold higher in tips grown in LD than in short-day (SD) conditions. The levels of SoGA20ox1 protein gradually decreased in tips when spinach plants grown in LD were transferred to SD conditions. SoGA20ox1 transcripts were detected by in situ hybridization in rapidly growing tissues--such as the shoot apical meristem, leaf and flower primordia, leaflets, and vascular tissues--but not in the expanding subapical region. In petioles, expression of SoGA20ox1 was detected in the companion cells.

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.

Gibberellins are involved in nodulation of Sesbania rostrata

Upon submergence, Azorhizobium caulinodans infects the semiaquatic legume Sesbania rostrata via the intercellular crack entry process, resulting in lateral root-based nodules. A gene encoding a gibberellin (GA) 20-oxidase, SrGA20ox1, involved in GA biosynthesis, was transiently up-regulated during lateral root base nodulation. Two SrGA20ox1 expression patterns were identified, one related to intercellular infection and a second observed in nodule meristem descendants. The infection-related expression pattern depended on bacterially produced nodulation (Nod) factors. Pharmacological studies demonstrated that GAs were involved in infection pocket and infection thread formation, two Nod factor-dependent events that initiate lateral root base nodulation, and that they were also needed for nodule primordium development. Moreover, GAs inhibited the root hair curling process. These results show that GAs are Nod factor downstream signals for nodulation in hydroponic growth.

Overexpression of ADC2 in Arabidopsis induces dwarfism and late-flowering through GA deficiency

We have obtained Arabidopsis thaliana transgenic plants constitutively overexpressing ADC2, one of the two genes encoding arginine decarboxylase (ADC) in Arabidopsis. These plants contained very high levels of putrescine (Put) but no changes were observed in spermidine and spermine contents. The results obtained from quantification of free and conjugated polyamines suggest that conjugation may be a limiting step for control of Put homeostasis within a non-toxic range for plant survival. Transgenic plants with increased levels of ADC2 transcript and elevated Put content showed dwarfism and late-flowering, and the phenotype was rescued by gibberellin A3 (GA3) application. The contents of bioactive GA4 and GA1, and of GA9 (a precursor of GA4), as well as the levels of AtGA20ox1, AtGA3ox1 and AtGA3ox3 transcripts (quantified by real-time PCR) were lower in the ADC2 overexpressor plants than in the wild type. No change in the expression of genes encoding earlier enzymes in the GA biosynthesis pathway was detected by microarray analysis. These results suggest that Put accumulation affects GA metabolism through the repression of biosynthetic steps catalyzed by GA 20-oxidase and GA 3-oxidase.

Thermoperiodic stem elongation involves transcriptional regulation of gibberellin deactivation in pea

The physiological basis of thermoperiodic stem elongation is as yet poorly understood. Thermoperiodic control of gibberellin (GA) metabolism has been suggested as an underlying mechanism. We have investigated the influence of different day and night temperature combinations on GA levels, and diurnal steady-state expression of genes involved in GA biosynthesis (LS, LH, NA, PSGA20ox1, and PsGA3ox1) and GA deactivation (PsGA2ox1 and PsGA2ox2), and related this to diurnal stem elongation in pea (Pisum sativum L. cv Torsdag). The plants were grown under a 12-h light period with an average temperature of 17 degrees C. A day temperature/night temperature combination of 13 degrees C/21 degrees C reduced stem elongation after 12 d by 30% as compared to 21 degrees C/13 degrees C. This was correlated with a 55% reduction of GA1. Although plant height correlated with GA1 content, there was no correlation between diurnal growth rhythms and GA1 content. NA, PsGA20ox1, and PsGA2ox2 showed diurnal rhythms of expression. PsGA2ox2 was up-regulated in 13 degrees C/21 degrees C (compared to 21 degrees C/13 degrees C), at certain time points, by up to 19-fold. Relative to PsGA2ox2, the expression of LS, LH, NA, PSGA20ox1, PsGA3ox1, and PsGA2ox1 was not or only slightly affected by the different temperature treatments. The sln mutant having a nonfunctional PsGA2ox1 gene product showed the same relative stem elongation response to temperature as the wild type. This supports the importance of PsGA2ox2 in mediating thermoperiodic stem elongation responses in pea. We present evidence for an important role of GA catabolism in thermoperiodic effect on stem elongation and conclude that PsGA2ox2 is the main mediator of this effect in 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.

