Inhibitory activity and action mode of apple polyphenols against Peronophythora litchii that causes litchi downy blight

Abstract

Litchi downy blight caused by Peronophythora litchii is one of the most devastating diseases in litchi fruit, and developing safe and effective anti-pathogenic agents is required. This study aimed to investigate the inhibitory effects and related mechanisms of apple polyphenols (AP), naturally occurring products, on P. litchii in vitro and in vivo. The results showed that AP inhibited mycelial growth and sporangia germination of P. litchii in vitro in dose-dependent manner. Moreover, plasma membrane integrity of P. litchii was severely damaged by AP treatment at 5 g L−1, which could account for the leakage of reducing sugar and soluble protein during incubation of pathogen. The destruction of P. litchii due to AP exposure was further confirmed by ultrastructural observation. Microscopic image displayed that AP induced the accumulation of reactive oxygen species (ROS) in P. litchii at early stage of treatment. Proteomic results exhibited that mitochondrion-related proteins in P. litchii were down-regulated by AP, which resulted in decreases in respiration rate and ATP production, consequently contributing to compromised pathogenicity of this pathogen. In vivo test demonstrated that 5 g L−1 AP delayed the disease development of litchi downy blight in P. litchii-inoculated litchi fruit. Suppression of disease in P. litchii-inoculated litchi fruit due to AP treatment was associated with reinforcement of host resistance, as indicated by enhanced activities of phenylalanine ammonia lyase (PAL) and β-1,3-glucanase (GLU), upregulated expressions of LcPAL and LcGLU and more accumulations of total phenolics and flavonoids in AP-treated fruit. The results suggest that AP application could ameliorate the occurrence of litchi downy blight by both the direct inhibition on P. litchii and the elicitation of resistance in litchi fruit.