背景回顾：The postembryonic formation of lateral roots (LRs) starts in internal root tissue, the pericycle.  

提出问题：An important question of LR development is how the connection of the primary root vasculature with that of the emerging LR is established and whether the pericycle and/or other cell types direct this process. 

主要发现：Here, using clonal analysis and time-lapse experiments, we show that both the procambium and pericycle of the primary root (PR) affect the LR vascular connectivity in a coordinated manner. 

结果1-木质部桥的形成：We show that during LR formation, procambial derivates switch their identity and become precursors of xylem cells. These cells, together with the pericycle-origin xylem, participate in the formation of what we call a “xylem bridge” (XB), which establishes the xylem connection between the PR and the nascent LR.  

结果2-木质部桥的可塑性：If the parental protoxylem cell fails to differentiate, XB is still sometimes formed but via a connection with metaxylem cells, highlighting that this process has some plasticity. 

结果3-木质部桥特化和分化的关键基因：Using mutant analyses, we show that the early specification of XB cells is determined by CLASS III HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIP III) transcription factors (TFs). Subsequent XB cell differentiation is marked by the deposition of secondary cell walls (SCWs) in spiral and reticulate/scalariform patterns, which is dependent on the VASCULAR-RELATED NAC-DOMAIN (VND) TFs. 

结果4-保守机制：XB elements were also observed in Solanum lycopersicum, suggesting that this mechanism may be more widely conserved in plants. 

结论：Together, our results suggest that plants maintain vascular procambium activity, which safeguards the functionality of newly established lateral organs by assuring the continuity of the xylem strands throughout the root system.

侧根的胚后形成起始于根内组织中柱鞘。关于侧根发育的一个关键的问题是，主根的维管系统是与正在形成的侧根维管系统之间的连接是如何建立的，以及中柱鞘和或其他细胞类型是否会参与这一过程。本文中，利用克隆分析和延迟实验，作者发现主根的原形成层和中柱鞘共同影响了侧根维管系统的连接性。作者发现在侧根形成过程中，原形成层子细胞会改变自身身份，变成木质部细胞的前体细胞。这些细胞与来源于中柱鞘的木质部一起，参与了我们称之为“木质部桥”的形成，从而建立起主根和相邻侧根之间的木质部连接。如果原木质部细胞不能够分化，“木质部桥”有时还是能够形成，但是是通过与后生木质部细胞的连接形成的，说明“木质部桥”的形成具有一定的可塑性。利用突变体分析，作者发现“木质部桥”细胞的早期特化是由HD-ZIP III转录因子所决定的。随后，螺旋状和网状/阶状次生细胞壁（SCW）的沉积标志着“木质部桥”细胞的分化，并且这一过程依赖于VND转录因子。在番茄中，作者也观察到了“木质部桥”，说明这一保守机制广泛存在于植物中。综上，本文的研究揭示了植物维持维管原形成层活性，通过保证整个根系统中的木质部连接性，从而确保新建立的侧向器官的功能性。