Feed-back regulation of gibberellin biosynthesis and gene expression in Pisum sativum L

Study on the Regulatory Effects of GA3 on Soybean Internode Elongation

Excessive plant height is an important factor that can lead to lodging, which is closely related to soybean yield. Gibberellins are widely used as plant growth regulators in agricultural production. Gibberellic acid (GA₃), one of the most effective active gibberellins, has been used to regulate plant height and increase yields. The mechanism through which GA₃ regulates internode elongation has been extensively investigated. In 2019 and 2020, we applied GA₃ to the stems, leaves, and roots of two soybean cultivars, Heinong 48 (a high-stalk cultivar) and Henong 60 (a dwarf cultivar), and GA₃ was also applied to plants whose apical meristem was removed or to girded plants to compare the internode length and stem GA₃ content of soybean plants under different treatments. These results suggested that the application of GA₃ to the stems, leaves, and roots of soybean increased the internode length and GA₃ content in the stems. Application of GA₃ decreased the proportion of the pith in the soybean stems and primary xylem while increasing the proportion of secondary xylem. The apical meristem is an important site of GA₃ synthesis in soybean stems and is involved in the regulation of stem elongation. GA₃ was shown to be transported acropetally through the xylem and laterally between the xylem and phloem in soybean stems. We conclude that the GA₃ level in stems is an important factor affecting internode elongation.

Development of a whole plant bioassay to test effects of potentized calcium carbonate in pillule formulation

OBJECTIVES

From a pharmaceutical point of view, we see a need to develop stable preclinical test systems to identify and investigate effects of potentized remedies as used in Anthroposophic Medicine and Homeopathy. We evaluated a plant bioassay regarding its capacity to distinguish homeopathic remedies from placebo, applied as sucrose pillules.

METHODS

Pea seed (Pisum sativum L) was soaked for 24 hours in water with dissolved homeopathic or placebo pillules, or in water only. Shoot length was measured 14 days after planting and treatment groups were compared by analysis of variance (ANOVA). The stability of the system was validated by systematic negative control experiments.

RESULTS

The system is suitable to test a common application form - sucrose pillules - of a potentized preparation without influence of the pharmaceutical carrier substance. A screening of 13 potentized preparations revealed Calcium carbonicum to affect pea shoot growth (p < 0.05). Three independent series of main experiments were performed with potentized Calcium carbonicum to assess reproducibility. Meta-analysis of all data revealed significant effects of Calcium carbonicum 12c and 30c on pea shoot growth (p < 0.05), which were however dependent on the date of experiment and/or the experimental series.

CONCLUSIONS

Potentized Calcium carbonicum, applied as sucrose pillules, influenced pea shoot growth in the assay investigated. However, due to the small effect size and due to the modulation of the effects by still unknown external factors, further optimization of this bioassay is necessary to be used in pharmaceutical quality control or in investigating the biological or pharmaceutical mode of action of potentized preparations.

Characterization of GA20ox genes in tall and dwarf types coconut (Cocos nucifera L.)

Coconuts (Cocos nucifera L.) are divided by the height into tall and dwarf types. In many plants the short phenotype was emerged by mutation of the GA20ox gene encoding the enzyme involved in gibberellin (GA) biosynthesis. Two CnGA20ox genes, CnGA20ox1 and CnGA20ox2, were cloned from tall and dwarf types coconut. The sequences, gene structures and expressions were compared. The structure of each gene comprised three exons and two introns. The CnGA20ox1 and CnGA20ox2 genes consisted of the coding region of 1110 and 1131 bp, encoding proteins of 369 and 376 amino acids, respectively. Their amino acid sequences are highly homologous to GA20ox1 and GA20ox2 genes of Elaeis guineensis, but only 57% homologous to each other. However, the characteristic amino acids two histidines and one aspartic acid which are the two iron (Fe²⁺) binding residues, and arginine and serine which are the substrate binding residues of the dioxygenase enzyme in the 20G-FeII_Oxy domain involved in GA biosynthesis, were found in the active site of both enzymes. The evolutionary relationship of their proteins revealed three clusters in vascular plants, with two subgroups in dicots and three subgroups in monocots. This result confirmed that CnGA20ox was present as multi-copy genes, and at least two groups CnGA20ox1 and CnGA20ox2 were found in coconut. The nucleotide sequences of CnGA20ox1 gene in both coconut types were identical but its expression was about three folds higher in the leaves of tall coconut than in those of dwarf type which was in good agreement with their height. In contrast, the nucleotide sequences of CnGA20ox2 gene in the two coconut types were different, but the expression of CnGA20ox2 gene could not be detected in either coconut type. The promoter region of CnGA20ox1 gene was cloned, and the core promoter sequences and various cis-elements were found. The CnGA20ox1 gene should be responsible for the height in coconut, which is different from other plants because no mutation was present in CnGA20ox1 gene of dwarf type coconut.

