Cell Wall Metabolism in Ripening Fruit

Fractionation of pectic polysaccharides from the juice of ripening ;Bartlett' pears (Pyrus communis) gave two general types of polyuronides. The major type was a homogalacturonan (HGA) whose molecular weight decreased upon ripening. The other type comprised heteropolymers composed of various amounts of arabinose, rhamnose, and galactose. Treatment of the major arabinose-containing heteropolymeric fraction of high molecular weight (400,000) with a pear exo-polygalacturonase to degrade contaminating HGA gave a polyuronide which was inert to tomato endopolygalacturonase. Glycosyl-linkage analysis of this arabinosyl-polyuronide gave results expected from a rhamnogalacturonan I-like polysaccharide with large, highly branched araban side chains (RG-I). A linkage between HGA and RG-I was not found. RG-I, in ripening pears, appeared to be degraded with the initial loss of much of its arabinose.