The initiation of new lateral roots in Arabidopsis occurs at predictable distances from the growing tip and depends on auxin transport and redistribution. However, the precise mechanism that regulates the positioning of new roots has remained elusive. Tom Beeckman and colleagues now show, on , how oscillating waves of auxin accumulation and response in the cells that exit the root meristem bring about the regular left-right alternating pattern of lateral root development along the main root axis. AUX1, an auxin influx carrier, is essential for this left-right patterning in response to gravity; it transports auxin from cells exiting the root tip back to those still in the root tip. Cells between the growing tip and the meristem display an oscillatory responsiveness to auxin with a periodicity of 15 hours. The authors demonstrate that this peak in auxin responsiveness corresponds precisely with the formation of a lateral root. Thus cells are primed for root development while still in the root tip.
