The Arabidopsis RGA gene encodes a transcriptional regulator repressing the gibberellin signal transduction pathway

Genetic analysis of growth-regulator-induced parthenocarpy in Arabidopsis

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

Rice gibberellin-insensitive dwarf mutant gene Dwarf 1 encodes the alpha-subunit of GTP-binding protein

A rice Dwarf 1 gene was identified by using a map-based cloning strategy. Its recessive mutant allele confers a dwarf phenotype. Linkage analysis revealed that a cDNA encoding the alpha-subunit of GTP-binding protein cosegregated with d1 in 3,185 d1 segregants. Southern hybridization analysis with this cDNA as a probe showed different band patterns in several d1 mutant lines. In at least four independent d1 mutants, no gene transcript was observed by Northern hybridization analysis. Sequencing analysis revealed that an 833-bp deletion had occurred in one of the mutant alleles, which resulted in an inability to express GTP-binding protein. A transgenic d1 mutant with GTP-binding protein gene restored the normal phenotype. We conclude that the rice Dwarf 1 gene encodes GTP-binding protein and that the protein plays an important role in plant growth and development. Because the d1 mutant is classified as gibberellin-insensitive, we suggest that the GTP-binding protein might be associated with gibberellin signal transduction.

'Green revolution' genes encode mutant gibberellin response modulators

World wheat grain yields increased substantially in the 1960s and 1970s because farmers rapidly adopted the new varieties and cultivation methods of the so-called 'green revolution'. The new varieties are shorter, increase grain yield at the expense of straw biomass, and are more resistant to damage by wind and rain. These wheats are short because they respond abnormally to the plant growth hormone gibberellin. This reduced response to gibberellin is conferred by mutant dwarfing alleles at one of two Reduced height-1 (Rht-B1 and Rht-D1) loci. Here we show that Rht-B1/Rht-D1 and maize dwarf-8 (d8) are orthologues of the Arabidopsis Gibberellin Insensitive (GAI) gene. These genes encode proteins that resemble nuclear transcription factors and contain an SH2-like domain, indicating that phosphotyrosine may participate in gibberellin signalling. Six different orthologous dwarfing mutant alleles encode proteins that are altered in a conserved amino-terminal gibberellin signalling domain. Transgenic rice plants containing a mutant GAI allele give reduced responses to gibberellin and are dwarfed, indicating that mutant GAI orthologues could be used to increase yield in a wide range of crop species.

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.

Gibberellin-responsive genes: high level of transcript accumulation in leaf sheath meristematic tissue from Zea mays L

In order to identify genes that are related to the gibberellin (GA) response in maize (Zea mays L.), mRNA species from wild-type and single-gene dwarf mutants, d5 and D8, were compared by fluorescent differential display. The d5 mutant is unable to produce biologically active GA, but recovers its tall stature on exogenous application of GA. D8 is insensitive to GA, despite the accumulation of a high level of endogenous GA, suggesting it to be a receptor mutant or a mutant in signal transduction pathway(s). After screening 7000 cDNA populations, one clone was isolated, for which transcripts were rare in d5 shoots but accumulated within 1 h after GA3 application. This clone, designated as ZmGR1a, encodes a polypeptide with a relative molecular mass of ca. 13 kDa, which shows significant homology to proline-rich proteins from several plant species. A similar experiment with D8 identified a clone, ZmGR2a, with low transcript levels, but accumulation within 6 h after GA3 treatment of d5 shoots. ZmGR2a encodes a polypeptide with a relative molecular mass of ca. 19 kDa, which shows no significant homology with any known protein. Southern blot analysis indicated that ZmGR1a and ZmGR2a form a small multigene family within the maize genome. In situ hybridization with wild-type seedlings showed transcripts on both to be abundant in leaf sheath meristematic tissue, in which GA enhances cell elongation and cell division.

EAF1 regulates vegetative-phase change and flowering time in Arabidopsis

We have identified a new locus that regulates vegetative phase change and flowering time in Arabidopsis. An early-flowering mutant, eaf1 (early flowering 1) was isolated and characterized. eaf1 plants flowered earlier than the wild type under either short-day or long-day conditions, and showed a reduction in the juvenile and adult vegetative phases. When grown under short-day conditions, eaf1 plants were slightly pale green and had elongated petioles, phenotypes that are observed in mutants altered in either phytochrome or the gibberellin (GA) response. eaf1 seed showed increased resistance to the GA biosynthesis inhibitor paclobutrazol, suggesting that GA metabolism and/or response had been altered. Comparison of eaf1 to other early-flowering mutants revealed that eaf1 shifts to the adult phase early and flowers early, similarly to the phyB (phytochrome B) and spy (spindly) mutants. eaf1 maps to chromosome 2, but defines a locus distinct from phyB, clf (curly leaf), and elf3 (early-flowering 3). These results demonstrate that eaf1 defines a new locus involved in an autonomous pathway and may affect GA regulation of flowering.

