Identification of a negative regulator of gibberellin action, HvSPY, in barley

The Arabidopsis protein SHI represses gibberellin responses in Arabidopsis and barley

The current model of gibberellin (GA) signal transduction is based on a derepressible system and a number of candidate negative regulators have been identified in Arabidopsis. We previously have reported the identification of the Arabidopsis gene SHORT INTERNODES (SHI) that causes suppression of GA responses when constitutively activated. In this paper, we show by using reporter gene analysis that the SHI gene is expressed in young organs, e.g. shoot apices and root tips. The model predicts a suppressor of GA responses to be active in these tissues to prevent premature growth or development. To study the effect of SHI on GA signaling, we used a functional assay that measures effects of signaling components on a well-defined GA response; the up-regulation of alpha-amylase in barley (Hordeum vulgare) aleurones in response to GA treatment. We found that SHI was able to specifically block the activity of a high-isoelectric point alpha-amylase promoter following GA(3) treatment, which further supports that SHI is a suppressor of GA responses. We have identified two putative loss-of-function insertion alleles of SHI and lines homozygous for either of the new alleles show no phenotypic deviations from wild type. Because SHI belongs to a gene family consisting of nine members, we suggest that SHI and the SHI-related genes are functionally redundant. We also show that a functional ERECTA allele is able to partly suppress the dwarfing effect of the shi gain-of-function mutation, suggesting that the erecta mutation harbored by the Landsberg erecta ecotype is an enhancer of the shi dwarf phenotype.

Dissection of abscisic acid signal transduction pathways in barley aleurone layers

Abscisic acid (ABA) induces genes that are highly expressed during late embryogenesis, but suppresses gibberellin (GA)-responsive genes essential for seed germination and seedling growth. Promoter elements necessary and sufficient for ABA up- and down-regulation of gene expression have been previously defined in barley aleurone layers. We have studied the effect of a protein phosphatase 2C, ABI1, an ABA-inducible protein kinase, PKABA1, and a transcription factor, VP1, on ABA action in a barley aleurone transient expression system. The observations have allowed us to dissect ABA signal transduction pathways leading to either induction or suppression of gene expression. The ABA induction of embryogenesis genes is highly inhibited in the presence of a mutated protein phosphatase 2C, encoded by the abi1-1 dominant mutant gene that is known to block ABA responses in Arabidopsis. However, the abi1-1 gene product has no effect on the ABA suppression of a GA-responsive alpha-amylase gene. On the other hand, PKABA1 suppresses the expression of alpha-amylase genes, but has little effect on ABA up-regulated genes. Therefore, it appears that ABA induction and suppression follow two separate signal transduction pathways with the former inhibited by ABI1 and the latter modulated by PKABA1. The presence of VP1 enhances the ABA induction of late embryogenesis genes, but also suppresses germination specific genes. A schematic model based on these observations is presented to explain the effect of these regulatory proteins on ABA-mediated gene expression.

The role of SPY and its TPR domain in the regulation of gibberellin action throughout the life cycle of Petunia hybrida plants

SPY acts as a negative regulator of gibberellin (GA) action in Arabidopsis, but its mode of action and regulation are still unknown. SPY over-expression in transgenic petunia plants affected various GA-regulated processes, including seed germination, shoot elongation, flower initiation, flower development and the expression of a GA-induced gene, GIP. A similar phenotype was obtained when wild-type petunia plants were treated with the GA-biosynthesis inhibitor, paclobutrazol. The N-terminus of SPY contains tetratricopeptide repeats (TPR). TPR motifs participate in protein-protein interactions, suggesting that SPY is part of a multiprotein complex. To test this hypothesis, we over-expressed the SPY's TPR region without the catalytic domain in transgenic petunia and generated a dominant-negative SPY mutant. The transgenic seeds were able to germinate on paclobutrazol, suggesting an enhanced GA signal. We cloned the petunia SPY homologue, PhSPY, and showed that its mRNA level is not affected by GA or ABA. The results of this study support the role of SPY as a negative regulator of GA action, suggest that the TPR domain is required for the interaction with other proteins to form an active complex and indicate that different plants use similar mechanisms to transduce the GA signal.

