The resistance of litchi fruit to Peronophythora litchii is associated with lignin biosynthesis and reactive oxygen species metabolism

Litchi downy blight is a destructive disease that severely reduces yield and causes postharvest losses in litchi fruit. The varieties ‘Heiye’ (HY) and ‘Guiwei’ (GW) were identified as highly resistant and susceptible to Peronophythora litchii infection, respectively. We further investigated the differences between resistant and susceptible varieties through transcriptomic and biochemical analyses to uncover the mechanisms underlying postharvest resistance to litchi downy blight in litchi fruit. RNA-seq analysis of postharvest litchi fruit infected with P. litchii revealed that differentially expressed genes between HY and GW were primarily associated with phenylpropanoid biosynthesis and reactive oxygen species (ROS)-related pathways. The analysis of key metabolites in phenylpropanoid biosynthesis showed a significant difference in lignin content between the two varieties. Fifteen upregulated genes (C4H-1, COMT-1, CAD-6, and POD-7) were identified by RNA-seq and qRT-PCR. These genes correspond to highly active enzymes of C4H, CAD and POD involved in lignin biosynthesis, which were detected in P. litchii-inoculated HY fruit compared to GW. These factors contributed to the higher lignin content in HY fruit. Additionally, the ROS scavenging ability of HY pericarp was superior to that of GW, as evidenced by reduced ROS and malondialdehyde (MDA) levels, increased activities of superoxide dismutase (SOD) and catalase (CAT), and increased expression of two SOD genes and two CAT genes following P. litchii infection. These findings suggest that lignin biosynthesis and ROS metabolism play crucial roles in maintaining litchi fruit resistance to P. litchii, providing valuable insights for breeding and developing resistant varieties.