Gibberellin signaling: biosynthesis, catabolism, and response pathways

HORMONOMETER: a tool for discerning transcript signatures of hormone action in the Arabidopsis transcriptome

Plant hormones regulate growth and responses to environmental change. Hormone action ultimately modifies cellular physiological processes and gene activity. To facilitate transcriptome evaluation of novel mutants and environmental responses, there is a need to rapidly assess the possible contribution of hormone action to changes in the levels of gene transcripts. We developed a vector-based algorithm that rapidly compares lists of transcripts yielding correlation values. The application as described here, called HORMONOMETER, was used to analyze hormone-related activity in a transcriptome of Arabidopsis (Arabidopsis thaliana). The veracity of the resultant analysis was established by comparison with cognate and noncognate hormone transcriptomes as well as with mutants and selected plant-environment interactions. The HORMONOMETER accurately predicted correlations between hormone action and biosynthetic mutants for which transcriptome data are available. A high degree of correlation was detected between many hormones, particularly at early time points of hormone action. Unforeseen complexity was detected in the analysis of mutants and in plant-herbivore interactions. The HORMONOMETER provides a diagnostic tool for evaluating the physiological state of being of the plant from the point of view of transcripts regulated by hormones and yields biological insight into the multiple response components that enable plant adaptation to the environment. A Web-based interface has been developed to facilitate external interfacing with this platform.

Abscisic acid represses growth of the Arabidopsis embryonic axis after germination by enhancing auxin signaling

Under unfavorable environmental conditions, the stress phytohormone ABA inhibits the developmental transition from an embryo in a dry seed into a young seedling. We developed a genetic screen to isolate Arabidopsis thaliana mutants whose early seedling development is resistant to ABA. Here, we report the identification of a recessive mutation in AUXIN RESISTANT1 (AUX1), encoding a cellular auxin influx carrier. Although auxin is a major morphogenesis hormone in plants, little is known about ABA-auxin interactions during early seedling growth. We show that aux1 and pin2 mutants are insensitive to ABA-dependent repression of embryonic axis (hypocotyl and radicle) elongation. Genetic and physiological experiments show that this involves auxin transport to the embryonic axis elongation zone, where ABA enhances the activity of an auxin-responsive promoter. We propose that ABA represses embryonic axis elongation by potentiating auxin signaling in its elongation zone. This involves repression of the AUXIN INDUCIBLE (Aux/IAA) gene AXR2/IAA7, encoding a key component of ABA- and auxin-dependent responses during postgerminative growth.

Towards a systems biology approach to understanding seed dormancy and germination

Seed germination is the first adaptive decision in the development of many land plants. Advances in genetics and molecular physiology have taught us much about the control of germination using the model plant Arabidopsis thaliana. Here we review the current state of the art with an emphasis on mechanistic considerations and explore the potential impact of a systems biology approach to the problem.

Gibberellin signaling in the endodermis controls Arabidopsis root meristem size

Plant growth is driven by cell proliferation and elongation. The hormone gibberellin (GA) regulates Arabidopsis root growth by controlling cell elongation, but it is currently unknown whether GA also controls root cell proliferation. Here we show that GA biosynthetic mutants are unable to increase their cell production rate and meristem size after germination. GA signals the degradation of the DELLA growth repressor proteins GAI and RGA, promoting root cell production. Targeting the expression of gai (a non-GA-degradable mutant form of GAI) in the root meristem disrupts cell proliferation. Moreover, expressing gai in dividing endodermal cells was sufficient to block root meristem enlargement. We report a novel function for GA regulating cell proliferation where this signal acts by removing DELLA in a subset of, rather than all, meristem cells. We suggest that the GA-regulated rate of expansion of dividing endodermal cells dictates the equivalent rate in other root tissues. Cells must double in size prior to dividing but cannot do so independently, because they are physically restrained by adjacent tissues with which they share cell walls. Our study highlights the importance of probing regulatory mechanisms linking molecular- and cellular-scale processes with tissue and organ growth responses.

Far-red light inhibits germination through DELLA-dependent stimulation of ABA synthesis and ABI3 activity

Under the canopy, far-red (FR) light represses seed germination by inactivating phytochrome photoreceptors. This elicits a decrease in gibberellins (GA) levels and an increase in abscisic acid (ABA) levels. GA promotes germination by enhancing the proteasome-mediated destruction of DELLA repressors. ABA prevents germination by stimulating the expression of ABI repressors. How phytochromes elicit changes in hormone levels or how GA- and ABA-dependent signals are coordinated to repress germination remains poorly understood. We show that repression of germination by FR light involves stabilized DELLA factors GAI, RGA and RGL2 that stimulate endogenous ABA synthesis. In turn, ABA blocks germination through the transcription factor ABI3. The role of PIL5, a basic helix-loop-helix transcription factor stimulating GAI and RGA expression, is significant, provided GA synthesis is high enough; otherwise, high GAI and RGA protein levels persist to block germination. Under white light, GAI and RGA driven by the RGL2 promoter can substitute for RGL2 to promote ABA synthesis and repress germination, consistent with the recent findings with RGL2. The three DELLA factors inhibit testa rupture whereas ABI3 blocks endosperm rupture.

Combined experimental and theoretical study of long-range interactions modulating dimerization and activity of yeast geranylgeranyl diphosphate synthase

We present here how two amino acid residues in the first helix distal from the main dimer interface modulate the dimerization and activity of a geranylgeranyl diphosphate synthase (GGPPs). The enzyme catalyzes condensation of farnesyl diphosphate and isopentenyl diphosphate to generate a C(20) product as a precursor for chlorophylls, carotenoids, and geranylgeranylated proteins. The 3D structure of GGPPs from Saccharomyces cerevisiae reveals an unique positioning of the N-terminal helix A, which protrudes into the other subunit and stabilizes dimerization, although it is far from the main dimer interface. Through a series of mutants that were characterized by analytic ultracentrifugation (AUC), the replacement of L8 and I9 at this helix with Gly was found sufficient to disrupt the dimer into a monomer, leading to at least 10(3)-fold reduction in activity. Molecular dynamics simulations and free energy decomposition analyses revealed the possible effects of the mutations on the protein structures and several critical interactions for maintaining dimerization. Further site-directed mutagenesis and AUC studies elucidated the molecular mechanism for modulating dimerization and activity by long-range interactions.

Effects of gibberellic acid on primary terpenoids and delta-tetrahydrocannabinol in Cannabis sativa at flowering stage

Plants synthesize an astonishing diversity of isoprenoids, some of which play essential roles in photosynthesis, respiration, and the regulation of growth and development. Two independent pathways for the biosynthesis of isoprenoid precursors coexist within the plant cell: the cytosolic mevalonic acid (MVA) pathway and the plastidial methylerythritol phosphate (MEP) pathway. However, little is known about the effects of plant hormones on the regulation of these pathways. In the present study we investigated the effect of gibberellic acid (GA(3)) on changes in the amounts of many produced terpenoids and the activity of the key enzymes, 1-deoxy-D-xylulose 5-phosphate synthase (DXS) and 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR), in these pathways. Our results showed GA(3) caused a decrease in DXS activity in both sexes that it was accompanied by a decrease in chlorophylls, carotenoids and Delta(9)-tetrahydrocannabinol (THC) contents and an increase in alpha-tocopherol content. The treated plants with GA(3) showed an increase in HMGR activity. This increase in HMGR activity was followed by accumulation of stigmasterol and beta-sitosterol in male and female plants and campestrol in male plants. The pattern of the changes in the amounts of sterols was exactly similar to the changes in the HMGR activity. These data suggest that GA(3) can probably influence the MEP and MVA pathways oppositely, with stimulatory and inhibitory effects on the produced primary terpenoids in MVA and DXS pathways, respectively.

