Agriculture Reference
In-Depth Information
2.7
Root Apices as “Brain-Like” Command Centres
Root apices show distinct cell files throughout their apices, a feature which
is especially prominent in root apices with closed meristems, such as maize
and Arabidopsis .Thecellfilesaredistinctbecausetheend-polesofadjacent
cellswithincellfilesaretightlyadhered,formingactin-basedplantsynapses
transporting auxin (Baluška et al. 2003a, b, 2004b, 2005). In contrast, the
shoot apices do not show distinct cells files and fail to show a DR5-signalled
auxin maximum (Aloni et al. 2003) which is characteristic for root apices
(Sabatini et al. 1999). Besides having synapses, root apices also exhibit
clear zonation whereby the apical meristem joins to a so-called transi-
tion zone which has sensory capabilities and where root curvatures are
initiated (Baluška et al. 2001c). As cells of the transition zone are not en-
gaged in any demanding activities, such as mitotic divisions or rapid cell
elongation, they are free to focus all their resources on the acquisition,
processing, and storing of information. As they are close to the phloem
unloading sites (Fig. 2.3), they are also flooded with sucrose (Stadler et
al. 2005), which allows them to perform ATP-dependent processes such
as robust ion channel activities, rapid vesicle recycling, and continuous
cytoskeleton rearrangements. Interestingly, the transition zone phloem el-
ements are of ancient type as they are lacking companion cells (van Bel
2003).
Postmitotic cells entering the transition zone reorganize actin filaments
from a previously diffuse perinuclear network into robust bundles which
extend from the nuclear surface towards non-growing end-poles (Baluška
et al. 1997; Volkmann and Baluška 1999; Voigt et al. 2005). After reaching
end-poles, the F-actin bundles anchor at these subcellular domains which
are specialized for the synaptic vesicle recycling which drives transcellular
auxin transport and which is also important for synaptic information pro-
cessing and storing (Baluška et al. 2003b, 2004b, 2005). Another dramatic
reorganization of the actin cytoskeleton is accomplished at the basal limit
of the transition zone when conical actin bundles, organized around the
centrally localized nucleus, become loosened, leave the nuclear surfaces,
and extend longitudinally between the non-growing end-poles. This second
reorganization of the actin cytoskeleton is essential for the onset of rapid
cell elongation (Baluška et al. 1997; Volkmann and Baluška 1999; Baluška et
al. 2000). In addition to the actin cytoskeleton, microtubules also undergo
rearrangements in cells which traverse the transition zone in such a way
that all cortical microtubules become transversely (with respect to the root
axis) oriented. This allows polarization of cell expansion with rapidly ex-
tending side walls but non-growing end-poles (Baluška et al. 1992, 1993;
Barlow and Baluška 2000).
 
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