Agriculture Reference
In-Depth Information
Surprisingly,
SHR
RNA is not expressed in the ground tissue cell lineage but in
the stele (pericycle and vascular cylinder) located immediately adjacent to it, sug-
gesting a non-cell autonomous mode of action of SHR (Fig. 8.7A) (Helariutta
et al.
,
2000). In order to approach this non-cell autonomous mechanism, Nakajima
et al.
(2001) compared the protein accumulation to the RNA accumulation pattern and
found that the SHR protein is transported from the stele to the adjacent endodermal
layer, probably through plasmodesmata. This indicates that SHR acts as a mobile
signal in exerting its functions as a transcriptional regulator. Although movement
of transcription factors had been observed before (Lucas
et al.
, 1995), this was the
first time a functional significance for such movement was demonstrated. Nakajima
et al.
(2001) also explored the outcome of introducing SHR ectopically to the en-
dodermal layer under the SCR promoter. In this case a highly specific increase of
ground tissue layers having endodermal characteristics was observed (Fig. 8.7B).
Based on the morphological analysis of the
scr
and
shr
mutants as well as on a
detailed spatio-temporal analysis of
SCR
and
SHR
gene expression, it is evident that
the patterning of ground tissue is established already during early embryogenesis.
8.4.5 Vascular patterning
The formation of the vascular network of a plant takes place continuously at the
meristematic regions of a plant during both shoot and root development. Even though
under normal conditions vascular development is highly predictable, when the situ-
ation arises it can also react and adapt to either localized or environmental stimuli.
As described in the context of distal patterning of the root, the establishment of the
vascular network has also been shown to involve auxin transport and auxin signalling
(Aloni, 1987; Sachs, 1991; Ye, 2002).
The vascular network in plants consists of transport tissues: xylem, which trans-
ports water and nutrients; and phloem, which transports photosynthates. Between
them there is a third vascular tissue type, the (pro)cambium, that consists of the
stem cells from which the xylem and phloem elements originate. There are several
distinct patterns in which these three tissues are organized in different plant species
(Ye, 2002).
Although xylem and phloem both are formed from cambial initial cells, their fate
is quite different. The phloem is a system of several cell types: sieve elements (SE),
companion cells (CC), phloem fibres and phloem parenchyma cells. The SE are
the actual transport cells. They undergo a partial autolysis (involving disintegration
of the nucleus) and in some regions of the plant deposit callose on its cell walls.
The CC support SE with macromolecules (Esau, 1977; Kuhn
et al.
, 1997; Oparka
and Turgeon, 1999). Xylem consists of tracheary elements (TE), xylem parenchyma
cells and xylem fibres. The differentiation of TE involves deposition of elaborate
cell wall thickenings (containing cellulose and lignin) and programmed cell death.
In contrast to other tissue types, the vascular pattern is established only during
the last stages of embryonic development after the vascular initials have divided to
first form a pattern of four near-identical poles, followed by a set of tangential and
