Wound-induced superoxide production and PAL activity decline with potato tuber age and wound healing ability

ABSTRACT

Wound healing of potato tubers involves the concerted action of several enzymes that facilitate polymerization of phenolics into suberin at the wound site. A decline in the efficiency of healing and resistance to pathogens with advancing tuber age was associated with reduced ability of older tubers to produce superoxide radicals (FRs) in response to wounding. Autophotographs of luminol-treated longitudinal sections of tissue from 6-, 18- and 30-month-old tubers revealed a substantial decline in superoxide production at the wound surface with advancing age. Older tubers were less able to respond to wounding by increasing phenylalanine ammonia lyase (PAL) activity. This enzyme produces t-cinnamic acid, which constitutes a component of the phenolic domain of suberin, and is normally induced by wounding and/or ethylene. Interestingly, the ability of wounded tissue to oxidize exogenous 1-aminocyclopropane-1-carboxylic acid (ACC) to C₂H₄ also decreased with advancing tuber age. The oxidation of ACC was inhibited by the FR scavenger, n-propyl gallate (PG), and inhibition was greatest in tissue from younger tubers, reflecting their greater ability to produce superoxide radicals upon wounding. Regardless of tuber age, 1-aminocyclobutane-1-carboxylic acid, an ACC oxidase inhibitor, did not inhibit C₂H₄ generation from exogenous ACC. Hence, C₂H₄ production from ACC by wounded tuber tissue is largely non-enzymatic and FR-driven, and thus serves as an indicator of the ability of wounded tissue to produce superoxide. Age-induced reduction in PAL activity and FR production at the wound surface probably limited the oxidative polymerization of phenolics into suberin during wound periderm formation. The age-induced loss in ability of wounded tissue to heal and resist pathogens is thus consistent with reduced synthesis and polymerization of phenolic adducts into suberin, a consequence of reduced FR and PAL activity at the wound surface.