Occurrence of internal browning in tuberous roots of sweetpotato and its related starch biosynthesis.

Although sweetpotato is an important crop worldwide, there has been almost no research on the occurrence of internal browning (IB) to date. In this study, we clarified the mechanism of occurrence of the disorder by using two types of cultivars with different IB susceptibility. In cells around the secondary vascular tissue, large size of starch grains accumulated in IB-susceptible cultivar compared with resistant one. Histochemical observation performed on cells around the secondary vascular tissues showed the presence of high levels of polyphenol oxidase activity, chlorogenic acid, and hydrogen peroxide in cells from the IB-affected regions in IB-susceptible cultivar. Likewise, high levels of starch content, hydrogen peroxide concentration, and polyphenol content were detected in the affected regions of IB-susceptible cultivar. In IB-susceptible cultivar, both the transcript levels of gens related starch and polyphenol biosynthesis were higher at an early stage of root maturation, while the levels in resistant cultivar were low at this stage and thereafter increased relatively more moderately. These observations suggest that the occurrence of IB disorder in sweetpotato largely depends on the morphology and timing of accumulated starch grain in cells around the secondary vascular tissues.

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.