Target metabolite and gene transcription profiling during the development of superficial scald in apple (Malus x domestica Borkh)

Fruit quality features resulting from ripening processes need to be preserved throughout storage for economical reasons. However, during this period several physiological disorders can occur, of which superficial scald is one of the most important, due to the development of large brown areas on the fruit skin surface. This study examined the variation in polyphenolic content with the progress of superficial scald in apple, also with respect to 1-MCP, an ethylene competitor interacting with the hormone receptors and known to interfere with this etiology. The change in the accumulation of these metabolites was further correlated with the gene set involved in this pathway, together with two specific VOCs (Volatile Organic Compounds), α-farnesene and its oxidative form, 6-methyl-5-hepten-2-one. Metabolite profiling and qRT-PCR assay showed these volatiles are more heavily involved in the signalling system, while the browning coloration would seem to be due more to a specific accumulation of chlorogenic acid (as a consequence of the activation of MdPAL and MdC3H), and its further oxidation carried out by a polyphenol oxidase gene (MdPPO). In this physiological scenario, new evidence regarding the involvement of an anti-apoptotic regulatory mechanism for the compartmentation of this phenomenon in the skin alone was also hypothesized, as suggested by the expression profile of the MdDAD1, MdDND1 and MdLSD1 genes. The results presented in this work represent a step forward in understanding the physiological mechanisms of superficial scald in apple, shedding light on the regulation of the specific physiological cascade.