Fig. 4. Regular lateral root spacing and oscillation in DR5::GUS activity in the basal meristem. (A) DR5::GUS-activity in a 7.5-hour time course in Col-0 (gray) and aux1-7 (white) expressed as percentage of seedlings with strong GUS staining at each time point. The green line highlights the oscillation in DR5 activity in Col-0. Labels a-d (green circles) mark the time points with high DR5 activity. ND, not determined. (B) Schematic representation of toner label experiment. CYCB1;1::GUS seedlings were labeled with toner particles (left) and after 30 hours stained for GUS (right). (C) Detail of seedling root with toner particles attached on top of the lateral root initiation site (^*). Arrowhead, adventitious root. (D) Percentage of seedlings with toner label on top of lateral root initiation site. Labels a-d (green circles) mark the time points with higher percentage of seedlings having toner label on top of lateral root initiation site. (E) Percentage of seedlings with 0-3 lateral root primordia at 30, 45 and 60 HAG. Labels a-c (blue circles) indicate time points at which the next lateral root appears at the earliest. (F) Timeline indicating the intervals between four successive maxima of DR5 activity (green circles) and the periodicity of initiation of the first three lateral roots after germination (blue circles). The 15-hour periodicity in the DR5::GUS expression matches the timing of lateral root initiation.