Glycan-dependent signaling: O-linked N-acetylglucosamine

The addition of O-linked N-acetylglucosamine (O-GlcNAc) to target proteins may serve as a signaling modification analogous to protein phosphorylation. Like phosphorylation, O-GlcNAc is a dynamic modification occurring in the nucleus and cytoplasm. Various analytical methods have been developed to detect O-GlcNAc and distinguish it from glycosylation in the endomembrane system. Many target molecules have been identified; these targets are typically components of supramolecular complexes such as transcription factors, nuclear pore proteins, or cytoskeletal components. The enzymes responsible for O-GlcNAc addition and removal are highly conserved molecules having molecular features consistent with a signaling role. The O-GlcNAc transferase and O-GlcNAcase are likely to act in consort with kinases and phosphatases generating various isoforms of physiological substrates. These isoforms may differ in such properties as protein-protein interactions, protein stability, and enzymatic activity. Since O-GlcNAc plays a critical role in the regulation of signaling pathways of higher plants, the glycan modification is likely to perform similar signaling functions in mammalian cells. Glucose and amino acid metabolism generates hexosamine precursors that may be key regulators of a nutrient sensing pathway involving O-GlcNAc signaling. Altered O-linked GlcNAc metabolism may also occur in human diseases including neurodegenerative disorders, diabetes mellitus and cancer.

Expression of Arabidopsis GAI in transgenic rice represses multiple gibberellin responses

Bioactive gibberellins (GAs) are essential endogenous regulators of plant growth. GA signaling is mediated via GAI, a nuclear member of the GRAS family of plant transcription factors. Previous experiments have suggested that GAI is a GA-derepressible repressor of plant growth. Here we test this hypothesis by examining the effects of the expression of Arabidopsis GAI in transgenic Basmati rice. High-level expression of GAI caused dwarfism and reduced GA responses, and the strength of this effect was correlated with the level of transgene expression. In particular, the expression of GAI abolished the GA-mediated induction of rice aleurone alpha-amylase activity, thus implicating GAI orthologs in the well-characterized cereal aleurone GA response. The GA derepressible repressor model predicts that high-level expression of GAI should confer dwarfism, and these observations are consistent with this prediction.

Ectopic expression of the tetratricopeptide repeat domain of SPINDLY causes defects in gibberellin response

The SPINDLY (SPY) protein of Arabidopsis is a negative regulator of gibberellin (GA) response. The SPY protein has 10 copies of the tetratricopeptide repeat (TPR) at the N terminus. TPR motifs function as protein-protein interaction domains. Several spy alleles are affected only in the TPR region suggesting that protein-protein interactions mediated by this domain are important for proper GA signaling. We have used a reverse genetics approach to further investigate the role of the TPR domain. The TPR domain of SPY was overexpressed in wild-type, gai, and spy plants. Expression of the TPR domain alone is not sufficient to rescue spy mutants. Expression of the TPR domain in a wild-type background produces phenotypes similar to those caused by loss-of-function spy mutants including resistance to GA biosynthesis inhibitors, short hypocotyl length, and early flowering. The dwarfing of the floral shoot internodes caused by the gai mutation was suppressed by expression of the TRP domain. Expression of the TPR domain had no effect on the abundance of endogenous SPY mRNA. The TPR domain was found to interact with SPY both in vitro and in yeast two-hybrid assays. These data indicate that the TPR domain of SPY can participate in protein-protein interactions and that these interactions are important for the proper functioning of SPY.

Altered expression of SPINDLY affects gibberellin response and plant development

Gibberellins (GAs) are plant hormones with diverse roles in plant growth and development. SPINDLY (SPY) is one of several genes identified in Arabidopsis that are involved in GA response and it is thought to encode an O-GlcNAc transferase. Genetic analysis suggests that SPY negatively regulates GA response. To test the hypothesis that SPY acts specifically as a negatively acting component of GA signal transduction, spy mutants and plants containing a 35S:SPY construct have been examined. A detailed investigation of the spy mutant phenotype suggests that SPY may play a role in plant development beyond its role in GA signaling. Consistent with this suggestion, the analysis of spy er plants suggests that the ERECTA (ER) gene, which has not been implicated as having a role in GA signaling, appears to enhance the non-GA spy mutant phenotypes. Arabidopsis plants containing a 35S:SPY construct possess reduced GA response at seed germination, but also possess phenotypes consistent with increased GA response, although not identical to spy mutants, during later vegetative and reproductive development. Based on these results, the hypothesis that SPY is specific for GA signaling is rejected. Instead, it is proposed that SPY is a negative regulator of GA response that has additional roles in plant development.

