Logistics of water and salt transport through the plant: structure and functioning of the xylem

The xylem is a long-distance transport system that is unique to higher plants. It evolved into a very sophisticated plumbing system ensuring controlled loading/unloading of ions and water and their effective translocation to the required sinks. The focus of this overview will be the intrinsic inter-relations between structural and functional features of the xylem. Taken together the xylem is designed to prevent cavitation (entry of air bubbles), induced by negative pressures under transpiration and to repair the cavitated vessels. Half-bordered pits between xylem parenchyma cells and xylem vessels are on the one hand the gates to the vessels but on the other hand a serious 'bottle-neck' for transport. Hence it becomes evident that special transport systems exist at the interface between the cells and vessels, which allow intensive fluxes of ions and water to and out of the xylem. The molecular identification and biophysical/biochemical characterization of these transporters has just started. Paradigms for the sophisticated mechanism of controlled xylem transport under changing environmental conditions are SKOR, a Shaker-like channel involved in K+-loading and SOS1, a Na+/H+ antiporter with a proposed dual function in Na+ transport. In view of the importance of plant water relations it is not surprising to find that water channels dominate the gate of access to xylem. Future studies will focus on the mechanism(s) that regulate water channels and ion transporters and on their physiological role in, for example, the repair of embolism. Clearly, progress in this specific field of research will greatly benefit from an integration of molecular and biophysical techniques aimed to understand 'whole-plant' behaviour under the ever-changing environmental conditions in the daily life of all plants.