ACETYL-ESTERIFICATION AND METHYL-ESTERIFICATION OF PECTINS OF FRIABLEAND COMPACT SUGAR-BEET CALLI - CONSEQUENCES FOR INTERCELLULAR-ADHESION

Monoclonal antibodies (2F4), specific for a conformational epitope of homopolygalacturonic acid induced by calcium ions, were used to compar e the nature and the distribution of the pectic polysaccharides in cel l walls of compact and friable sugar-beet (Beta vulgaris L. var. altis sima) calli, at the electron-microscope level. Labelings performed bef ore or after de-esterification pre-treatments of callus sections enabl ed three major types of pectic polysaccharides to be distinguished wit hin compact calli: (i) acidic pectins, probably with few acetyl ester groups, detected without any de-esterification treatment in expanded a reas of cell separation but never on middle lamellae between tightly a ssociated cells; (ii) highly methyl-esterified pectins with an expecte d low acetyl ester content, recognized by the 2F4 antibodies after pec tin methylesterase de-esterification, and mostly located on intercellu lar junctions and on middle lamellae in the central zones of the calli ; (iii) highly methyl-esterified and largely acetylated pectins, only localized after alkaline de-esterification, in all primary walls of th e compact calli. By contrast, all pectins of friable calli were highly methyl- and acetyl-esterified, This was consistent with an average de gree of methyl-esterification of about 60% measured in both calli, and a higher average degree of acetylation for the friable callus line (8 5%) compared to the compact one (60%). Accordingly, the pectic fractio n (acid-soluble) predominant in both calli was acetyl-esterified to 85 % in friable callus and to 22% in compact callus cell walls. Friabilit y of sugar-beet callus is thus correlated with an increase in acetylat ion of its pectin. Labelings of the Golgi apparatus indicate that the pectic polymers of both callus types are synthesized in dictyosomes in a highly methyl-esterified form and are probably subsequently acetyl- esterified.