Cell wall development in the velamen of the orchid Miltoniopsis investigated by confocal microscopy

The velamen is a multicelled epidermal layer found in orchid roots. This layer consists of dead cells that have helical secondary wall thickenings and is usually thicker in epiphytic orchids with aerial roots than terrestrial orchids, consistent with the velamen’s role in water capture. While velamen struture has been widely studied as a taxonomic character, few studies have looked at its development. We used confocal microscopy to investigate cell wall formation in the epiphytic orchid Miltoniopsis. Development of the helical thickenings begins with microtubule-dependant cellulose deposition but as the secondary wall ridges develop, parallel microtubules bind either side of the ridge. Cellulose organization was also investigated using pontamine staining which showed bifluorescence of the cellulose strands. These data may provide insight into the mechanical efficiency of the velamen layer in orchid roots.