HSI2/VAL1 and HSL1/VAL2 function redundantly to regulate seed dormancy by controlling DOG1 expression in Arabidopsis

Abstract

DELAY OF GERMINATION1 (DOG1) represents a major quantitative locus for the genetic regulation of seed dormancy in Arabidopsis. Accumulation of DOG1 in seeds leads to deep dormancy and delayed germination. Here, we report that the conserved B3 DNA binding domains of the transcriptional repressors HIGH-LEVEL EXPRESSION OF SUGAR INDICIBLE GENE2/ VIVIPAROUS-1/ABSCISIC ACID INSENSITIVE 3-LIKE1 (HSI2/VAL1) and HSI2-LIKE1/ VIVIPAROUS-1/ABSCISIC ACID INSENSITIVE 3-LIKE2 (HSL1/VAL2), which play critical roles in the developmental transition from seed maturation to seedling growth, interact with RY elements in the DOG1 proximal promoter leading to repression of DOG1 transcription during germination and seedling establishment. DOG1 expression is partially de-repressed in hsi2/val1 (hsi2-2) but not in hsl1/val2 (hsl1-1) knockout mutants and is strongly upregulated in a hsi2/val1 hsl1/val2 double mutant, indicating that HSI2/VAL1 and HSL1/VAL2 act redundantly to repress DOG1 expression. HSI2/VAL1 and HSL1/VAL2 form homo- and hetero-dimers in vivo, and dimerization is dependent on the HSI2/VAL1 PHD-like domain. Complementation of hsi2-2 with HSI2/VAL1 harboring a disrupted plant homeodomain (PHD)-like domain results in stronger de-repression of DOG1 expression than the hsi2-2 knockout, indicating that the PHD-like domain plays a critical role in mediating functional interactions between HSI2/VAL1 and HSL1/VAL2. Both HSI2/VAL1 and HSL1/VAL2 interact with components of polycomb repressive complex 2 (PRC2), including CURLY LEAF and MULTICOPY SUPPRESSOR OF IRA1 (MSI1), along with LIKE HETERCHROMATIN PROTEIN 1 (LHP1), which are involved in the deposition and expansion of histone H3 lysine 27 trimethylation (H3K27me3) marks in repressive chromatin. Thus, HSI2/VAL1 HSL1/VAL2-dependent recruitment of PRC2 leads to silencing of DOG1 through the deposition of H3K27me3.