Aerial and terrestrial root habits influence the composition of the cell walls of Vanilla phaeantha (Orchidaceae)

In response to the restrictions imposed by their epiphytic habit, orchids have developed structural traits that allow greater efficiency in water uptake and use, such as a complex adventitious root system with velamen. The composition of cell wall of this specialized epidermis can be altered according to the substrate to which it is fixed, influencing wall permeability, absorption, and storage of water in roots. The current study aimed to evaluate the cell wall composition of adventitious roots of Vanilla phaeantha (Orchidaceae) that grow attached to the phorophyte, fixed in the soil, or hung free. Immunocytochemical analyses were used to determine the protein, hemicellulose, and pectin composition of the cell walls of aerial and terrestrial roots. We observed that pectins are present in the different tissues of the aerial roots, while in the terrestrial roots, they are concentrated in the cortical parenchyma. The deposition of xyloglucans, extensins, and arabinogalactans was greater in the epidermis of the free side of the roots attached to the phorophyte. The strong labeling of pectins in aerial roots may be related to the influx of water and nutrients, which are generally scarce in this environment. The arrangement of hemicelluloses and proteins with the pectins may be associated with increased cell rigidity and sustainability, a feature of interest for the aerial roots. In summary, the habit of roots can interfere with the non-cellulosic composition of the cell walls of V. phaeantha, possibly related to changes in cell functionality.