Overexpression of Polyphenol Oxidase Gene in Strawberry Fruit Delays the Fungus Infection Process

Polyphenols are secondary metabolites widely present in plants which benefit to human health. In the present study we analyzed the changes of polyphenol contents during strawberry fruit development as well as changes of polyphenol oxidase (PPO). The results depicted that the polyphenol content showed a decreasing trend with the fruit development. The pH value impacts the PPO activity, and in strawberry fruit the optimal pH for the PPO activity was 4.5. Meanwhile, PPO activity kept decreasing with the development of the fruit flesh and achenes. The damaged fruit enhanced the PPO activity. We found four PPO genes encoding the PPO in the strawberry that had different expression levels in tissues. The overexpression of the FaPPO1 genes improved the PPO activity in strawberry fruit and delays the fungus infection process. The FaPPO1 gene expression changes had affected the pathogen-related gene expression, such as PAL, SOD, POD, BG, and Chitinase genes. The fruit damage induced the FaPPO1 gene expression, and the abscisic acid and methyl jasmonic were also involved in the regulation of FaPPO1 gene expression. The FaPPO1 and FaPPO2 gene expressions were regulated both by abiotic stresses of low temperature, NaCl, and H₂O₂ and biotic stresses of powdery mildew and gray mold. Understanding the regulation mechanism of PPO will be helpful and provide meaningful ideas in future for strawberry breeders.