The development of xylem tracheary elements (TEs)--the hydro-mineral sap conducting cells--has been an evolutionary breakthrough to enable long distance nutrition and upright growth of vascular land plants. To allow sap conduction, TEs form hollow laterally reinforced cylinders by combining programmed cell death and secondary cell wall formation. To ensure their structural resistance for sap conduction, TE cell walls are reinforced with the phenolic polymer lignin, which is deposited after TE cell death by the cooperative supply of monomers and other substrates from the surrounding living cells.
Keywords: lignin; non-cell autonomous process; post-mortem lignification; secondary cell wall; tracheary elements; xylem/wood vessels.