Early development of epiphytic roots: perspectives based on the composition of the velamen cell wall

ABSTRACT The velamen, a root structure of some epiphytic species for water uptake, usually is stratified epidermis consisting of dead cells. In general, its cell walls exhibit variation during development, including in thickness and amount and type of impregnated substances. These changes result in diverse physical and chemical properties that can serve in water and nutrient uptake, as well as in mechanical support and protection. On this basis, the main objective of the current study was to describe the composition of the cell walls of the velamen during the development of the roots of four species of Cattleya. Anatomical, histochemical and immunocytochemical analyses were performed with samples (n=3 individuals per species) of meristematic, developing, and mature regions of the root. The development of the primary wall led to the deposition of pectins as highly methylesterified homogalacturonans, which were demetihylesterified with maturation of the velamen. The deposition of lipids (and subsequently lignins) in velamen cells marked a transition stage to the formation of the secondary wall, which gives rigidity to the tissue. For the first time, we showed that the deposition of lipids and lignins began close to the exodermis in the direction of the epivelamen.

摘要 根被（velamen）是部分附生植物用于水分吸收的根系结构，通常为由死细胞构成的分层表皮组织。一般而言，其细胞壁在发育过程中会发生变化，涉及厚度、浸渍物质的含量与类型等方面。这些变化赋予其多样的物理与化学特性，可用于水分与养分吸收，同时兼具机械支撑与保护功能。基于此，本研究的核心目标是描述卡特兰属（Cattleya）4个物种根系发育过程中根被的细胞壁组成。本研究对4个物种根部分生区、发育区与成熟区的样本（每个物种3个个体）开展了解剖学、组织化学与免疫细胞化学分析。初生壁发育阶段会沉积高度甲酯化的同聚半乳糖醛酸类果胶，随着根被成熟，这些果胶会发生脱甲酯化修饰。根被细胞内脂质（随后是木质素）的沉积标志着向次生壁形成的过渡阶段，次生壁可为组织提供刚性支撑。本研究首次证实，脂质与木质素的沉积起始于靠近外皮层（exodermis）、朝向外根被层（epivelamen）的区域。