Overexpression of polyphenol oxidase in transgenic sugarcane results in darker juice and raw sugar

Color intensity of raw sugar is, in part, a result of the activity of the enzyme polyphenol oxidase (PPO) acting on phenolic compounds to produce dark colored polymers when sugarcane (Saccharum spp.) is crushed to release the juice. Paler colored sugar has a potential market premium over darker sugar. In an attempt to alter the level of PPO activity in transgenic plants, sense and antisense constructs containing the native sugarcane PPO gene were introduced into sugarcane by biolistics. In a series of field experiments, it was demonstrated that PPO activity among clones correlated significantly with juice color. In laboratory crystallizations of raw sugar using juice derived from clones with high and low PPO activity, the juice with the higher PPO activity produced darker colored crystals. PPO activity was elevated and juice color was darker in all types of transgenic plants. However, clones derived from a sense construct had higher PPO activity than the other transgenic clones, tissue culture control clones, or cultivars. Furthermore, northern blot analysis demonstrated that PPO sense transgenics had much higher levels of PPO transcripts in the stem than other clones. This is the first targeted manipulation of an endogenous metabolic enzyme-encoding gene in sugarcane that leads to altered enzyme activity. Although low PPO lines with good agronomic performance were not generated, this research demonstrates the principle that juice and sugar color are correlated with PPO activity, consistent with the hypothesis that lowering PPO activity in sugarcane could reduce the color intensity of juice and raw sugar.