Agriculture Reference
In-Depth Information
cells isolated from leaves of
Zinnia
can be induced by appropriate hormonal manip-
ulations to undergo terminal differentiation to form tracheary elements (Fukada &
Komamine, 1980). During this process, a number of proteins and carbohydrates
are secreted into the medium (Stacey
et al.
, 1995) and specific changes in gene
expression occur (Milioni
et al.
, 2002). Motose
et al.
(2001) reported that a secreted
compound facilitates tracheary element differentiation and that an active fraction
identified through their analysis contained AGP. Whether AGP is the active principle
in this fraction has to be substantiated, but these data are again indicative of AGPs
playing a functional role in differentiation.
The identification of genes encoding the core proteins of AGPs has opened the
door to the use of molecular genetic tools to further investigate AGP function. The
first data from these approaches are now being published and have proved infor-
mative. For example, an AGP has been described in cucumber whose expression is
gibberellin-induced (Park
et al.
, 2003). Ectopic expression of this gene in transgenic
tobacco plants led to increased AGP accumulation and increased internode elonga-
tion, suggesting that the influence of gibberellin on plant growth might be, at least in
part, mediated via AGPs. However, the interpretation of gene function based purely
on ectopic expression can be problematic and more definitive data can be obtained
from experiments in which gene expression is abrogated. In this context, the first
reported phenotypes of plants with mutations in AGP-encoding genes are intriguing,
and at the same time somewhat surprising. Van Hengel and Roberts (2003) reported
on the characterisation of an insertional mutant in an
Arabidopsis
gene encoding
AtAGP30, leading to the loss of expression of this specific AGP. Plant growth and
development was apparently normal, but
in vitro
root regeneration was suppressed,
providing a link to previous data implicating AGPs in aspects of root growth (Van
Hengel & Roberts, 2002). In addition, the mutant seeds showed an altered response
to abscisic acid, a hormone correlated with various elements of seed development
and germination. How exactly loss of AtAGP30 function might lead to the observed
phenotypes is not yet clear, but the data provide some of the first molecular genetic
data on AGP function.
In another report on loss of AGP function, Shi
et al.
(2003) were investigat-
ing
Arabidopsis
mutants altered in their response to salt stress. One of the genes
identified by this approach (
SOS5
) encoded a protein that contained AGP-like do-
mains but was distinct from previously characterised AGP sequences. Under normal
growth condition, the mutant appeared normal, but after salt stress the root tips be-
came swollen and cellular organisation was disrupted. The authors proposed that the
SOS5 AGP-like protein plays a role in cell wall architecture and adhesion and that
mutation of the gene leads to at least some cells becoming prone to abnormal expan-
sion. In particular, it is possible that the SOS5 protein (and other AGP-like proteins)
is involved in linking the plasma membrane and cell wall. This idea is interesting,
especially bearing in mind the recent data showing that AGPs can interact with wall-
associated kinases (WAKs) (Gens
et al.
, 2000). WAKs (which will be examined in
more detail later in this chapter) have been proposed as potential bridges between
the plasma membrane and the cell wall and to be involved in the transduction of
