Transcriptomic Study on the Crispness Maintenance of ‘Honeycrisp' Apple Fruit

Fruit softening has long been a focus of postharvest studies with the long-term goal of delaying its occurrence. However, there are fruit, like the ‘Honeycrisp’ apple, that has the ability to remain crisp during months of cold storage. Understanding how fruit like ‘Honeycrisp’ retain crispness would enable us to learn how to delay postharvest fruit softening. In order to study the molecular mechanisms behind the crispness maintenance trait, RNA-Seq technology was used to compare differential gene expression between fresh and stored fruit of ‘Honeycrisp’ and ‘MN1764’ (fruit lose firmness). In the comparison of fresh and stored fruit, 686 differentially expressed genes (DEGs) were identified for ‘Honeycrisp’, while 1048 DEGs were identified for ‘MN1764’ based on a p-value < 0.01 and a log2 fold change >2 or <-2. The DEGs were further categorized using the Gene Ontology (GO) database into cellular components, biological processes and molecular functions. DEGs encoding cellular components were mostly associated with membrane, cell wall and intracellular components. Biological process-related DEGs were mostly associated with oxidation-reduction reactions and carbohydrate metabolism, while molecular function-related DEGs were mostly associated with protein binding and catalytic activity. Among the DEGs related to cell wall modification, there were three up-regulated cellulose synthases, six down-regulated xyloglucan endotransglucosylases and three down-regulated β- galactosidase genes found only in ‘Honeycrisp’ but not in ‘MN1764’. DEGs identified for ‘MN176’ included two up-regulated polygalacturonase genes, one up-regulated expansin gene, and three down-regulated cellulose synthase genes. These results will be used to focus future research efforts.