VARIATIONS IN THE STRUCTURE OF NEUTRAL SUGAR CHAINS IN THE PECTIC POLYSACCHARIDES OF MORPHOLOGICALLY DIFFERENT CARROT CALLI AND CORRELATIONS WITH THE SIZE OF CELL CLUSTERS

Carrot (Daucus carota L.) embryogenic callus (EC) loses its embryogeni c competence and becomes nonembryogenic callus (NC) during long-term c ulture. With the loss of embryogenic competence, the cell clusters bec ome smaller and the extent of intercellular attachments is reduced. Pe ctic fractions prepared from EC and NC were separated into two subfrac tions by gel filtration. A difference in sugar composition between EC and NC was found only in the high-molecular-mass (ca. 1300 kDa) subfra ction, and the ratio of the amount of arabinose to that of galactose ( Ara/Gal) was strongly and positively correlated with the size of cell clusters in several different cultures. From the results of sugar-comp osition and methylation analyses, and the results of treatment with ex o-arabinanase, models of the neutral sugar chains of pectins from EC a nd NC are proposed. Both neutral sugar chains are composed of three re gions. The basal region is composed of linearly linked arabinan 5-Ara( f)) moieties in both types of callus. The middle galactan region is co mposed of 6-linked galactose, some of which branches at the 3 and 4 po sitions, and this region is larger and more frequently branched in NC than in EC. Finally, the terminal arabinan region is composed of 5-lin ked arabinose, branched at the 3 position, and the size of the termina l arabinan is larger in EC than in NC. The significance of the neutral sugar chains of pectins in the interaction of cell wall components an d intercellular attachment is discussed.