The shoot apical meristem functions to generate the lateral organs of a plant throughout the vegetative and reproductive phases. Homeobox genes play key roles in controlling such developmental programs, but their modes of action have not been well defined. Here we describe isolation and biological functions of a novel tobacco homeobox gene, designated NTH15 (Nicotiana tabacum homeobox 15), from a tobacco shoot apex cDNA library. NTH15 encodes a polypeptide of 342 amino acids, its homeodomain is very similar to the class 1 KNOTTED-type homeodomains. NTH15 mRNA is mainly localized in corpus cells in the tobacco shoot apical meristem, but not in tunica layers nor in differentiated lateral organs. The NTH15 cDNA was fused to the cauliflower mosaic virus 35S promoter and used to generate transgenic tobacco plants. Almost all transgenic tobacco plants showed abnormal leaf and/or flower morphology, and were categorized into three groups depending on severity of the leaf phenotype. In transgenic leaves, drastic decrease of GA1 and increase of cytokinin were observed, while the levels of other phytohormones were only slightly changed. Taken together, our results suggest NTH15 is involved in tobacco morphogenesis and abnormal leaf morphology in transgenic plants results from altered hormone levels.

The rice (Oryza sativa L.) homeobox gene OSH1 causes morphological alterations when ectopically expressed in transgenic rice, Arabidopsis thaliana, and tobacco (Nicotiana tabacum L.) and is therefore believed to function as a morphological regulator gene. To determine the relationship between OSH1 expression and morphological alterations, we analyzed the changes in hormone levels in transgenic tobacco plants exhibiting abnormal morphology. Levels of the plant hormones indole-3-acetic acid, abscisic acid, gibberellin (GA), and cytokinin (zeatin and trans-zeatin [Z]) were measured in leaves of OSH1-transformed and wild-type tobacco. Altered plant morphology was found to correlate with changes in hormone levels. The more severe the alteration in phenotype of transgenic tobacco, the greater were the changes in endogenous hormone levels. Overall, GA 1 and GA 4 levels decreased and abscisic acid levels increased compared with wild-type plants. Moreover, in the transformants, Z (active form of cytokinin) levels were higher and the ratio of Z to Z riboside (inactive form) also increased. When GA 3 was supplied to the shoot apex of transformants, internode extension was restored and normal leaf morphology was also partially restored. However, such GA 3 -treated plants still exhibited some morphological abnormalities compared with wild-type plants. Based on these data, we propose the hypothesis that OSH1 affects plant hormone metabolism either directly or indirectly and thereby causes changes in plant development.The molecular mechanisms underlying organ morphogenesis from undifferentiated cells represent one of the most important biological questions. Genes involved in eukaryotic development were first isolated from Drosophila (Garber et al., 1983) and later from several other animal species. The products of these genes share a unique and homologous structure, the homeobox (Gehring, 1987).In animals cellular differentiation occurs only in the early stages of development, whereas in higher plants undifferentiated cells are maintained as meristems throughout the life of the plant and successively give rise to leaves and floral organs. Recently, the possibility that homeobox genes also play a part in the development and morphogenesis of higher plants has been suggested.The maize (Zea mays L.) gene KN1 (knotted-1) was the first plant gene shown to encode a homeodomain-containing protein (Hake et al., 1989). Ectopic expression of KN1 in maize or tobacco (Nicotiana tabacum L.) causes altered morphology in the transformed plants (Smith et al., 1992; Sinha et al., 1993). We have also observed that ectopic expression of the rice (Oryza sativa L.) homeobox gene OSH1 causes altered morphology in rice, Arabidopsis (Matsuoka et al., 1993(Matsuoka et al., , 1995, and tobacco (Kano-Murakami et al., 1993). For example, OSH1-transformed tobacco plants exhibit abnormally shaped leaves, flowers, and loss of apical dominance. These observations suggest that the OSH1 gene product may regulate the expression of genes related to morphogen...

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We previously reported that overexpression of the rice homeobox gene OSH1 led to altered morphology and hormone levels in transgenic tobacco (Nicotiana tabacum L.) plants. Among the hormones whose levels were changed, GA 1 was dramatically reduced. Here we report the results of our analysis on the regulatory mechanism(s) of OSH1 on GA metabolism. GA 53 and GA 20 , precursors of GA 1 , were applied separately to transgenic tobacco plants exhibiting severely changed morphology due to overexpression of OSH1. Only treatment with the end product of GA 20-oxidase, GA 20 , resulted in a striking promotion of stem elongation in transgenic tobacco plants. The internal GA 1 and GA 20 contents in OSH1-transformed tobacco were dramatically reduced compared with those of wild-type plants, whereas the level of GA 19 , a mid-product of GA 20-oxidase, was 25% of the wild-type level. We have isolated a cDNA encoding a putative tobacco GA 20-oxidase, which is mainly expressed in vegetative stem tissue. RNA-blot analysis revealed that GA 20-oxidase gene expression was suppressed in stem tissue of OSH1-transformed tobacco plants. Based on these results, we conclude that overexpression of OSH1 causes a reduction of the level of GA 1 by suppressing GA 20-oxidase expression.The regulatory mechanisms controlling plant morphogenesis constitute one of the most important questions in plant biology. The homeobox gene knotted-1, which is involved in maize leaf development, was isolated in 1989 (Hake et al., 1989). Many plant homeobox genes have subsequently been isolated and it is believed that these genes play a role in regulating morphogenesis (Kerstetter et al., 1994). The homeobox gene products share a unique and homologous structure, the homeodomain (Gehring, 1987). Homeodomain proteins possess a helix-turn-helix motif, and recognize and bind to specific DNA sequences, resulting in altered expression of the target gene (Scott et al., 1989). Accordingly, plant homeobox genes are thought to control plant morphogenesis through the regulation of expression of genes involved in plant development.It has been reported that ectopic expression of the rice homeobox gene OSH1 causes morphological changes in rice, Arabidopsis, tobacco (Nicotiana tabacum L.), and kiwifruit (Kano-Murakami et al., 1993; Matsuoka et al., 1993; Kusaba et al., 1995). For example, OSH1-transformed tobacco plants exhibit abnormal-shaped leaves and flowers, and loss of apical dominance. These observations suggest that the OSH1 gene product may regulate the expression of genes involved in plant morphogenesis. Kano-Murakami et al. (1993) suggested that OSH1 need not be expressed continuously or throughout the entire plant to result in morphological aberrations. These results indicate that OSH1 may be a morphological regulator acting at an early stage of tissue or organ differentiation. However, the molecular mechanism(s) by which OSH1 regulates plant morphogenesis are unknown.Plant morphogenesis is thought to be regulated by various physiological factors, including gene...

