Comparative transcriptome reveals molecular mechanism in apple genotypes differing in CO2 tolerance in CA storage

Internal browning is physiological storage disorder in many apples often associated with high CO2 during controlled atmosphere (CA) storage. However the underlying molecular mechanisms leading to internal browning are poorly understood. This study examined changes in the apple transcriptome associated with internal browning related to high CO2 in CA storage by comparing the responses of a CO2 sensitive cultivar 'Han Fu' and the CO2 tolerant cultivar 'Golden Delicious'. The apples were harvested at optimum maturity for long term storage and stored in a CA of 3 % O2 and 6 % CO2 at 4 ℃ for 60 d. The CO2 sensitive 'Han Fu' showed severe browning symptoms within 30 d storage in CA, while no internal browning symptoms were detected in 'Golden Delicious' even after 60 d storage under the same storage conditions. PPO activity was higher in 'Han Fu' apples after 30 d storage. Differences in the expression profile between cultivars were assessed using RNA-sequencing techniques to identify candidate genes associated with internal browning in a cultivar specific manner. The RNA sequencing results showed that higher expression of the related to apetala 2 (RAP 2) and pyruvate decarboxylase (PDC) encoding genes in 'Golden Delicious' apples were associated with higher CO2 tolerance in this cultivar. Conversely, the higher expression of key genes from lactate production (lactate dehydrogenase, LDH), lipid catabolic (patatin-like protein, PLP), polyphenol biosynthesis (phenylalanine ammonia-lyase, PAL), polyphenol oxidase (PPO) were associated with the internal browning development in 'Han Fu' apple in CA storage condition. A conceptual model elucidating the molecular mechanism of browning development in 'Han Fu' and CO2 acclimation in 'Golden Delicious' during storage in CA is proposed. The further elucidation of this mechanism will lead to a theory to optimize cultivar breeding in apple.