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.

Involvement of multiple cis-elements in the regulation of GA responsive promoters: Definition of a new cis-element in the Amy32b gene promoter of barley (Hordeum vulgare)

The Amy32b gene is a member of the low-pI alpha-amylase gene family of barley, whose expression is tightly regulated by hormones in the aleurone layer. Four cis-elements are known to be important for the GA induction of this gene: GARE, amylase box, pyrimidine box, and O2S. These sequences are located between -101 and -149 relative to the transcription start site. In the present work, we have created a series of Amy32b promoter-GUS reporter constructs introducing mutations in the -79 to -93 region. Using a transient expression system, we have functionally defined an additional region (-81 to -89) essential for the GA activation of the Amy32b promoter. This region is highly conserved among barley, wheat, and wild oat low-pI alpha-amylase promoters. Interestingly, in contrast with the variability in the relative distances among other cis-elements, this region maintains a nearly constant distance to GARE, which suggests that the function of these elements might be coupled. The involvement of this and other sequences in the transactivation of Amy32b by a transcription factor, GAMyb, has also been studied. Our results indicate that the only indispensable element for the GAMyb transactivation of the alpha-amylase promoter is GARE. The present work brings new evidence to the proposed model that considers the GAMyb-GARE interaction as a critical point for the GA induction of alpha-amylase genes, but also strengthens the notion that multiple sequences are required for full regulation of alpha-amylase promoters.

Gibberellin/abscisic acid antagonism in barley aleurone cells: site of action of the protein kinase PKABA1 in relation to gibberellin signaling molecules

The antagonism between gibberellins (GA) and abscisic acid (ABA) is an important factor regulating the developmental transition from embryogenesis to seed germination. In barley aleurone layers, the expression of genes encoding alpha-amylases and proteases is induced by GA but suppressed by ABA. It has been shown that an ABA-induced protein kinase, PKABA1, mediates the ABA suppression of alpha-amylase expression. Using a barley aleurone transient expression system, we have now localized the site of action of PKABA1 relative to other signal transduction components governing the expression of alpha-amylase. The expression of alpha-amylase can be transactivated by the transcription factor GAMyb, which is itself induced by GA. A truncated GAMyb containing the DNA binding domain but lacking the transactivation domain prevents the GA induction of alpha-amylase, further supporting the notion that GAMyb mediates the GA induction of alpha-amylase expression. Although ABA and PKABA1 strongly inhibit the GA induction of alpha-amylase, they have no effect on GAMyb-transactivated alpha-amylase expression. Using a GAMyb promoter--beta-glucuronidase construct, we also show that both ABA and PKABA1 repress the GA induction of GAMyb. In the slender mutant, GAMyb and alpha-amylase are highly expressed, even in the absence of GA. However, this constitutive expression can still be inhibited by ABA, PKABA1, or an inhibitor of cGMP synthesis. On the basis of these observations, we suggest that PKABA1 acts upstream from the formation of functional GAMyb but downstream from the site of action of the Slender gene product. Because PKABA1 inhibits the GA induction of the GAMyb promoter--beta-glucuronidase construct, it appears that at least part of the action of PKABA1 is to downregulate GAMyb at the transcriptional level.

Gibberellin/abscisic acid antagonism in barley aleurone cells: site of action of the protein kinase PKABA1 in relation to gibberellin signaling molecules

The antagonism between gibberellins (GA) and abscisic acid (ABA) is an important factor regulating the developmental transition from embryogenesis to seed germination. In barley aleurone layers, the expression of genes encoding alpha-amylases and proteases is induced by GA but suppressed by ABA. It has been shown that an ABA-induced protein kinase, PKABA1, mediates the ABA suppression of alpha-amylase expression. Using a barley aleurone transient expression system, we have now localized the site of action of PKABA1 relative to other signal transduction components governing the expression of alpha-amylase. The expression of alpha-amylase can be transactivated by the transcription factor GAMyb, which is itself induced by GA. A truncated GAMyb containing the DNA binding domain but lacking the transactivation domain prevents the GA induction of alpha-amylase, further supporting the notion that GAMyb mediates the GA induction of alpha-amylase expression. Although ABA and PKABA1 strongly inhibit the GA induction of alpha-amylase, they have no effect on GAMyb-transactivated alpha-amylase expression. Using a GAMyb promoter--beta-glucuronidase construct, we also show that both ABA and PKABA1 repress the GA induction of GAMyb. In the slender mutant, GAMyb and alpha-amylase are highly expressed, even in the absence of GA. However, this constitutive expression can still be inhibited by ABA, PKABA1, or an inhibitor of cGMP synthesis. On the basis of these observations, we suggest that PKABA1 acts upstream from the formation of functional GAMyb but downstream from the site of action of the Slender gene product. Because PKABA1 inhibits the GA induction of the GAMyb promoter--beta-glucuronidase construct, it appears that at least part of the action of PKABA1 is to downregulate GAMyb at the transcriptional level.

