Compositional and structural modification of the cell wall of cauliflower ( Brassica oleracea L. var botrytis) during tissue development and plant maturation

Abstract

The purpose of this study was to investigate maturation-related changes in cell-wall polysaccharides in cauliflower ( Brassica oleracea L.) florets and stem tissues. Alcohol insoluble residues (AIRs) prepared from these were analysed for their carbohydrate composition. The cell walls of the immature tissues contained arabinose- and galactose-rich pectic polysaccharides and the major hemicelluloses consisted of xyloglucans. Maturation was accompanied by a basipetal decrease in the galactose- and arabinose-containing pectic polymers and a decrease in their degree of methyl-esterification. At the same time, there was a considerable increase in (1→4)-linked xylose residues and cellulosic glucose, particularly in the lower stem, and an increase in Klason lignin. More precise information on polymer structure was obtained from sequential extraction (with water, cyclohexane- trans-1,2-diamine- N, N, N′, N′-tetra acetate, Na 2CO 3 and KOH) of the AIRs of upper and lower stem tissues. The cellulose-rich residues remaining contained significant amounts of pectic polysaccharides, a proportion of which was released on neutralization of the residues. The greatest differences between the immature and mature stem cell-wall polymers occurred in the 0.5 M KOH fractions and these were investigated in more detail. Glycosidic linkage analysis showed that the insoluble residues from the immature stem contained much larger amounts of arabinose-rich pectic polysaccharides, and lower amounts of acidic xylans, compared with the mature stem. GC-MS linkage analysis of the polysaccharide fractions recovered from graded EtOH precipitation of the soluble fractions showed a large proportion of the maturation-related increase in (1→4)-linked xylose comprised glucuronoxylans closely associated or complexed, with pectic polysaccharides and xyloglucans. The possible relationship between such complexes and the onset of lignification is discussed.