Polysaccharide distribution and cell wall deposition in the endosperm and outer layers of developing sorghum grains

Despite its climate resilience and nutritional value, sorghum remains less studied than most major cereals. Cell walls are key to grain quality and processing. Yet, little is known about cell wall deposition during sorghum grain development. By immunofluorescence labeling and biochemical approaches, an overview of the spatiotemporal distribution of cell wall polysaccharides in sorghum grain has been obtained from the end of the cellularization step to maturity, both in the endosperm and outer layers. For the first time, the presence of minor polysaccharides, such as xyloglucans, pectins, and heteromannans were detected. By analysing the oligosaccharides released by xyloglucan-specific endoglucanase treatment using high-performance anion-exchange chromatography, we demonstrated that xyloglucans were galactoxyloglucans with a XXXG core motif, mainly including XLXG and XLLG units. Consistent with biochemical and immunolabeling results, an important proportion of the cell wall related genes identified by RNAseq analysis exhibited significant differential expression with higher expression at earlier developmental stages whatever the tissue. Many cell wall genes, including cellulose and hemicellulose biosynthesis genes, were more highly expressed in the outer layers than in the endosperm, except at the last stage. Overall, these results have improved our understanding of sorghum grain quality by providing new insights into cell wall formation.