The plant extracellular matrix and signalling - Intercellular Communication in Plants - page 91

A

B C

Figure 4.2 Arabinogalactan protein (AGP) structure and potential for signal generation. (A) A typical
AGP consists of a protein backbone that is extensively modified by the attachment of complex
carbohydrates. The AGP may be anchored to a membrane via a lipid anchor. (B) The AGP may be
detached from the lipid anchor via action of a phospholipase, generating a free AGP with signalling
potential. (C) Further processing of AGPs containing N -acetylglucosamine oligomers in the
carbohydrate side chains may occur via action of chitinases. Both the modified AGP and the
N -acetylglucosamine oligomers released could act as signals.

cells above the quiescent centre and subsequently appears in files of cells more prox-
imal to the root tip (Dolan et al. , 1995). Other AGP epitopes were found to show
tissue-specific patterns in flowers of peas (Pennel & Roberts, 1990), developing
carrot roots (Knox et al. , 1991) as well as in pistils (Du et al. , 1996), and germi-
nating pollen tubes (Li et al. , 1995). Such cell-specific patterns were also found in
in vitro cultured plant tissues, most notably those associated with somatic embryo-
genesis. For example, analysis of carrot suspension cultures revealed that cells that
possessed embryogenic potential were distinguishable from their non-embryogenic
counterparts by the presence of a particular AGP epitope recognised by the antibody
JIM8 (McCabe et al. , 1997). Such correlative observations led to the hypothesis that
AGPs might act as markers for particular developmental fates and, moreover, that
they might actually function as cell-wall-based signals to communicate cell iden-
tity between neighbouring cells and/or to influence tissue growth. There are some
functional data to support this hypothesis.
Firstly, the supply of free AGPs to in vitro systems has been shown to influence
tissue growth and differentiation. Thus, AGPs added to suspension cultures were
found to inhibit cell division (Thompson & Knox, 1998) and exogenous AGPs were
shown to influence the embryogenic potential of suspension culture cells (Toonan
et al. , 1997; Van Hengel et al. , 2001). These last data are reminiscent of the findings of
McCabe et al. (1997), who showed that non-embryogenic cells secreted a compound
into the medium that facilitated embryogenesis from embryogenic cells. Although
the nature of this compound was not demonstrated, correlative data suggested that
the factor might be an AGP.
Further data supporting a functional role for AGPs in differentiation has come
from analysis of another in vitro system, xylogenesis in Zinnia elegans . Mesophyll