Oxidative stress via the Maillard reaction is associated with the occurrence of internal browning in roots of sweetpotato (Ipomoea batatas)

Internal browning (IB) of sweetpotato is a physiological disorder that causes great losses not only for growers but also for consumers. Differences in sugar metabolism in tissues were compared for normal and disorderly regions of roots of an IB-susceptible cultivar. Compared to the normal region, the disorderly region had higher content of reducing sugar. Likewise, higher levels of genes related to sugar biosynthesis were detected in the disorderly region than in the normal region. Through immunohistochemical screening of IB-susceptible and -resistant cultivars, we identified markers of oxidative stress, including due to the Maillard reaction, in the disorderly region. 8-Nitroguanosine, hexanoyl-lysin, acrolein, and dityrosine were markedly accumulated, especially in parenchyma cells around secondary vascular tissue (SVT) in the disorderly region of the IB-susceptible cultivar. Likewise, in this region, marked accumulation of methyglyoxal and 3-DG imidazolone was observed in parenchymal cells around SVT. These results show that reactive oxygen species in roots of growing sweetpotato may be caused by non-enzymatic reactions via the biological Maillard reaction due to sugar accumulation within the cells and that this phenomenon is related to IB occurrence.