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.

Triacontanol and jasmonic acid differentially modulate the lipid organization as evidenced by the fluorescent probe behavior and 31P nuclear magnetic resonance shifts in model membranes

Fluorescence resonance energy transfer (FRET), time-resolved fluorescence and anisotropy decays were determined in large unilamellar vesicles (LUVs) of egg phosphatidylcholine with the FRET pair N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dipalmitoyl-sn-glycero-3-phospho-ethanolamine as donor and lissamine rhodamine B 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine as acceptor, using 2-ps pulses from a Ti:sapphire laser on LUVs with incorporated plant growth regulators: triacontanol (TRIA) and jasmonic acid (JA). FRET efficiency, energy transfer rate, rotation correlation time, microviscosity, and diffusion coefficient of lateral diffusion of lipids were calculated from these results. It was observed that TRIA and JA differentially modulated all parameters studied. The effect of JA in such modulations was always partially reversed by TRIA. Also, the generalized polarization of laurdan fluorescence indicated that JA enhances the degree of hydration in lipid bilayers to a larger extent than does TRIA. Solid-state (31)P magic-angle spinning nuclear magnetic resonance spectra of LUVs showed two chemical shifts, at 0.009 and -11.988 ppm, at low temperatures (20 degrees C), while at increasing temperatures (20-60 degrees C) only one (at -11.988 ppm) was prominent and the other (0.009 ppm) gradually became obscure. However, LUVs with TRIA exhibited only one of the shifts at 0.353 ppm even at lower temperatures and JA did not affect the chemical shifts.

Proteome analysis of Norway maple (Acer platanoides L.) seeds dormancy breaking and germination: influence of abscisic and gibberellic acids

BACKGROUND

Seed dormancy is controlled by the physiological or structural properties of a seed and the external conditions. It is induced as part of the genetic program of seed development and maturation. Seeds with deep physiological embryo dormancy can be stimulated to germinate by a variety of treatments including cold stratification. Hormonal imbalance between germination inhibitors (e.g. abscisic acid) and growth promoters (e.g. gibberellins) is the main cause of seed dormancy breaking. Differences in the status of hormones would affect expression of genes required for germination. Proteomics offers the opportunity to examine simultaneous changes and to classify temporal patterns of protein accumulation occurring during seed dormancy breaking and germination. Analysis of the functions of the identified proteins and the related metabolic pathways, in conjunction with the plant hormones implicated in seed dormancy breaking, would expand our knowledge about this process.

RESULTS

A proteomic approach was used to analyse the mechanism of dormancy breaking in Norway maple seeds caused by cold stratification, and the participation of the abscisic (ABA) and gibberellic (GA) acids. Forty-four proteins showing significant changes were identified by mass spectrometry. Of these, eight spots were identified as water-responsive, 18 spots were ABA- and nine GA-responsive and nine spots were regulated by both hormones. The classification of proteins showed that most of the proteins associated with dormancy breaking in water were involved in protein destination. Most of the ABA- and GA-responsive proteins were involved in protein destination and energy metabolism.

CONCLUSION

In this study, ABA was found to mostly down-regulate proteins whereas GA up-regulated proteins abundance. Most of the changes were observed at the end of stratification in the germinated seeds. This is the most active period of dormancy breaking when seeds pass from the quiescent state to germination. Seed dormancy breaking involves proteins of various processes but the proteasome proteins, S-adenosylmethionine synthetase, glycine-rich RNA binding protein, ABI3-interacting protein 1, EF-2 and adenosylhomocysteinase are of particular importance. The effect of exogenously applied hormones was not a determining factor for total inhibition (ABA) or stimulation (GA) of Norway maple seed dormancy breaking and germination but proteomic data has proven these hormones play a role.

Dual role for 14-3-3 proteins and ABF transcription factors in gibberellic acid and abscisic acid signalling in barley (Hordeum vulgare) aleurone cells

The balance of gibberellins [gibberellic acid (GA)] and abscisic acid (ABA) is a determining factor during transition of embryogenesis and seed germination. Recently, we showed that 14-3-3 proteins are important in ABA signalling in barley aleurone cells. Using 14-3-3 RNAi constructs in the barley aleurone transient expression system, we demonstrate here that silencing of each 14-3-3 isoform suppresses GA induction of the alpha-amylase gene. 14-3-3 Proteins interact with ABA-responsive element (ABRE) binding factors HvABF1, 2 and 3, and here we show that these transcription factors also interact with the ABA-responsive kinase PKABA1, a kinase that mediates cross-talk between the GA and ABA pathway. ABF1 and ABF2 have a function in both signalling pathways as: (1) ectopic expression of wild-type ABF1 and mutant ABF2, lacking the 14-3-3 interaction domain, transactivates the ABA inducible HVA1 gene; and (2) GA induction of the alpha-amylase gene is repressed by ectopic expression of wild-type ABF1 and 2. Mutant ABF1 and 2 were still effective repressors of GA signalling. In summary, our data provide evidence that 14-3-3 proteins and members of the ABF transcription factor family have a regulatory function in the GA pathway and suggest that PKABA1 and ABF transcription factors are cross-talk intermediates in ABA and GA signalling.

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.

Forward and reverse genetics of rapid-cycling Brassica oleracea

Seeds of rapid-cycling Brassica oleracea were mutagenized with the chemical mutagen, ethylmethane sulfonate. The reverse genetics technique, TILLING, was used on a sample population of 1,000 plants, to determine the mutation profile. The spectrum and frequency of mutations induced by ethylmethane sulfonate was similar to that seen in other diploid species such as Arabidopsis thaliana. These data indicate that the mutagenesis was effective and demonstrate that TILLING represents an efficient reverse genetic technique in B. oleracea that will become more valuable as increasing genomic sequence data become available for this species. The extensive duplication in the B. oleracea genome is believed to result in the genetic redundancy that has been important for the evolution of morphological diversity seen in today's B. oleracea crops (broccoli, Brussels sprouts, cauliflower, cabbage, kale and kohlrabi). However, our forward genetic screens identified 120 mutants in which some aspect of development was affected. Some of these lines have been characterized genetically and in the majority of these, the mutant trait segregates as a recessive allele affecting a single locus. One dominant mutation (curly leaves) and one semi-dominant mutation (dwarf-like) were also identified. Allelism tests of two groups of mutants (glossy and dwarf) revealed that for some loci, multiple independent alleles have been identified. These data indicate that, despite genetic redundancy, mutation of many individual loci in B. oleracea results in distinct phenotypes.

