Candidate Gene Transcriptional Signature Unravels the Reprogramming Occurring in the Peel of Apple Fruit of ‘Granny Smith’ During Postharvest Storage

After harvest fruit are stored to preserve the quality features established during the on-tree development and maturation, ensuring thus a continuous availability of fresh fruit on the market. For certain fruit species like apple, storage can last for almost a year, especially when coupled with several strategies, such as the reduction of the oxygen concentration or the application of ethylene competitor molecules, like 1-methylcyclopropene (1-MCP). To guarantee the maintenance of the highest quality, the monitoring of the physiological processes ongoing during the postharvest ripening is compelling. For this purpose, 16 genes belonging to key fruit ripening pathways, such as the ethylene and the sugar/fermentation metabolism, have been chosen as potential markers for the molecular characterization of the major changes occurring in the fruit during storage. Among these genes, ACS, PPO, PG1, RAP2-like, and ADH exhibited the most significant differential expression across the various samples. Based on the transcriptional pattern, this set of genes constitutes a valuable molecular tool for a precise and reliable RNA-based monitoring of the postharvest ripening progression and fermentation process in apples. PPO, together with S6PDH, were furthermore employed to inspect the onset of the superficial scald in apple and resulted to correlate with the evaluation of the incidence of this disorder and the accumulation of the sugar alcohol sorbitol, known to play important protecting roles to chilling injuries. The assessment of the transcriptional signature of these elements can facilitate the development of gene expression markers suitable for a more informed investigation of the physiological progression of the postharvest ripening in apples, ultimately leading to the promotion of high-quality stored apples, extending storage time while minimizing postharvest disorders and fruit loss.