Fig. 7. Hypothetical model for auxin signaling in the basal meristem and subsequent lateral root initiation. (A,B) Possible presence of an auxin (response) gradient based on the staining patterns of auxin reporters. In accordance with the auxin gradient in the quiescent center and surrounding initials, the lateral root initiation `stem cells' are triggered by the neighboring xylem cell showing high auxin concentration and/or response (A). When this gradient disappears (as is the case of auxin transport inhibition), auxin is distributed equally (even into the pericycle) and no lateral root initiation can take place (B). Light to dark blue shades reflect low to high auxin content. Xylem pole pericycle cells that will initiate a lateral root are in red. (C) Scheme of two adjacent pericycle cells on the same cell file undergoing early developmental steps prior to lateral root initiation: from priming by auxin in the basal meristem (associated with migration of nuclei as indicated with the dotted arrows) to the auxin response required for asymmetric cell division. (D) Hypothetical scheme showing the longitudinal progression of pericycle cells in time and space in the main root with the indication of the major developmental steps toward lateral root initiation. First, auxin (blue arrows) is targeted to the basal meristem (BM) from the root apical meristem (RAM) via AUX1-mediated transport in the lateral root cap (dark gray). Subsequently, reflux (PIN-mediated) is presumably involved in generating the auxin maximum in the protoxylem cells (blue strands) adjacent to the pericycle cells (green). Later, in the differentiation zone, the primed pericycle cells (light blue) are exposed again to auxin response and signaling mechanisms (Aux/IAAs, for instance IAA14/SLR) before CYCB1;1 becomes expressed and the actual division occurs (light-blue cells with dark-blue center)