背景回顾：The plant vasculature delivers phloem sap to the growth apices of sink organs, the meristems, via the interconnected sieve elements of the protophloem. In the A. thaliana root meristem, the stem cells form two files of protophloem sieve elements (PPSEs), whose timely differentiation requires a set of positive genetic regulators. In corresponding loss-of-function mutants, signaling of secreted CLAVATA3/EMBRYO SURROUNDING REGION 45 (CLE45) peptide through the BARELY ANY MERISTEM 3 (BAM3) receptor is hyperactive and interferes with PPSE differentiation. This can be mimicked by an external CLE45 application to wild type. 

提出问题：Because developing PPSEs express CLE45-BAM3 pathway components from early on until terminal differentiation, it remains unclear how they escape the autocrine inhibitory CLE45 signal. 

结果1-pH依赖性的CLE45响应：Here, we report that the wild type becomes insensitive to CLE45 treatment on neutral to alkaline pH media, as well as upon simultaneous treatment with a specific proton pump inhibitor at a standard pH of 5.7. We find that these observations can be explained by neither pH-dependent CLE45 uptake nor pH-dependent CLE45 charge. 

结果2-CLE45 R4：Moreover, pH-dependent perception specifically requires the CLE45 R4 residue and is not observed for the redundant PPSE-specific CLE25 and CLE26 peptides. 

结果3-PPSE发育状态与CLE45响应：Finally, pH-dependent CLE45 response in developing PPSEs as opposed to pH-independent response in neighboring cell files indicates that late-developing PPSEs can no longer sense CLE45. This is consistent with an apoplastic acidic to alkaline pH gradient we observed along developing PPSE cell files. 

结论：In summary, we conclude that developing PPSEs self-organize their transition to differentiation by desensitizing themselves against autocrine CLE45 signaling through an apoplastic pH increase.

植物通过相互连接的原生韧皮部筛分子，将韧皮部汁液运输至“库”器官的生长顶端，即分生组织。在拟南芥的根分生组织中，干细胞形成两队列的原生韧皮部筛分子（PPSEs），而其适时分化需要一系列的正遗传调控子。在相应的功能缺失突变体研究中，发现分泌肽CLE45及其受体BAM3的信号转导十分活跃，并且会干扰PPSE的分化。而通过对野生型外部施加CLE45小肽，也能够模拟这一表型。发育中的PPSE会从早期到末端分化都会持续表达CLE45-BAM3通路组分，因此尚不清楚PPSE分化是如何逃脱其自分泌的CLE45抑制信号的。本文中，作者发现野生型在pH呈中性或碱性的培养基上，会变得对CLE45处理不敏感，并且在pH为5.7的培养基上通过特定的质子泵抑制剂处理也会变得对CLE45处理不敏感。作者发现这些试验结果均不能用pH依赖性的CLE45吸收或者pH依赖性的CLE45生成来解释。此外，pH依赖性的特异性感知需要CLE45 R4残基，并且在PPSE特异性的CLE25和CLE26多肽中并未观察到这一现象。最终，作者发现在发育中的PPSE中存在pH依赖性CLE45响应，但是邻近队列细胞中表现为相反的pH非依赖性响应，表明PPSEs发育后期不能再感知CLE45。这与作者观察到的质外体pH随着发育中的PPSE细胞队列，从酸性过渡到碱性的梯度相一致。

** Christian S. Hardtke **