Physiological adjustment of pomegranate pericarp responding to sunburn and its underlying molecular mechanisms

Sunburn is common in pomegranate, and sunburned fruits have poor appearance and low marketability. However, the physiological and metabolic responses to sunburn and their underlying molecular mechanisms in pomegranate fruit are little understood. Fruit of sunburn-sensitive cultivar 'Hongyushizi' was used to carry out physiological parameter detection and widely-targeted metabolomics and transcriptome study. Malondialdehyde and relative conductivity increased with the severity of sunburn, which indicated increased membrane injury. Meanwhile, the content of antioxidants (total phenols and flavonoids), which reduce and repair membrane damage, increased and were accompanied by increases in total antioxidant capacity. In sunburned fruits compared with controls, 129 metabolites changed (including naringenin, pelargonidin and kaempferol) and 447 differentially expressed genes including CHI (Pgr25966.1), F3'5'H (Pgr26644.1), and CHS (Pgr005566.1) may have contributed to these changes. Transcription factors, such as NAC 5 (Pgr008725.1), MYB 93 (Pgr001791.1), and MYB 111 (Pgr027973.1) may be involved in phenylpropanoid and flavonoid biosynthesis by regulating the CHI, F3'5'H, and CHS etc. CONCLUSIONS: These findings provide insight into the sunburn mechanisms of pomegranate, and also into the genetic improvement of fruit sunburn.