背景回顾：A key innovation in land plants was the evolution of meristems with stem cells possessing multiple cutting faces (division planes) from which three-dimensional growth is derived in both haploid (gametophyte) and diploid (sporophyte) generations. Within each meristem exists a pool of stem cells that must be maintained at a relatively constant size for development to occur appropriately. 

提出问题：In flowering plants, stem cells of the diploid generation are maintained by CLAVATA3/EMBRYO SURROUNDING REGION-related (CLE) peptide signaling. In the liverwort Marchantia polymorpha, the haploid body undergoes dichotomous branching, an ancestral characteristic of growth derived from the meristem, in which two equivalent body axes are developed via stem cell division, regulated by unknown molecular mechanisms. 

结果1-CLE2处理和去除表型：We show here that in M. polymorpha, treatment with MpCLE2/CLAVATA3 (CLV3) peptide resulted in the accumulation of undifferentiated cells, marked by MpYUC2 expression, in the apical meristem. Removal of MpCLE2 peptide resulted in multichotomous branching from the accumulated cells. 

结果2-MpCLE2受体MpCLV1：Genetic analysis demonstrated that the CLAVATA1 (MpCLV1) receptor, but not the WUSCHEL-related HOMEOBOX (MpWOX) transcription factor, is responsible for MpCLE2 peptide signaling. 

结果3-MpCLE2介导细胞间通信：In the apical meristem, MpCLV1 was expressed broadly in the central region, including the MpYUC2-positive area, whereas MpCLE2 was expressed in a largely complementary manner compared to MpYUC2, suggesting MpCLE2 mediates local cell-to-cell communication. 

结论：CLV3/CLE peptide, a negative regulator of diploid stem cells in flowering plants, acts as a haploid stem cell-promoting signal in M. polymorpha, implicating a critical role for this pathway in the evolution of body plan in land plants. 

 摘 要 

陆地植物的一个关键创新就是演化出带有干细胞的分生组织，这些干细胞能够在孢子体（二倍体）和配子体（单倍体）世代的三维生长过程中，形成多个方向的分裂板。在每个分生组织中都存在一个始终维持在一定大小的干细胞池，从而来控制发育的适当进行。在有花植物中，二倍体世代的干细胞是由CLE多肽信号途径来维持的。在地钱（Marchantia polymorpha）中，单倍体的株体会经历二叉分枝，这是一种源自分生组织的祖先生长特征，可以通过分生组织的分裂形成两个对称的体轴，但是潜在的分子调控机制仍不清晰。本文中，作者发现利用MpCLE2多肽处理地钱，会导致顶端分生组织中积累未分化细胞的积累，这可以通过MpYUC2的表达来反映。而去除掉MpCLE2多肽后，这些在顶端分生组织中积累的未分化细胞会进行多叉分枝。遗传分析显示，MpCLV1受体，而非MpWOX转录因子作用于MpCLE2多肽的信号转导。在顶端分生组织中，MpCLV1在包括MpYUC2表达区域的中心区广泛表达，但是MpCLE2的表达正好与MpYUC2的相反，说明MpCLE2介导了局域的细胞间通信。CLV3/CLE多肽在有花植物中是二倍体干细胞的负调控子，但是在单倍体的地钱中却发挥着促进干细胞的作用，说明该通路在陆地植物的体轴演化过程中发挥了至关重要的作用。

 Yuki Hirakawa