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Published on: September 15, 2015
Allelic variation in the thioredoxin TRX-M4 influences carbon partitioning to control soluble sugar content in apple
Jing Yuan1, Hongtao Wang1, Wenyi Lin1
1State Key Laboratory for Crop Stress Resistance and High-Efficiency Production, Shaanxi Key Laboratory of Apple, College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China.
Abstract:
Sugar content is a key determinant of fruit quality in apple (Malus × domestica), but the genetic factors that control sugar composition are poorly understood. Here, we performed quantitative trait locus (QTL) mapping of a 'Honeycrisp' × 'Qinguan' F1 population and identified THIOREDOXIN M-TYPE 4 (TRX-M4) as a genetic switch that governs carbon partitioning during fruit maturation. A coding-region SNP in TRX-M4 is associated with contrasting carbohydrate profiles: high starch/low sugar in 'Qinguan' versus high sugar/low starch in 'Honeycrisp.' The TRX-M4Y139 allele in 'Qinguan' exhibits enhanced thioredoxin activity, and transgenic lines carrying this allele show increased activities of two starch synthesis enzymes, ADP-glucose pyrophosphorylase and starch synthase 1 (SS1). TRX-M4Y139 thus promotes starch synthesis by diverting photoassimilates into a large transient starch pool and reducing sugar accumulation in developing fruits. By contrast, the TRX-M4F139 allele shows lower catalytic efficiency and weaker promotion of ADP-glucose pyrophosphorylase and SS1 activities, leading to lower starch accumulation and allowing a greater proportion of carbon to bypass the starch pool and accumulate as soluble sugars in fruit cells. This study advances our understanding of carbon flux in fruits and demonstrates how a naturally occurring thioredoxin variant functions as a molecular switch for carbon partitioning, ultimately determining fruit sweetness. Furthermore, we developed a derived cleaved amplified polymorphic sequence (dCAPS) marker to enable rapid marker-assisted selection for fruit sweetness and precision editing, providing immediate applications in apple breeding.
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