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.