Prompt post-harvest softening of fruits is one of the major bottleneck constraints of peach industry development.Therefore,the study of fruit maturation and softening mechanism of peach is of great significance for breeding peach cultivars with better preservability.High concentrations of indole-3-acetic acid(IAA)are required for climacteric ethylene biosynthesis to cause fruit softening in melting flesh peaches at the late ripening stage.In contrast,the fruits of stony hard peach cultivars do not soften and produce little ethylene due to the low IAA concentrations.To investigate the regulation of IAA accumulation during peach ripening [the transition from stage S3 to stage S4 Ⅲ(climacteric)],a digital gene expression(DGE)analysis was performed.We compared the expression patterns of auxin-homeostasis-related genes in fruits of the melting flesh peachGoldhoney 3and the stony hard flesh peach Yumyeong during the ripening stage.Here,we reveal that a YUCCA flavin monooxygenase gene(PpYUC11,ppa008176m),a key gene in auxin biosynthesis,displayed an identical differential expression profile as the profiles of IAA accumulation and PpACS1 transcription: the mRNA transcripts increased at the late ripening stage in melting flesh peaches but were below the limit of detection in mature fruits of stony hard peaches.In addition,we describe the strong association between intron TC microsatellite genotypes of PpYUC11 and the flesh texture(normal or stony hard)in 43 peach varieties,indicating that this locus may be responsible for the stony hard phenotype in peach.These findings suggest that PpYUC11 may play an essential role in auxin biosynthesis during peach fruit ripening and is a candidate gene for the control of the stony hard phenotype in peach.