Target Metabolite and Gene Transcription Profiling during the Development of Superficial Scald in Granny Smith Apple (Malus x domestica Borkh)

Superficial scald is one of the most severe postharvest disorders, affecting apple fruit during cold storage (-1 to 4°C). The main symptom of this physiopathy is represented by a diffuse browning coloration, generally limited to the skin and the underlying six-cell layer, arising after the re-establishment of room temperature conditions. This disorder heavily affect the apples of Granny Smith cultivar, leading to a severe fruit quality decay.

In this study, in order to step forward in the elucidation of superficial scald etiology, a comparative approach was adopted correlating the polyphenols accumulation with the expression of a set of candidate genes involved this pathway, also with respect to the application of 1-MCP (1-methylcyclopropene), an ethylene competitor known to interfere with this physiology.

Moreover two specific volatiles were also analyzed, α-farnesene and its oxidative form, 6-methyl-5-hepten-2-one, thought as the causes of superficial scald development. Metabolite profiling and qPCR assay revealed as these volatiles are more involved in the signaling system, while the appearance of scald was assigned to the accumulation of specific polyphenol compound such as chlorogenic acid (as consequence of the activation of MdPAL and MdC3H), and its further oxidation by polyphenol oxidase gene (MdPPO). In addition, a new group of genes with anti-apoptotic function (MdDAD1, MdDND1, MdLSD1) was assessed, suggesting their role in the compartimentation of this phenomenon in the skin. In the end it is also proposed that the functional interplay between these mechanisms lead to prevent the progression of the brown coloration in inner tissues.