Life beyond death: The formation of xylem sap conduits

Abstract

Xylem is the vascular tissue conducting water and minerals in plants. The conduction of the hydro-mineral sap in this tissue is enabled by specific conduit cells named tracheary elements (TEs). These vascular cells undergo a distinct differentiation programme which requires programmed cell death (PCD) to functionalise the cell for sap conduction: PCD empties the cell lumen leaving a hollow corpse delimited only by its cell wall to form the future vascular cylinder. In contrast to many other cell types, PCD initiates the ‘physiological life’ of TEs to enable the cell to conduct the hydro-mineral sap. This central role of PCD appeared as the first distinct differentiation event of TE ancestor cells during plant evolution. Breakthrough studies combining real-time live-cell imaging and TE differentiation in cell suspension cultures enabled to define the temporal succession of the pre-mortem TE differentiation events’cellulose and hemicellulose depositions in the secondary cell wall’and the post-mortem events including cell wall lignification and the clearing of the residual protoplast. The coordination between these different events and the exact timing of PCD is controlled by specific signalling molecules.