A novel two-component hybrid molecule regulates vascular morphogenesis of the Arabidopsis root

The developmental ontogeny of the vascular system (consisting of xylem, phl oem and [pro]cambium) is poorly understood despite its central role in plan t physiology. We show that in the Arabidopsis root meristem, xylem cell lin eages are specified early, whereas phloem and procambium are established th rough a set of asymmetric cell divisions. These divisions require the WOODE N LEG (WOL) gene. The WOL gene encodes a novel two-component signal transdu cer with an unusual tandem arrangement of two receiver domains. It is expre ssed specifically in the vasculature from the early stages of embryogenesis on, consistent with a role as a sensor for vascular morphogenesis.