Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan.
Development. 2012 Apr;139(8):1391-8. doi: 10.1242/dev.074492. Epub 2012 Feb 29.
Plant growth is directed by the activity of stem cells within meristems. The first meristems are established during early embryogenesis, and this process involves the specification of both stem cells and their organizer cells. One of the earliest events in root meristem initiation is marked by re-specification of the uppermost suspensor cell as hypophysis, the precursor of the organizer. The transcription factor MONOPTEROS (MP) is a key regulator of hypophysis specification, and does so in part by promoting the transport of the plant hormone auxin and by activating the expression of TARGET OF MP (TMO) transcription factors, both of which are required for hypophysis specification. The mechanisms leading to the activation of these genes by MP in a chromatin context are not understood. Here, we show that the PHD-finger proteins OBERON (OBE) and TITANIA (TTA) are essential for MP-dependent embryonic root meristem initiation. TTA1 and TTA2 are functionally redundant and function in the same pathway as OBE1 and OBE2. These PHD-finger proteins interact with each other, and genetic analysis shows that OBE-TTA heterotypic protein complexes promote embryonic root meristem initiation. Furthermore, while MP expression is unaffected by mutations in OBE/TTA genes, expression of MP targets TMO5 and TMO7 is locally lost in obe1 obe2 embryos. PHD-finger proteins have been shown to act in initiation of transcription by interacting with nucleosomes. Indeed, we found that OBE1 binds to chromatin at the TMO7 locus, suggesting a role in its MP-dependent activation. Our data indicate that PHD-finger protein complexes are crucial for the activation of MP-dependent gene expression during embryonic root meristem initiation, and provide a starting point for studying the mechanisms of developmental gene activation within a chromatin context in plants.
植物的生长是由分生组织内干细胞的活性所决定的。最早的分生组织是在胚胎早期建立的，这个过程涉及到干细胞及其组织者细胞的特化。根分生组织起始的最早事件之一是，最上面的悬浮细胞重新特化为下胚轴，即组织者的前体。转录因子 MONOPTEROS（MP）是下胚轴特化的关键调节因子，它通过促进植物激素生长素的运输和激活 TARGET OF MP（TMO）转录因子的表达来实现这一点，这两者都是下胚轴特化所必需的。MP 在染色质背景下激活这些基因的机制尚不清楚。在这里，我们表明 PHD 指蛋白 OBERON（OBE）和 TITANIA（TTA）对于 MP 依赖的胚胎根分生组织起始是必不可少的。TTA1 和 TTA2 在功能上是冗余的，与 OBE1 和 OBE2 一起作用于同一途径。这些 PHD 指蛋白相互作用，遗传分析表明 OBE-TTA 异型蛋白复合物促进胚胎根分生组织起始。此外，虽然 OBE/TTA 基因突变不影响 MP 的表达，但 MP 靶标 TMO5 和 TMO7 的表达在 obe1obe2 胚胎中局部丢失。已经表明 PHD 指蛋白通过与核小体相互作用来参与转录起始。事实上，我们发现 OBE1 在 TMO7 基因座上与染色质结合，表明其在 MP 依赖性激活中起作用。我们的数据表明，PHD 指蛋白复合物对于胚胎根分生组织起始过程中 MP 依赖的基因表达的激活至关重要，并为在植物染色质背景下研究发育基因激活的机制提供了一个起点。
