Genome-wide identification of the CsPAL gene family and functional analysis for strengthening green mold resistance in citrus fruit

Phenylalanine aminolyase (PAL) is a key rate-limiting enzyme. It catalyzes the initial step of the phenylpropane metabolic pathway. PAL is involved in the regulation of fruit diseases, but the specific function of the PAL gene in the regulation of postharvest fruit diseases is unknown. In this study, we identified four CsPAL members in citrus, including CsPAL1-CsPAL4. Based on physicochemical properties, gene structure, protein structure, protein sequence, and phylogenetic relationship analysis, all four CsPALs are predicted to have catalytic activity and take part in the disease resistance of citrus fruit to green mold. In addition, using Agrobacterium-mediated transient overexpression, all four CsPALs were shown to improve the tolerance of citrus fruit to green mold. Measurements of secondary metabolites in peel samples from the four transient overexpression groups showed a significant increase in both flavonoids and total phenols and a significant decrease in lignin content. For the first time, four CsPAL members were identified from the citrus genome, demonstrating that they are all implicated in the gene function of citrus fruit resistance to green mold through the elevation of flavonoids and total phenols but not lignin.