背景回顾：Vascular cambium contains bifacial stem cells, which produce secondary xylem to one side and secondary phloem to the other. 

提出问题：However, how these fate decisions are regulated is unknown. 

主要发现：Here we show that the positioning of an auxin signalling maximum within the cambium determines the fate of stem cell daughters. The position is modulated by gibberellin-regulated, PIN1-dependent polar auxin transport. 

结果：Gibberellin treatment broadens auxin maximum from the xylem side of the cambium towards the phloem. As a result, xylem-side stem cell daughter preferentially differentiates into xylem, while phloem-side daughter retains stem cell identity. Occasionally, this broadening leads to direct specification of both daughters as xylem, and consequently, adjacent phloem-identity cell reverts to being stem cell. Conversely, reduced gibberellin levels favour specification of phloem-side stem cell daughter as phloem. 

结论：Together, our data provide a mechanism by which gibberellin regulates the ratio of xylem and phloem production.

维管形成层包含，其向内分化形成次生木质部，向外分化形成次生韧皮部。但是，形成层干细胞的分化命运是如何被调控的还不清楚。本文中，作者发现维管形成层中生长素信号最大值点的位置决定了干细胞子代的命运。而生长素极大值点的位置是由赤霉素调控的PIN1依赖性的极性生长素运输所决定的。赤霉素处理会增强生长素由木质部侧向韧皮部侧的生长素极大值点。因此，木质部侧的干细胞子代会优先分化形成木质部，而韧皮部侧的干细胞子代仍然维持干细胞的特性。有时候，这种增强会导致干细胞的两个子细胞都分化形成木质部，结果会导致相邻的韧皮部细胞重新回到了干细胞状态。相反，降低赤霉素水平会使得韧皮部侧的干细胞子代优先分化形成韧皮部。综上，本文的研究结果揭示了一个由赤霉素介导的木质部和韧皮部比例调控的分子机制。

** Ari Pekka Mähönen **