Arabidopsis HIT 4, a regulator involved in heat‐triggered reorganization of chromatin and release of transcriptional gene silencing, relocates from chromocenters to the nucleolus in response to heat stress

Summary Arabidopsis HIT 4 is known to mediate heat‐induced decondensation of chromocenters and release from transcriptional gene silencing ( TGS ) with no change in the level of DNA methylation. It is unclear whether HIT 4 and MOM 1, a well‐known DNA methylation‐independent transcriptional silencer, have overlapping regulatory functions. A hit4‐1/mom1 double mutant strain was generated. Its nuclear morphology and TGS state were compared with those of wild‐type, hit4‐1 , and mom1 plants. Fluorescent protein tagging was employed to track the fates of HIT 4, hit4‐1 and MOM 1 in vivo under heat stress. HIT 4‐ and MOM 1‐mediated TGS were distinguishable. Both HIT 4 and MOM 1 were localized normally to chromocenters. Under heat stress, HIT 4 relocated to the nucleolus, whereas MOM 1 dispersed with the chromocenters. hit4‐1 was able to relocate to the nucleolus under heat stress, but its relocation was insufficient to trigger the decompaction of chromocenters. The hypersensitivity to heat associated with the impaired reactivation of TGS in hit4‐1 was not alleviated by mom1 ‐induced release from TGS . HIT 4 delineates a novel and MOM 1‐independent TGS regulation pathway. The involvement of a currently unidentified component that links HIT 4 relocation and the large‐scale reorganization of chromatin, and which is essential for heat tolerance in plants is hypothesized.