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Fig. 8. A speculative model for GNOM action – canalising auxin fluxes. (A)
Heart-stage embryos of wild-type (top) and strong gnom allele
(bottom). Yellow lines delineate the apical (a), central (c) and basal (b)
regions of the embryos and their relation to the body pattern of the seedling
(right). Black arrows indicate auxin flow from sources in the apical part of
the embryo to the sink in the basal part. Presumed auxin gradients are shown
at the left. (B) Relationship between localisation of PIN1 efflux carrier
(red) and auxin-response gradients (blue) in lateral root primordium
development. Arrows indicate auxin canalisation by gradual re-orientation of
individual transport polarities of cells. Red stubs touching a given cell
boundary mark the cell to which the respective PIN1 label is thought to
belong. (C) Presumptive critical step for the canalisation of auxin flow
during lateral root formation. Stage II lateral root primordium immediately
after division is shown at the left, with the two daughter cell layers
displaying opposite polarities. Gradual, GNOM-dependent, relocalisation of
efflux carriers might be guided by weak polarising cues from adjacent tissues,
supplying more auxin to the inner layer, which then imposes its auxin
transport polarity on the outer layer. Arrows indicate direction of auxin
flux; auxin efflux carriers (PIN1; in red); GN, GNOM-positive endosomes
involved in recycling auxin carriers.
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