ONSEN as a model transposon to study host and parasitic DNA interplay

ONSEN is a Copia -type long terminal repeat (LTR) retrotransposon in Arabidopsis thaliana that serves as a powerful model for investigating host-transposon interactions. Its activation is directly coupled to the heat stress response via cis -acting elements within its LTRs, which recruit heat shock transcription factors. Under basal conditions, ONSEN is epigenetically silenced through DNA methylation and histone modifications. Heat stress partially relaxes this repression, yet productive mobilization remains limited by host countermeasures, including small RNAs, m 6 A RNA modification, and post-stress restoration of DNA methylation. These layered controls make ONSEN an ideal system for dissecting stress-responsive epigenetic regulation. Moreover, ONSEN insertions preferentially target euchromatic, gene-rich regions and can alter gene expression, splicing, and stress responsiveness. The conserved heat activation across plant retrotransposons further underscores its evolutionary relevance. Together, ONSEN offers a tractable framework for understanding how transposons and host genomes co-evolve and respond to environmental challenges. This review highlights how ONSEN provides insight into the dynamic balance between transposon activation and host genome defense.