Establishment of Embryonic Stem Cell Niches in Maize

在模式植物拟南芥中，导致胚胎分生组织建立的干细胞前体细胞的指定已经在放射状16细胞皮肤原阶段发生。细胞调控所需的基因调控网络包括生长素诱导的单翅目昆虫(MP)-BODENLOS(BDL)模块，该模块由ARF转录因子MP及其抑制子BDL以及WUSCHEL(WUS)和WUS相关同源框(WOX)转录因子组成。与拟南芥相比，关于谷类胚胎发生过程中分生组织是如何建立和组织的，几乎一无所知。拟南芥胚胎发生过程中细胞命运的决定严格依赖于常规的细胞分裂模式，而谷物胚胎发生中的细胞分裂平面则表现为混沌和随机的。那么，分生组织前体和干细胞是如何在谷物中被指定的呢？通过原位杂交研究WUS/WOX同源物的激素转运和反应以及基因表达模式的少数描述性报道表明，这些过程在玉米和拟南芥之间并不保守。玉米分生组织发生的时间要晚得多，在胚胎已经由数百个细胞组成的过渡阶段对称性破坏之后。此外，我们的初步数据预测，胚乳中产生的生长素可以触发对称性破坏，这种细胞规格最初可能被胚胎周围区域(ESR)阻止，ESR是拟南芥中缺乏的一种专门的胚乳结构，不同于MP-BDL的ARF-IAA模块调节胚胎分生组织的建立，这也涉及谷物特有的分子因子。因此，为了了解谷物分生组织的建立，这个雄心勃勃的项目旨在(I)鉴定和研究在玉米胚胎过渡阶段表达的生长素调控的ARF-IAA模块的作用，(Ii)阐明ESR在早期胚胎模式形成中的作用，(Iii)了解ZmWUS1/ZmWOX5基因调控网络在建立胚胎干细胞生态位方面的作用，以及(Iv)确定在玉米胚胎顶端胚区不对称分泌的EAL1信号肽的功能和靶标。

Specification of stem cell precursor cells leading to the establishment of embryonic meristems occurs already at the radial 16-celled dermatogen stage in the model plant Arabidopsis. Gene regulatory networks required for cell specification include the auxin-induced MONOPTEROS (MP)-BODENLOS (BDL) module consisting of the ARF transcription factor MP and its repressor BDL as well as WUSCHEL (WUS) and WUS-related homeobox (WOX) transcription factors. Compared with Arabidopsis, almost nothing is known about how meristems are established and organized during embryogenesis in cereals. Cell fate decisions during embryogenesis in Arabidopsis are strictly coupled to stereotypic cell division patterns, while cell division planes in cereal embryogenesis appear chaotic and random. Thus, how are meristem precursor and stem cells specified in cereals? The few descriptive reports studying hormone transport and responses as well as gene expression pattern of WUS/WOX homologs by in situ hybridization suggest that these processes are not conserved between maize and Arabidopsis. Meristem initiation occurs much later in maize, after symmetry breaking at the transition stage when the embryo consists already of hundreds of cells. Moreover, our preliminary data predict that auxin generated in the endosperm can trigger symmetry breaking, that this cell specification might initially be prevented by the embryo surrounding region (ESR), a specialized endosperm structure lacking in Arabidopsis, that ARF-IAA modules different from MP-BDL regulate embryonic meristem establishment, and that this involves also cereal-specific molecular players. Thus, to understand meristem establishment in cereals, this ambitious project aims (i) to identify and study the role of auxin-regulated ARF-IAA modules expressed at the transition stage of maize embryos, (ii) to elucidate the role of the ESR in early embryo patterning, (iii) to understand the role of the ZmWUS1/ZmWOX5 gene regulatory network for the establishment of embryonic stem cell niches and (iv) to identify the function and targets of the cereal-specific EAL1 signal peptide that is secreted asymmetrically at the apical embryo domain at the transition stage.