Flower development of the lateral buds was accelerated in Japanese pear [Pyrus pyrifolia (Burm.) Nak.] when vertical shoots were bent at a 45° angle in late June. The indole-3-acetic acid (IAA) concentration in lateral buds on vertical (control) shoots increased in mid-July, while remaining nearly constant in bent shoots. The abscisic acid (ABA) concentration of buds in bent shoots rose between 4 July and 15 Aug., whereas control shoots exhibited an increase in concentration followed by a decline. Gibberellin4+7 (GA4+7) concentration was high on 16 June, and then declined by 4 July, with the decline being greatest in bent shoots. Gibberellin4+7 concentration was higher in the buds on vertical shoots than in those on bent shoots for much of July. The concentrations of zeatin-type cytokinins (CKs) in lateral buds were higher in bent shoots than in vertical shoots. Bending of pear shoots may weaken competition between buds and other organs through altering hormone levels in lateral buds, resulting in acceleration of flower development.

Lettuce big-vein virus (LBVV) is the type species of the genus Varicosavirus and is a two-segmented negative-sense single-stranded RNA virus. The larger LBVV genome segment (RNA1) consists of 6797 nt and encodes an L polymerase that resembles that of rhabdoviruses. Here, the nucleotide sequence of the second LBVV genome segment (RNA2) is reported. LBVV RNA2 consisted of 6081 nt and contained antisense information for five major ORFs: ORF1 (nt 210-1403 on the viral RNA), ORF2 (nt 1493-2494), ORF3 (nt 2617-3489), ORF4 (nt 3843-4337) and ORF5 (nt 4530-5636), which had coding capacities of 44, 36, 32, 19 and 41 kDa, respectively. The gene at the 39 end of the viral RNA encoded a coat protein, while the other four genes encoded proteins of unknown functions. The 39-terminal 11 nt of LBVV RNA2 were identical to those of LBVV RNA1, and the 59-terminal regions of LBVV RNA1 and RNA2 contained a long common nucleotide stretch of about 100 nt. Northern blot analysis using probes specific to the individual ORFs revealed that LBVV transcribes monocistronic RNAs. Analysis of the terminal sequences, and primer extension and RNase H digestion analysis of LBVV mRNAs, suggested that LBVV utilizes a transcription termination/initiation strategy comparable with that of rhabdoviruses.

The ECTOPICALLY PARTING CELLS 1 (EPC1) gene encodes a putative retaining glycosyltransferase of the GT64 family, and epc1-1 mutant plants have a severely dwarfed phenotype. A new mutant allele of this gene, epc1-2, has been isolated. Reduced cell adhesion that has previously been reported for the epc1-1 mutant was not observed for either the epc1-1 or epc1-2 mutants grown in our conditions, suggesting that EPC1 does not affect cell adhesion but is involved in some other process affecting plant growth and development. It is shown that the epc1-2 mutant exhibits hypersensitivity to the phytohormone abscisic acid in germination and root elongation assays, however it shows an unaltered response to gibberellin, epi-brassinosteroid, auxin, or ethylene. An EPC1:YFP fusion protein is localized to small motile structures within the cytosol that are similar in size and number to the Golgi apparatus. Analysis of cell wall pectins revealed that levels of beta-(1,4)-galactan in the epc1-2 mutant are reduced by 50%, whilst other pectic polysaccharides (homogalacturonan, arabinan, and rhamnogalacturonan II) are unchanged.

To characterize the gibberellin (GA) 20-oxidase gene in apple, the genomic and cDNA clone from "Fuji" apple (accession no. AB037114) was isolated. The deduced amino acid sequence of this cDNA showed 71% and 66% identity to those of GA 20-oxidase cloned from French bean and Arabidopsis, respectively. The transcript of this gene was detected mainly in immature seeds between 1-3 months after full bloom. These results suggested that this apple GA 20-oxidase gene might be involved in GA biosynthesis in developing apple seed.