A LINE-type retrotransposon active in meristem stem cells causes heritable transpositions in the sweet potato genome

We isolated a LINE-type retrotransposon, LIb, which showed high transposition activity in sweet potato callus. A copy transposed in the callus was 6303 bp in length and showed key features of a LINE element. Apparently full-length copies sharing the 5' UTR sequence with the 6303-bp copy increased dramatically in the callus as several original copies in the sweet potato genome. These apparently full-length copies had almost identical sequences to other transposed copies, many of which were truncated at the 5' end upon transposition. These results indicate that active LIb is confined to a single LINE family, and that members containing a long functional 5' UTR are present in limited numbers in the sweet potato genome. This is despite their copy numbers being estimated at over 100. The transcription of LIb was not completely suppressed, even in wild-type plants. Spontaneous transpositions were found among local variant lines of the cultivar Koukei14, from which the callus with high LIb activity was derived. Meristem culture of this cultivar appeared to facilitate transpositions of LIb in a mericlone plant. This is the first experimental demonstration of retrotransposition in a plant species without the imposition of cell differentiation. LIb transpositions appear to occur in single founder cells in the meristem because the LIb insertion was found throughout mericlone plant tissues. Transpositional activities in meristem cells might be essential characteristics of plant retrotransposons that cause heritable changes in host plant genomes and genetic systems.