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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)
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