背景回顾：Root growth dynamics is an outcome of complex hormonal crosstalk. The primary root meristem size for example, is determined by antagonizing actions of cytokinin and auxin. 

主要发现：Here we show that RAV1, a member of the AP2/ERF family of transcription factors, mediates cytokinin signalling in roots to regulate meristem size. 

结果1-rav1突变体表型：The rav1 mutants have prominently longer primary roots, with a meristem that is significantly enlarged and contain higher cell numbers, compared to wild type. The mutant phenotype could be restored on exogenous cytokinin application or by inhibiting auxin transport. 

结果2-RAV1与CK信号转导：At the transcript level, primary cytokinin-responsive genes like ARR1, ARR12 were significantly downregulated in the mutant root, indicating impaired cytokinin signalling. 

结果3-RAV1与Auxin：In concurrence, cytokinin induced regulation of SHY2, an Aux/IAA gene, and auxin efflux carrier PIN1 was hindered in rav1, leading to altered auxin transport and distribution. This effectively altered root meristem size in the mutant. 

结果4-RAV1-CRF1-CK信号转导：Notably, CRF1 another member of the AP2/ERF family implicated in cytokinin signalling, is transcriptionally repressed by RAV1 to promote cytokinin response in roots. Further associating RAV1 with cytokinin signalling, our results demonstrate that cytokinin upregulates RAV1 expression through ARR1, during post-embryonic root development. Regulation of RAV1 expression is a part of secondary cytokinin response that eventually represses CRF1 to augment cytokinin signalling. 

结论：To conclude, RAV1 functions in a branch pathway downstream to ARR1 that regulates CRF1 expression to enhance cytokinin action during primary root development in Arabidopsis.

 摘 要 

根生长动态是一个复杂激素交联的结果。比如说，主根分生组织的大小是由细胞分裂素和生长素之间的拮抗作用所决定的。本文中，作者发现AP2/ERF家族成员RAV1介导了根中的细胞分裂素信号转导，从而调控根分生组织的大小。与野生型相比，rav1突变体具有明显更长的主根，分生组织显著扩大，包含更多的细胞数量。通过外施细胞分裂素或者抑制生长素转运，可以恢复rav1突变体的异常表型。在转录层面，ARR1和ARR12等细胞分裂素响应基因在突变体的根中显著下调，说明细胞分裂素信号转导存在缺陷。同时，细胞分裂素诱导的一个Aux/IAA基因SHY2和生长素外流基因PIN1的调控在rav1突变体中被阻碍，导致了生长素转运和分布的改变。这改变了突变体中的分生组织大小。有趣的是，参与细胞分裂素信号转导的AP2/ERF家族的另一个成员CRF1，其在转录水平受到RAV1的抑制，从而促进根中的细胞分裂素响应。进一步研究RAV1与细胞分裂素信号转导的关联，作者发现细胞分裂素能够在胚后根发育过程中，通过ARR1上调RAV1的表达。RAV1表达的调控是次级细胞分裂素响应的一部分，最终会抑制CRF1的表达，从而增强细胞分裂素信号传导。综上，拟南芥RAV1作用于ARR1下游的一个分支途径上，通过调控CRF1的表达来增强主根发育过程中细胞分裂素的作用。

** Ronita Nag Chaudhuri **