Regulation of Pectic Polysaccharide Domains in Relation to Cell Development and Cell Properties in the Pea Testa

The occurrence of pectic polysaccharide epitopes in cells and tissues of the pea testa during late stages of seed development have been examined in relation to anatomy and cell properties. Homogalacturonan, in a highly methyl-esterified form, was present throughout late development in all pea testa cell walls, including the thickened cell walls of the outer macrosclereid layer. Two epitopes, characteristic of the side-chains of the rhamnogalacturonan-I domain of pectic polysaccharides, occurred in restricted and separate cell layers of the pea testa. A (1-->4)-beta-D-galactan epitope was restricted to regions of the outer cell wall of the testa and to inner regions of the macrosclereid layer at 20 DAA and was absent from the osteosclereid and parenchyma cell walls. By 25 DAA the (1-->4)-beta-D-galactan epitope occurred only in the outer epidermal cell walls. A (1-->5)-alpha-L-arabinan epitope was also dependent on the developmental stage of the seed and was found with greatest abundance in the walls of the inner parenchyma cells. Cell separation studies indicated that, although calcium cross-links were involved in the maintenance of the link between the macrosclereid layer and proximal cell layers, most cell-to-cell adhesion in the testa was not due to calcium- or ester-based bonds.