Suppression of lateral root initiation in Barley in response to transient root water shortage: a novel role for ABA in lateral root initiation?

The plasticity of branching patterns of plant roots plays a crucial role in the capture iof resources from unpredictable, heterogeneous and dynamic soil environement. This plasticity is largely achieved through the regulation of the site and time of initiation of lateral roots. The seminal root of Hordeum v. seedlings grown in aeroponics lack branches in the segment which is formed during transient mild root water shortage episodes.This response is highly reproductible and is not due to changes of mineral concentrations in the root zone that might result from the treatment. Surprisingly in view of abscissic acid effects in Arabidopsis root development, transient application of 50μM ABA produces the same response as the water shortage. A miscrocopy approach was perfromed to characterise the stage at which primordium development is blocked by this treatment. We shown that LR formation in Barley is restricted to the apical region of the seminal root and follows an acropetal sequence. The first asymmetric division, though to be the earliest visible stages of LR initiation, takes place between 10 and 15 mm from the tip. No primordia were found in the zone devoid in LR (under water stress or ABA treatment). In addition, careful time lapse imaging of the seminal root reveals that the region devoid of LR under water stress or ABA treatment is offset by about 8 mm proximal to the SR segment formed during the treatment. These results suggest a checkpoint-like regulation of lateral initiation operating at about 8 mm from the tip, i.e. in the very early phases of LR initiation (before or around the first asymmetric divisions). Experiments to validate and elucidate the role of ABA in this process are under way.