EFFECT OF ANTISENSE SUPPRESSION OF ENDOPOLYGALACTURONASE ACTIVITY ON POLYURONIDE MOLECULAR-WEIGHT IN RIPENING TOMATO FRUIT AND IN FRUIT HOMOGENATES

Fruit of tomato (Lycopersicon esculentum Mill.) in which endopolygalac turonase (PG) activity had been suppressed to <1% of wild-type levels were slightly firmer than nontransgenic controls later in ripening. En zymically inactive cell walls were prepared from these ripening fruit using Tris-buffered phenol. When extracted with chelator followed by N a2CO3, the amounts of pectin solubilized from cell walls of nontransge nic control or from transgenic antisense PC fruit were similar. Size-e xclusion chromatography analysis showed that, relative to controls, in antisense PG fruit polyuronide depolymerization was delayed in the ch elator-soluble fraction throughout ripening and reduced in the Na2CO3- soluble fraction at the overripe stage. Reduced pectin depolymerizatio n rather than altered extractability thus may have contributed to enha nced fruit firmness. Substantially larger effects of suppressed PC act ivity were detected in tomato fruit homogenates processed to paste. In control paste the majority of the polyuronide was readily soluble in water and was very highly depolymerized. In antisense PC paste the pro portion of polyuronide solubilized by water was reduced, and polyuroni des retained a high degree of polymerization. The suppression of fruit PC activity thus has a small effect on polyuronide depolymerization i n the fruit but a much larger effect in paste derived from these fruit . This indicates that in the cell wall PC-mediated degradation of poly uronide is normally restricted but that in tissue homogenates or in is olated cell walls this restriction is removed and extensive pectin dis assembly results unless PC is inactivated.