Harpin enhances antioxidant nutrient accumulation and decreases enzymatic browning in stored soybean sprouts

Enzymatic browning causes quality losses in the soybean sprout industry. Herein, the effects of harpin, a proteinaceous bacterial elicitor isolated from Erwinia amylovora, in regulating enzymatic browning and antioxidant nutrient accumulation in stored soybean sprout were investigated. Enhanced soybean sprout growth (evaluated by sprout length and fresh weight) occurred after spraying three times (0, 2, and 4 days after sowing) with 30 mg L-1 harpin during the growth stage. The decline in vitamin C and total phenolic contents and total antioxidant capacity (evaluated by Fe3+ reducing power) were attenuated by harpin during storage. Harpin increased phenylalanine ammonia-lyase, NADPH oxidase (NOX), superoxide dismutase, and catalase activities and inhibited polyphenol oxidase activity and enzymatic browning in soybean sprouts during storage. However, this harpin-promoted sprout growth, enhanced antioxidant accumulation and enzyme activity and improved sprout quality could be partly, but drastically, abolished using diphenyleneiodonium chloride, a specific inhibitor of NOX. Moreover, the mechanisms by which harpin influences antioxidant accumulation and enzymatic browning in soybean sprouts during storage were discussed from the perspective of NOX-mediated H2O2 signalling.