Abstract
Superficial scald is a major physiological disorder in apple fruit that is induced by cold storage and is mainly expressed as brown necrotic patches on peel tissue. However, a global view of the gene-protein-metabolite interactome underlying scald prevention/sensitivity is currently missing. Herein, we have found for the first time that cold storage in an atmosphere enriched with ozone (O3) induced scald symptoms in ‘Granny Smith’ apple fruits during subsequent ripening at room temperature. In contrast, treatment with the ethylene perception inhibitor 1-methylcyclopropene (1-MCP) reversed this O3-induced scald effect. Amino acids, including branched-chain amino acids, were the most strongly induced metabolites in peel tissue of 1-MCP treated fruits. Proteins involved in oxidative stress and protein trafficking were differentially accumulated prior to and during scald development. Genes involved in photosynthesis, flavonoid biosynthesis and ethylene signaling displayed significant alterations in response to 1-MCP and O3. Analysis of regulatory module networks identified putative transcription factors (TFs) that could be involved in scald. Subsequently, a transcriptional network of the genes-proteins-metabolites and the connected TFs was constructed. This approach enabled identification of several genes coregulated by TFs, notably encoding glutathione S-transferase (GST) protein(s) with distinct signatures following 1-MCP and O3 treatments. Overall, this study is an important contribution to future functional studies and breeding programs for this fruit, aiding to the development of improved apple cultivars to superficial scald.
Highlights
Apple (Malus × domestica Borkh.) is one of the most marketable and popular fruit crops; over 80 Mt are produced per year throughout the temperate regions of the world (FAO statistic, http://faostat3.fao.org)
It has been recently proposed that cold-induced oxidative stress in the peel of ‘Granny Smith’ are linked to the accumulation of chlorogenic acid in the vacuole, which in turn could react with polyphenol oxidase (PPO), leading to the peel browning that is typical of scald (Busatto et al, 2018)
Phenotypic (Figure 1A; the same fruits are presented at different time points) and scald evaluation (Figure 1B) indicated that control and especially ozone-treated fruits exhibited scald symptoms at 0 d ripening
Summary
Apple (Malus × domestica Borkh.) is one of the most marketable and popular fruit crops; over 80 Mt are produced per year throughout the temperate regions of the world (FAO statistic, http://faostat3.fao.org). The long-term cold storage of apples leads to development of superficial scald, a major physiological disorder that is characterized by necrosis of the hypodermal cortical tissue These symptoms could be developed during cold storage and become more evident during subsequent ripening at room temperature, reducing the market quality of susceptible apple cultivars, such as ‘Granny Smith’ (Lurie and Watkins, 2012). Studies linked the oxidation of afarnesene, a natural volatile compound present in the wax of the fruit, and its oxidation into conjugated trienols (CTols) and ketone 6-methyl-5-hepten-2-one (MHO) during scald development (Rowan et al, 1995). Inhibition of ethylene perception by 1-MCP has been shown to stimulate the production of antioxidant compounds to scavenge reactive oxygen species (ROS), the synthesis of fatty acids to stabilize plastid and vacuole membranes against cold, and the accumulation of sorbitol that can act as a cryoprotectant (Busatto et al, 2018)
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