Structure and properties of pectin gels in plant cell walls

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Abstract
This review deals with recent advances in the structural characterization of pectins and the gels which they form, in relation to auxin‐induced extension growth, the ripening of fruit, and cellular recognition. Pectins are block polysaccharides. Heavily branched, largely methyl‐esterified blocks alternate with unbranched blocks of varying degrees of esterification. The unbranched, non‐esterified blocks can aggregate through calcium binding to form the junction zones that hold a gel together. The aggregates are of two, or possibly four, chains at low calcium levels, and larger with excess calcium. The fall in wall pH during auxin‐induced growth activates glycanase enzymes. These may attack some components of the pectic fraction, as well as xyloglucans. Pectin‐bound calcium ions may be displaced but this probably has little effect on gel strength. Pectins may be cross‐linked by diferulate esters when growth stops. The softening of ripe fruit is due to loss of cohesion in the pectin gel. In apples this results from replacement of the pectins by more esterified forms. In many other fruits it results from depolymerization by polygalacturonases, assisted by pectinesterases, so that the remaining segments are too short for effective calcium binding. Pectins have a further role in the recognition reactions between plant cells and some of their bacterial and fungal pathogens.