Karrikins discovered in smoke trigger Arabidopsis seed germination by a mechanism requiring gibberellic acid synthesis and light

Discovery of the primary seed germination stimulant in smoke, 3-methyl-2H-furo[2,3-c]pyran-2-one (KAR1), has resulted in identification of a family of structurally related plant growth regulators, karrikins. KAR1 acts as a key germination trigger for many species from fire-prone, Mediterranean climates, but a molecular mechanism for this response remains unknown. We demonstrate that Arabidopsis (Arabidopsis thaliana), an ephemeral of the temperate northern hemisphere that has never, to our knowledge, been reported to be responsive to fire or smoke, rapidly and sensitively perceives karrikins. Thus, these signaling molecules may have greater significance among angiosperms than previously realized. Karrikins can trigger germination of primary dormant Arabidopsis seeds far more effectively than known phytohormones or the structurally related strigolactone GR-24. Natural variation and depth of seed dormancy affect the degree of KAR1 stimulation. Analysis of phytohormone mutant germination reveals suppression of KAR1 responses by abscisic acid and a requirement for gibberellin (GA) synthesis. The reduced germination of sleepy1 mutants is partially recovered by KAR1, which suggests that germination enhancement by karrikin is only partly DELLA dependent. While KAR1 has little effect on sensitivity to exogenous GA, it enhances expression of the GA biosynthetic genes GA3ox1 and GA3ox2 during seed imbibition. Neither abscisic acid nor GA levels in seed are appreciably affected by KAR1 treatment prior to radicle emergence, despite marked differences in germination outcome. KAR1 stimulation of Arabidopsis germination is light-dependent and reversible by far-red exposure, although limited induction of GA3ox1 still occurs in the dark. The observed requirements for light and GA biosynthesis provide the first insights into the karrikin mode of action.

New resources for studying the rose flowering process

Knowledge of the flowering process - an important trait in ornamental plants such as roses - is necessary for efficient control of flowering. This study was carried out to develop and characterize new resources to gain further insight into floral control in rose. We studied floral initiation in a nonrecurrent blooming rose (hybrid of Rosa wichurana) and a recurrent blooming rose (Rosa hybrida Black Baccara. In Black Baccara, floral initiation took place rapidly after bud burst, whereas in the greenhouse R. wichurana remained vegetative. During floral initiation, the apex enlarged and domed quickly and concomitantly. This is the first description of this transition between the vegetative and floral bud stages in rose. From these vegetative and pre-floral tissues, two cDNA libraries were constructed and 5,000 ESTs sequenced. By collecting our ESTs and those available in public databases, we developed a comprehensive database representing approximately 5,000 unique sequences after clustering. By screening this database for candidate genes involved in the flowering process, we identified 13 genes potentially involved in gibberellic acid signalling, photoperiod pathways, and floral development. Based on expression data, we put forward different hypotheses on the control of flowering in rose (photoperiod control and involvement of gibberellins) relative to what is already known in Arabidopsis.

After-ripening alters the gene expression pattern of oxidases involved in the ethylene and gibberellin pathways during early imbibition of Sisymbrium officinale L. seeds

After-ripening (AR) in Sisymbrium officinale seeds altered SoACS7, SoACO2, SoGA20ox2, SoGA3ox2, and SoGA2ox6 gene expression. Except for SoGA20ox2 expression, which sharply diminished, the expression of the other genes rose during development, particularly that of SoACS7. In contrast, only the SoACO2 and SoGA2ox6 transcripts increased with seed desiccation; the others decreased. AR increased the SoGA3ox2 transcript in dry seed, but dramatically decreased the SoACS7 transcript. At the onset of imbibition, AR inhibited SoACS7 and SoACO2 expression and stimulated that of SoGA20ox2, SoGA3ox2, and SoGA2ox6, demonstrating that the participation of ethylene (ET) and gibberellins (GAs) differs in after-ripened and non-after-ripened seeds. The inhibition of SoACO2 expression in the presence of GA(4+7), paclobutrazol (PB), inhibitors of ET synthesis and signalling (IESS), and notably ET+GA(4+7) indicated ET-GA cross-talk in non-after-ripened seeds. A positive effect of AR in reversing this inhibition was found. The idea of ET-GA cross-talk is also supported by the effect of ET on SoGA3ox2 expression, notably induced by the AR process. In contrast, SoGA20ox2 expression did not appear to be susceptible to AR. SoGA2ox6 expression, poorly known in seeds, suggests that AR prompted an up-regulation under all treatments studied, whereas in non-after-ripened seeds expression was down-regulated. On the other hand, the beta-mannanase (MAN) activity dramatically increased in dry after-ripened seed, being significantly boosted by ET. The absence of MAN inhibition by IESS suggests that although ET seems to be one of the factors controlling MAN, its presence did not appear to be essential. GA(4+7) only increased MAN in seeds which were after-ripened. Here, it is proposed that ET and GAs participate actively in establishing the AR process.

Releasing the brakes of plant growth: how GAs shutdown DELLA proteins

Bioactive gibberellins (GAs) are tetracyclic diterpenoid plant hormones that promote important processes of plant growth and development, such as seed germination, growth through elongation, and floral transition. Thus, mutant plants that are affected in GA biosynthesis or signalling exhibit altered seed germination and, at the adult stage, are dwarf and dark green and also show delayed flowering. The components of the GA metabolism and signalling pathways are reviewed here and recent findings regarding the regulation and possible mode of action of DELLA proteins are discussed.

Wheat cysteine proteases triticain alpha, beta and gamma exhibit mutually distinct responses to gibberellin in germinating seeds

We cloned three novel papain-type cysteine proteases (CPs), triticain alpha, beta and gamma, from 1-d-germinating wheat seeds. Triticain alpha, beta and gamma were constituted with 461, 472 and 365 amino acid residues, respectively, and had Cys-His-Asn catalytic triads as well as signal and propeptide sequences. Triticain gamma contained a putative vacuole-sorting sequence. Phylogenetic analysis showed that these CPs were divided into mutually different clusters. Triticain alpha and gamma mRNAs were expressed in seeds at an early stage of maturation and at the stage of germination 2d after imbibition, while triticain beta mRNA appeared shortly after imbibition. The expression of mRNAs for triticain alpha and gamma was suppressed by uniconazol, a gibberellin synthesis inhibitor. All the three CP mRNAs were strongly expressed in both embryo and aleurone layers. These results suggest that triticain alpha, beta and gamma play differential roles in seed maturation as well as in digestion of storage proteins during germination.