Gibberellin metabolism and signaling

Gibberellins (GAs) are a family of plant hormones controlling many aspects of plant growth and development including stem elongation, germination, and the transition from vegetative growth to flowering. Cloning of the genes encoding GA biosynthetic and inactivating enzymes has led to numerous insights into the developmental regulation of GA hormone accumulation that is subject to both positive and negative feedback regulation. Genetic and biochemical analysis of GA-signaling genes has revealed that posttranslational regulation of DELLA protein accumulation is a key control point in GA response. The highly conserved DELLA proteins are a family of negative regulators of GA signaling that appear subject to GA-stimulated degradation through the ubiquitin-26S proteasome pathway. This review discusses the regulation of GA hormone accumulation and signaling in the context of its role in plant growth and development.

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.

Expression analysis of a GA 20-oxidase in embryos from two sorghum lines with contrasting dormancy: possible participation of this gene in the hormonal control of germination

The role of GAs in promoting seed germination is well known and experiments with seeds from different species have suggested the requirement of de novo synthesis of GAs upon imbibition for germination. There are also strong indications that the enhancement of GA synthesis is part of the mechanism through which environmental signals (i.e. light) induce germination. Since along the GA biosynthetic pathway, oxidation at C-20 carried out by GA 20-oxidases is thought to be a site of regulation, a cDNA clone encoding a GA 20-oxidase was isolated from embryos of sorghum (SbGA 20ox). Expression analysis of this gene in embryos within imbibed caryopses with low dormancy showed detectable amounts of the specific mRNA early upon incubation, increasing thereafter. In contrast, it remained barely detectable in embryos from dormant caryopses. Changes in endogenous GA4 levels were in agreement with those of SbGA 20ox mRNA, suggesting that GA production might be regulated differentially at the level of transcription of this gene. The expression of SbGA 20ox was enhanced in incubated embryos isolated from either type of caryopses, illustrating a physiological control exerted by the surrounding seed tissues on gene expression. The results also show that ABA leads to a suppression of transcription of this gene.

Auxin regulation of the gibberellin pathway in pea

The auxin indole-3-acetic acid (IAA) has been shown to promote the biosynthesis of the active gibberellin (GA(1)) in shoots of pea (Pisum sativum). We used northern analysis to investigate the timing of IAA-induced changes in transcript levels of PsGA3ox1 (Mendel's LE), PsGA2ox1, PsGA2ox2, and PsGA20ox1, key genes for the later stages of GA(1) biosynthesis and metabolism in pea. Rapid (2-4 h) changes occurred in the transcript levels of PsGA3ox1, PsGA2ox1, and PsGA2ox2 after treatment with IAA. [(14)C]GA(1) metabolism studies in decapitated shoots indicated that IAA inhibits GA(1) deactivation, consistent with the suppression of PsGA2ox1 (SLN) transcript levels by IAA. Studies with the sln mutant also indicated that PsGA2ox1 activity is involved in GA(1) deactivation in decapitated shoots. Culture of excised internode tissue in the presence of auxin clearly demonstrated that internode tissue is a site of GA(1) biosynthesis per se. Excised internode tissue cultured in the presence/absence of cycloheximide showed that de novo protein synthesis is required for IAA-induced increases in PsGA3ox1 transcript levels. Auxin dose response studies indicated that IAA concentration is a critical determinant of GA(1) biosynthesis over 1 to 2 orders of magnitude, and a range of auxins was shown to affect the GA pathway.

Control of gibberellin levels and gene expression during de-etiolation in pea

Gibberellin A(1) (GA(1)) levels drop significantly in wild-type pea (Pisum sativum) plants within 4 h of exposure to red, blue, or far-red light. This response is controlled by phytochrome A (phyA) (and not phyB) and a blue light receptor. GA(8) levels are increased in response to 4 h of red light, whereas the levels of GA(19), GA(20), and GA(29) do not vary substantially. Red light appears to control GA(1) levels by down-regulating the expression of Mendel's LE (PsGA3ox1) gene that controls the conversion of GA(20) to GA(1), and by up-regulating PsGA2ox2, which codes for a GA 2-oxidase that converts GA(1) to GA(8). This occurs within 0.5 to 1 h of exposure to red light. Similar responses occur in blue light. The major GA 20-oxidase gene expressed in shoots, PsGA20ox1, does not show substantial light regulation, but does show up-regulation after 4 h of red light, probably as a result of feedback regulation. Expression of PsGA3ox1 shows a similar feedback response, whereas PsGA2ox2 shows a feed-forward response. These results add to our understanding of how light reduces shoot elongation during de-etiolation.

