High levels of auxin signalling define the stem-cell organizer of the vascular cambium

First author: Ondřej Smetana; Affiliations: University of Helsinki (赫尔辛基大学): Helsinki, Finland

Corresponding author: Ari Pekka Mähönen

Wood, a type of xylem tissue, originates from cell proliferation of the vascular cambium. Xylem is produced inside, and phloem outside, of the cambium. 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 unknown. 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 (单列的) vascular cambium. 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.

木材，是一类木质部组织，起源于维管形成层的细胞增殖。形成层向内分化形成木质部，向外分化形成韧皮部。植物中的形态发生是由“组织者”细胞协调调控的，该类细胞会指导毗邻干细胞的细胞分裂和分化。维管形成层的干细胞和其“组织者”细胞都还不是很清楚。本文，作者通过谱系追踪和分子遗传研究鉴定了拟南芥根中一类带有木质部特征的细胞会直接指导毗邻维管形成层细胞的分裂和发挥干细胞的作用。因此，这些具有木质部特征的细胞构成了“组织者”细胞。植物激素生长素局部最大化以及HD-ZIP III转录因子的随之表达促进了木质部特征形成的以及“组织者”细胞的细胞静止。另外，“组织者”细胞通过非细胞自主的方式维持韧皮部的特征。与“组织者”细胞双元功能一致的是，木质部和韧皮部均源自于同一个干细胞，该干细胞在不同径向（向内或向外）上会分化成不同的细胞类型，进而证明了经典的单列维管形成层理论。高水平的生长素信号异位激活会使得细胞分化为木质部导管，并且在邻近细胞中诱导细胞分化和形成层、韧皮部标记的表达，从而说明了局部的生长素信号最大化能够诱导干细胞“组织者”的形成。尽管维管形成层在植物分生组织中具有独特的功能，但该组织中的干细胞“组织者”与根及茎分生组织中的干细胞“组织者”具有类似的性质。

通讯：Ari Pekka Mähönen (http://www.biocenter.helsinki.fi/bi/mahonen/labmembers/index.htm)

研究方向：利用拟南芥的根作为模型研究植物的形成层发育。

doi: https://doi.org/10.1038/s41586-018-0837-0

Journal: Nature

Published date: 09 January, 2019