A new structural element containing glycine-rich proteins and rhamnogalacturonan I in the protoxylem of seed plants.

The water pipes of elongating plant organs are the result of programmed cell death and are formed by the walls of dead and empty protoxylem elements. These protoxylem elements are passively elongated many times by the surrounding tissue before they are replaced and collapse. Well-known adaptations for this unique task include the characteristic secondary wall thickenings, forming rings and helices.

Protoxylem: the deposition of a network containing glycine-rich cell wall proteins starts in the cell corners in close association with the pectins of the middle lamella.

Antibodies were used to localise polysaccharide and protein networks in the protoxylem of etiolated soybean (Glycine max L.) hypocotyls. The deposition of glycine-rich proteins (GRPs) starts in the cell corners between protoxylem elements and xylem parenchyma cells.