Abstract：
The evolution of the plant vascular system is a key process in Earth history because it enabled plants to conquer land and transform the terrestrial surface. Among the vascular tissues, the phloem is particularly intriguing because of its complex functionality. In angiosperms, its principal components are the sieve elements, which transport phloem sap, and their neighboring companion cells. Together, they form a functional unit that sustains sap loading, transport, and unloading. The developmental trajectory of sieve elements is unique among plant cell types because it entails selective organelle degradation including enucleation. Meticulous analyses of primary, so-called protophloem in the Arabidopsis thaliana root meristem have revealed key steps in protophloem sieve element formation at single-cell resolution. A transcription factor cascade connects specification with differentiation and also orchestrates phloem pole patterning via noncell-autonomous action of sieve element-derived effectors. Reminiscent of vascular tissue patterning in secondary growth, these involve receptor kinase pathways, whose antagonists guide the progression of sieve element differentiation. Receptor kinase pathways may also safeguard phloem formation by maintaining the developmental plasticity of neighboring cell files. Our current understanding of protophloem development in the A. thaliana root has reached sufficient detail to instruct molecular-level investigation of phloem formation in other organs.
摘要：
植物维管系统的进化是地球历史上的关键过程，因为它使植物能够征服土地并改变陆地表面。在血管组织中，韧皮部是特别有趣的，因为它的复杂的功能。在被子植物中，它的主要成分是筛子元素，运输韧皮部汁液，和它们的相邻伴侣细胞。一起,它们形成一个维持汁液加载的功能单元，运输,和卸载。筛子元素的发育轨迹在植物细胞类型中是独特的，因为它需要选择性的细胞器降解，包括去核。对初级的细致分析，拟南芥根分生组织中所谓的原韧体已经揭示了在单细胞分辨率下形成原韧体筛元素的关键步骤。转录因子级联将规格与分化联系起来，并且还通过筛子元素衍生的效应子的非细胞自主作用来协调韧皮部极的形成。让人联想到次级生长中的血管组织图案，这些涉及受体激酶途径，其拮抗剂指导筛元素分化的进展。受体激酶途径还可以通过维持邻近细胞文件的发育可塑性来保护韧皮部形成。我们目前对拟南芥根中原韧皮部发育的理解已经足够详细，可以指导其他器官韧皮部形成的分子水平研究。