Production of dwarf lettuce by overexpressing a pumpkin gibberellin 20-oxidase gene

We investigated the effect of overexpressing a pumpkin gibberellin (GA) 20-oxidase gene encoding an enzyme that forms predominantly biologically inactive products on GA biosynthesis and plant morphology in transgenic lettuce (Lactuca sativa cv Vanguard) plants. Lettuce was transformed with the pumpkin GA 20-oxidase gene downstream of a strong constitutive promoter cassette (El2-35S-Omega). The transgenic plants in which the pumpkin gene was detected by polymerase chain reaction were dwarfed in the T(2) generation, whereas transformants with a normal growth phenotype did not contain the transgene. The result of Southern-blot analysis showed that the transgene was integrated as a single copy; the plants segregated three dwarfs to one normal in the T(2) generation, indicating that the transgene was stable and dominant. The endogenous levels of GA(1) and GA(4) were reduced in the dwarfs, whereas large amounts of GA(17) and GA(25), which are inactive products of the pumpkin GA 20-oxidase, accumulated in these lines. These results indicate that a functional pumpkin GA 20-oxidase is expressed in the transgenic lettuce, resulting in a diversion of the normal pathway of GA biosynthesis to inactive products. Furthermore, this technique may be useful for controlling plant stature in other agricultural and horticultural species.

The ectopic overexpression of a citrus gibberellin 20-oxidase enhances the non-13-hydroxylation pathway of gibberellin biosynthesis and induces an extremely elongated phenotype in tobacco

Transgenic plants of Nicotiana tabacum overexpressing a gibberellin (GA) 20-oxidase cDNA (CcGA20ox1) from citrus, under the control of the 35S promoter, were taller (up to twice) and had larger inflorescences and longer flower peduncles than those of control plants. Hypocotyls of transgenic seedlings were also longer (up to 4 times), and neither the seedlings nor the growing plants elongated further after application of GA3. Hypocotyl and stem lengths were reduced by application of paclobutrazol, and this inhibition was reversed by exogenous GA3. The ectopic overexpression of CcGA20ox1 enhanced the non-13-hydroxylation pathway of GA biosynthesis leading to GA4, apparently at the expense of the early-13-hydroxylation pathway. The level of GA4 (the active GA from the non-13-hydroxylation pathway) in the shoot of transgenic plants was 3-4 times higher than in control plants, whereas that of GA1, formed via the early-13-hydroxylation pathway (the main GA biosynthesis pathway in tobacco), decreased or was not affected. GA4 applied to the culture medium or to the expanding leaves was found to be at least equally active as GA1 on stimulating hypocotyl and stem elongation of tobacco plants. The results suggest that the tall phenotype of tobacco transgenic plants was due to their higher content of GA4, and that the GA response was saturated by the presence of the transgene.

Gibberellin metabolism: new insights revealed by the genes

The identification of most of the genes involved in the metabolic pathways for gibberellin hormones has helped us to understand these pathways and their regulation. Many of these enzymes are multifunctional and therefore fewer enzymes than might be expected are required to synthesize the various gibberellin structures. However, several of the enzymes are encoded by multiple genes that are regulated differently, adding unexpected genetic complexity. Several endogenous and environmental factors modify the expression of gibberellin biosynthesis genes, including developmental stage, hormonal status and light. A future challenge will be to dissect the complex, interacting pathways that mediate the regulation of gibberellin metabolism.

Regulation of transcript levels of a potato gibberellin 20-oxidase gene by light and phytochrome B

Up to three gibberellin (GA) 20-oxidase genes have now been cloned from several species including Arabidopsis, bean (Phaseolus vulgaris), and potato (Solanum tuberosum). In each case the GA 20-oxidase genes exhibit different patterns of tissue expression. We have performed extensive northern analysis on one of the potato GA 20-oxidase genes (StGA20ox1), which is the only one that shows significant transcript levels in leaves. We show that levels of StGA20ox1 transcript are elevated in transgenic antisense plants that have reduced levels of phytochrome B (PHYB) compared with wild-type plants, implicating PHYB in the control of GA biosynthesis. We show that StGA20ox1 transcript levels vary in leaves of different age throughout the plant and cycle throughout the day, furthermore they are up-regulated by light and down-regulated in the dark. The degree of the response to the light-on signal is similar in potato plants deficient in phytochrome A or PHYB and wild-type plants. The induction of StGA20ox1 by blue light raises the possibility that a blue light receptor may be involved in the control of this gene by light.

