Nitric Oxide Stress-Induced Genetic And Epigenetic Variations Of Transposable Elements In Rice

Nitric oxide (NO) as a signal molecule is involved in growth, development and defense responses in plants; however, when reaching to a certain concentration, it constitutes a kind of abiotic stress. With industrialization and urbanization, more and more NO is released for human activity which accumulates in air or soil. By forming an abiotic stress, NO can affect crop growth and development.The activity of transposable elements (TEs) can be induced by certain stress conditions. Enhanced TE activity may cause genomic and phenotypic instability, but under certain circumstances can be beneficial to the host's adaptation.We explored the impact of NO stress on genome stability in rice. We identified 8 plants from the second selfed generation of NO-stressed rice (cv. Matsumae) plants, which showed mobilization of a MITE in the rice genome, mPing. We found 32 loci wherein mPing showed transpositional activity, including 15 loci from which mPing showed excisions, and 17 loci to which mPing showed insertions. The autonomous transposon Pong, a putative transposase donor for mPing, was also found activated in these plants. Both excised and inserted mPing copies were stably inherited to the next generation. Three LTR-retrotransposons, Tos17, Osr23 and Osr35, showed genetic changes and/or alterations in cytosine methylation, but did not show any evidence for transposition. However, expression of at least one of these retrotransposons, i.e., Tos17, was up-regulated in the NO-derived plants, which was accompanied by decrease in DNA demethylation at the LTR regions. This suggests that either the transcript level did not reach to the threshold required for transpositional activation or the transcripts were probably degraded by si-RNA mediated mechanisms. The later possibility was hinted by the observation that there exists distinct difference in the methylation patterns between the retrotranspon (Tos17) and its flanking regions.