Structural basis for gibberellin recognition by its receptor GID1

Gibberellins (GAs) are phytohormones essential for many developmental processes in plants. A nuclear GA receptor, GIBBERELLIN INSENSITIVE DWARF1 (GID1), has a primary structure similar to that of the hormone-sensitive lipases (HSLs). Here we analyse the crystal structure of Oryza sativa GID1 (OsGID1) bound with GA(4) and GA(3) at 1.9 A resolution. The overall structure of both complexes shows an alpha/beta-hydrolase fold similar to that of HSLs except for an amino-terminal lid. The GA-binding pocket corresponds to the substrate-binding site of HSLs. On the basis of the OsGID1 structure, we mutagenized important residues for GA binding and examined their binding activities. Almost all of them showed very little or no activity, confirming that the residues revealed by structural analysis are important for GA binding. The replacement of Ile 133 with Leu or Val-residues corresponding to those of the lycophyte Selaginella moellendorffii GID1s-caused an increase in the binding affinity for GA(34), a 2beta-hydroxylated GA(4). These observations indicate that GID1 originated from HSL and was further modified to have higher affinity and more strict selectivity for bioactive GAs by adapting the amino acids involved in GA binding in the course of plant evolution.

Gibberellin signaling

This review covers recent advances in gibberellin (GA) signaling. GA signaling is now understood to hinge on DELLA proteins. DELLAs negatively regulate GA response by activating the promoters of several genes including Xerico, which upregulates the abscisic acid pathway which is antagonistic to GA. DELLAs also promote transcription of the GA receptor, GIBBERELLIN INSENSITIVE DWARF 1 (GID1) and indirectly regulate GA biosynthesis genes enhancing GA responsiveness and feedback control. A structural analysis of GID1 provides a model for understanding GA signaling. GA binds within a pocket of GID1, changes GID1 conformation and increases the affinity of GID1 for DELLA proteins. GA/GID1/DELLA has increased affinity for an F-Box protein and DELLAs are subsequently degraded via the proteasome. Therefore, GA induces growth through degradation of the DELLAs. The binding of DELLA proteins to three of the PHYTOCHROME INTERACTING FACTOR (PIF) proteins integrates light and GA signaling pathways. This binding prevents PIFs 3, 4, and 5 from functioning as positive transcriptional regulators of growth in the dark. Since PIFs are degraded in light, these PIFs can only function in the combined absence of light and presence of GA. New analyses suggest that GA signaling evolved at the same time or just after the plant vascular system and before plants acquired the capacity for seed reproduction. An analysis of sequences cloned from Physcomitrella suggests that GID1 and DELLAs were the first to evolve but did not initially interact. The more recently diverging spike moss Selaginella has all the genes required for GA biosynthesis and signaling, but the role of GA response in Selaginella physiology remains a mystery.

The DDF1 transcriptional activator upregulates expression of a gibberellin-deactivating gene, GA2ox7, under high-salinity stress in Arabidopsis

High-salinity stress affects plant growth and development. We have previously reported that overexpression of the salinity-responsive DWARF AND DELAYED FLOWERING 1 (DDF1) gene, encoding an AP2 transcription factor of the DREB1/CBF subfamily, causes dwarfism mainly by levels of reducing bioactive gibberellin (GA) in transgenic Arabidopsis. Here, we found that the GA 2-oxidase 7 gene (GA2ox7), which encodes a C20-GA deactivation enzyme, is strongly upregulated in DDF1-overexpressing transgenic plants. A loss-of-function mutation of GA2ox7 (ga2ox7-2) suppressed the dwarf phenotype of DDF1-overexpressing plants, indicating that their GA deficiency is due to overexpression of GA2ox7. Transient overexpression of DDF1 activated the promoter of GA2ox7 in Arabidopsis leaves. A gel shift assay showed that DDF1 binds DRE-like motifs (GCCGAC and ATCGAC) in the GA2ox7 promoter. In Arabidopsis under high-salinity stress, six GA2ox genes, including GA2ox7, were upregulated. Furthermore, the ga2ox7-2 mutant was less growth retarded than wild-type Col under high-salinity stress. These results demonstrate that, under salinity stress, Arabidopsis plants actively reduce endogenous GA levels via the induction of GA 2-oxidase, with the result that growth is repressed for stress adaptation.

A novel class of gibberellin 2-oxidases control semidwarfism, tillering, and root development in rice

Gibberellin 2-oxidases (GA2oxs) regulate plant growth by inactivating endogenous bioactive gibberellins (GAs). Two classes of GA2oxs inactivate GAs through 2beta-hydroxylation: a larger class of C(19) GA2oxs and a smaller class of C(20) GA2oxs. In this study, we show that members of the rice (Oryza sativa) GA2ox family are differentially regulated and act in concert or individually to control GA levels during flowering, tillering, and seed germination. Using mutant and transgenic analysis, C(20) GA2oxs were shown to play pleiotropic roles regulating rice growth and architecture. In particular, rice overexpressing these GA2oxs exhibited early and increased tillering and adventitious root growth. GA negatively regulated expression of two transcription factors, O. sativa homeobox 1 and TEOSINTE BRANCHED1, which control meristem initiation and axillary bud outgrowth, respectively, and that in turn inhibited tillering. One of three conserved motifs unique to the C(20) GA2oxs (motif III) was found to be important for activity of these GA2oxs. Moreover, C(20) GA2oxs were found to cause less severe GA-defective phenotypes than C(19) GA2oxs. Our studies demonstrate that improvements in plant architecture, such as semidwarfism, increased root systems and higher tiller numbers, could be induced by overexpression of wild-type or modified C(20) GA2oxs.

The gibberellic acid signaling repressor RGL2 inhibits Arabidopsis seed germination by stimulating abscisic acid synthesis and ABI5 activity

Seed germination is antagonistically controlled by the phytohormones gibberellic acid (GA) and abscisic acid (ABA). GA promotes seed germination by enhancing the proteasome-mediated destruction of RGL2 (for RGA-LIKE2), a key DELLA factor repressing germination. By contrast, ABA blocks germination by inducing ABI5 (for ABA-INSENSITIVE5), a basic domain/leucine zipper transcription factor repressing germination. Decreased GA synthesis leads to an increase in endogenous ABA levels through a stabilized RGL2, a process that may involve XERICO, a RING-H2 zinc finger factor promoting ABA synthesis. In turn, increased endogenous ABA synthesis is necessary to elevate not only ABI5 RNA and protein levels but also, critically, those of RGL2. Increased ABI5 protein is ultimately responsible for preventing seed germination when GA levels are reduced. However, overexpression of ABI5 was not sufficient to repress germination, as ABI5 activity requires phosphorylation. The endogenous ABI5 phosphorylation and inhibition of germination could be recapitulated by the addition of a SnRK2 protein kinase to the ABI5 overexpression line. In sleepy1 mutant seeds, RGL2 overaccumulates; germination of these seeds can occur under conditions that produce low ABI5 expression. These data support the notion that ABI5 acts as the final common repressor of germination in response to changes in ABA and GA levels.

