Mobilization Of Transposons MPing, Ping And Pong And Heritable Alterations In DNA Methylation Pattern Induced By Laser Irradiation In Rice

Mobile elements or transposons are genetic elements that can move, and sometimes spread, within their host genomes. Identification of specific inducible conditions whereby certain dormant elements can be activated to transpose is important to further understand the biological roles of mobile elements. Of equal importance is to investigate the transmission genetics of the active state of these elements. We found in this study that He-Ne laser-irradiated rice seeds produced plants with apparent mutations in several phenotypic traits including dispersed spikes, earlier-riping and elongated kernels. Southern blot analysis on randomly selected mutant individuals indicated that mPing, a recently discovered endogenous MITE (miniature inverted repeat transposable element) in rice, together with its putative taansposase-encoding partners, Ping and Pong, was efficiently mobilized in somatic cells of these rice plants. Further transposon-display (TD) assay on these treated plants (M0 generation) and their progenies (M1 to M3) revealed polymorphic banding patterns consistent with mobilization of all three elements. Various mPing excisions and de novo insertions, as identified by element-bracketing, locus-specific PCR assays, were isolated from the irradiated plants and characterized. Pong excisions were also determined.Sequencing at 35 mPing empty donor sites isolated from the irradiatied plants indicated properties characteristic of the element excision. Sequence-based mapping of 11 identified mPing de novo insertions from the irradiated plants or their progenies showed that the insertion site of mPing are distributed in different chromosoms. The highest frequencies of mPing insertion and excision are respectively 17.9 % and 18.4%. Analysis of the treated plants (M0) and their successive selfed generations indicated that the activated state of mPing, Ping and Pong can be transmited to several (at least to M3) orgnismal progenies via meiotic divisions, though in some cases immediate repression on element activity in the treated generation (M0) was evident. The same set of morphological mutants was analyzed for another active mobile element, the reptrotransposon Tos17. It was found that in some of the plants, Tos17 was alsos transpositionally activated. AFLP analysis indicated that activity of mobile elements was accompanied by a high level of genome-wide genetic instability.We also found that DNA methylation patterns of 11 selected low-copy mobile elements and 9 nuclear genes were altered in some of the laser-irradiated rice M0 plants. This was evidenced by methylation-sensitive gel blot analysis, which showed that the transposons mainly manifested hypo-methylation, whereas both hypo- and hyper-methylation were detected for the nuclear genes. Genome-wide fingerprinting on methylation patterns by the MSAP method revealed that the methylation alterations were actually widespred, and also maily represent demethylation. Progeny analysis indicated that the altered MSAP patterns also were largely meiotically heritable, at least to the M3 generation.Based upon the above observations, we conclude that He-Ne laser irradiation represents a potent mutagen for rice, not only from the classic genetic point of view, but also from an epigenetic perspective by mobilizing otherwise quiescent mobile elements and by inducing...