Lipid Peroxidation Induced by Reactive Oxygen Species via NADPH-Dependent Oxidative Burst Triggers the Occurrence of Internal Browning in Radish Root

The occurrence of internal browning in radish root is closely related to lipid peroxidation caused by excessive ROS production via NADPH-dependent ROS burst during the latter half of maturation. Japanese radish (Raphanus sativus L.) roots with differing severities of internal browning (IB) disorder grown under different soil temperatures were collected, and expression analysis of genes involved in NADPH-dependent ROS burst, mitochondrial electron transport chain, and ascorbate-glutathione cycle was performed. Roots with IB disorder were frequently found in higher soil temperature plots but rarely in the low soil temperature plot. Accumulation of metabolic products derived from the Maillard reaction and oxidative stress response was greater at higher soil temperatures. Although the content of hydrogen peroxide increased at higher soil temperatures, no clear correlation was found for sugar content among the experimental plots. Higher gene expressions related to NADPH-dependent ROS burst were detected at higher soil temperatures. Comparison of gene expression related to the mitochondrial electron transport chain in roots grown under different soil temperatures showed that the expression of complexes I- and III-related genes did not differ among the experimental plots. Furthermore, higher gene expressions related to the ascorbate-glutathione cycle were detected at higher soil temperatures. These facts indicate that the activation of NADPH-dependent oxidative burst induced by high temperatures increases lipid peroxidation, resulting in IB disorder in radish root.