Susceptibility of apple cultivars to watercore disorder is associated with expression of bidirectional sugar transporter gene MdSWEET12a

Watercore is a physiological disorder that presents as water-soaked tissue in mature apple fruits, and is occasionally accompanied by flesh browning or tissue breakdown during storage. Excessive accumulation of sorbitol in the intercellular space caused by a defect in sugar transport is considered to induce watercore. In this study, we performed GWAS for watercore using 2,739 individuals, identified a major QTL on chromosome 14, and found that the allele derived from 'Delicious' increases watercore susceptibility. Through RNA-seq analysis, we screened all 775 genes located in linkage disequilibrium blocks that included the QTL and identified 11 genes with significantly different expression between watercore-susceptible cultivars that carry the effective allele ('Fuji' and 'Haruka') and resistant ones that do not ('Ralls Janet' and 'Shinano Gold'). Among the 11 genes, only MdSWEET12a (MD14G1151300) was differentially expressed between other sets of susceptible ('Fuji' and 'Kotaro') and resistant ('Golden Delicious' and 'Aori 27') cultivars, with a pattern of upregulated expression over the season consistent with the pattern of watercore occurrence. MdSWEET12a is a sugar transporter responsible for sugar efflux from the cell, which might transport sugars from the phloem to the intercellular space, resulting in the accumulation of sorbitol and occurrence of watercore. A DNA marker for MdSWEET12a was developed for the prediction of watercore-susceptible individuals.