Asymmetric cell division (ACD) and positional signals play critical roles in the tissue patterning process. In the Arabidopsis root meristem, two major phloem cell types arise via ACDs of distinct origins: one for companion cells (CCs) and the other for proto- and metaphloem sieve elements (SEs). The molecular mechanisms underlying each of these processes have been reported; however, how these are coordinated has remained elusive. Here, we report a novel phloem development process coordinated via the SHORTROOT (SHR) transcription factor. The well-characterized movement of SHR into the endodermis regulates the ACD for CC formation by activating microRNA165/6, while SHR moving into the phloem regulates the ACD involved in the generation of the two phloem SEs. In the phloem, SHR sequentially activates NARS1 and SND2, encoding NAC domain transcription factors, and these three together form a positive feedforward loop. Under this regulatory scheme, NARS1, generated in the CCs of the root differentiation zone, establishes a top-down signal to drive the ACD for phloem SEs in the meristem. SND2 appears to function downstream to amplify NARS1 via positive feedback. This novel regulatory mechanism expands our understanding of the sophisticated vascular tissue patterning processes occurring during the postembryonic root development.-

不对称细胞分裂（ACD）和位置信号在组织构图过程中起关键作用。在拟南芥的根分生组织中，两种主要的韧皮部细胞类型通过来源不同的ACD产生：一种用于伴生细胞（CCs），另一种用于原韧皮部和后生韧皮部筛分元件（SEs）。已经报道了这些过程的分子机制。然而，如何协调这些仍然难以捉摸。在这里，我们报告了通过SHORTROOT（SHR）转录因子协调的新型韧皮部发育过程。SHR进入内胚层的特征充分，通过激活microRNA165 / 6调节ACD形成CC，而SHR进入韧皮部则调节参与两个韧皮部SE生成的ACD。在韧皮部中，SHR依次激活NARS1和SND2，编码NAC域转录因子，并且这三个一起形成正向前馈环。在这种调节方案下，在根分化区的CC中产生的NARS1建立了一个自上而下的信号，以驱动分生组织中韧皮部SE的ACD。SND2似乎在下游起作用，以通过正反馈放大NARS1。这种新颖的调节机制扩大了我们对胚后根发育过程中发生的复杂血管组织构图过程的理解。