Proteomic changes in 'Ambrosia' apple fruit during cold storage and in response to delayed cooling treatment

'Ambrosia' apple is one of the emerging apple cultivars that have been gaining popularity in North America due to its superior eating quality. An effective postharvest storage regime has been established for optimal storage and quality maintenance. Unfortunately, a postharvest physiological disorder has often been reported, which shows symptoms similar to low temperature induced soft scald in 'Honeycrisp'. Therefore, a delayed cooling strategy prior to storage has been developed and implemented as a successful and economical method to prevent this disorder. In order to reveal the molecular mechanism of soft scald development and delayed cooling in combating this low temperature induced disorder, a quantitative proteomic investigation employing stable isotope dimethylation labeling by peptides was conducted on 'Ambrosia' apples from three commercial orchards in BC, Canada. Quantitative changes in protein abundance in association with disorder development and in response to delayed cooling treatment after one and three month storage were found. Among the quantified proteins, 495 and 575 proteins were commonly presented in three biological replicates after one and three month storage; respectively. Among them, the abundance of 78 and 88 proteins were found to be significantly changed in response to delayed cooling as compared with normal cold storage for one and three months, respectively. These identified proteins were functionally annotated using MAPMAN software, which identified glycolysis, lipid metabolism, amino acids (including GABA shunt), hormone response, stress and signaling, redox and glutathione metabolism as the major pathways influenced by the delayed cooling treatment. A principal component analysis (PCA) revealed groups of proteins that play a major role in response to disorder development and a negative response to delayed cooling. In addition, an ABC transporter protein F family is correlated to the delayed cooling treatment. This study demonstrates the potential mechanism of the biological effect of delayed cooling on apple fruit at the proteomic level. It also provides in-depth insight on molecular mechanisms of the delayed cooling treatment in apples.