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Background  Polyphenol oxidase (PPO), often encoded by a multi-gene family, causes oxidative browning, a significant problem in many food products. Low-browning potatoes were produced previously through suppression of PPO gene expression, but the contribution of individual PPO gene isoform to the oxidative browning process was unknown. Here we investigated the contributions of different PPO genes to total PPO protein activity, and the correlations between PPO protein level, PPO activity and tuber tissue browning potential by suppression of all previously characterized potato PPO genes, both individually and in combination using artificial microRNAs (amiRNAs) technology.    Results  Survey of the potato genome database revealed 9 PPO-like gene models, named  StuPPO1 to StuPPO9  in this report.  StuPPO1 ,  StuPPO2 ,  StuPPO3  and  StuPPO4  are allelic to the characterized  POTP1/P2 ,  POT32 ,  POT33  and  POT72 , respectively. Fewer ESTs were found to support the transcriptions of  StuPPO5  to  StuPPO8. StuPPO9  related ESTs were expressed at significant higher levels in pathogen-infected potato tissues. A series of browning phenotypes were obtained by suppressing  StuPPO1 to StuPPO4  genes alone and in combination. Down-regulation of one or several of the PPO genes did not usually cause up-regulation of the other PPO genes in the transgenic potato tubers, but resulted in reduced PPO protein levels. The different PPO genes did not contribute equally to the total PPO protein content in the tuber tissues, with StuPPO2 accounting for ~ 55% as the major contributor, followed by StuPPO1, ~ 25-30% and StuPPO3 and StuPPO4 together with less than 15%. Strongly positive correlations between PPO protein level, PPO activity and browning potential were demonstrated in our analysis. Low PPO activity and low-browning potatoes were produced by simultaneous down-regulation of  StuPPO2  to  StuPPO4 , but the greatest reduction occurred when  StuPPO1  to  StuPPO4  were all suppressed.    Conclusion   StuPPO1  to  StuPPO4  genes contributed to browning reactions in tuber tissues but their effect was not equal. Different PPO genes may be regulated independently reflecting their diversified functions. Our results show that amiRNAs can be used to suppress closely related members of highly conserved multi-gene family. This approach also suggests a new strategy for breeding low-browning crops using small DNA inserts.

Reduced polyphenol oxidase gene expression and enzymatic browning in potato (Solanum tuberosum L.) with artificial microRNAs