Ethylene in vegetative development: a tale with a riddle

The vegetative development of plants is strongly dependent on the action of phytohormones. For over a century, the effects of ethylene on plants have been studied, illustrating the profound impact of this gaseous hormone on plant growth, development and stress responses. Ethylene signaling is under tight self-control at various levels. Feedback regulation occurs on both biosynthesis and signaling. For its role in developmental processes, ethylene has a close and reciprocal relation with auxin, another major determinant of plant architecture. Here, we discuss, in view of novel findings mainly in the reference plant Arabidopsis, how ethylene is distributed and perceived throughout the plant at the organ, tissue and cellular levels, and reflect on how plants benefit from the complex interaction of ethylene and auxin, determining their shape. Furthermore, we elaborate on the implications of recent discoveries on the control of ethylene signaling.

Stamen-derived bioactive gibberellin is essential for male flower development of Cucurbita maxima L

Gibberellin (GA) signalling during pumpkin male flower development is highly regulated, including biosynthetic, perception, and transduction pathways. GA 20-oxidases, 3-oxidases, and 2-oxidases catalyse the final part of GA synthesis. Additionally, 7-oxidase initiates this part of the pathway in some cucurbits including Cucurbita maxima L. (pumpkin). Expression patterns for these GA-oxidase-encoding genes were examined by competitive reverse transcription-PCR (RT-PCR) and endogenous GA levels were determined during pumpkin male flower development. In young flowers, GA20ox3 transcript levels are high in stamens, followed by high levels of the GA precursor GA(9). Later, just before flower opening, transcript levels for GA3ox3 and GA3ox4 increase in the hypanthium and stamens, respectively. In the stamen, following GA3ox4 expression, bioactive GA(4) levels rise dramatically. Accordingly, catabolic GA2ox2 and GA2ox3 transcript levels are low in developing flowers, and increase in mature flowers. Putative GA receptor GID1b and DELLA repressor GAIPb transcript levels do not change in developing flowers, but increase sharply in mature flowers. Emasculation arrests floral development completely and leads to abscission of premature flowers. Application of GA(4) (but not of its precursors GA(12)-aldehyde or GA(9)) restores normal growth of emasculated flowers. These results indicate that de novo GA(4) synthesis in the stamen is under control of GA20ox3 and GA3ox4 genes just before the rapid flower growth phase. Stamen-derived bioactive GA is essential and sufficient for male flower development, including the petal and the pedicel 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.

Regulation of the gibberellin pathway by auxin and DELLA proteins

The synthesis and deactivation of bioactive gibberellins (GA) are regulated by auxin and by GA signalling. The effect of GA on its own pathway is mediated by DELLA proteins. Like auxin, the DELLAs promote GA synthesis and inhibit its deactivation. Here, we investigate the relationships between auxin and DELLA regulation of the GA pathway in stems, using a pea double mutant that is deficient in DELLA proteins. In general terms our results demonstrate that auxin and DELLAs independently regulate the GA pathway, contrary to some previous suggestions. The extent to which DELLA regulation was able to counteract the effects of auxin regulation varied from gene to gene. For Mendel's LE gene (PsGA3ox1) no counteraction was observed. However, for another synthesis gene, a GA 20-oxidase, the effect of auxin was weak and in WT plants appeared to be completely over-ridden by DELLA regulation. For a key GA deactivation (2-oxidase) gene, PsGA2ox1, the up-regulation induced by auxin deficiency was reduced to some extent by DELLA regulation. A second pea 2-oxidase gene, PsGA2ox2, was up-regulated by auxin, in a DELLA-independent manner. In Arabidopsis also, one 2-oxidase gene was down-regulated by auxin while another was up-regulated. Monitoring the metabolism pattern of GA(20) showed that in Arabidopsis, as in pea, auxin can promote the accumulation of bioactive GA.

