KNOX II Transcription Factor HOS59 Regulates <i>Gypsy</i> Transposable Elements to Modulate Panicle Development in Rice

The KNOX transcription factor family plays important roles in plant growth and development. However, the precise functions and regulatory mechanisms of KNOX II genes in reproductive tissues remain to be elucidated. Thus, this study integrated complementary techniques, including ChIP-seq, IP-MS, and transcriptome sequencing, to investigate the chromatin distribution and functional features of HOS59, a member of the KNOX II subclass. Notably, a higher enrichment of HOS59 deposition in Gypsy TEs was observed in rice panicles. Regions where HOS59 colocalised in Gypsy TEs were highly enriched with H3K9me2 modification and were preferentially distributed in pericentromeric regions. PPI analysis suggested that HOS59 may affect heterochromatin stability through the RdDM pathway and H3K9me2 modification. Meanwhile, overexpression of HOS59 promoted the maintenance of heterochromatin stability. Altered expression patterns of cell cycle-related genes in hos59 mutants indicated a key regulatory role for HOS59 in silencing Gypsy TEs in pericentromeric regions, as well as in cell division and panicle development. Moreover, the HOS59-OsSPL13-SRS5 module regulating grain size was identified. In summary, our findings revealed that HOS59 may bind to and repress Gypsy TEs to affect grain development, which provides novel insights into the regulatory networks of KNOX family genes in rice.