The major tissues of the cereal endosperm are the starchy endosperm (SE) in the inner and the aleurone layer (AL) at the outer periphery. The fates of the cells that comprise these tissues are determined according to positional information; however, our understanding of the underlying molecular mechanisms remains limited. Here, we conducted a high-resolution spatiotemporal analysis of the rice endosperm transcriptome during early cellularization. In rice, endosperm cellularization proceeds in a concentric pattern from a primary alveolus cell layer, such that developmental progression can be defined by the number of cell layers. Using laser-capture microdissection to obtain precise tissue sections, transcriptomic changes were followed through five histologically defined stages of cellularization from the syncytial to 3-cell layer (3 L) stage. In addition, transcriptomes were compared between the inner and the outermost peripheral cell layers. Large differences in the transcriptomes between stages and between the inner and the peripheral cells were found. SE attributes were expressed at the alveolus-cell-layer stage but were preferentially activated in the inner cell layers that resulted from periclinal division of the alveolus cell layer. Similarly, AL attributes started to be expressed only after the 2 L stage and were localized to the outermost peripheral cell layer. These results indicate that the first periclinal division of the alveolus cell layer is asymmetric at the transcriptome level, and that the cell-fate-specifying positional cues and their perception system are already operating before the first periclinal division. Several genes related to epidermal identity (i.e., type IV homeodomain-leucine zipper genes and wax biosynthetic genes) were also found to be expressed at the syncytial stage, but their expression was localized to the outermost peripheral cell layer from the 2 L stage onward. We believe that our findings significantly enhance our knowledge of the mechanisms underlying cell fate specification in rice endosperm.

谷物胚乳的主要组织是内部的淀粉质胚乳 (SE) 和外围的糊粉层 (AL)。组成这些组织的细胞的命运是根据位置信息确定的；然而，我们对潜在分子机制的理解仍然有限。在这里，我们在早期细胞化过程中对水稻胚乳转录组进行了高分辨率时空分析。在水稻中，胚乳细胞化从初级肺泡细胞层以同心模式进行，因此发育进程可以通过细胞层数来定义。使用激光捕获显微切割获得精确的组织切片，转录组的变化通过从合胞体到 3 细胞层 (3 L) 阶段的五个组织学定义的细胞化阶段进行跟踪。此外，还比较了内部和最外周细胞层之间的转录组。发现阶段之间以及内部和外围细胞之间的转录组存在很大差异。SE 属性在肺泡细胞层阶段表达，但优先在由肺泡细胞层的斜周分裂引起的内细胞层中被激活。类似地，AL 属性仅在 2 L 阶段后才开始表达，并定位于最外周细胞层。这些结果表明肺泡细胞层的第一次周缘分裂在转录组水平上是不对称的，并且在第一次周缘分裂之前，细胞命运特定的位置线索及其感知系统已经在运作。几个与表皮特性相关的基因（即，还发现 IV 型同源域-亮氨酸拉链基因和蜡生物合成基因）在合胞阶段表达，但从 2 L 阶段开始，它们的表达定位于最外周细胞层。我们相信，我们的发现显着增强了我们对水稻胚乳细胞命运特化机制的了解。