AtSDR4L and DIG1 interact with VAL2 to promote seed-to-seedling transition

During the embryonic-to-vegetative transition in plants, many genes that were active in seeds become transcriptionally turned off. Recently, we reported a new transcriptional corepressor of the embryonic program, SEED DORMANCY 4-LIKE (AtSDR4L). Multiple lines of evidence suggest that AtSDR4L plays an essential role in regulating the transition from seed to seedling, possibly by collaborating with components of Polycomb Repressive Complexes (PRCs) and PRC-associated proteins VIVIPAROUS1/ABI3-LIKE (VAL) proteins. Through yeast-two-hybrid (Y2H) assays and genome-wide binding analysis, we demonstrated that AtSDR4L and DIG1 physically interact with VAL2 but not with VAL1. The interactions depend on the C-terminal halves of AtSDR4L and DIG1, and the N-terminal half of VAL2. AtSDR4L binds to target loci in germinating seeds, preceding reduced accumulation of H3K27me3 Atsdr4l seedlings at a few loci important for seed development. These findings suggest AtSDR4L, and potentially DIG1, facilitate the recruitment of PRC2 through VAL2 to inactivate seed development genes, thus promoting the seed-to-seedling transition. Chromatin immunoprecipitation DNA-seq (ChIP-seq) was performed to map the histone modification H3K27me3 in 1 and 3-day-old seedlings of Arabidopsis wilde-type and Atsdr4l mutants. Additionally, ChIP-seq was conducted to investigate AtSDR4L binding in germinating seeds treated with 5 uM ABA.

在植物胚胎向营养生长的转换过程中，诸多在种子中活跃表达的基因会发生转录性沉默。此前我们曾报道一种全新的胚胎发育程序转录共抑制因子——种子休眠4类似蛋白（SEED DORMANCY 4-LIKE，AtSDR4L）。多项实验证据表明，AtSDR4L在调控种子向幼苗的转换过程中发挥核心作用，其机制可能是与多梳抑制复合体（Polycomb Repressive Complexes, PRCs）的组分以及PRC相关蛋白VIVIPAROUS1/ABI3-LIKE（VAL）家族蛋白协同发挥功能。通过酵母双杂交（yeast-two-hybrid, Y2H）实验与全基因组结合分析，我们证实AtSDR4L与DIG1可与VAL2发生物理互作，但无法与VAL1结合。该互作依赖于AtSDR4L与DIG1的C端半区，以及VAL2的N端半区。AtSDR4L可结合萌发种子中的靶位点；而在Atsdr4l突变体幼苗中，少数与种子发育相关位点的H3K27me3（组蛋白H3赖氨酸27三甲基化）积累量显著降低。上述研究结果表明，AtSDR4L（以及潜在的DIG1）可通过VAL2招募PRC2复合体，从而沉默种子发育相关基因，最终促进种子向幼苗的转换。本研究通过染色质免疫沉淀测序（Chromatin immunoprecipitation DNA-seq, ChIP-seq），对野生型及Atsdr4l突变体拟南芥在萌发后1天和3天的幼苗中的组蛋白修饰H3K27me3进行了全基因组图谱绘制。此外，本研究还利用ChIP-seq技术，探究了经5 μM脱落酸（abscisic acid, ABA）处理的萌发种子中AtSDR4L的结合情况。