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.

Analyses of GA20ox- and GID1-over-expressing aspen suggest that gibberellins play two distinct roles in wood formation

Gibberellins (GAs) are involved in many aspects of plant development, including shoot growth, flowering and wood formation. Increased levels of bioactive GAs are known to induce xylogenesis and xylem fiber elongation in aspen. However, there is currently little information on the response pathway(s) that mediate GA effects on wood formation. Here we characterize an important element of the GA pathway in hybrid aspen: the GA receptor, GID1. Four orthologs of GID1 were identified in Populus tremula x P. tremuloides (PttGID1.1-1.4). These were functional when expressed in Arabidopsis thaliana, and appear to present a degree of sub-functionalization in hybrid aspen. PttGID1.1 and PttGID1.3 were over-expressed in independent lines of hybrid aspen using either the 35S promoter or a xylem-specific promoter (LMX5). The 35S:PttGID1 over-expressors shared several phenotypic traits previously described in 35S:AtGA20ox1 over-expressors, including rapid growth, increased elongation, and increased xylogenesis. However, their xylem fibers were not elongated, unlike those of 35S:AtGA20ox1 plants. Similar differences in the xylem fiber phenotype were observed when PttGID1.1, PttGID1.3 or AtGA20ox1 were expressed under the control of the LMX5 promoter, suggesting either that PttGID1.1 and PttGID1.3 play no role in fiber elongation or that GA homeostasis is strongly controlled when GA signaling is altered. Our data suggest that GAs are required in two distinct wood-formation processes that have tissue-specific signaling pathways: xylogenesis, as mediated by GA signaling in the cambium, and fiber elongation in the developing xylem.

Auxin regulation of gibberellin biosynthesis in the roots of pea (Pisum sativum)

Auxin promotes GA biosynthesis in the aboveground parts of plants. However, it has not been demonstrated previously that this interaction occurs in roots. To understand the interactions between auxin and GAs in these organs, we treated wild-type pea (Pisum sativum L.) roots with the inhibitors of auxin action, p-chlorophenoxyisobutyric acid (PCIB) and yokonolide B (YkB), and with the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA). These compounds generally downregulated GA synthesis genes and upregulated GA deactivation genes, and reduced the level of the bioactive GA₁. These effects indicate that in pea roots, auxin at normal endogenous levels stimulates GA biosynthesis. We show also that supra-optimal levels of exogenous auxin reduce the endogenous level of bioactive GA in roots, although the effect appears too small to account for the strong growth-inhibitory effect of high auxin levels.

Expression of gibberellin 20-oxidase1 (AtGA20ox1) in Arabidopsis seedlings with altered auxin status is regulated at multiple levels

Bioactive gibberellins (GAs) affect many biological processes including germination, stem growth, transition to flowering, and fruit development. The location, timing, and level of bioactive GA are finely tuned to ensure that optimal growth and development occur. The balance between GA biosynthesis and deactivation is controlled by external factors such as light and by internal factors that include auxin. The role of auxin transport inhibitors (ATIs) and auxins on GA homeostasis in intact light-grown Arabidopsis thaliana (L.) Heynh. seedlings was investigated. Two ATIs, 1-N-naphthylthalamic acid (NPA) and 1-naphthoxyacetic acid (NOA) caused elevated expression of the GA biosynthetic enzyme AtGA20-oxidase1 (AtGA20ox1) in shoot but not in root tissues, and only at certain developmental stages. It was investigated whether enhanced AtGA20ox1 gene expression was a consequence of altered flow through the GA biosynthetic pathway, or was due to impaired GA signalling that can lead to enhanced AtGA20ox1 expression and accumulation of a DELLA protein, Repressor of ga1-3 (RGA). Both ATIs promoted accumulation of GFP-fused RGA in shoots and roots, and this increase was counteracted by the application of GA(4). These results suggest that in ATI-treated seedlings the impediment to DELLA protein degradation may be a deficiency of bioactive GA at sites of GA response. It is proposed that the four different levels of AtGA20ox1 regulation observed here are imposed in a strict hierarchy: spatial (organ-, tissue-, cell-specific) > developmental > metabolic > auxin regulation. Thus results show that, in intact auxin- and auxin transport inhibitor-treated light-grown Arabidopsis seedlings, three other levels of regulation supersede the effects of auxin on AtGA20ox1.