Regulation of gibberellin 20-oxidase and gibberellin 3beta-hydroxylase transcript accumulation during De-etiolation of pea seedlings

Gibberellin (GA) 20-oxidase (GA 20-ox) and GA 3beta-hydroxylase (GA 3beta-hy) are enzymes that catalyze the late steps in the formation of active GAs, and are potential control points in the regulation of GA biosynthesis by light. We have investigated the photoregulation of the GA 20-ox and GA 3beta-hy transcript levels in pea (Pisum sativum L.). The GA 20-ox transcript level was higher in light-grown seedlings than in etiolated seedlings, whereas GA 3beta-hy mRNA accumulation was higher in etiolated seedlings. However, transfer of etiolated seedlings to light led to a 5-fold increase in the expression of both transcripts 4 h after transfer. GA 20-ox mRNA accumulation is regulated by both phytochromes A and B. Transfer to light also resulted in a 6-fold decrease in GA(1) levels within 2 h. These results suggest that the light-induced drop in GA(1) level is not achieved through regulation of GA 20-ox and GA 3beta-hy mRNA accumulation. The application of exogenous GA(1) to apical buds of etiolated seedlings prior to light treatments inhibited the light-induced accumulation of both GA 20-ox and GA 3beta-hy mRNA, suggesting that negative feedback regulation is an important mechanism in the regulation of GA 20-ox and GA 3beta-hy mRNA accumulation during de-etiolation of pea seedlings.

Cloning and molecular analyses of a gibberellin 20-oxidase gene expressed specifically in developing seeds of watermelon

To understand the biosynthesis and functional role of gibberellins (GAs) in developing seeds, we isolated Cv20ox, a cDNA clone from watermelon (Citrullus lanatus) that shows significant amino acid homology with GA 20-oxidases. The complementary DNA clone was expressed in Escherichia coli as a fusion protein, which oxidized GA(12) at C-20 to the C(19) compound GA(9), a precursor of bioactive GAs. RNA-blot analysis showed that the Cv20ox gene was expressed specifically in developing seeds. The gene was strongly expressed in the integument tissues, and it was also expressed weakly in inner seed tissues. In parthenocarpic fruits induced by 1-(2-chloro-4-pyridyl)-3-phenylurea treatment, the expression pattern of Cv20ox did not change, indicating that the GA 20-oxidase gene is expressed primarily in the maternal cells of developing seeds. The promoter of Cv20ox was isolated and fused to the beta-glucuronidase (GUS) gene. In a transient expression system, beta-glucuronidase staining was detectable only in the integument tissues of developing watermelon seeds.

The gene encoding tobacco gibberellin 3beta-hydroxylase is expressed at the site of GA action during stem elongation and flower organ development

Gibberellin 3beta-hydroxylase catalyzes the final step in the biosynthetic pathway leading to the plant hormone gibberellin (GA) and, therefore, the in vivo localization of this enzyme should give a direct indication of the site of synthesis of bioactive GAs in plants. We have isolated a cDNA clone, Nty (Nicotiana tabacum GA 3beta-hydroxylase), which encodes a putative GA 3beta-hydroxylase, by RT-PCR using RNA from tobacco shoot apices. Functional analysis, using an NTY protein expressed in Escherichia coli, revealed that Nty encoded an active GA 3beta-hydroxylase. A high expression level of Nty was observed in shoot apices, flowers, roots, young internodes but not in leaves or seeds. We performed more detailed expression analyses using in situ hybridization and histochemical analyses of the GUS activity in transgenic tobacco plants carrying an Nty promoter:GUS fusion gene. These studies revealed that expression of Nty was restricted to specific regions, including actively dividing and elongating cells in the various organs; rib meristem and elongation zones of shoot apices, tapetum and pollen grains in developing anthers and root tips, which are consistent with the sites of GA action. It is proposed that GA actions depend on the modulation of endogenous bioactive GA levels through the regulation of GA 3beta-hydroxylase expression in situ.

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.

