Side chains of pectic polysaccharides are regulated in relation to cell proliferation and cell differentiation

The occurrence and function of the side chains occurring in the rhamnogalac turonan I domain of pectic poly- saccharides have been investigated during carrot cell development using monoclonal antibodies to defined epitopes of (1 --> 4)-beta-D-galactan and (1 --> 5)-alpha-L-arabinan. Immunolocalizatio n studies of carrot root apices indicated that cell walls in the central re gion of the meristem contained higher levels of (1 --> 5)-alpha-arabinan th an the cell walls of surrounding cells. In contrast (1 --> 4)-beta-galactan was absent from the cell walls of the central meristematic cells but appea red abundantly at a certain point during root cap cell differentiation and also appeared in cell walls of differentiating stele and cortical cells. Th is developmental pattern of epitope occurrence was also reflected in a susp ension-cultured carrot cell line that can be induced to switch from prolife ration to elongation by altered culture conditions. (1 --> 4)-beta-galactan occurred at a low level in cell walls of proliferating cells but accumulat ed rapidly in cell walls following induction, before any visible cell elong ation, while (1 --> 5)-alpha-arabinan was present in cell walls of prolifer ating cells but was absent from cell walls of elongated cells. Immunochemic al assays of the cultured cells confirmed the early appearance of (1 --> 4) -beta-galactan during the switch from cell proliferation to cell elongation . Anion-exchange chromatography confirmed that (1 --> 4)-beta-galactan was attached to acidic pectic domains and also indicated that it was separate f rom a distinct homogalacturonan-rich component. These results indicate that the neutral components of pectic polysaccharides may have important roles in plant cell development.