Extensive solubilization and depolymerization of cell wall polysaccharides during avocado (Perseaamericana) ripening involves concerted action of polygalacturonase and pectinmethylesterase

Abstract

During the ripening of avocado (Persea americana Mill.) fruit, water-soluble polyuronides increased dramatically, concomitant with marked downshifts in molecular mass. Treatment of cell walls from pre-ripe fruit with purified avocado polygalacturonase (PG, EC 3.2.1.15) promoted the release and molecular mass downshift of polyuronides. The polyuronides released by PG were similar in size distribution to water-soluble polyuronides from fruit at intermediate stages of ripening. Polyuronides released from pre-ripe fruit by PG, although of relatively high molecular mass, were not further degraded upon additional incubation with fresh enzyme. Similarly, water-soluble polyuronides prepared from fruit at intermediate stages of ripening were largely resistant to the action of purified PG in vitro. When polyuronides derived from fruit at intermediate stages of ripening were treated with weak alkali or pectinmethylesterase (PME, EC 3.1.1.11), extensive molecular mass downshifts occurred in response to incubation with PG. These results suggest that PG plays the central role in polyuronide degradation in ripening avocado fruit cell walls and that partial de-esterification is necessary for the increase in the susceptibility of polyuronides to PG. Differences in the patterns of polyuronide depolymerization in avocado fruit compared with the more thoroughly characterized tomato fruit are discussed.