Plants have evolved many mechanisms to cope with soil drying. One of the physiological mechanisms, which has recently received much attention, is that when plants are subjected to soil drying, shoot physiology is largely regulated by a root-derived chemical signal, in addition to the developing shoot water deficit which is traditionally considered mainly responsible. The nature and function of this signal have been studied over one decade and our earlier work has shown that abscisic acid (ABA) is the most possible candidate of such a signal. In this review, our recent work on the sources, metabolism, transport pathway, fate of the root-derived ABA and its relations to shoot physiological processes when plants are subjected to water deficit is described.