Upregulation of an Arabidopsis RING-H2 gene, XERICO, confers drought tolerance through increased abscisic acid biosynthesis

RING (really interesting new gene) zinc-finger proteins have important regulatory roles in the development of a variety of organisms. The XERICO gene encodes a small protein (162 amino acids) with an N-terminal trans-membrane domain and a RING-H2 zinc-finger motif located at the C-terminus. In silico gene-expression analysis indicated that XERICO is induced by salt and osmotic stress. Compared with wild-type (WT) Arabidopsis plants, transgenic plants overexpressing XERICO (35S::XERICO) exhibited hypersensitivity to salt and osmotic stress and exogenous abscisic acid (ABA) during germination and early seedling growth. When subjected to a drought treatment, transcriptional upregulation of a key ABA-biosynthesis gene, AtNCED3, was much faster and stronger in 35S::XERICO plants compared with WT plants. Further, upregulation of XERICO substantially increased cellular ABA levels. The adult 35S::XERICO plants, in contrast to early seedling growth, showed a marked increase in their tolerance to drought stress. Yeast two-hybrid screening indicated that XERICO interacts with an E2 ubiquitin-conjugating enzyme (AtUBC8) and ASK1-interacting F-box protein (AtTLP9), which is involved in the ABA-signaling pathway. Affymetrix GeneChip array analysis showed that the expressions of many of the genes involved in the biosynthesis of plant hormones (e.g. ethylene, brassinosteroid, gibberellic acid) were significantly changed in the 35S::XERICO plants. These results suggest that the homeostasis of various plant hormones might be altered in 35S::XERICO plants, possibly by overaccumulation of ABA.

Regulation of the early GA biosynthesis pathway in pea

The early steps in the gibberellin (GA) biosynthetic pathway are controlled by single copy genes or small gene families. In pea (Pisum sativum L.) there are two ent-kaurenoic acid oxidases, one expressed only in the seeds, while ent-copalyl synthesis and ent-kaurene oxidation appear to be controlled by single copy genes. None of these genes appear to show feedback regulation and the only major developmental regulation appears to be during seed development. During shoot maturation, transcript levels do not change markedly with the result that all the three genes examined are expressed in mature tissue, supporting recent findings that these tissues can synthesise GAs. It therefore appears that the regulation of bioactive GA levels are determined by the enzymes encoded by the 2-oxoglutarate-dependent dioxygenase gene families controlling the later steps in GA biosynthesis. However the early steps are nonetheless important as a clear log/linear relationship exists between elongation and the level of GA1 in a range of single and double mutants in genes controlling these steps.

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.

The auxin transport inhibitor response 3 (tir3) allele of BIG and auxin transport inhibitors affect the gibberellin status of Arabidopsis

The Arabidopsis gene BIG (formerly DOC1/TIR3/UMB1/ASA1) is known to encode a huge calossin-like protein that is required for polar auxin transport (PAT). Mutations at this locus, in addition to reducing PAT, can alter the sensitivity of plants to several hormones and light. The tir3-1 allele of BIG reduces the response of plants to application of the gibberellin (GA) precursors ent-kaurenoic acid and GA12 and its semidwarf phenotype is partially reversed by C19-GAs. The effects of auxin transport inhibitors (ATIs) on GA 20-oxidation was examined in wild-type and tir3-1 seedlings. 1-N-naphthylphthalamic acid (NPA) and triiodobenzoic acid lead to overexpression of the GA-biosynthetic gene AtGA20ox1 comparable in magnitude to the overexpression observed in seedlings treated with paclobutrazol, a GA biosynthesis inhibitor. In contrast to that of AtGA20ox1, overexpression of AtGA20ox2 is pronounced only in paclobutrazol-treated Col and Ler, and is less in tir3-1 and in all NPA-treated seedlings. Thus the effects of BIG and ATIs on the expression of genes encoding GA 20-oxidases are complex, and suggest that at least in some tissues ATIs, directly or indirectly, may reduce the level of bioactive GA and/or alter GA signal transduction.