A pollen-specific novel calmodulin-binding protein with tetratricopeptide repeats

Calcium is essential for pollen germination and pollen tube growth. A large body of information has established a link between elevation of cytosolic Ca(2+) at the pollen tube tip and its growth. Since the action of Ca(2+) is primarily mediated by Ca(2+)-binding proteins such as calmodulin (CaM), identification of CaM-binding proteins in pollen should provide insights into the mechanisms by which Ca(2+) regulates pollen germination and tube growth. In this study, a CaM-binding protein from maize pollen (maize pollen calmodulin-binding protein, MPCBP) was isolated in a protein-protein interaction-based screening using (35)S-labeled CaM as a probe. MPCBP has a molecular mass of about 72 kDa and contains three tetratricopeptide repeats (TPR) suggesting that it is a member of the TPR family of proteins. MPCBP protein shares a high sequence identity with two hypothetical TPR-containing proteins from Arabidopsis. Using gel overlay assays and CaM-Sepharose binding, we show that the bacterially expressed MPCBP binds to bovine CaM and three CaM isoforms from Arabidopsis in a Ca(2+)-dependent manner. To map the CaM-binding domain several truncated versions of the MPCBP were expressed in bacteria and tested for their ability to bind CaM. Based on these studies, the CaM-binding domain was mapped to an 18-amino acid stretch between the first and second TPR regions. Gel and fluorescence shift assays performed with CaM and a CaM-binding synthetic peptide further confirmed MPCBP binding to CaM. Western, Northern, and reverse transcriptase-polymerase chain reaction analysis have shown that MPCBP expression is specific to pollen. MPCBP was detected in both soluble and microsomal proteins. Immunoblots showed the presence of MPCBP in mature and germinating pollen. Pollen-specific expression of MPCBP, its CaM-binding properties, and the presence of TPR motifs suggest a role for this protein in Ca(2+)-regulated events during pollen germination and growth.

Gibberellin signal transduction

Recent studies using biochemical and genetic approaches have identified a number of components, including several negative regulators, of the gibberellin (GA) signal transduction pathway in higher plants. The basal state of GA signaling is likely to be repressive, and the GA signal seems to activate the pathway by de-repression to allow GA-stimulated growth and development.

New members of a cold-responsive group-3 Lea/Rab-related Cor gene family from common wheat (Triticum aestivum L.)

A Cor (cold-responsive) cDNA that belongs to the group-3 Lea (late embryogenesis abundant)/Rab (responsive to abscisic acid, ABA) family was isolated from a winter-hardy cultivar of common wheat (Triticum aestivum L.). Screening of a cold-acclimated cDNA library was performed using an ABA- and other stress-responsive barley cDNA clone, Hva1, as a probe. A wheat cDNA clone (designated as Wrab19) putatively encoded a basic (pI = 10.3) and hydrophobic protein with 179 amino acids. The deduced protein showed characteristics of the group-3 LEA/RAB protein family. In contrast to the single copy barley Hva1, Wrab19 belonged to a multigene family in the hexaploid wheat genome and six loci were assigned to the homoeologous group 1 chromosomes. Using Wrab19 as a probe, four homologous cDNAs (designated as Wrab17) were isolated that encoded acidic (pI = 4.6-4.7) and hydrophobic proteins, all with 166 amino acids. The deduced proteins showed high homology (a mean of 84% identity) with a barley gibberellic acid (GA3)-inducible protein, ES2A, and several other group-3 LEA/RAB proteins. Wrab17 was considered to be a three-copy gene and each copy was assigned to chromosome 5A, 4B or 4D of hexaploid wheat. Transcripts of both Wrab19 and Wrab17 accumulated within 1 day of cold acclimation at 4 degrees C. They were responsive to ABA and/or GA3, but showed some cultivar differences in their response to these plant hormones. We conclude that the two genes are new members of the group-3 Lea/Rab-related Cor gene family in wheat.

