Wood, a type of xylem tissue, originates from cell proliferation of the vascular cambium. Xylem is produced inside, and phloem outside, of the cambium1. Morphogenesis in plants is typically coordinated by organizer cells that direct the adjacent stem cells to undergo programmed cell division and differentiation. The location of the vascular cambium stem cells and whether the organizer concept applies to the cambium are currently unknown2. Here, using lineage-tracing and molecular genetic studies in the roots of Arabidopsis thaliana, we show that cells with a xylem identity direct adjacent vascular cambial cells to divide and function as stem cells. Thus, these xylem-identity cells constitute an organizer. A local maximum of the phytohormone auxin, and consequent expression of CLASS III HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIP III) transcription factors, promotes xylem identity and cellular quiescence of the organizer cells. Additionally, the organizer maintains phloem identity in a non-cell-autonomous fashion. Consistent with this dual function of the organizer cells, xylem and phloem originate from a single, bifacial stem cell in each radial cell file, which confirms the classical theory of a uniseriate vascular cambium3. Clones that display high levels of ectopically activated auxin signalling differentiate as xylem vessels; these clones induce cell divisions and the expression of cambial and phloem markers in the adjacent cells, which suggests that a local auxin-signalling maximum is sufficient to specify a stem-cell organizer. Although vascular cambium has a unique function among plant meristems, the stem-cell organizer of this tissue shares features with the organizers of root and shoot meristems.In the roots of Arabidopsis thaliana, cells with xylem identity and high levels of auxin signalling function as organizer cells that direct neighbouring vascular cambial cells to act as stem cells.

中文翻译：

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高水平的生长素信号定义了血管形成层的干细胞组织者

木材是一种木质部组织，起源于维管形成层的细胞增殖。木质部在形成层内部产生，韧皮部在形成层外部产生。植物的形态发生通常由组织者细胞协调，这些组织者细胞指导相邻的干细胞进行程序化的细胞分裂和分化。目前尚不清楚血管形成层干细胞的位置以及组织者概念是否适用于形成层。在这里，我们使用拟南芥根部的谱系追踪和分子遗传研究，表明具有木质部特性的细胞直接引导相邻的血管形成层细胞分裂并作为干细胞发挥作用。因此，这些木质部身份细胞构成了一个组织者。植物激素生长素的局部最大值，以及随之而来的 CLASS III HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIP III) 转录因子的表达，促进组织者细胞的木质部特性和细胞静止。此外，组织者以非细胞自主的方式保持韧皮部的身份。与组织者细胞的这种双重功能一致，木质部和韧皮部起源于每个径向细胞文件中的单个双面干细胞，这证实了单列维管形成层的经典理论。表现出高水平异位激活生长素信号的克隆分化为木质部血管；这些克隆诱导细胞分裂以及相邻细胞中形成层和韧皮部标志物的表达，这表明局部生长素信号传导最大值足以指定干细胞组织者。虽然维管形成层在植物分生组织中具有独特的功能，