Histochemical observations and gene expression changes related to internal browning in tuberous roots of sweet potato (Ipomea batatas)

The mechanism underlying internal browning (IB), or brown discoloration, of the central region of tuberous roots of sweet potato (Ipomoea batatas) was examined. IB disorder begins in roots from approx. 90 days after transplanting, and the severity increases significantly with time. IB damage initially occurs in cells around the secondary vascular tissue, and the area per cell occupied by starch grains in this region was larger than in the unaffected region. High levels of reducing sugars, polyphenol oxidase (PPO) activities, chlorogenic acid, and hydrogen peroxide (H2O2) were detected in cells from the IB damaged regions. The content of sugar and polyphenols was higher in disks (transverse sections) with larger amounts of damaged tissues than in disks of sound root. The transcript levels of acid invertase (IbAIV) tended to be higher with greater IB severity, whereas fluctuation patterns of ADP-glucose pyrophosphorylase (IbAGPase), granule bound starch synthase (IbGBSS), and starch branching enzyme 1 (IbSBE1) were lower with higher IB severity. These observations suggest that the incidence of IB disorder in sweet potato is largely dependent on the excessive generation of reactive oxygen species (ROS) in cells around the secondary vascular tissues due to the abundant accumulation of sugar and/or starch grains during the root maturation period.