SUGAR-INDUCED SIGNAL TRANSDUCTION IN PLANTS

Sugars have important signaling functions throughout all stages of the plant's life cycle. This review presents our current understanding of the different mechanisms of sugar sensing and sugar-induced signal transduction, including the experimental approaches used. In plants separate sensing systems are present for hexose and sucrose. Hexokinase-dependent and -independent hexose sensing systems can further be distinguished. There has been progress in understanding the signal transduction cascade by analyzing the function of the SNF1 kinase complex and the regulatory PRL1 protein. The role of sugar signaling in seed development and in seed germination is discussed, especially with respect to the various mechanisms by which sugar signaling controls gene expression. Finally, recent literature on interacting signal transduction cascades is discussed, with particular emphasis on the ethylene and ABA signal transduction pathways.

Fusion genetic analysis of gibberellin signaling mutants

A fusion genetic strategy was used to identify gibberellin (GA) signaling mutants in transgenic Arabidopsis expressing the beta-glucuronidase (GUS) and firefly luciferase (LUC) reporter genes under control of the GA-responsive GASA1 promoter. Initial analyses determined the spatial and temporal patterns of reporter expression, and showed that reporter induction by GA was antagonized by ABA. gamma-Irradiated M2 progeny with altered reporter activities were identified by LUC bioimaging followed by GUS assays and northern hybridization of the endogenous GASA1 mRNA. Genetic analysis showed that three mutants, which overexpressed both reporters and endogenous GASA1, were caused by recessive (goe1 and goe2, for GASA over-expressed) and semi-dominant (goe3) mutations at different loci. These mutants are altered in their sensitivity to GA and the GA biosynthetic inhibitor paclobutrazol, and in the expression of several GA signaling related genes.

Gibberellins and seed development in maize. II. Gibberellin synthesis inhibition enhances abscisic acid signaling in cultured embryos

Abscisic acid (ABA) is required for seed maturation in maize (Zea mays L.) and other plants. Gibberellins (GAs) are also present in developing maize embryos, and mutual antagonism of GAs and ABA appears to govern the choice between precocious germination or quiescence and maturation. Exogenous ABA can also induce quiescence and maturation in immature maize embryos in culture. To examine the role of GAs versus ABA in regulating maize embryo maturation, the effects of modulating GA levels were compared with those of ABA in embryos cultured at successive stages of development. The effects of GA synthesis inhibition or exogenous GA application differed markedly in embryos at different stages of development, indicating changes in both endogenous GA levels and in the capacity for GA synthesis as embryogenesis and maturation progress. In immature embryos, the inhibition of GA synthesis mimicked the effects of exogenous ABA, as shown by the suppression of germination, the acquisition of anthocyanin pigments, and the accumulation of a variety of maturation-phase mRNAs. We suggest that GA antagonizes ABA signaling in developing maize embryos, and that the changing hormone balance provides temporal control over the maturation phase.

Gibberellin and abscisic acid signalling in aleurone

The plant hormones gibberellin and abscisic acid regulate gene expression, secretion and cell death in aleurone. The emerging picture is of gibberellin perception at the plasma membrane whereas abscisic acid acts at both the plasma membrane and in the cytoplasm - although gibberellin and abscisic acid receptors have yet to be identified. A range of downstream-signalling components and events has been implicated in gibberellin and abscisic acid signalling in aleurone. These include the Galpha subunit of a heterotrimeric G protein, a transient elevation in cGMP, Ca2+-dependent and Ca2+-independent events in the cytoplasm, reversible protein phosphory-lation, and several promoter cis-elements and transcription factors, including GAMYB. In parallel, molecular genetic studies on mutants of Arabidopsis that show defects in responses to these hormones have identified components of gibberellin and abscisic acid signalling. These two approaches are yielding results that raise the possibility that specific gibberellin and abscisic acid signalling components perform similar functions in aleurone and other tissues.

