Agriculture Reference
In-Depth Information
A
B
Figure 1.6
Auxin transport and distribution during organogenesis. Sites of auxin accumulation are
shadowed. Presumptive routes of auxin transport are depicted by arrows. (A) Lateral root primordium:
Auxin is provided by PIN1-dependent auxin transport through the primordium interior toward the tip,
where it accumulates. From here, part of the auxin is retrieved through the outer layers. (B) Auxin is
provided to the primordium tip through the outer layers. From the tip, auxin is drained through a
gradually established transport route toward the vasculature. Reproduced from Benkova
et al
. (2003),
with permission.
already initiated primordia, which transport auxin into their interior and further
toward already differentiated vasculature (Benkova
et al
., 2003). Therefore, auxin
is depleted from the surroundings of the primordia (sink function) and its highest
concentration remains at the most distant position, where a new primordium is ini-
tiated. Thus, a positive feedback represented by auxin accumulation in combination
with lateral inhibition provides a mechanism for reiterativity and stability of the
spiral or other types of phylotactic pattern (Reinhardt
et al.
, 2003). In the root,
auxin also accumulates by a PAT-dependent mechanism at sites of organ initiation
(Benkova
et al
., 2003), but how it is precisely regulated remains so far unclear.
Once sites of organ initiation are selected, cell division is activated and the organ
primordium develops along a new growth axis. In all types of primordia, PIN polar
localization reorganizes and the new direction of PIN-mediated auxin transport
determines the growth axis of the developing organ, establishing an auxin gradient
with its maximum at the tip (Benkova
et al.
, 2003). Interestingly, the main auxin
transport routes appear to be reversed, when lateral root primordia are compared
with other types of organs. In aerial organs, auxin is supplied through the outer layer
and accumulates at the primordium tip (Fig. 1.6B). From the tip, auxin is transported
into the interior of the primordium. In the case of lateral roots, auxin is provided to
the tip through the inner cells and distributed away through the primordium surface
(Fig. 1.6A). Regardless of these differences, a common auxin-gradient-dependent
mechanism seems to underlie organ initiation as well as primordium development
in all plant organs regardless of their mature morphology or developmental origin.
1.5.4 Root meristem maintenance
Plants, in contrast to animals, not only can initiate new organs postembryonically,
but also possess specialized, permanently dividing and differentiating tissues called
