CAS Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla, Yunnan, China.
Center of Plant Ecology, Core Botanical Gardens, Chinese Academy of Sciences, Mengla, Yunnan, China.
Ann Bot. 2020 Jul 24;126(2):261-275. doi: 10.1093/aob/mcaa077.
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 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).
We combined the use of transmission electron microscopy, X-ray micro-tomography 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.
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 upregulated. 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 was involved in the final execution of cell death and autolysis.
Our results showed 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.
根表皮外表面发育的根被是某些植物科（特别是附生兰花）吸收和保留水分的重要特征，有助于它们在缺水环境中生存。根被细胞来源于初生根分生组织；然而，在这种发育之后，根被细胞通过不完全理解的程序性细胞死亡（PCD）过程死亡。
我们结合使用透射电子显微镜、X 射线微断层扫描和转录组学方法，对典型附生兰花建兰根被细胞发育和死亡过程中发生的主要解剖和分子变化进行了特征描述，以确定 PCD 是如何发生的。
在建兰根被细胞发育过程中观察到典型的 PCD 解剖学和基因表达变化。在 PCD 的起始阶段，我们发现细胞和液泡的大小都增加了，并且几个参与油菜素内酯和乙烯途径的基因被上调。在次生细胞壁形成阶段，显著的解剖学变化包括 DNA 降解、细胞质变薄、细胞器减少、液泡破裂和细胞壁增厚。与纤维素和木质素生物合成相关的基因表达发生变化，这些基因在次生细胞壁形成中起作用，受细胞骨架相关因子和苯丙氨酸解氨酶调节。在 PCD 的最后阶段，从根被的外部到内部终止了细胞自溶。与自噬、液泡加工酶、半胱氨酸蛋白酶和拟南芥胱天蛋白酶相关的基因的调节参与了细胞死亡和自溶的最终执行。
我们的结果表明，附生兰花根被的发育受 PCD 过程的控制，包括 PCD 的起始、次生细胞壁的形成以及细胞死亡后自溶的执行。