Flow cytometry and surface plasmon resonance analyses demonstrate that the monoclonal antibody JIM19 interacts with a rice cell surface component involved in abscisic acid signalling in protoplasts

Abscisic acid (ABA) is a plant hormone involved in many developmental and physiological processes, but as yet, no ABA receptor has been identified. Flow cytometry of rice protoplasts and immunoblotting of purified plasma membranes (PMs) have been used to demonstrate that the monoclonal antibody JIM19 recognizes carbohydrate epitopes of cell surface glycoproteins. Using surface plasmon resonance technology specific binding of PMs to JIM19 was observed. Such interaction was antagonized significantly by ABA, but not by the biologically inactive ABA catabolite phaseic acid. These in vitro interactions were correlated with the biological activities of JIM19, ABA and phaseic acid on activation of the ABA-inducible Em promoter using two different transient reporter gene assays, beta-glucuronidase/luciferase and quantitative flow cytometry of Aequoria green fluorescent protein. Pre-treatment with JIM19 resulted in significant inhibition of ABA-inducible gene expression. Taken together, these data suggest that JIM19 interacts with a functional PM complex involved in ABA signalling.

Hormonal regulation of a cysteine proteinase gene, EPB-1, in barley aleurone layers: cis- and trans-acting elements involved in the co-ordinated gene expression regulated by gibberellins and abscisic acid

The synthesis of EPB, a cysteine proteinase responsible for the degradation of seed endosperm storage proteins in barley (Hordeum vulgare), is induced by gibberellins (GA) and repressed by abscisic acid (ABA). The EPB gene family consists of two very similar members, EPB-1 and EPB-2, with the former being more highly induced by GA. We have functionally characterized the cis-acting elements in the EPB-1 promoter and determined that a gibberellin response element (GARE), a pyrimidine box and an upstream element are necessary for GA induction. By comparison with the promoters of alpha-amylase genes, which are also induced by GA, we suggest that GARE is coupled with the upstream element and the pyrimidine box to form a GA response complex. In addition, we have shown that the 3'-untranslated/untranscribed region of the EPB-1 gene is required for a low background expression in the absence of GA. Constitutive expression of a transcription factor, GAMyb, in the absence of GA leads to the transactivation of EPB-1 expression in a dosage dependent manner with the highest level comparable to that in fully GA-induced tissue. Co-expression of a truncated version of GAMyb containing only the DNA binding domain blocks the GA-induction of EPB-1, further supporting the role of GAMyb in the regulation of gene expression. Although ABA is very effective in blocking the GA induction of EPB-1, it has no effect on the GAMyb-mediated expression of EPB-1. We suggest that ABA acts upstream of the formation of functional GAMyb which co-ordinates the hormonal regulation of a diverse group of genes in cereal aleurone layers, including those encoding EPB and alpha-amylases.

The Arabidopsis dwarf mutant shi exhibits reduced gibberellin responses conferred by overexpression of a new putative zinc finger protein

shi (for short internodes), a semidominant dwarfing mutation of Arabidopsis caused by a transposon insertion, confers a phenotype typical of mutants defective in the biosynthesis of gibberellin (GA). However, the application of GA does not correct the dwarf phenotype of shi plants, suggesting that shi is defective in the perception of or in the response to GA. In agreement with this observation, the level of active GAs was elevated in shi plants, which is the result expected when feedback control of GA biosynthesis is reduced. Cloning of the SHI gene revealed that in shi, the transposon is inserted into the untranslated leader so that a cauliflower mosaic virus 35S promoter in the transposon reads out toward the SHI open reading frame. This result, together with mRNA analysis, suggests that the phenotype of the shi mutant is a result of overexpression of the SHI open reading frame. The predicted amino acid sequence of SHI has acidic and glutamine-rich stretches and shows sequence similarity over a putative zinc finger region to three presumptive Arabidopsis proteins. This suggests that SHI may act as a negative regulator of GA responses through transcriptional control.

Gibberellin dose-response curves and the characterization of dwarf mutants of barley

Dose-response curves relating gibberellin (GA) concentration to the maximal leaf-elongation rate (LERmax) defined three classes of recessive dwarf mutants in the barley (Hordeum vulgare L.) 'Himalaya. ' The first class responded to low (10(-8)-10(-6) M) [GA3] (as did the wild type). These grd (GA-responsive dwarf) mutants are likely to be GA-biosynthesis mutants. The second class of mutant, gse (GA sensitivity), differed principally in GA sensitivity, requiring approximately 100-fold higher [GA3] for both leaf elongation and alpha-amylase production by aleurone. This novel class may have impaired recognition between the components that are involved in GA signaling. The third class of mutant showed no effect of GA3 on the LERmax. When further dwarfed by treatment with a GA-biosynthesis inhibitor, mutants in this class did respond to GA3, although the LERmax never exceeded that of the untreated dwarf. These mutants, called elo (elongation), appeared to be defective in the specific processes that are required for elongation rather than in GA signaling. When sln1 (slender1) was introduced into these different genetic backgrounds, sln was epistatic to grd and gse but hypostatic to elo. Because the rapid leaf elongation typical of sln was observed in the grd and gse backgrounds, we inferred that rapid leaf elongation is the default state and suggest that GA action is mediated through the activity of the product of the Sln gene.