Release of the repressive activity of rice DELLA protein SLR1 by gibberellin does not require SLR1 degradation in the gid2 mutant

The rice (Oryza sativa) DELLA protein SLR1 acts as a repressor of gibberellin (GA) signaling. GA perception by GID1 causes SLR1 protein degradation involving the F-box protein GID2; this triggers GA-associated responses such as shoot elongation and seed germination. In GA-insensitive and GA biosynthesis mutants, SLENDER RICE1 (SLR1) accumulates to high levels, and the severity of dwarfism is usually correlated with the level of SLR1 accumulation. An exception is the GA-insensitive F-box mutant gid2, which shows milder dwarfism than mutants such as gid1 and cps even though it accumulates higher levels of SLR1. The level of SLR1 protein in gid2 was decreased by loss of GID1 function or treatment with a GA biosynthesis inhibitor, and dwarfism was enhanced. Conversely, overproduction of GID1 or treatment with GA(3) increased the SLR1 level in gid2 and reduced dwarfism. These results indicate that derepression of SLR1 repressive activity can be accomplished by GA and GID1 alone and does not require F-box (GID2) function. Evidence for GA signaling without GID2 was also provided by the expression behavior of GA-regulated genes such as GA-20oxidase1, GID1, and SLR1 in the gid2 mutant. Based on these observations, we propose a model for the release of GA suppression that does not require DELLA protein degradation.

Interactions of two transcriptional repressors and two transcriptional activators in modulating gibberellin signaling in aleurone cells

Gibberellins (GAs) regulate many aspects of plant development, such as germination, growth, and flowering. The barley (Hordeum vulgare) Amy32b alpha-amylase promoter contains at least five cis-acting elements that govern its GA-induced expression. Our previous studies indicate that a barley WRKY gene, HvWRKY38, and its rice (Oryza sativa) ortholog, OsWRKY71, block GA-induced expression of Amy32b-GUS. In this work, we investigated the functional and physical interactions of HvWRKY38 with another repressor and two activators in barley. HvWRKY38 blocks the inductive activities of SAD (a DOF protein) and HvGAMYB (a R2R3 MYB protein) when either of these proteins is present individually. However, SAD and HvGAMYB together overcome the inhibitory effect of HvWRKY38. Yet, the combination of HvWRKY38 and BPBF (another DOF protein) almost diminishes the synergistic effect of SAD and HvGAMYB transcriptional activators. Electrophoretic mobility shift assays indicate that HvWRKY38 blocks the GA-induced expression of Amy32b by interfering with the binding of HvGAMYB to the cis-acting elements in the alpha-amylase promoter. The physical interaction of HvWRKY38 and BPBF repressors is demonstrated via bimolecular fluorescence complementation assays. These data suggest that the expression of Amy32b is modulated by protein complexes that contain either activators (e.g. HvGAMYB and SAD) or repressors (e.g. HvWRKY38 and BPBF). The relative amounts of the repressor or activator complexes binding to the Amy32b promoter regulate its expression level in barley aleurone cells.

ARABIDOPSIS THALIANA HOMEOBOX GENE1 establishes the basal boundaries of shoot organs and controls stem growth

Apical meristems play a central role in plant development. Self-renewing cells in the central region of the shoot meristem replenish the cell population in the peripheral region, where organ primordia emerge in a predictable pattern, and in the underlying rib meristem, where new stem tissue is formed. While much is known about how organ primordia are initiated and their lateral boundaries established, development at the interface between the stem and the meristem or the lateral organs is poorly understood. Here, we show that the BELL-type ARABIDOPSIS THALIANA HOMEOBOX GENE1 (ATH1) is required for proper development of the boundary between the stem and both vegetative and reproductive organs and that this role partially overlaps with that of CUP-SHAPED COTYLEDON genes. During the vegetative phase, ATH1 also functions redundantly with light-activated genes to inhibit growth of the region below the shoot meristem. Consistent with a role in inhibiting stem growth, ATH1 is downregulated at the start of inflorescence development and ectopic ATH1 expression prevents growth of the inflorescence stem by reducing cell proliferation. Thus, ATH1 modulates growth at the interface between the stem, meristem, and organ primordia and contributes to the compressed vegetative habit of Arabidopsis thaliana.

Gibberellin homeostasis and plant height control by EUI and a role for gibberellin in root gravity responses in rice

The rice Eui (ELONGATED UPPERMOST INTERNODE) gene encodes a cytochrome P450 monooxygenase that deactivates bioactive gibberellins (GAs). In this study, we investigated controlled expression of the Eui gene and its role in plant development. We found that Eui was differentially induced by exogenous GAs and that the Eui promoter had the highest activity in the vascular bundles. The eui mutant was defective in starch granule development in root caps and Eui overexpression enhanced starch granule generation and gravity responses, revealing a role for GA in root starch granule development and gravity responses. Experiments using embryoless half-seeds revealed that RAmy1A and GAmyb were highly upregulated in eui aleurone cells in the absence of exogenous GA. In addition, the GA biosynthesis genes GA3ox1 and GA20ox2 were downregulated and GA2ox1 was upregulated in eui seedlings. These results indicate that EUI is involved in GA homeostasis, not only in the internodes at the heading stage, but also in the seedling stage, roots and seeds. Disturbing GA homeostasis affected the expression of the GA signaling genes GID1 (GIBBERELLIN INSENSITIVE DWARF 1), GID2 and SLR1. Transgenic RNA interference of the Eui gene effectively increased plant height and improved heading performance. By contrast, the ectopic expression of Eui under the promoters of the rice GA biosynthesis genes GA3ox2 and GA20ox2 significantly reduced plant height. These results demonstrate that a slight increase in Eui expression could dramatically change rice morphology, indicating the practical application of the Eui gene in rice molecular breeding for a high yield potential.

Arabidopsis seed germination responses to osmotic stress involve the chromatin modifier PICKLE

pkl mutant seed germination is hypersensitive to ABA treatment due to abnormally high and persistent ABI3 and ABI5 expression. PKL, a putative chromatin modifier, is instrumental to associate ABI3 and ABI5 with silent chromatin in response to ABA. Thus, PKL prevents exaggerated germination arrest responses by shutting off ABI3 and ABI5 expression in response to mild stresses.

Evolutionarily conserved DELLA-mediated gibberellin signaling in plants

Gibberellins (GAs) play important roles in many essential plant growth and development processes. A family of nuclear growth-repressing DELLA proteins is the key component in GA signaling. GA perception is mediated by GID1, and the key event of GA signaling is the degradation of DELLA proteins via the 26S proteasome pathway. DELLA proteins integrating other plant hormones signaling and environmental cue modulating plant growth and development have been revealed. GA turning on the de-DELLA-repressing system is conserved, and independently establishes step-by-step recruitment of GA-stimulated GID1-DELLA interaction and DELLA growth-repression functions during land plant evolution. These discoveries open new prospects for the understanding of GA action and DELLA-mediated signaling in plants.

