Agriculture Reference
In-Depth Information
leaf epidermal identity (Becraft
et al.
, 1996) and the
ARC4
gene in
Arabidopsis
is
involved in cellular organisation during growth of the ovule integument (Gifford
et al.
, 2003). Although it seems likely that peptide factors are involved in these
signalling processes, the factors have not yet been identified. Similarly, the finding
that mutations in subtilisin-like proteases influence epidermal differentiation and
stomatal patterning also strongly indicates the presence of as yet uncharacterised
peptide based signalling components that are processed within the ECM (Berger &
Altmann, 2000) (see also Chapter 2).
These examples have arisen from the analysis of higher plants, but work with
other experimental systems has also implicated the cell wall as a key element in
determining cellular differentiation. In particular, research using the marine alga
Fucus
has led to novel insights.
Fertilised eggs of
Fucus
undergo a series of conserved cell divisions to generate
apical thallus (shoot-like) and basal rhizoid (root-like) organs. This process occurs
in the free-living zygote and the early events of cell growth and division can easily be
followed under the microscope. This is in contrast to the situation in angiosperms
where such events occur hidden deep within the surrounding sporophytic tissue.
The
Fucus
system has thus been used as experimental model to gain insight into
the earliest events of plant embryogenesis (Brownlee & Berger, 1995). Using novel
tools of laser ablation, Brownlee and colleagues undertook a series of elegant exper-
iments to decipher the outcome on embryo development of the selected removal of
particular cell types (Berger
et al.
, 1994). One of the most informative experiments
involved the destruction of the rhizoid protoplast to leave a presumptive thallus
cell subtended by fragments of the rhizoid wall. A variety of growth patterns were
observed, but if thallus-cell growth led (fortuitously) to contact with the remnants
of the rhizoid cell wall, the thallus cell underwent a transdifferentiation to form a
rhizoid-like structure. The conclusion from this experiment is that the rhizoid cell
wall must contain some type of information that specifies rhizoid cell fate. The
molecular character of this information is as yet unknown. Nevertheless, the experi-
ments clearly indicate the potential of cell wall epitopes as determinants of cell fate
and suggest that such cell wall determinants might partake in a cross-talk between
adjacent cells, leading to the appropriate differentiation of each cell type. Such two-
way cellular discussions are common in animal systems and are characterised by the
Notch/delta system involving the proteolytic cleavage and sensing of signals within
the ECM (Artavanis-Tsakonas
et al.
, 1999; Fortini, 2001). Obvious counterparts to
Notch/delta components have not yet been reported in plants.
To summarise, a number of intriguing experiments and observations indicate the
presence of many as yet uncharacterised chemical signals in plants. For some of these
there are hints and leads as to their identity. For example, the genomic data indicating
the presence of novel peptide-type signalling components provide a strong synergy
with biochemical and molecular genetic data that have led to the identity of peptide-
based signals (Fletcher
et al.
, 1999; Yang
et al.
, 1999) (see Chapter 2). Although the
precise linkage between signal processing system, signal factor and specific receptor
has generally not yet been achieved, rapid and significant progress in this area can