Impact of altered gibberellin metabolism on biomass accumulation, lignin biosynthesis, and photosynthesis in transgenic tobacco plants

Gibberellins (GAs) are involved in regulation of many aspects during plant development. To investigate the impact of altered GA levels on plant growth and metabolism, transgenic tobacco (Nicotiana tabacum) plants have been engineered to express either a GA20-oxidase (AtGA20-ox) or a GA2-oxidase (AtGA2-ox) gene from Arabidopsis under control of the cauliflower mosaic virus 35S promoter. Resulting plants were characterized by elongated or stunted shoot growth, respectively, indicating changes in the content of bioactive GAs. In accordance with the effect on plant growth, biomass production was increased or decreased in AtGA20-ox or AtGA2-ox plants, respectively, and was found to be positively correlated with the rate of photosynthesis as determined at the whole plant level. Differences in dry matter accumulation were most likely due to changes in lignin deposition as indicated by histochemical staining and quantitative measurements. Altered lignification of transgenic plants was paralleled by up- or down-regulation of the expression of lignin biosynthetic genes. Short-term GA3 feeding of excised petioles induced lignin formation in the absence of a transcriptional activation of pathway-specific genes. Thus, short-term GA treatment mediates lignin deposition most likely by polymerization of preformed monomers, whereas long-term effects on lignification involve elevated production of precursors by transcriptional stimulation of the biosynthetic pathway. Interestingly, analysis of stem cross sections revealed a differential effect of GA on the formation of xylem and pith cells. The number of lignified vessels was increased in AtGA20-ox plants pointing to a stimulation of xylem formation while the number of pith cells declined indicating a negative regulation.

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.

The gibberellic-acid insensitive dwarfing gene sdw3 of barley is located on chromosome 2HS in a region that shows high colinearity with rice chromosome 7L

In this study, comparative high resolution genetic mapping of the GA-insensitive dwarfing gene sdw3 of barley revealed highly conserved macrosynteny of the target region on barley chromosome 2HS with rice chromosome 7L. A rice contig covering the sdw3-orthologous region was identified and subsequently exploited for marker saturation of the target interval in barley. This was achieved by (1) mapping of rice markers from the orthologous region of the rice genetic map, (2) mapping of rice ESTs that had been physically localized on the rice contig, or (3) mapping of barley ESTs that show strong sequence similarity to coding sequences present in the rice contig. Finally, the sdw3 gene was mapped to an interval of 0.55 cM in barley, corresponding to a physical distance of about 252 kb in rice, after employing orthologous EST-derived rice markers. Three putative ORFs were identified in this interval in rice, which exhibited significant sequence similarity to known signal regulator genes from different species. These ORFs can serve as starting points for the map-based isolation of the sdw3 gene from barley.

Molecular mechanism of gibberellin signaling in plants

The hormone gibberellin (GA) plays an important role in modulating diverse processes throughout plant development. In recent years, significant progress has been made in the identification of upstream GA signaling components and trans- and cis-acting factors that regulate downstream GA-responsive genes in higher plants. GA appears to derepress its signaling pathway by inducing proteolysis of GA signaling repressors (the DELLA proteins). Recent evidence indicates that the DELLA proteins are targeted for degradation by an E3 ubiquitin ligase SCF complex through the ubiquitin-26S proteasome pathway.

Where do gibberellin biosynthesis and gibberellin signaling occur in rice plants?

To identify where gibberellin (GA) biosynthesis and signaling occur, we analyzed the expression of four genes involved in GA biosynthesis, GA 20-oxidase1 and GA 20-oxidase2 (OsGA20ox1 and OsGA20ox2), and GA 3-oxidase1 and GA 3-oxidase2 (OsGA3ox1 and OsGA3ox2), and two genes involved in GA signaling, namely, the gene encoding the alpha-subunit of the heterotrimeric GTP-binding protein (Galpha), and SLENDER RICE1 (SLR1), which encodes a repressor of GA signaling. At the vegetative stage, the expression of OsGA20ox2, OsGA3ox2, Galpha, and SLR1 was observed in rapidly elongating or dividing organs and tissues, whereas the expression of OsGA20ox1 or OsGA3ox1 could not be detected. At the inflorescence or floral stage, the expression of OsGA20ox2, OsGA3ox2, Galpha, and SLR1 was also observed in the shoot meristems and stamen primordia. The overlapping expression of genes for GA biosynthesis and signaling indicates that in these tissues and organs, active GA biosynthesis occurs at the same site as does GA signaling. In contrast, no GA-biosynthesis genes were expressed in the aleurone cells of the endosperm; however, the two GA-signaling genes were actively expressed, indicating that the aleurone does not produce bioactive GAs, but can perceive GAs. The expression of OsGA20ox1 and OsGA3ox1 was observed only in the epithelium of the embryo and the tapetum of the anther. Based on the specific expression pattern of OsGA20ox1 and OsGA3ox1 in these tissues, we discuss the unique nature of the epithelium and the tapetum in terms of GA biosynthesis. The epithelium and the tapetum are considered to be an important source of bioactive GAs for aleurone and other organs of the flower, respectively.