Gibberellin homeostasis in tobacco is regulated by gibberellin metabolism genes with different gibberellin sensitivity

Gibberellins are phytohormones that regulate growth and development of plants. Gibberellin homeostasis is maintained by feedback regulation of gibberellin metabolism genes. To understand this regulation, we manipulated the gibberellin pathway in tobacco and studied its effects on the morphological phenotype, gibberellin levels and the expression of endogenous gibberellin metabolism genes. The overexpression of a gibberellin 3-oxidase (biosynthesis gene) in tobacco (3ox-OE) induced slight variations in phenotype and active GA(1) levels, but we also found an increase in GA(8) levels (GA(1) inactivation product) and a conspicuous induction of gibberellin 2-oxidases (catabolism genes; NtGA2ox3 and -5), suggesting an important role for these particular genes in the control of gibberellin homeostasis. The effect of simultaneous overexpression of two biosynthesis genes, a gibberellin 3-oxidase and a gibberellin 20-oxidase (20ox/3ox-OE), on phenotype and gibberellin content suggests that gibberellin 3-oxidases are non-limiting enzymes in tobacco, even in a 20ox-OE background. Moreover, the expression analysis of gibberellin metabolism genes in transgenic plants (3ox-OE, 20ox-OE and hybrid 3ox/20ox-OE), and in response to application of different GA(1) concentrations, showed genes with different gibberellin sensitivity. Gibberellin biosynthesis genes (NtGA20ox1 and NtGA3ox1) are negatively feedback regulated mainly by high gibberellin levels. In contrast, gibberellin catabolism genes which are subject to positive feedback regulation are sensitive to high (NtGA2ox1) or to low (NtGA2ox3 and -5) gibberellin concentrations. These two last GA2ox genes seem to play a predominant role in gibberellin homeostasis under mild gibberellin variations, but not under large gibberellin changes, where the biosynthesis genes GA20ox and GA3ox may be more important.

GID1-mediated gibberellin signaling in plants

Gibberellin (GA) perception is mediated by GID1 (GA-INSENSITIVE DWARF1), a receptor that shows similarity to hormone-sensitive lipases. A key event in GA signaling is the degradation of DELLA proteins, which are negative regulators of GA response that interact with GID1 in a GA-dependent manner. This GID1-DELLA GA-perception system is conserved among vascular plants but is not found in the moss Physcomitrella patens. The identification of factors in GA signaling downstream of DELLA and the development of a new concept of DELLA function beyond its role as a repressor of GA signaling are important advances. DELLA proteins appear to have at least two other distinct roles: maintaining GA homeostasis and regulating cross-talk between GA and other plant hormones.

Barley grain maturation and germination: metabolic pathway and regulatory network commonalities and differences highlighted by new MapMan/PageMan profiling tools

Plant seeds prepare for germination already during seed maturation. We performed a detailed transcriptome analysis of barley (Hordeum vulgare) grain maturation, desiccation, and germination in two tissue fractions (starchy endosperm/aleurone and embryo/scutellum) using the Affymetrix Barley1 GeneChip. To aid data evaluation, Arabidopsis thaliana MapMan and PageMan tools were adapted to barley. The analyses allow a number of conclusions: (1) Cluster analysis revealed a smooth transition in transcription programs between late seed maturation and germination within embryo tissues, but not in the endosperm/aleurone. (2) More than 12,000 transcripts are stored in the embryo of dry barley grains, many of which are presumably activated during germination. (3) Transcriptional activation of storage reserve mobilization events occurs at an early stage of germination, well before radicle protrusion. (4) Key genes of gibberellin (GA) biosynthesis are already active during grain maturation at a time when abscisic acid peaks suggesting the formation of an endogenous store of GA in the aleurone. This GA probably acts later during germination in addition to newly synthesized GA. (5) Beside the well-known role of GA in gene activation during germination spatiotemporal expression data and cis-element searches in homologous rice promoters confirm an equally important gene-activating role of abscisic acid during this developmental period. The respective regulatory webs are linked to auxin and ethylene controlled networks. In summary, new bioinformatics PageMan and MapMan tools developed in barley have been successfully used to investigate in detail the transcriptome relationships between seed maturation and germination in an important crop plant.

High temperature-induced abscisic acid biosynthesis and its role in the inhibition of gibberellin action in Arabidopsis seeds

Suppression of seed germination at supraoptimal high temperature (thermoinhibiton) during summer is crucial for Arabidopsis (Arabidopsis thaliana) to establish vegetative and reproductive growth in appropriate seasons. Abscisic acid (ABA) and gibberellins (GAs) are well known to be involved in germination control, but it remains unknown how these hormone actions (metabolism and responsiveness) are altered at high temperature. Here, we show that ABA levels in imbibed seeds are elevated at high temperature and that this increase is correlated with up-regulation of the zeaxanthin epoxidase gene ABA1/ZEP and three 9-cis-epoxycarotenoid dioxygenase genes, NCED2, NCED5, and NCED9. Reverse-genetic studies show that NCED9 plays a major and NCED5 and NCED2 play relatively minor roles in high temperature-induced ABA synthesis and germination inhibition. We also show that bioactive GAs stay at low levels at high temperature, presumably through suppression of GA 20-oxidase genes, GA20ox1, GA20ox2, and GA20ox3, and GA 3-oxidase genes, GA3ox1 and GA3ox2. Thermoinhibition-tolerant germination of loss-of-function mutants of GA negative regulators, SPINDLY (SPY) and RGL2, suggests that repression of GA signaling is required for thermoinibition. Interestingly, ABA-deficient aba2-2 mutant seeds show significant expression of GA synthesis genes and repression of SPY expression even at high temperature. In addition, the thermoinhibition-resistant germination phenotype of aba2-1 seeds is suppressed by a GA biosynthesis inhibitor, paclobutrazol. We conclude that high temperature stimulates ABA synthesis and represses GA synthesis and signaling through the action of ABA in Arabidopsis seeds.

The Maize Viviparous8 locus, encoding a putative ALTERED MERISTEM PROGRAM1-like peptidase, regulates abscisic acid accumulation and coordinates embryo and endosperm development

We describe a mutant of Zea mays isolated from a W22 inbred transposon population, widow's peak mutant1 (wpk1), with an altered pattern of anthocyanin synthesis and aleurone cell differentiation in endosperm. In addition, a failure of the developing mutant embryo to form leaf initials is associated with decreased expression of a subset of meristem regulatory genes that includes Abphyl1 and Td1. We show that the viviparous8 (vp8) mutant has a similar pleiotropic phenotype in the W22 inbred background in contrast to the viviparous embryo phenotype exhibited in the standard genetic background, and we confirmed that wpk1 is allelic to vp8. Further genetic analysis revealed that the standard vp8 stock contains an unlinked, partially dominant suppressor of the vp8 mutation that is not present in W22. Consistent with the early-onset viviparous phenotype of vp8, expression of several embryonic regulators, including LEC1/B3 domain transcription factors, was reduced in the mutant embryo. Moreover, reduced abscisic acid (ABA) content of vp8/wpk1 embryos was correlated with altered regulation of ABA biosynthesis, as well as ABA catabolic pathways. The ABA biosynthetic gene Vp14 was down-regulated in the nonsuppressed background, whereas the ZmABA8'oxA1a ABA 8'-hydroxylase gene was strongly up-regulated in both genetic backgrounds. Molecular analysis revealed that Vp8 encodes a putative peptidase closely related to Arabidopsis thaliana ALTERED MERISTEM PROGRAM1. Because the Vp8 regulates meristem development as well as seed maturation processes, including ABA accumulation, we propose that VP8 is required for synthesis of an unidentified signal that integrates meristem and embryo formation in seeds.

