Expansin genes are candidate markers for the control of fruit weight in peach

Fruit weight is an important target trait in peach breeding and culture. Although many studies have investigated quantitative trait loci (QTL) associated with fruit weight and cell wall development, no causal genes have been identified to date. Expansins (EXPs) are plant cell wall-loosening proteins that modulate the enlargement and softening of fruit, but their role in peach development is unknown. To address this issue, we evaluated the expression profiles of all EXP genes in the peach genome. We identified 29 genes that could be classified into six groups according to their temporal expression patterns during fruit development. A genome-wide association analysis showed that six of these genes were located in the confidence intervals of QTL regions of fruit weight. The expression of two of the genes, ppa017982m and ppa010443m, was positively correlated with fruit diameter, suggesting that they control fruit weight by regulating cell enlargement. After screening the single nucleotide polymorphism (SNP) distribution in varieties of peach that bear small or large fruit (n = 30 each), an SNP in chromosome 5 (nucleotide 5,026,380) was identified in the promoter region of ppa017982m. This SNP can serve as a molecular marker in breeding programs designed to eliminate small fruit varieties.