Integration of metabolome, histochemistry and transcriptome analysis provides insights into lignin accumulation in oleocellosis-damaged flavedo of citrus fruit

Oleocellosis, a physiological disorder that affects the appearance quality of citrus fruit, is usually accompanied by the lignification of fruit flavedo. However, the mechanism of lignin accumulation in citrus flavedo in response to oleocellosis remains largely unknown. Here, the molecular mechanism of flavedo lignification caused by oleocellosis in citrus was dissected by metabolome, histochemistry and transcriptome analysis of healthy and oleocellosis disorder green lemon (Citrus limon Burm. f. Eureka) fruit. The metabolome analysis showed a remarkable decrease in primary metabolite content while an increase in secondary metabolite content in oleocellosis-damaged flavedo. The histochemical analysis revealed that oleocellosis disorder accelerated flavedo lignification and resulted in lignin accumulation. The transcriptomic analysis showed that in oleocellosis-damaged flavedo, phenylpropanoid biosynthesis was significantly activated, and 79 differentially expressed genes (DEGs) related to lignin accumulation were identified, among which 65 (82.3 % of the total DEGs) were significantly up-regulated, particularly the PAL, C4H, CCR, F5H, POD and UGT72E genes. The results of penicillium inoculation showed that oleocellosis disorder could enhance fruit pathogen resistance. In conclusion, the occurrence of oleocellosis may enhance the pathogen resistance of citrus fruit by lignin accumulation.