Procera is a putative DELLA mutant in tomato (Solanum lycopersicum): effects on the seed and vegetative plant

The procera (pro) mutant of tomato exhibits a well-characterized constitutive gibberellic acid (GA) response phenotype. The tomato DELLA gene LeGAI in the pro mutant background contains a point mutation that results in an amino acid change in the conserved VHVID putative DNA-binding domain in LeGAI to VHEID. This same point mutation is in four different genetic backgrounds exhibiting the pro phenotype, suggesting that this mutation co-segregates with the pro phenotype. Complementation of the mutant with a constitutively expressed wild-type LeGAI gene sequence was not conclusive due to the infertility of transgenic plants. The pro mutation alters tomato branching architecture through differential suppression of axillary bud development, indicating a role for DELLA proteins in the regulation of plant structure. Isolated gib-1 pro double mutant embryo axes, which are unable to synthesize GA, germinate faster than their wild-type counterparts, and exert greater embryo growth potential. The pro mutation is therefore regulating GA responses within the tomato embryo. Transient expression of a LeGAI-GFP (green fluorescent protein) fusion protein in onion epidermis results in its location to the nucleus, and this protein is rapidly degraded by the proteasome in the presence of GA.

Functional analysis of rice HOMEOBOX4 (Oshox4) gene reveals a negative function in gibberellin responses

The homeodomain-leucine zipper (HD-Zip) putative transcription factor genes are divided into 4 families. In this work, we studied the function of a rice HD-Zip I gene, H OME O BO X4 (Oshox4). Oshox4 transcripts were detected in leaf and floral organ primordia but excluded from the shoot apical meristem and the protein was nuclear localized. Over-expression of Oshox4 in rice induced a semi-dwarf phenotype that could not be complemented by applied GA3. The over-expression plants accumulated elevated levels of bioactive GA, while the GA catabolic gene GA2ox3 was upregulated in the transgenic plants. In addition, over-expression of Oshox4 blocked GA-dependent alpha-amylase production. However, down-regulation of Oshox4 in RNAi transgenic plants induced no phenotypic alteration. Interestingly, the expression of YAB1 that is involved in the negative feedback regulation of the GA biosynthesis was upregulated in the Oshox4 over-expressing plants. One-hybrid assays showed that Oshox4 could interact with YAB1 promoter in yeast. In addition, Oshox4 expression was upregulated by GA. These data together suggest that Oshox4 may be involved in the negative regulation of GA signalling and may play a role to fine tune GA responses in rice.

Hormone interactions and regulation of PsPK2::GUS compared with DR5::GUS and PID::GUS in Arabidopsis thaliana

The putative pea PINOID homolog, PsPK2, is expressed in all growing plant parts and is positively regulated by auxin, gibberellin, and cytokinin. Here, we studied hormonal regulation of PsPK2::GUS expression compared with DR5::GUS and PID::GUS in Arabidopsis. PsPK2::GUS, DR5::GUS, and PID::GUS expression in Arabidopsis shoots is mainly localized in the stipules, hydathodes, veins, developing leaves, and cotyledons. Unlike DR5::GUS, PsPK2::GUS, and PID::GUS are weakly expressed in root tips. Both DR5::GUS and PsPK2::GUS are induced by different auxins and are more sensitive to methyl indole acetic acid, 4-chloro-indole acetic acid, and α-naphthalene acetic acid than others. GA(3) has no significant effect on GUS activity in DR5::GUS-transformed seedlings compared to the control, but induction by auxin and gibberellin in combination is synergistic. Cytokinin increases auxin transport in Arabidopsis seedlings. Auxin, gibberellin, and cytokinin all increase GUS activity in shoots of PsPK2::GUS transformed plants compared to the control. However, only auxin and gibberellin increase GUS activity in PID::GUS shoots. In conclusion, auxin, gibberellin, and cytokinin positively regulate PsPK2 expression in shoots, but not in roots. Auxin and gibberellin also upregulate AtPIN1 and LEAFY expression, which is similar to PsPIN1 and Uni in pea. With minor exceptions, the orthologous genes from both species are regulated similarly.

Potential sites of bioactive gibberellin production during reproductive growth in Arabidopsis

Gibberellin 3-oxidase (GA3ox) catalyzes the final step in the synthesis of bioactive gibberellins (GAs). We examined the expression patterns of all four GA3ox genes in Arabidopsis thaliana by promoter-beta-glucuronidase gene fusions and by quantitative RT-PCR and defined their physiological roles by characterizing single, double, and triple mutants. In developing flowers, GA3ox genes are only expressed in stamen filaments, anthers, and flower receptacles. Mutant plants that lack both GA3ox1 and GA3ox3 functions displayed stamen and petal defects, indicating that these two genes are important for GA production in the flower. Our data suggest that de novo synthesis of active GAs is necessary for stamen development in early flowers and that bioactive GAs made in the stamens and/or flower receptacles are transported to petals to promote their growth. In developing siliques, GA3ox1 is mainly expressed in the replums, funiculi, and the silique receptacles, whereas the other GA3ox genes are only expressed in developing seeds. Active GAs appear to be transported from the seed endosperm to the surrounding maternal tissues where they promote growth. The immediate upregulation of GA3ox1 and GA3ox4 after anthesis suggests that pollination and/or fertilization is a prerequisite for de novo GA biosynthesis in fruit, which in turn promotes initial elongation of the silique.

Identification, genetic characterization, GA response and molecular mapping of Sdt97: a dominant mutant gene conferring semi-dwarfism in rice (Oryza sativa L.)

Semi-dwarfism is an important agronomic trait in rice breeding programmes. sd-1, termed the ‘Green Revolution gene’, confers semi-dwarf stature, increases harvest index, improves lodging resistance, and is associated with increased responsiveness to nitrogen fertilizer. It has contributed substantially to the significant increase in rice production. In this paper, a novel semi-dwarf mutant in rice is reported. Genetic analysis revealed that only a single dominant gene locus non-allelic to sd-1, temporarily designated Sdt97, is involved in the control of semi-dwarfism of the mutant. The semi-dwarfism of the mutant could be partly restored to the tall wild-type by application of exogenous GA3, suggesting that the mutant gene Sdt97 may be involved in the gibberellin (GA) synthesis pathway and not the GA response pathway in rice. A residual heterozygous line (RHL) population derived from a recombinant inbred line (RIL) was developed. Simple sequence repeat (SSR) and bulked segregation analysis (BSA) combined with recessive class analysis (RCA) techniques were used to map Sdt97 to the long arm of chromosome 6 at the interval between two STS markers, N6 and TX5, with a genetic distance of 0·2 cM and 0·8 cM, respectively. A contig map was constructed based on the reference sequence aligned by the Sdt97 linked markers. The physical map of the Sdt97 locus was defined to a 118 kb interval, and 19 candidate genes were detected in the target region. This is the first time that a dominant semi-dwarf gene has been reported in rice. Cloning and functional analysis of gene Sdt97 will help us to learn more about molecular mechanism of rice semi-dwarfism.

