Comparative study of cell walls during wheat and rice grain development

The cell wall polysaccharides of wheat and rice endosperm represent an important
source of dietary fibre with up to 50% of the dietary fibre intake in western diets
deriving from cereal consumption. Plant cell walls are complex structures composed
of many interlinking polysaccharide chains as well as lignin, phenolics and some
proteins. The significance of many of these molecules has yet to be elucidated,
however cell walls have been shown to be rapidly modified during growth and
differentiation demonstrating that the cell wall is a dynamic structure modified in
muro to adapt to changing biological constraints. Wheat and rice present near
synchronous developmental cycles and significantly different endosperm cell wall
compositions, allowing the localization of these polysaccharides to be related to
developmental changes. Monosaccharide analysis has been widely used on mature
endosperm and flour sample in cereal grains to provide an overview of cell wall
composition, but no previous studies have considered different developmental
stages. In both wheat and rice four distinct phases of cell wall deposition were
detected, with 4-8 days after anthesis (DAA) and 12-20 DAA showing the greatest
levels of depositions in all monosaccharides. After 20 DAA significant deposition of
pectic polysaccharides was detected in both species, which may reflect preparations
for grain dehydration. Monoclonal antibodies specific to cell wall polysaccharides
and immunofluorescence microscopy were used to determine the spatial and
temporal locations of these polysaccharides. A conserved sequence of
polysaccharide deposition during cellularisation was also seen in both species,
matching that reported in barley grains. Arabinogalactan-petides (AGPs) are a
significant component of wheat and rice grains and through the use of novel wheat
AGP monoclonal antibodies they were shown to be localised either at the plasma
membrane or in the cytoplasm, contrary to previous hypotheses that they may be
cell wall proteins.