Arabinoxylan, β-glucan and pectin in barley and malt endosperm cell walls: a microstructure study using CLSM and cryo-SEM.

Abstract

The architecture of endosperm cell walls in Hordeum vulgare (barley) differs remarkably from that of other grass species and is affected by germination or malting. Here, the cell wall microstructure is investigated using (bio)chemical analyses, cryogenic scanning electron microscopy (cryo-SEM) and confocal laser scanning microscopy (CLSM) as the main techniques. The relative proportions of β-glucan, arabinoxylan and pectin in cell walls were 61, 34 and 5%, respectively. The average thickness of a single endosperm cell wall was 0.30µm, as estimated by the cryo-SEM analysis of barley seeds, which was reduced to 0.16µm after malting. After fluorescent staining, 3D confocal multiphoton microscopy (multiphoton CLSM) imaging revealed the complex cell wall architecture. The endosperm cell wall is composed of a structure in which arabinoxylan and pectin are colocalized on the outside, with β-glucan depositions on the inside. During germination, arabinoxylan and β-glucan are hydrolysed, but unlike β-glucan, arabinoxylan remains present in defined cell walls in malt. Integrating the results, an enhanced model for the endosperm cell walls in barley is proposed.