Department of Plant Biology, University of Georgia, Athens, GA 30602, USA.
Curr Biol. 2011 Aug 9;21(15):1268-76. doi: 10.1016/j.cub.2011.06.049. Epub 2011 Jul 28.
The division of plant zygotes is typically asymmetric, generating daughter cells with different developmental fates. In Arabidopsis, the apical daughter cell produces the proembryo, whereas the basal daughter cell forms the mostly extraembryonic suspensor. Establishment of apical and basal fates is known to depend on the YODA (YDA) mitogen-associated protein (MAP) kinase cascade and WUSCHEL-LIKE HOMEOBOX (WOX) homeodomain transcription factors.
Mutations in GROUNDED (GRD) cause anatomical defects implying a partial loss of developmental asymmetry in the first division. Subsequently, suspensor-specific WOX8 expression disappears while proembryo-specific ZLL expression expands in the mutants, revealing that basal fates are confounded. GRD encodes a small nuclear protein of the RWP-RK family and is broadly transcribed in the early embryo. Loss of GRD eliminates the dominant effects of hyperactive YDA variants, indicating that GRD is required for YDA-dependent signaling in the embryo. However, GRD function is not regulated via direct phosphorylation by MAP kinases, and forced expression of GRD does not suppress the effect of yda mutations. In a strong synthetic interaction, grd;wox8;wox9 triple mutants arrest as zygotes or one-cell embryos lacking apparent polarity.
The predicted transcription factor GRD acts cooperatively with WOX homeodomain proteins to establish embryonic polarity in the first division. Like YDA, GRD promotes zygote elongation and basal cell fates. GRD function is required for YDA-dependent signaling but apparently not regulated by the YDA MAP kinase cascade. Similarity of GRD to Chlamydomonas MID suggests a conserved role for small RWP-RK proteins in regulating the transcriptional programs of generative cells and the zygote.
植物合子的分裂通常是不对称的，产生具有不同发育命运的子细胞。在拟南芥中，顶端子细胞产生原胚，而基底子细胞形成主要的胚外悬浮胚。顶端和基底部命运的建立依赖于 YODA（YDA）有丝分裂原相关蛋白（MAP）激酶级联和 WUSCHEL-LIKE HOMEOBOX（WOX）同源域转录因子。
GRD（GROUNDED）突变导致解剖缺陷，暗示第一次分裂中部分丧失了发育不对称性。随后，在突变体中，悬浮胚特异性的 WOX8 表达消失，而原胚特异性的 ZLL 表达扩大，揭示了基底部命运的混淆。GRD 编码 RWP-RK 家族的一种小核蛋白，在早期胚胎中广泛转录。GRD 的缺失消除了超活性 YDA 变体的显性效应，表明 GRD 是胚胎中 YDA 依赖信号所必需的。然而，GRD 功能不是通过 MAP 激酶的直接磷酸化来调节的，并且强制表达 GRD 并不能抑制 yda 突变的影响。在强烈的合成相互作用中，grd;wox8;wox9 三重突变体停滞在没有明显极性的合子或单细胞胚胎中。
预测的转录因子 GRD 与 WOX 同源域蛋白协同作用，在第一次分裂中建立胚胎极性。与 YDA 一样，GRD 促进合子伸长和基底部细胞命运。GRD 功能是 YDA 依赖信号所必需的，但显然不受 YDA MAP 激酶级联的调节。GRD 与衣藻 MID 的相似性表明，小 RWP-RK 蛋白在调节生殖细胞和合子的转录程序中具有保守作用。
