Epsilon-poly-L-lysine increases disease resistance of citrus against postharvest green mold by activating amino acid metabolism and phenolic compounds biosynthesis

As a famous fruit worldwide, citrus is susceptible to green mold caused by Penicillium digitatum, which causes large economic losses every year. ε-Poly-L-Lysine (ε-PL) is a novel preservative with strong inhibitory effects on fungi, and has the capacity to induce disease resistance in fruit, but the mechanism was rarely reported, especially in citrus. The present study, 800 μg mL -1 ε-PL and P. digitatum spores were inoculated in two different wounds on the citrus pericarp interval of 24 h. The results revealed that ε-PL inhibited the development of green mold without direct contact with P. digitatum, indicating the disease resistance of citrus was activated. Transcriptome analysis revealed that ε-PL activated amino acid metabolism and phenylpropanoid biosynthesis. Besides, the accumulation of glutamic acid, proline, arginine, serine, lysine, phenylalanine, and tyrosine were changed during storage. In the phenylpropanoid biosynthesis, ε-PL increased PAL, C4H, and 4CL activities and total phenolic and flavonoid contents. Importantly, among these phenolic compounds, ε-PL promoted the accumulation of individual phenolic compounds including ferulic acid, chlorogenic acid, p-coumaric acid, caffeic acid, gallic acid, catechins, epicatechin, and narirutin. In conclusion, ε-PL enhanced resistance of citrus through amino acid metabolism and phenolic compounds accumulation. These results improved the knowledge on the mechanism of ε-PL induced disease resistance, and also provided a fresh theoretical basis for the use of ε-PL in post-harvest citrus preservation.