In multicellular organisms, the balance between cell division and differentiation determines organ size, and represents a central unknown in developmental biology. In Arabidopsis roots, this balance is mediated between cytokinin and auxin through a regulatory circuit converging on the IAA3/SHORT HYPOCOTYL 2 (SHY2) gene. Here, we show that crosstalk between brassinosteroids (BRs) and auxin occurs in the vascular transition zone to promote root meristem development. We found that BR increases root meristem size by up‐regulating expression of the PINFORMED 7 (PIN7) gene and down‐regulating expression of the SHY2 gene. In addition, BES1 could directly bind to the promoter regions of both PIN7 and SHY2, indicating that PIN7 and SHY2 mediate the BR‐induced growth of the root meristem by serving as direct targets of BES1. Moreover, the PIN7 overexpression and loss‐of‐function SHY2 mutant were sensitive to the effects of BR and could partially suppress the short‐root phenotypes associated with deficient BR signaling. Interestingly, BRs could inhibit the accumulation of SHY2 protein in response to cytokinin. Taken together, these findings suggest that a complex equilibrium model exists in which regulatory interactions among BRs, auxin, and cytokinin regulate optimal root growth.

中文翻译：

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SHY2是生长素，油菜素内酯和细胞分裂素在调节根分生组织发育中的一个节点。

在多细胞生物中，细胞分裂与分化之间的平衡决定了器官的大小，并代表了发育生物学中的一个主要未知领域。在拟南芥根中，这种平衡是通过收敛于IAA3 / SHORT HYPOCOTYL 2（SHY2）基因上的调节回路在细胞分裂素和生长素之间介导的。在这里，我们显示了油菜素类固醇（BRs）和生长素之间的串扰发生在血管过渡区，以促进根分生组织的发育。我们发现，BR通过的上调表达增加根分生组织大小PINFORMED 7（PIN7）基因和的下调表达SHY2基因。此外，BES1可以直接与PIN7和SHY2的启动子区域结合，表明PIN7和SHY2通过充当BES1的直接靶点介导BR诱导的根分生组织的生长。此外，PIN7过表达和功能丧失的SHY2突变体对BR的影响很敏感，并且可以部分抑制与BR信号不足相关的短根表型。有趣的是，BRs可以抑制SHY2蛋白对细胞分裂素的响应。综上所述，这些发现表明存在一个复杂的平衡模型，其中BR，生长素和细胞分裂素之间的调节相互作用调节了最佳的根系生长。