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.

Semidwarf (sd-1), "green revolution" rice, contains a defective gibberellin 20-oxidase gene

The introduction of semidwarf rice (Oryza sativa L.) led to record yield increases throughout Asia in the 1960s. The major semidwarfing allele, sd-1, is still extensively used in modern rice cultivars. The phenotype of sd-1 is consistent with dwarfism that results from a deficiency in gibberellin (GA) plant growth hormones. We propose that the semidwarf (sd-1) phenotype is the result of a deficiency of active GAs in the elongating stem arising from a defective 20-oxidase GA biosynthetic enzyme. Sequence data from the rice genome was combined with previous mapping studies to locate a putative GA 20-oxidase gene (Os20ox2) at the predicted map location of sd-1 on chromosome 1. Two independent sd-1 alleles contained alterations within Os20ox2: a deletion of 280 bp within the coding region of Os20ox2 was predicted to encode a nonfunctional protein in an indica type semidwarf (Doongara), whereas a substitution in an amino acid residue (Leu-266) that is highly conserved among dioxygenases could explain loss of function of Os20ox2 in a japonica semidwarf (Calrose76). The quantification of GAs in elongating stems by GC-MS showed that the initial substrate of GA 20-oxidase activity (GA53) accumulated, whereas the content of the major product (GA20) and of bioactive GA1 was lower in semidwarf compared with tall lines. We propose that the Os20ox2 gene corresponds to the sd-1 locus.

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.

HOW GIBBERELLIN REGULATES PLANT GROWTH AND DEVELOPMENT: A Molecular Genetic Analysis of Gibberellin Signaling

Gibberellins are hormones that control growth and a wide variety of other plant developmental processes. In recent years, significant progress has been made on the biochemistry of gibberellin biosynthesis and on the mechanisms by which gibberellin levels are regulated in plants. There have also been major advances in the understanding of gibberellin signaling, with several key genes being cloned. This review discusses our current understanding of gibberellin signaling, as seen from the perspective of molecular genetic analysis, and relates these observations to previous biochemical studies. In particular, we highlight an important conclusion of recent years: that GAI/RGA and orthologs play major roles in gibberellin signaling in diverse plant species, and that gibberellin probably stimulates growth by derepression of GAI/RGA.

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.

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.

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.

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.

Extragenic suppressors of the Arabidopsis gai mutation alter the dose-response relationship of diverse gibberellin responses

Active gibberellins (GAs) are endogenous factors that regulate plant growth and development in a dose-dependent fashion. Mutant plants that are GA deficient, or exhibit reduced GA responses, display a characteristic dwarf phenotype. Extragenic suppressor analysis has resulted in the isolation of Arabidopsis mutations, which partially suppress the dwarf phenotype conferred by GA deficiency and reduced GA-response mutations. Here we describe detailed studies of the effects of two of these suppressors, spy-7 and gar2-1, on several different GA-responsive growth processes (seed germination, vegetative growth, stem elongation, chlorophyll accumulation, and flowering) and on the in planta amounts of active and inactive GA species. The results of these experiments show that spy-7 and gar2-1 affect the GA dose-response relationship for a wide range of GA responses and suggest that all GA-regulated processes are controlled through a negatively acting GA-signaling pathway.

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.

Gibberellin signaling

Recent findings provide insights into the gibberellin signaling system in plants. Genes for gibberellin biosynthetic enzymes have been cloned, and an emerging theme is that gibberellin biosynthesis is negatively regulated by gibberellin responses. Mutants defective in gibberellin signaling have been analyzed, and an important finding is that gibberellin represses growth inhibition. The list of intracellular gibberellin signal-transduction elements has been expanded to include G-proteins and protein kinases.

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.

