背景回顾：Grass leaves develop from a ring of primordial initial cells within the periphery of the shoot apical meristem, a pool of organogenic stem cells that generates all of the organs of the plant shoot. At maturity, the grass leaf is a flattened, strap-like organ comprising a proximal supportive sheath surrounding the stem and a distal photosynthetic blade. The sheath and blade are partitioned by a hinge-like auricle and the ligule, a fringe of epidermally derived tissue that grows from the adaxial (top) leaf surface. Together, the ligule and auricle comprise morphological novelties that are specific to grass leaves. 

提出问题：Understanding how the planar outgrowth of grass leaves and their adjoining ligules is genetically controlled can yield insight into their evolutionary origins. 

主要发现：Here we use single-cell RNA-sequencing analyses to identify a ‘rim’ cell type present at the margins of maize leaf primordia. 

结果1-叶和叶舌：Cells in the leaf rim have a distinctive identity and share transcriptional signatures with proliferating ligule cells, suggesting that a shared developmental genetic programme patterns both leaves and ligules. 

结果2-WOX3：Moreover, we show that rim function is regulated by genetically redundant Wuschel-like homeobox3 (WOX3) transcription factors. Higher-order mutations in maize Wox3 genes greatly reduce leaf width and disrupt ligule outgrowth and patterning. 

结论：Together, these findings illustrate the generalizable use of a rim domain during planar growth of maize leaves and ligules, and suggest a parsimonious model for the homology of the grass ligule as a distal extension of the leaf sheath margin.

禾本科植物的叶是由茎尖分生组织外围的一圈原生初始细胞发育而来，植株所有的地上器官都源自于一个器官性干细胞池。待叶发育成熟后，禾本科的叶是一个扁平的带状器官，其近轴处是一个包裹干细胞的叶鞘，主要起支撑作用，而远端则是一个负责光合作用的叶片。叶鞘和叶片主要由叶耳和叶舌区隔开，而叶舌是一个从近轴顶端叶表面长出的舌状突起。叶舌和叶耳一起组成了禾本科植物叶的新颖形态特征。了解禾本科叶及相邻叶舌的平面生长遗传调控机制有助于理解其起源和演化历史。本文中，作者利用单细胞转录组测序技术在玉米的叶原基边缘鉴定到了一个“rim”（边沿）细胞类型。在叶边沿的细胞具有一个与众不同的特征，与正在增殖中的叶舌细胞享有类似的转录特征，说明叶和叶舌可能共享了一套发育遗传调控程序。此外，作者还发现叶边沿的功能受到转录因子WOX3的冗余调控。玉米高阶Wox3基因突变体能够极大程度上减少叶宽，并且扰乱叶舌的向外生长和模式建成。综上，本文的研究结果揭示了玉米叶和叶舌的平面生长都涉及到了“边沿”域的参与，并且支持了禾本科植物的叶舌作为叶鞘边缘的远端延伸的简约模型。

** Michael J. Scanlon **