What are tilosomes? An update and new perspectives

ABSTRACT Characterization of tilosomes and study of their development revealed that they are similar to the cell wall ingrowths of transfer cells. Roots from Anathallis sclerophylla were used to examine the development, histochemistry and ultrastructure of tilosomes using light, confocal and electron microscopy (both SEM and TEM). Tilosomes initiate as cellulosic papillae in the root elongation zone, increasing the apo/symplast surface throughout the wall-membrane apparatus. Both the development and structure of tilosomes and velamen strips appear similar to the reticulated and flange cell wall ingrowths of Vicia faba transfer cells. We suggest two functional phases for tilosomes: a) in young tissues they increase the symplast connection and thus improve outside-inside transport; and b) in mature tissues they direct solutes to passage cells. Both the physiology and the genetic mechanisms controlling the ontogeny of tilosomes remain obscure.