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.

Expression of a gibberellin-induced leucine-rich repeat receptor-like protein kinase in deepwater rice and its interaction with kinase-associated protein phosphatase

We identified in deepwater rice (Oryza sativa L.) a gene encoding a leucine-rich repeat receptor-like transmembrane protein kinase, OsTMK (O. sativa transmembrane kinase). The transcript levels of OsTMK increased in the rice internode in response to gibberellin. Expression of OsTMK was especially high in regions undergoing cell division and elongation. The kinase domain of OsTMK was enzymatically active, autophosphorylating on serine and threonine residues. A cDNA encoding a rice ortholog of a kinase-associated type 2C protein phosphatase (OsKAPP) was cloned. KAPPs are putative downstream components in kinase-mediated signal transduction pathways. The kinase interaction domain of OsKAPP was phosphorylated in vitro by the kinase domain of OsTMK. RNA gel-blot analysis indicated that the expression of OsTMK and OsKAPP was similar in different tissues of the rice plant. In protein-binding assays, OsKAPP interacted with a receptor-like protein kinase, RLK5 of Arabidopsis, but not with the protein kinase domains of the rice and maize receptor-like protein kinases Xa21 and ZmPK1, respectively.

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.

The GRAS gene family in Arabidopsis: sequence characterization and basic expression analysis of the SCARECROW-LIKE genes

Mutations at the SCARECROW (SCR) locus in Arabidopsis thaliana result in defective radial patterning in the root and shoot. The SCR gene product contains sequences which suggest that it is a transcription factor. A number of Arabidopsis Expressed Sequence Tags (ESTs) have been identified that encode gene products bearing remarkable similarity to SCR throughout their carboxyl-termini, indicating that SCR is the prototype of a novel gene family. These ESTs have been designated SCARECROW-LIKE (SCL). The gene products of the GIBBERELLIN-INSENSITIVE (GAI) and the REPRESSOR of ga1-3 (RGA) loci show high structural and sequence similarity to SCR and the SCLs. Sequence analysis of the products of the GRAS (GAI, RGA, SCR) gene family indicates that they share a variable amino-terminus and a highly conserved carboxyl-terminus that contains five recognizable motifs. The SCLs have distinct patterns of expression, but all of those analyzed show expression in the root. One of them, SCL3, has a tissue-specific pattern of expression in the root similar to SCR. The importance of the GRAS gene family in plant biology has been established by the functional analyses of SCR, GAI and RGA.

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.

Genetic control of branching in Arabidopsis and tomato

The patterns of axillary bud formation and the growth characteristics of side-shoots determine to a large extent the form of plants. Characterization of mutants in the monopodial plant Arabidopsis thaliana and in the sympodial tomato, as well as cloning of some of the respective genes, contributes to a better understanding of side-shoot development. Genes have been identified that influence the initiation of axillary meristems and the pattern of their subsequent development.

The Lateral suppressor (Ls) gene of tomato encodes a new member of the VHIID protein family

The ability of the shoot apical meristem to multiply and distribute its meristematic potential through the formation of axillary meristems is essential for the diversity of forms and growth habits of higher plants. In the lateral suppressor mutant of tomato the initiation of axillary meristems is prevented, thus offering the unique opportunity to study the molecular mechanisms underlying this important function of the shoot apical meristem. We report here the isolation of the Lateral suppressor gene by positional cloning and show that the mutant phenotype is caused by a complete loss of function of a new member of the VHIID family of plant regulatory proteins.

Gibberellin: inhibitor of an inhibitor of...?

Gibberellin is an endogenous plant growth regulator. Here, we describe our present understanding of how gibberellin regulates plant growth, using recent results gained from studies of gibberellin-signalling mutants of Arabidopsis. These results show that a signalling pathway represses plant growth and that gibberellin releases this repression. In consequence, the well-known growth-promoting properties of gibberellin are due to its activity as an "inhibitor of an inhibitor" [Brian Pw. Sym Soc. Exp Bio 1957; 11:166-182 (Ref. 1)] of plant growth.

