DDM1 (decrease in DNA methylation) genes in rice (Oryza sativa).

Regulation of cytosine methylation in the plant genome is of pivotal in determining the epigenetic states of chromosome regions. Relative tolerance of plant to deficiency in cytosine methylation provides unparalleled opportunities to study the mechanism for regulation of cytosine methylation. The Decrease in DNA Methylation 1 (DDM1) of Arabidopsis thaliana is one of the best characterized plant epigenetic regulators that are necessary for maintenance of cytosine methylation in genomic DNA. Although cytosine methylation could affect various aspects of plant growth and development including those related to agricultural importance, orthologs of DDM1 in plants other than Arabidopsis has not been studied in detail. In this study, we identified two rice genes with similarity to Arabidopsis DDM1 and designated them OsDDM1a and OsDDM1b. Both of the rice DDM1 homologs are transcribed during development and their amino acid sequences are 93 % identical to each other. Transgenic rice lines expressing the OsDDM1a cDNA in the antisense orientation exhibited genomic DNA hypomethylation. In those lines, repeated sequences were more severely affected than a single copy sequence as is the case in Arabidopsis ddm1 mutants. Transcripts derived from endogenous transposon-related loci were up-regulated in the antisense OsDDM1 lines, opening a possibility to identify and utilize potentially active transposons for rice functional genomics.

Isolation of a regulatory gene of anthocyanin biosynthesis in tuberous roots of purple-fleshed sweet potato.

Many transcriptional factors harboring the R2R3-MYB domain, basic helix-loop-helix domain, or WD40 repeats have been identified in various plant species as regulators of flavonoid biosynthesis in flowers, seeds, and fruits. However, the regulatory elements of flavonoid biosynthesis in underground organs have not yet been elucidated. We isolated the novel MYB genes IbMYB1 and IbMYB2s from purple-fleshed sweet potato (Ipomoea batatas L. Lam. cv Ayamurasaki). IbMYB1 was predominantly expressed in the purple flesh of tuberous roots but was not detected (or only scarcely) in other anthocyanin-containing tissues such as nontuberous roots, stems, leaves, or flowers. IbMYB1 was also expressed in the tuberous roots of other purple-fleshed cultivars but not in those of orange-, yellow-, or white-fleshed cultivars. Although the orange- or yellow-fleshed cultivars contained anthocyanins in the skins of their tuberous roots, we could not detect IbMYB1 transcripts in these tissues. These results suggest that IbMYB1 controls anthocyanin biosynthesis specifically in the flesh of tuberous roots. The results of transient and stable transformation experiments indicated that expression of IbMYB1 alone was sufficient for induction of all structural anthocyanin genes and anthocyanin accumulation in the flesh of tuberous roots, as well as in heterologous tissues or heterologous plant species.

Strong expression of the rice catalase gene CatB promoter in protoplasts and roots of both a monocot and dicots.

The rice (Oryza sativa L.) catalase (EC 1.11.1.6) gene CatB is expressed in roots and cultured cells. We examined the promoter activity of its 5'-flanking region in a monocot and in two dicots. Transient expression assays in rice Oc and tobacco BY-2 suspension cell protoplasts showed that CatB's 5'-flanking DNA fragments (nucleotides -1066 to +298) had about 20 and 3-4 times as much promoter activity, respectively, as the CaMV 35S promoter. Serial deletion analyses of the CatB promoter region revealed that the shortest fragment (-56 to +298) still had about 10 times as much promoter activity as the CaMV 35S promoter in rice protoplasts. In tobacco protoplasts, the activity of the fragment (-56 to +298) was about half of the CaMV 35S promoter. Transgenic rice and Arabidopsis plants carrying GUS genes driven by the 5'-truncated CatB promoters were generated and their GUS activity was examined. The region ranging from -329 to +298 showed preferential expression in the roots of rice and Arabidopsis, and in the shoot apical meristems of Arabidopsis. In situ hybridization revealed that CatB was highly expressed in branch root primordia and root apices of rice. Fusion of the GUS gene to the region (-329 to +298) conferred strong expression in these same areas, indicating that the presence of this region was sufficient to express CatB specifically in the roots. There may be new regulatory element(s) in this region, because it contained no previously known cis-regulatory elements specific for gene expression in roots.