| Research Abstract |
Exposure of normal, tall rice (Oryza sativa) seedlings to 5-azacytidine, a powerful inhibitor of DNA methylation in vivo, induced both demethylation of genomic DNA and dwarf plants. Genes that had been affected by treatment were identified by differential screening of a DNA library, and a gene encoding a small GTP-binding protein, rgp1, was subsequently. The cDNA or rgp1 was found to encode a deduced protein of 226 amino acids (24.8kDa) with highest sequence similarity to the ras-related ypt3 protein of fission yeast. The rgp1 protein, rgp1-p25, expressed in transformed E.coli, clearly showed GTP-binding activity. To determine the physiological role of rgp1-p25, the coding region of rgp1 was introduced into tobacco plants in both sense and anti-sense orientations. Transformants, with rgp1 in either orientation, showed distinct phenotypic changes; reduced apical dominance, dwarfism and abnormal flower development. The expression of tgp1, a tobacco homologue of rgp1, was markedly reduced in anti-sense transformants, indicating that tha phenotypic changes may have been mediated by interference with tgp1, mRNA. In sense transformants, the changes may have been induced by over-production of rgp1-p25. We speculated that the increased tillering may be related to abnormal phytohormone metabolism or response pathways. A preliminaly experiment clearly showed that the content of zeation, one of the active cytokinins in higher plants, differed significantly between transformants and controls. Furthermore, transformants were found to respond differentially to externally-applied cytokinins. These observations indicate that the rgp1-p25 may be involved in either the regulation of cytokinin biosynthesis or, more probably, in its intracellular transport.
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