Suppression of lateral root initiation occurs in response to transient root water shortage and reveals the presence a new checkpoint

The plasticity of branching patterns of plants plays a crucial role in the capture of resources from unpredictable, heterogeneous and fl uctuating environments. This plasticity is achieved by regulating the site and timing of initiation and controling the further steps of the developmental program of new branches. We show in Hordeum vs seedlings grown in aeroponics that the distal seminal root segment formed during a transient mild root water shortage is devoid of lateral roots (LR). Since LR formation in barley occurs in the apical region of the seminal root (SR) and follows an acropetal sequence, this suggests that the treatment suppresses LR initiation. The response is highly reproducible and is not due to changes of mineral concentrations in the root zone that might result from the treatment. It is, however, very sensitive to the atmospheric relative humidity and to the number of elongating leaves and roots. Tip excision experiments show that the SR meristem is involved in the response. In addition,the characterization of the developmental stages of LR primordia along treated SR indicates that the suppression occurs before the fi rst divisions of LR founder cells. Interestingly, careful time lapse imaging of the SR reveals that the zone devoid of LR is localized about 8 mm proximal to the SR segment formed during the treatment. This study suggests the presence of a checkpoint operating in the early phases of LR initiation and allows to predict its site of action.