背景回顾：Post-embryonic organogenesis is critical for plant development. 

提出问题：Underground, lateral roots (LR) form the bulk of mature root systems, yet the ontogeny of the LRP is not clear. 

主要研究：Here, we use single-cell RNA-seq through the first four stages of LR formation in Arabidopsis. 

结果1：Our analysis leads to a model in which a single group of precursor cells, with different cell identity from their pericycle origins, rapidly reprograms and splits into a mixed ground tissue/stem cell niche fate and a vascular precursor fate.

结果2：The ground tissue and stem cell niche fates soon separate and a subset of more specialized vascular cells form sucrose transporting phloem cells that appear to connect to the primary root. 

结果3：We did not detect cells resembling epidermis or root cap, suggesting that outer tissues may form later preceding LR emergence.

结果4：At this stage, some remaining initial precursor cells form the primordium flanks while the rest create a reservoir of pluripotent cells able to replace the LR if damaged. 

结果5：Laser ablation of the central and lateral LRP regions shows that remaining cells restart the sequence of tissue initiation to form a LR. 

结论：Our analysis reveals an ontological hierarchy for LR formation with an early and sequential split of main root tissues and stem cells.

 摘 要 

胚后器官发育对于植物发育来说至关重要。地下的侧根形成了植物成熟的根系统，但是有关侧根原基的形态发生还不清楚。本文中，作者利用单细胞RNA-seq技术，对拟南芥侧根形成的前四个阶段进行了分析。作者的分析结果形成一个侧根发育模型，在该模型中，一群与中柱鞘原始细胞存在不同细胞特性的前体细胞，会经历快速的重编程，进而分裂成一个由具基本组织/干细胞龛命运的细胞和具维管命运的细胞组成的混合细胞群。接着，基本组织和干细胞龛命运会很快分离，并且一个更加特化的维管细胞亚群形成了能够运输蔗糖的韧皮部细胞，且似乎连接到初生根。作者没有检测到类似表皮或根冠的细胞，表明外层组织可能在之后，也就是侧根出现之前才会形成。在这一阶段，一些残留的原始前体细胞形成原基侧翼，而其余的则形成一个多能干细胞库，一旦侧根组织受损，这些细胞能够替换形成新的侧根。激光烧蚀侧根原基中央和外侧区域的实验显示，剩余的细胞会重启组织起始序列，形成新的侧根。本文的研究揭示了侧根形成的形态发生层序，存在主要根组织和干细胞的早期、顺序分裂模式。