GENETIC ANALYSIS OF HORMONE SIGNALING

Phytohormones influence many diverse developmental processes ranging from seed germination to root, shoot, and flower formation. Recently, mutational analysis using the model plant Arabidopsis thaliana has been instrumental in determining the individual components of specific hormone signal transduction pathways. Moreover, epistasis and suppressor studies are beginning to explain how these genes and their products relate to one another. While no hormone transduction pathway is completely understood, the genes identified to date suggest that simple molecular rules can be established to explain how plant hormone signals are transduced. This review describes some of the shared characteristics of plant hormone signal transduction pathways and the properties for informational transfer common to many of the genes that specify the transduction of the signal.

Trichome cell growth in Arabidopsis thaliana can be derepressed by mutations in at least five genes

Leaf trichomes in Arabidopsis are unicellular epidermal hairs with a branched morphology. They undergo successive endoreduplication rounds early during cell morphogenesis. Mutations affecting trichome nuclear DNA content, such as triptychon or glabra3, alter trichome branching. We isolated new mutants with supernumerary trichome branches, which fall into three unlinked complementation groups: KAKTUS and the novel loci, POLYCHOME and RASTAFARI. They map to chromosomes IV, II, and V, respectively. The trichomes of these mutants presented an increased DNA content, although to a variable extent. The spindly-5 mutant, which displays a constitutive gibberellin response, also produces overbranched trichomes containing more nuclear DNA. We analyzed genetic interactions using double mutants and propose that two independent pathways, defined by SPINDLY and TRIPTYCHON, act to limit trichome growth. KAKTUS and POLYCHOME might have redundant actions mediating gibberellin control via SPINDLY. The overall leaf polysomaty was not notably affected by these mutations, suggesting that they affect the control of DNA synthesis in a tissue- or cell type-specific manner. Wild-type tetraploids also produce overbranched trichomes; they displayed a shifted polysomaty in trichomes and in the whole leaf, suggesting a developmental program controlling DNA increases via the counting of endoreduplication rounds.

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.

An abscisic acid-induced protein kinase, PKABA1, mediates abscisic acid-suppressed gene expression in barley aleurone layers

The phytohormone abscisic acid (ABA) induces genes-encoding proteins involved in desiccation tolerance and dormancy in seeds, but ABA also suppresses gibberellin (GA)-responsive genes encoding hydrolytic enzymes essential for postgermination growth. A unique serine/threonine protein kinase, PKABA1 mRNA, up-regulated by ABA in seeds, has been identified. In this report, the effect of PKABA1 on the signal transduction pathway mediating ABA induction and suppression of genes has been determined in aleurone layers of barley seeds. Two groups of gene constructs were introduced to barley aleurone layers by using particle bombardment: the reporter constructs containing the coding sequence of beta-glucuronidase gene linked to hormone-responsive promoters and the effector constructs containing the coding region of protein kinases linked to a constitutive promoter. Constitutive expression of PKABA1 drastically suppressed expression of low- and high-pI alpha-amylase and protease genes induced by GA. However, the presence of PKABA1 had only a small effect on the ABA induction of a gene encoding a late embryogenesis abundant protein, HVA1. Our results indicate that PKABA1 acts as a key intermediate in the signal transduction pathway leading to the suppression of GA-inducible gene expression in cereal aleurone layers.

ABA signal transduction

Recent advances in the study of abscisic acid signal transduction include the identification of cyclic ADP-ribose as a central mediator of abscisic acid responses. The characterisation of type 2C protein phosphatases, ABI1 and ABI2, implicates negative control and redundant action on the signal pathway of this hormone. In addition, abscisic acid-mediated inhibition of gibberellin-stimulated responses seems to depend on the activation of a phospholipase D during induction of alpha-amylase in barley aleurone cells as well as on a putative acetyltransferase involved in elongation growth.