Feedback control and diurnal regulation of gibberellin 20-oxidase transcript levels in potato

Tuber formation in potato (Solanum tuberosum) is promoted by short photoperiods and is inhibited by gibberellins (GAs). Endogenous levels of GA1 were shown to decrease in stolons and leaves of potato plants induced to tuberize, which suggests that photoperiodic regulation of GA biosynthesis may play a role in tuber induction. We report the isolation of three potato cDNA clones (StGA20ox1-3) encoding GA 20-oxidase, a key regulatory enzyme in the GA-biosynthetic pathway. Using northern analysis, we detected a differential pattern of tissue-specific expression of the mRNAs corresponding to these clones. StGA20ox mRNAs were also very abundant in leaves of the potato ga1 mutant, which is blocked in the 13-hydroxylation step, and were strongly down-regulated by gibberellic acid, suggesting a feedback regulation of these genes. In plants grown in short-day (inductive) conditions, levels of the StGA20ox transcripts in leaves fluctuated during a 24-h period, with a peak of accumulation observed about 4 h after the lights were turned off. Interruption of the night with a 30-min "night break" of light (noninductive conditions) did not have a marked effect on the levels of accumulation of the three GA 20-oxidase mRNAs during the day, but it induced a second peak of expression of StGA20ox1 and StGA20ox3 transcripts late in the night. This observation, together with the finding that StGA20ox1 mRNA is expressed at high levels in leaves, suggests that night-break induction of this gene might play a role in the control of tuberization by regulating endogenous levels of GAs in response to daylength conditions.

Regulation of gibberellin biosynthesis genes during flower and early fruit development of tomato

Gibberellins (GAs) are essential for the development of fertile flowers in tomato, and may also be required immediately after fertilization. In the GA-biosynthetic pathway, the reactions catalyzed by GA 20-oxidases have been implicated as site of regulation. To study the regulation of GA biosynthesis in flower and early fruit development, we isolated three tomato GA 20-oxidase cDNA clones, Le20ox-1, -2 and -3. The three genes showed different organ-specific patterns of mRNA accumulation. Analysis of the transcript levels of the three GA 20-oxidase genes, as well as those of copalyl diphosphate synthase (LeCPS) and GA 3 beta-hydroxylase (Le3OH-2) during flower bud and early fruit development, revealed temporally distinct patterns of mRNA accumulation. Up until anthesis, transcripts were observed for LeCPS, Le20ox-1, -2 and Le3OH-2, with an accumulation of Le20ox-1 mRNA. In contrast to the high level of Le3OH-2 transcripts in the fully open flower, mRNA levels of Le20ox-1, -2 and LeCPS were reduced at this stage. After anthesis, LeCPS and Le20ox-1 transcripts increased again. In addition, Le20ox-3transcripts increased whereas the transcripts of Le3OH-2 decreased to an undetectable level. In situ hybridization results demonstrated that during early stages of bud development, Le20ox-2 transcripts were localized in the tapetum and placenta. The presented results supply novel data about localization of GA biosynthesis gene transcripts, and indicate that transcript levels of GA biosynthesis genes are all highly regulated during flower bud development.

Phytochrome regulates gibberellin biosynthesis during germination of photoblastic lettuce seeds

Germination of lettuce (Lactuca sativa L.) seed is regulated by phytochrome. The requirement for red light is circumvented by the application of gibberellin (GA). We have previously shown that the endogenous content of GA1, the main bioactive GA in lettuce seeds, increases after red-light treatment. To clarify which step of GA1 synthesis is regulated by phytochrome, cDNAs encoding GA 20-oxidases (Ls20ox1 and Ls20ox2, for L. sativa GA 20-oxidase) and 3beta-hydroxylases (Ls3h1 and Ls3h2 for L. sativa GA 3beta-hydroxylase) were isolated from lettuce seeds by reverse-transcription polymerase chain reaction. Functional analysis of recombinant proteins expressed in Escherichia coli confirmed that the Ls20ox and Ls3h encode GA 20-oxidases and 3beta-hydroxylases, respectively. Northern-blot analysis showed that Ls3h1 expression was dramatically induced by red-light treatment within 2 h, and that this effect was canceled by a subsequent far-red-light treatment. Ls3h2 mRNA was not detected in seeds that had been allowed to imbibe under any light conditions. Expression of the two Ls20ox genes was induced by initial imbibition alone in the dark. The level of Ls20ox2 mRNA decreased after the red-light treatment, whereas that of Ls20ox1 was unaffected by light. These results suggest that red light promotes GA1 synthesis in lettuce seeds by inducing Ls3h1 expression via phytochrome action.

