Environmental stress influences genetic and epigenetic regulation in plant genomes. We previously reported that heat stress activated a copia-like retrotransposon named ONSEN (Ito et al., 2011). To investigate the heat sensitivity and transgenerational activation of ONSEN, we analyzed the stress response by temperature shift and multi-treatments of heat stress. ONSEN was activated at 37℃, and the newly inserted ONSEN was transcriptionally activate and mobile to the next generation subjected to heat stress, indicating that the regulation of ONSEN is independent from positional effects on the chromosome. Reciprocal crosses with activated ONSEN revealed that the transgenerational transposition was inherited from both sexes, indicating that the transposition is suppressed independent of gametophytic regulation. We showed previously that ONSEN was transposed in mutants deficient in siRNA biogenesis, including nrpd2 and rdr2, but not dcl3. To define the functional redundancy of DCL proteins in Arabidopsis, we analyzed ONSEN activation in mutants deficient in Dicer-like proteins, including dcl2, dcl3 and dcl4. ONSEN was nearly immobile in a single Dicer mutant; however, some transgenerational transpositions were observed in dcl2/dcl3/dcl4 triple mutants subjected to heat stress. This indicated that the Dicer family is redundant for ONSEN transposition. To examine the activation of ONSEN in undifferentiated cells, ONSEN transcripts and synthesized DNA were analyzed in heat-stressed callus tissue. In contrast with vegetative tissue, high accumulation of the transcripts and amplified DNA copies of ONSEN were detected in callus. This result indicated that ONSEN activation is controlled by cell-specific regulatory mechanisms.