Plant Genome III Conference Plant Genome III Conference
Retrotransposons are a class of transposable elements whose structure resemble the integrated copies of retroviruses. Recent studies have shown that retrotransposons are ubiquitous and major transposable elements in plants. For example, we have identified at least twenty families of retrotransposons in the rice genome. Phylogenetic analysis of retrotransposon sequences showed that some retrotransposons may have been horizontally transmitted between plant species. Recent findings supporting this transfer will be reported. In contrast to yeast and Drosophila retrotransposons, most of plant retrotransposons seem inactive under normal growth conditions. Some of the retrotransposons were activated under stress conditions. Retrotransposons of tobacco (Ttol, Tto2, Tto5) were activated by wounding, virus infection, and tissue culture. The activation by virus infection was shown to be mediated by salicylic acid, which is induced during virus infection and regulates a defense response. Some retrotransposons of rice were also activated by tissue culture. Those regulatory mechanisms of plant retrotransposons support the hypothesis, proposed by McClintock, that transposable elements may play an important role in the evolution by inducing mutations under traumatic conditions The presence of retrotransposons activated by tissue culture in both tobacco and rice plants suggests that similar retrotransposons are widely distributed in plant species and retrotransposons may be responsible for the tissue culture-induced mutation (somaclonal variation) observed in many plant species.
We also report our efforts to use retrotransposons activated by stress as tools for genome analysis, such as gene tagging and site- selected mutagenesis.