Gibberellin dose-response regulation of GA4 gene transcript levels in Arabidopsis

The gibberellins (GAs) are a complex family of diterpenoid compounds, some of which are potent endogenous regulators of plant growth. As part of a feedback control of endogenous GA levels, active GAs negatively regulate the abundance of mRNA transcripts encoding GA biosynthesis enzymes. For example, Arabidopsis GA4 gene transcripts encode GA 3beta-hydroxylase, an enzyme that catalyzes the conversion of inactive to active GAs. Here we show that active GAs regulate GA4 transcript abundance in a dose-dependent manner, and that down-regulation of GA4 transcript abundance is effected by GA4 (the product of 3beta-hydroxylation) but not by its immediate precursor GA9 (the substrate). Comparison of several different GA structures showed that GAs active in promoting hypocotyl elongation were also active in regulating GA4 transcript abundance, suggesting that similar GA:receptor and subsequent signal transduction processes control these two responses. It is interesting that these activities were not restricted to 3beta-hydroxylated GAs, being also exhibited by structures that were not 3beta-hydroxylated but that had another electronegative group at C-3. We also show that GA-mediated control of GA4 transcript abundance is disrupted in the GA-response mutants gai and spy-5. These observations define a sensitive homeostatic mechanism whereby plants may regulate their endogenous GA levels.

Mendel's dwarfing gene: cDNAs from the Le alleles and function of the expressed proteins

The major gibberellin (GA) controlling stem elongation in pea (Pisum sativum L.) is GA1, which is formed from GA20 by 3beta-hydroxylation. This step, which limits GA1 biosynthesis in pea, is controlled by the Le locus, one of the original Mendelian loci. Mutations in this locus result in dwarfism. We have isolated cDNAs encoding a GA 3beta-hydroxylase from lines of pea carrying the Le, le, le-3, and led alleles. The cDNA sequences from le and le-3 each contain a base substitution resulting in single amino acid changes relative to the sequence from Le. The cDNA sequence from led, a mutant derived from an le line, contains both the le "mutation" and a single-base deletion, which causes a shift in reading frame and presumably a null mutation. cDNAs from each line were expressed in Escherichia coli. The expression product for the clone from Le converted GA9 to GA4, and GA20 to GA1, with Km values of 1.5 microM and 13 microM, respectively. The amino acid substitution in the clone from le increased Km for GA9 100-fold and reduced conversion of GA20 to almost nil. Expression products from le and le-3 possessed similar levels of 3beta-hydroxylase activity, and the expression product from led was inactive. Our results suggest that the 3beta-hydroxylase cDNA is encoded by Le. Le transcript is expressed in roots, shoots, and cotyledons of germinating pea seedlings, in internodes and leaves of established seedlings, and in developing seeds.

Cloning gibberellin dioxygenase genes from pumpkin endosperm by heterologous expression of enzyme activities in Escherichia coli

Gibberellin (GA) plant hormones are biosynthesized via complex pathways, the final steps of which are catalyzed by 2-oxoglutarate-dependent dioxygenases. Here, the cloning of two such enzymes, the GA 7-oxidase and the GA 20-oxidase, is reported using a novel approach, namely, by screening for GA dioxygenase activities expressed as T7 gene 10 fusion proteins in recombinant Escherichia coli. In vitro translation products of mRNA from endosperm of immature pumpkin seeds contained three GA dioxygenase activities, including 7-oxidase, 20-oxidase, and 3beta-hydroxylase. A cDNA expression library was prepared from the endosperm mRNA in lambdaMOSElox. An aliquot of the amplified library was converted to plasmids in vivo and used for transformation of E. coli BL21(DE3), which thereafter expressed recombinant fusion proteins containing 7-oxidase, 20-oxidase, and 3beta-hydroxylase activities. By screening for specific GA dioxygenase expression, clones harboring 7-oxidase and 20-oxidase cDNA were isolated. The ORF of the 7-oxidase cDNA is 945 bp long, encoding for 314 amino acid residues with a calculated Mr of 35,712 and pI of 5.7. Recombinant GA 7-oxidase oxidizes GA12-aldehyde to GA12 and GA14-aldehyde to GA14. Evidence was obtained for further metabolism of GA12 by the 7-oxidase to four products, two of which are monohydroxylated GA12. The ORF of the 20-oxidase is-apart from seven changes, resulting in four amino acid substitutions-identical to the 20-oxidase cDNA previously cloned from pumpkin cotyledon mRNA; both 20-oxidases have the same catalytic properties.

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.