BACKGROUND AND AIMS Development of the velamen radicum on the outer surface of the root epidermis is an important characteristic for water uptake and retention in some plant families, particularly in epiphytic orchids, for survival under water-limited environments. Velamen radicum cells derive from the primary root meristem, however, following this development, velamen radicum cells die by incompletely understood processes of programmed cell death (PCD). METHODS We combined the use of transmission electron microscopy, x-ray microtomography, and transcriptome methods to characterize the major anatomical and molecular changes that occur during the development and death of velamen radicum cells of Cymbidium tracyanum, a typical epiphytic orchid, to determine how PCD occurs. KEY RESULTS Typical changes of PCD in anatomy and gene expression were observed in the development of velamen radicum cells. During the initiation of PCD, we found that both cell and vacuole size increased, and several genes involved in brassinosteroid and ethylene pathways were up-regulated. In the stage of secondary cell wall formation, significant anatomical changes included DNA degradation, cytoplasm thinning, organelle decrease, vacuole rupture and cell wall thickening. Changes were found in the expression of genes related to the biosynthesis of cellulose and lignin, which are instrumental in the formation of secondary cell walls, and are regulated by cytoskeleton-related factors and phenylalanine ammonia-lyase. In the final stage of PCD, cell autolysis was terminated from the outside to the inside of the velamen radicum. The regulation of genes related to autophagy, vacuolar processing enzyme, cysteine proteases and metacaspase were involved in the final execution of cell death and autolysis. CONCLUSIONS Our results found that the development of the root velamen radicum in an epiphytic orchid was controlled by the process of PCD, which included initiation of PCD, followed by formation of the secondary cell wall, and execution of autolysis following cell death.

中文翻译：

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控制附生兰花根部维拉门程序性细胞死亡的过程

背景和目的根表皮外表面上的维拉门根的发育是一些植物科，特别是附生兰花中水分吸收和保持的重要特征，以便在水分有限的环境中生存。Velamen radicum 细胞来源于初级根分生组织，然而，在这种发展之后，Velamen radicum 细胞因不完全了解的程序性细胞死亡 (PCD) 过程而死亡。方法我们结合使用透射电子显微镜、X 射线显微断层扫描和转录组方法来表征在典型的附生兰花蓝花蕙兰的 velamen radicum 细胞发育和死亡过程中发生的主要解剖和分子变化，以确定 PCD 是如何发生的。发生。主要结果 在维拉门细胞发育过程中观察到 PCD 在解剖学和基因表达方面的典型变化。在 PCD 的起始过程中，我们发现细胞和液泡的大小都增加了，并且参与油菜素类固醇和乙烯途径的几个基因被上调。在次生细胞壁形成阶段，显着的解剖学变化包括 DNA 降解、细胞质变薄、细胞器减少、液泡破裂和细胞壁增厚。发现与纤维素和木质素生物合成相关的基因表达发生变化，这些基因有助于次生细胞壁的形成，并受细胞骨架相关因子和苯丙氨酸解氨酶的调节。在PCD的最后阶段，细胞自溶从外向内终止。自噬、液泡加工酶、半胱氨酸蛋白酶和metacaspase相关基因的调控参与了细胞死亡和自溶的最终执行。结论 我们的结果发现附生兰花中根部维拉门根的发育受 PCD 过程的控制，包括 PCD 的启动，随后是次生细胞壁的形成，以及细胞死亡后自溶的执行。