Phytochrome regulation and differential expression of gibberellin 3beta-hydroxylase genes in germinating Arabidopsis seeds

Despite extensive studies on the roles of phytochrome in photostimulated seed germination, the mechanisms downstream of the photoreceptor that promote germination are largely unknown. Previous studies have indicated that light-induced germination of Arabidopsis seeds is mediated by the hormone gibberellin (GA). Using RNA gel blot analyses, we studied the regulation of two Arabidopsis genes, GA4 and GA4H (for GA4 homolog), both of which encode GA 3beta-hydroxylases that catalyze the final biosynthetic step to produce bioactive GAs. The newly isolated GA4H gene was expressed predominantly during seed germination. We show that expression of both GA4 and GA4H genes in imbibed seeds was induced within 1 hr after a brief red (R) light treatment. In the phytochrome B-deficient phyB-1 mutant, GA4H expression was not induced by R light, but GA4 expression still was, indicating that R light-induced GA4 and GA4H expression is mediated by different phytochromes. In contrast to the GA4 gene, the GA4H gene was not regulated by the feedback inhibition mechanism in germinating seeds. Our data demonstrate that expression of GA 3beta-hydroxylase genes is elevated by R light, which may result in an increase in biosynthesis of active GAs to promote seed germination. Furthermore, our results suggest that each GA 3beta-hydroxylase gene plays a unique physiological role during light-induced seed germination.

Gibberellin signaling

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

HRT, a novel zinc finger, transcriptional repressor from barley

A barley gene encoding a novel DNA-binding protein (HRT) was identified by southwestern screening with baits containing a gibberellin phytohormone response element from an alpha-amylase promoter. The HRT gene contains two introns, the larger of which (5722 base pairs (bp)) contains a 3094-bp LINE-like element with homology to maize Colonist1. In vitro mutagenesis and zinc- and DNA-binding assays demonstrate that HRT contains three unusual zinc fingers with a CX8-9CX10CX2H consensus sequence. HRT is targeted to nuclei, and homologues are expressed in other plants. In vivo, functional tests in plant cells indicate that full-length HRT can repress expression from certain promoters including the Amy1/6-4 and Amy2/32 alpha-amylase promoters. In contrast, truncated forms of HRT containing DNA-binding domains can activate, or derepress, transcription from these promoters. Northern hybridizations indicate that HRT mRNA accumulates to low levels in various tissues. Roles for HRT in mediating developmental and phytohormone-responsive gene expression are discussed.

Nitrate transport: a key step in nitrate assimilation

The nitrate assimilation pathway has been the matter of intensive research during the past decade. Many genes involved in low and high affinity nitrate uptake have been identified in fungi, algae and, more recently, in plants. The plant genes so far isolated are transcriptionally regulated; their inducibility by nitrate seems to be a common feature, shared by their homologs in fungi and algae. A number of questions remain to be elucidated regarding the physiological roles of these transporters and the regulation of their expression.

Gibberellins promote trichome formation by Up-regulating GLABROUS1 in arabidopsis

Trichome development is dependent on gibberellin (GA) signaling in Arabidopsis thaliana. Using the GA-deficient mutant ga1-3, the GA-response mutant spy-5, and uniconazol (a GA-biosynthesis inhibitor), we show that the GA level response correlates positively with both trichome number and trichome branch number. Two genes, GL1 and TTG, are required for trichome initiation. In ga1-3, coexpression of GL1 and R, the maize TTG functional homolog, under control of the constitutive 35S promoter, restored trichome development, whereas overexpression of neither GL1 nor R alone was sufficient to significantly suppress the glabrous phenotype. We next focused on GL1 regulation by GAs. In the double mutant the gl1-1 glabrous phenotype is epistatic to the spy-5 phenotype, suggesting that GL1 acts downstream of the GA signal transduction pathway. The activity of a beta-glucuronidase reporter gene driven by the GL1 promoter was decreased in the wild type grown on uniconazol and showed a clear GA-dependent activation in ga1-3. Finally, quantification of GL1 transcript levels by reverse transcriptase-polymerase chain reaction demonstrated that relative to wild type, ga1-3 plants contained less transcript. These data support the hypothesis that GAs induce trichome development through up-regulation of GL1 and possibly TTG genes.

Dissecting the gibberellin response pathway

The recent cloning of three Arabidopsis genes that regulate the response to gibberellin - one of the five 'classical' plant hormones - provides the first glimpse of possible molecular mechanisms operating in gibberellin signal transduction in plants.