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...
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...
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