The velamen radicum— a spongy tissue usually composed of multiple layers of dead cells on the outer surface of the epidermis of plant roots—is found throughout the monocotyledons, but is more common in epiphytic orchids. However, the developmental process of the velamen radicum remains poorly understood. In the velamen radicum, there have been no studies indicating the position of the developmental process in root tips. 

In a study published in Annals of Botany, researchers found that the development of the root velamen radicum in an epiphytic orchid was controlled by the process of programmed cell death (PCD), which included initiation of PCD, followed by formation of the secondary cell wall, and execution of autolysis following cell death. 

The study was jointly completed by researchers from Xishuangbanna Tropical Botanical Garden (XTBG) and Kunming Institute of Botany (KIB). 

The researchers 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 occurred. 

By using transmission electron and light microscopy, they found that cell and vacuole size increased, DNA degraded, cytoplasm and organelles decreased, the vacuoles ruptured and cell walls thickened with the development of the velamen radicum. 

Using x-ray micro-computerized tomography, they showed that the final process of autolysis occurred gradually but exponentially from the outside to the inside of the velamen radicum. 

Changes in the expression of genes also played important roles in molecular control of the processes: initiation of PCD, formation of the secondary cell wall, and execution of autolysis in velamen radicum development. 

"These results therefore improve our understanding of specific progress of this process involved in developmental PCD in the velamen radicum of epiphytic orchids", said Prof. ZHANG Jiaolin, a correspondence author of the study.