Transcriptome and Metabolome Analyses Provide Insights Into the Composition and Biosynthesis of Grassy Aroma Volatiles in White-Fleshed Pitaya

Aroma is one of the
major inherent quality characteristics in fruits.
Understanding the composition of aroma volatiles and their biosynthesis
mechanism is crucial to improving fruit quality. However, the biosynthesis
mechanism of aroma volatiles has not been characterized yet in white-fleshed
pitaya (Hylocereus undatus). This study
was performed to investigate aroma volatiles and related gene expression
patterns in the pulp of “mild grassy” and “strong
grassy” aroma cultivars. Analysis of volatile composition and
concentration showed that aldehydes, alcohols, esters, and alkenes
were predominant in both cultivars. However, comparative analysis
revealed a significant difference in the concentration of several
metabolites, particularly hexanal and 1-hexanol. The results of the
comparative transcriptome identified a large number of aroma-related
differentially expressed genes. The majority of these genes were enriched
in fatty acid and isoleucine degradation pathways. According to integrative
analyses, changes in the expression of lipoxygenase pathway genes,
specifically FAD, LOXs, HPLs, and ADHs, probably lead to the difference in strength of “grassy”
aroma between both cultivars. The qRT-PCR of 18 aroma-related genes
was performed to validate the transcriptome analysis. Our results
identified key genes and pathways connected with the biosynthesis
of aroma volatiles in white-fleshed pitaya. These results will be
useful to dissect the genetic mechanism of fruit aroma in white-fleshed
pitaya.