Over-expression of a tobacco homeobox gene, NTH15, decreases the expression of a gibberellin biosynthetic gene encoding GA 20-oxidase

Ectopic expression of the homeobox gene, NTH15 (Nicotiana tabacum homeobox 15) in transgenic tobacco leads to abnormal leaf and flower morphology, accompanied by a decrease in the content of the active gibberellin, GA1. Quantitative analysis of intermediates in the GA biosynthetic pathway revealed that the step from GA19 to GA20 was blocked in transgenic tobacco plants overexpressing NTH15. To investigate the relationship between the expression of NTH15 and genes involved in GA biosynthesis, we isolated three cDNA clones from tobacco encoding two types of GA 20-oxidase and a 3 beta-hydroxylase. RNA gel blot analysis revealed that the expression of one gene (Ntc12, encoding GA 20-oxidase), which in wild-type tobacco plants was abundantly expressed in leaves, was strongly suppressed in the transformants. The expression level of Ntc12 decreased with increasing severity of phenotype of transgenic tobacco leaves. The abnormal leaf morphology was largely overcome by treatment with GA20 or GA1 but not by GA19. These data strongly suggest that overexpression of NTH15 inhibits the expression of Ntc12, resulting in reduced levels of active GA and abnormal leaf morphology in transgenic tobacco plants. In situ hybridization in wild-type tobacco revealed that expression of Ntc12 occurred mainly in the rib meristem, cells surrounding the procambium and in leaf primordia. Expression was not seen in the tunica, corpus and procambium, tissues in which NTH15 was predominantly expressed. The contrasting expression patterns of these genes may reflect their antagonistic functions in the formation of lateral organs from the shoot apical meristem.

Decreased GA1 content caused by the overexpression of OSH1 is accompanied by suppression of GA 20-oxidase gene expression

We previously reported that overexpression of the rice homeobox gene OSH1 led to altered morphology and hormone levels in transgenic tobacco (Nicotiana tabacum L.) plants. Among the hormones whose levels were changed, GA1 was dramatically reduced. Here we report the results of our analysis on the regulatory mechanism(s) of OSH1 on GA metabolism. GA53 and GA20, precursors of GA1, were applied separately to transgenic tobacco plants exhibiting severely changed morphology due to overexpression of OSH1. Only treatment with the end product of GA 20-oxidase, GA20, resulted in a striking promotion of stem elongation in transgenic tobacco plants. The internal GA1 and GA20 contents in OSH1-transformed tobacco were dramatically reduced compared with those of wild-type plants, whereas the level of GA19, a mid-product of GA 20-oxidase, was 25% of the wild-type level. We have isolated a cDNA encoding a putative tobacco GA 20-oxidase, which is mainly expressed in vegetative stem tissue. RNA-blot analysis revealed that GA 20-oxidase gene expression was suppressed in stem tissue of OSH1-transformed tobacco plants. Based on these results, we conclude that overexpression of OSH1 causes a reduction of the level of GA1 by suppressing GA 20-oxidase expression.

Function and substrate specificity of the gibberellin 3beta-hydroxylase encoded by the Arabidopsis GA4 gene

cDNA corresponding to the GA4 gene of Arabidopsis thaliana L. (Heynh. ) was expressed in Escherichia coli, from which cell lysates converted [14C]gibberellin (GA)9 and [14C]GA20 to radiolabeled GA4 and GA1, respectively, thereby confirming that GA4 encodes a GA 3beta-hydroxylase. GA9 was the preferred substrate, with a Michaelis value of 1 microm compared with 15 microm for GA20. Hydroxylation of these GAs was regiospecific, with no indication of 2beta-hydroxylation or 2,3-desaturation. The capacity of the recombinant enzyme to hydroxylate a range of other GA substrates was investigated. In general, the preferred substrates contained a polar bridge between C-4 and C-10, and 13-deoxy GAs were preferred to their 13-hydroxylated analogs. Therefore, no activity was detected using GA12-aldehyde, GA12, GA19, GA25, GA53, or GA44 as the open lactone (20-hydroxy-GA53), whereas GA15, GA24, and GA44 were hydroxylated to GA37, GA36, and GA38, respectively. The open lactone of GA15 (20-hydroxy-GA12) was hydroxylated but less efficiently than GA15. In contrast to the free acid, GA25 19,20-anhydride was 3beta-hydroxylated to give GA13. 2,3-Didehydro-GA9 and GA5 were converted by recombinant GA4 to the corresponding epoxides 2, 3-oxido-GA9 and GA6.

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.