Transcriptome and metabolite profiling analyses provide insight into volatile compounds of the apple cultivar ‘Ruixue’ and its parents during fruit development

Aroma is one the most crucial inherent quality attributes of fruit. 'Ruixue' apples were selected from a cross between 'Pink Lady' and 'Fuji', a later ripening yellow new cultivar. However, there is little known about the content and composition of aroma compounds in 'Ruixue' apples or the genetic characters of 'Ruixue' and its parents. In addition, the metabolic pathways for biosynthesis of aroma volatiles and aroma-related genes remain poorly understood. Volatile aroma compounds were putatively identified using gas chromatography-mass spectrometry (GC-MS). Our results show that the profile of volatile compounds changes with ripening. Aldehydes were the dominant volatile compounds in early fruit development, with alcohols and esters increasing dramatically during maturation. On the basis of a heatmap dendrogram, these aroma compounds clustered into seven groups. In ripe fruit, esters and terpenoids were the main aroma volatiles in ripening fruit of 'Pink Lady' and 'Fuji' apples, and they included butyl 2-methylbutanoate; propanoic acid, hexyl ester; propanoic acid, hexyl ester; hexanoic acid, hexyl ester; acetic acid, hexyl ester and (Z, E)-[alpha]-farnesene. Interestingly, aldehydes and terpenoids were the dominant volatile aroma compounds in ripening fruit of 'Ruixue', and they mainly included hexanal; 2-hexenal; octanal; (E)-2-octenal; nonanal and (Z, E)-[alpha]-farnesene. By comparing the transcriptome profiles of 'Ruixue' and its parents fruits during development, we identified a large number of aroma-related genes related to the fatty acid, isoleucine and sesquiterpenoid metabolism pathways and transcription factors that may volatile regulate biosynthesis. Our initial study facilitates a better understanding of the volatile compounds that affect fruit flavour as well as the mechanisms underlying differences in flavour between 'Ruixue' and its parents.