To identify genes targeted by the tobacco KNOX homeodomain protein, Nicotiana tabacum homeobox 15 (NTH15), we have generated an inducible system using the human glucocorticoid receptor. In this system, steroid treatment strictly induced NTH15 function and immediately suppressed the expression of a gibberellin (GA) biosynthetic gene encoding GA 20-oxidase (Ntc12) and also resulted in a decrease in bioactive GA levels. The repression of Ntc12 was observed even when indirect effects were blocked by cycloheximide.
A major catabolic pathway for gibberellin (GA) is initiated by 2-hydroxylation, a reaction catalyzed by GA 2-oxidase. We have isolated and characterized a cDNA, designated Oryza sativa GA 2-oxidase 1 (OsGA2ox1) from rice (Oryza sativa L. cv Nipponbare) that encodes a GA 2-oxidase. The encoded protein, produced by heterologous expression in Escherichia coli, converted GA 1 , GA 4 , GA 9 , GA 20 , and GA 44 to the corresponding 2-hydroxylated products GA 8 , GA 34 , GA 51 , GA 29 , and GA 98 , respectively. Ectopic expression of the OsGA2ox1 cDNA in transgenic rice inhibited stem elongation and the development of reproductive organs. These transgenic plants were deficient in endogenous GA 1 . These results indicate that OsGA2ox1 encodes a GA 2-oxidase, which is functional not only in vitro but also in vivo. OsGA2ox1 was expressed in shoot apex and roots but not in leaves and stems. In situ hybridization analysis revealed that OsGA2ox1 mRNA was localized in a ring at the basal region of leaf primordia and young leaves. This ring-shaped expression around the shoot apex was drastically decreased after the phase transition from vegetative to reproductive growth. It was absent in the floral meristem, but it was still present in the lateral meristem that remained in the vegetative phase. These observations suggest that OsGA2ox1 controls the level of bioactive GAs in the shoot apical meristem; therefore, reduction in its expression may contribute to the early development of the inflorescence meristem.
The 'green revolution' was fueled by the introduction of the semi-dwarf trait into cereal crop cultivars. The semi-dwarf cultivars--which respond abnormally to the plant growth hormone gibberellin (GA)--are more resistant to wind and rain damage and thus yield more grain when fertilized. To generate dwarf rice plants using a biotechnological approach, we modified the level of GA by overproduction of a GA catabolic enzyme, GA 2-oxidase. When the gene encoding GA 2-oxidase, OsGA2ox1, was constitutively expressed by the actin promoter, transgenic rice showed severe dwarfism but failed to set grain because GA is involved in both shoot elongation and reproductive development. In contrast, OsGA2ox1 ectopic expression at the site of bioactive GA synthesis in shoots under the control of the promoter of a GA biosynthesis gene, OsGA3ox2 (D18), resulted in a semi-dwarf phenotype that is normal in flowering and grain development. The stability and inheritance of these traits shows the feasibility of genetic improvement of cereal crops by modulation of GA catabolism and bioactive GA content.
SummaryThe effects of chitosan coating on respiration, ethylene production, and storage of peach (Prunus persica Batsch. var. vulgaris Maxim cv. Hakuho), Japanese pear (Pyrus pyrifolia Nakai cv. Shinko and Housui), and kiwifruit (Actinidia deliciosa L. cv. Hayward) during storage were investigated. Acetic acid was found to be a suitable solvent of chitosan for coating fresh fruits. Chitosan coating significantly reduced the respiration rate of peach and 'Shinko' pear in storage and of kiwifruit after storage. Coating reduced the ethylene production of the peach markedly, increased the internal CO2, and decreased the internal O2 levels of 'Shinko' pear. Chitosan significantly inhibited the growth of Botrytis cinerea and fungi isolated from decaying 'Housui' pear. This resulted in a decrease of post-harvested spoilage of peach, 'Housui' pear, and kiwifruit. Chitosan-coated peach and 'Shinko' pear were markedly firmer and less mature at the end of storage , indicating that the treatment could be an alternative means of preserving fresh fruits.
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