BackgroundPostharvest ripening of apple (Malus x domestica) can be slowed down by low temperatures, and a combination of low O2 and high CO2 levels. While this maintains the quality of most fruit, occasionally storage disorders such as flesh browning can occur. This study aimed to explore changes in the apple transcriptome associated with a flesh browning disorder related to controlled atmosphere storage using RNA-sequencing techniques. Samples from a browning-susceptible cultivar (‘Braeburn’) were stored for four months under controlled atmosphere. Based on a visual browning index, the inner and outer cortex of the stored apples was classified as healthy or affected tissue.ResultsOver 600 million short single-end reads were mapped onto the Malus consensus coding sequence set, and differences in the expression profiles between healthy and affected tissues were assessed to identify candidate genes associated with internal browning in a tissue-specific manner. Genes involved in lipid metabolism, secondary metabolism, and cell wall modifications were highly modified in the affected inner cortex, while energy-related and stress-related genes were mostly altered in the outer cortex. The expression levels of several of them were confirmed using qRT-PCR. Additionally, a set of novel browning-specific differentially expressed genes, including pyruvate dehydrogenase and 1-aminocyclopropane-1-carboxylate oxidase, was validated in apples stored for various periods at different controlled atmosphere conditions, giving rise to potential biomarkers associated with high risk of browning development.ConclusionsThe gene expression data presented in this study will help elucidate the molecular mechanism of browning development in apples at controlled atmosphere storage. A conceptual model, including energy-related (linked to the tricarboxylic acid cycle and the electron transport chain) and lipid-related genes (related to membrane alterations, and fatty acid oxidation), for browning development in apple is proposed, which may be relevant for future studies towards improving the postharvest life of apple.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-014-0328-x) contains supplementary material, which is available to authorized users.

Braeburn' is an apple cultivar susceptible to the occurrence of internal browning (Braeburn Browning Disorder; BBD) during storage. This physiological disorder is characterised by the development of brown spots inside the fruit, eventually resulting in the formation of cavities. The objective of this study was to investigate the effects of the preharvest application of calcium, potassium and triazole fungicides on the postharvest 2 primary metabolites of 'Braeburn' fruit, and to offer a better understanding of the biochemical processes behind internal browning. The primary metabolites of 'Braeburn' cortex samples at harvest and after 2 weeks, weeks, 4 months and 8 months of storage at browning-inducing conditions were analysed using GC-MS. No significant difference in the primary metabolites was observed between the different levels of the applied preharvest applications. Early during storage, fruit developed browning, with the severity increasing with storage duration. This was correlated to a group of primary metabolites that showed either an increase (e.g., alanine, galactose, mannitol, sorbitol, xylose) or a decrease (e.g., malate, sucrose) in concentration with time. Radial distribution of the metabolites in the fruit tissue was also observed; some metabolites (e.g., galactose, mannitol) were higher in concentration in the inner cortex, while the concentrations of other metabolites (e.g., mannose, sucrose) were higher in the outer cortex.

The effects of several pre-and postharvest factors on apple cv. Braeburn browning disorder (BBD) incidence using a nine-factor experimental design has been investigated. The design allowed the determination of the effects of single factors as well as their interaction with growing season and storage time. BBD increased in severity with storage duration. BBD incidence was reduced with calcium and potassium fertilizers application, while it was increased when triazoles were used. Delayed controlled atmosphere (DCA) application resulted in less BBD in storage, while treatment with 1-methylcyclopropene (1-MCP) increased BBD incidence. More BBD was observed in fruit stored at above optimal CO 2 levels. BBD incidence was increased when O 2 concentration in CA was increased from 1 kPa to 3 kPa (optimum CA) or 6 kPa. Finally, the various factors showed a different effect for different growing seasons and storage time. These findings suggest a possible mechanism for the development of BBD. Further work should focus on extending the experimental design to include the interactions between the different pre-and postharvest factors.Keywords: Braeburn browning disorder; fertilizers; triazoles; controlled atmosphere; 1-methylcyclopropene Malus domestica Borkh., cv. Braeburn is a popular apple cultivar worldwide. To facilitate year-round availability of fruit, apples are stored at low temperature under controlled atmosphere (CA) conditions. Under the commercial storage conditions, apples can be stored well as a result of their reduced metabolic rate. The storability of cv. Braeburn is, however, limited by the development of internal browning (Elgar et al. 1999). Affected fruit show patches of brown tissue which can result in cavity formation (Elgar et al. 1998). The combination of brown flesh and off flavours makes brown apples unacceptable in the market, resulting in economic losses.Different apple varieties vary in their susceptibility to browning (Streif 2008). Cv. Braeburn has a high skin resistance (Rajapakse et al. 1990) turning the skin into a stronger diffusion barrier as compared to some other apple cultivars. Together with a dense cellular organization this results in relative high internal CO 2 concentration (Herremans et

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