RETROTRANSPOSONS OF RICE INVOLVED IN MUTATIONS INDUCED BY TISSUE-CULTURE

Five retrotransposon families of rice (Tos1-Tos5) have been reported p reviously. Here we report 15 new retrotransposon families of rice (Tos 6-Tos20). In contrast to yeast and Drosophila retrotransposons, all of the rice retrotransposons examined appear inactive (or almost inactiv e) under normal growth conditions. Three of the rice retrotransposons (Tos10, Tos17, and Tos19) are activated under tissue culture condition s. The most active one, Tos17, was studied in detail. The copy number of Tos17 increased with prolonged culture period. In all of the plants regenerated from tissue cultures, including transgenic plants, 5 to 3 0 transposed Tos17 copies were detected. The transcript of Tos17 was o nly detected under tissue culture conditions, indicating that the tran sposition of Tos17 is mainly regulated at the transcriptional level. T o examine the target-site specificity of Tos17 transposition, sequence s flanking transposed Tos17 copies were analyzed. At least four out of eight target sites examined are coding regions. Other target sites ma y also be in genes because two out of four were transcribed. The regen erated plants with Tos17-insertions in the phytochrome A gene and the S-receptor kinase-related gene were identified. These results indicate that activation of Tos17 is an important cause of tissue culture-indu ced mutations. Tissue culture-induced activation of Tos17 may be a use ful tool for insertional mutagenesis and functional analysis of genes.