Gibberellin metabolism, perception and signaling pathways in Arabidopsis

Bioactive gibberellins (GAs) are diterpene phytohormones that modulate growth and development throughout the whole life cycle of the plant. Arabidopsis genes encoding most GA biosynthesis and catabolism enzymes, as well as GA receptors (GIBBERELLIN INSENSITIVE DWARF1, GID1) and early GA signaling components have been identified. Expression studies on the GA biosynthesis genes are beginning to reveal the potential sites of GA biosynthesis during plant development. Biochemical and genetic analyses demonstrate that GA de-represses its signaling pathway by binding to GID1s, which induce degradation of GA signaling repressors (DELLAs) via an ubiquitin-proteasome pathway. To modulate plant growth and development, the GA pathway is also regulated by endogenous signals (other hormones) and environmental cues (such as light, temperature and salt stress). In many cases, these internal and external cues directly affect GA metabolism and bioactive GA levels, and indirectly alter DELLA accumulation and GA responses. Importantly, direct negative interaction between DELLA and PIF3 and PIF4 (2 phytochrome interacting transcription factors) appears to integrate the effects of light and GA on hypocotyl elongation.

Molecular biology of gibberellins signaling in higher plants

Gibberellins (GAs), a large family of tetracyclic, diterpenoid plant hormones, play an important role in regulating diverse processes throughout plant development. In recent years, significant advances have been made in the isolation of GA signaling components and GA-responsive genes. All available data have indicated that DELLA proteins are an essential negative regulator in the GA signaling pathway and GA derepresses DELLA-mediated growth suppression by inducing degradation of DELLA proteins through the ubiquitin-26S proteasome proteolytic pathway. Identification of GID1, a gene encoding an unknown protein with similarity to hormone-sensitive lipases, has revealed that GID1 acts as a functional GA receptor with a reasonable binding affinity to biologically active GAs. Furthermore, the GID1 receptor interacts with DELLA proteins in a GA-dependent manner. These results suggest that formation of a GID1-GA-DELLA protein complex targets DELLA protein into the ubiquitin-26S proteasome pathway for degradation.

Genes for control of plant stature and form

Here we summarize progress in identification of three classes of genes useful for control of plant architecture: those affecting hormone metabolism and signaling; transcription and other regulatory factors; and the cell cycle. We focus on strong modifiers of stature and form that may be useful for directed modification of plant architecture, rather than the detailed mechanisms of gene action. Gibberellin (GA) metabolic and response genes are particularly attractive targets for manipulation because many act in a dose-dependent manner; similar phenotypic effects can be readily achieved in heterologous species; and induced pleiotropic effects--such as on nitrogen assimilation, photosynthesis, and lateral root production--are usually positive with respect to crop performance. Genes encoding transcription factors represent strong candidates for manipulation of plant architecture. For example, AINTEGUMENTA, ARGOS (auxin-regulated gene controlling organ size), and growth-regulating factors (GRFs) are strong modifiers of leaf and/or flower size. Plants overexpressing these genes had increased organ size and did not display negative pleiotropic effects in glasshouse environments. TCP-domain genes such as CINCINNATA, and the associated regulatory miRNAs such as miRJAW, may provide useful means to modulate leaf curvature and other foliage properties. There are considerable opportunities for comparative and translational genomics in nonmodel plant systems.

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.

Gibberellin metabolism and its regulation

Bioactive gibberellins (GAs) are diterpene plant hormones that are biosynthesized through complex pathways and control diverse aspects of growth and development. Biochemical, genetic, and genomic approaches have led to the identification of the majority of the genes that encode GA biosynthesis and deactivation enzymes. Recent studies have highlighted the occurrence of previously unrecognized deactivation mechanisms. It is now clear that both GA biosynthesis and deactivation pathways are tightly regulated by developmental, hormonal, and environmental signals, consistent with the role of GAs as key growth regulators. In some cases, the molecular mechanisms for fine-tuning the hormone levels are beginning to be uncovered. In this review, I summarize our current understanding of the GA biosynthesis and deactivation pathways in plants and fungi, and discuss how GA concentrations in plant tissues are regulated during development and in response to environmental stimuli.

EFFECTOR OF TRANSCRIPTION2 is involved in xylem differentiation and includes a functional DNA single strand cutting domain

EFFECTORS OF TRANSCRIPTION2 (ET) are plant-specific regulatory proteins characterized by the presence of two to five C-terminal DNA- and Zn-binding repeats, and a highly conserved cysteine pattern. We describe the structural characterization of the three member Arabidopsis thaliana ET gene family and reveal some allelic sequence polymorphisms. A mutation analysis showed that AtET2 affects the expression of various KNAT genes involved in the maintenance of the undifferentiated state of cambial meristem cells. It also plays a role in the regulation of GA5 (gibberellin 3-beta-dioxygenase) and the cell-cycle-related GASA4. A correlation was established between AtET2 expression and the cellular differentiation state. AtET-GFP fusion proteins shuttle between the cytoplasm and nucleus, with the AtET2 product prevented from entering the nucleus in non-differentiating cells. Within the nucleus, AtET2 probably acts via a single strand cutting domain. A more general regulatory role for ET factors is proposed, governing cell differentiation in cambial meristems, a crucial process for the development of plant vascular tissues.

The GID1-mediated gibberellin perception mechanism is conserved in the Lycophyte Selaginella moellendorffii but not in the Bryophyte Physcomitrella patens

In rice (Oryza sativa) and Arabidopsis thaliana, gibberellin (GA) signaling is mediated by GIBBERELLIN-INSENSITIVE DWARF1 (GID1) and DELLA proteins in collaboration with a GA-specific F-box protein. To explore when plants evolved the ability to perceive GA by the GID1/DELLA pathway, we examined these GA signaling components in the lycophyte Selaginella moellendorffii and the bryophyte Physcomitrella patens. An in silico search identified several homologs of GID1, DELLA, and GID2, a GA-specific F-box protein in rice, in both species. Sm GID1a and Sm GID1b, GID1 proteins from S. moellendorffii, showed GA binding activity in vitro and interacted with DELLA proteins from S. moellendorffii in a GA-dependent manner in yeast. Introduction of constitutively expressed Sm GID1a, Sm G1D1b, and Sm GID2a transgenes rescued the dwarf phenotype of rice gid1 and gid2 mutants. Furthermore, treatment with GA(4), a major GA in S. moellendorffii, caused downregulation of Sm GID1b, Sm GA20 oxidase, and Sm GA3 oxidase and degradation of the Sm DELLA1 protein. These results demonstrate that the homologs of GID1, DELLA, and GID2 work in a similar manner in S. moellendorffii and in flowering plants. Biochemical studies revealed that Sm GID1s have different GA binding properties from GID1s in flowering plants. No evidence was found for the functional conservation of these genes in P. patens, indicating that GID1/DELLA-mediated GA signaling, if present, differs from that in vascular plants. Our results suggest that GID1/DELLA-mediated GA signaling appeared after the divergence of vascular plants from the moss lineage.