Transcriptome analysis reveals the effect of pre-harvest CPPU treatment on the volatile compounds emitted by kiwifruit stored at room temperature

Kiwifruits are rich in nutrients beneficial to humans. Because forchlorfenuron (CPPU)-treatment after full bloom can enlarge fruit size, and significantly increase the income of farmers, it has been extensively used. However, CPPU might also influence fruit sugar and acid content, and storage performance. This study analyzed the differences in volatile emissions between CPPU-treated and water-treated kiwifruits after two, four, six, or eight days of storage, and differential gene expression related to these compounds using high-throughput sequencing. The number of volatile compounds was relatively high at the first two days of storage for both treated and control fruits, decreased in the following days, and increased again, although less significantly in CPPU-treated than in control fruits. Aldehydes in the control and treated,groups showed a trend of stability vs. down-regulation, alcohols or terpenes showed high-low-high vs. down-regulation, and esters showed up-regulation vs. high-low-high, respectively. Only 60.12-66.68% of the genes obtained were mapped and 3370 new genes were annotated. Genes related to terpene biosynthesis were enriched, and, in CPPU-treated fruits, several genes related to hormone signal transduction were found in aldehydes, alcohols, and terpenes biosynthetic pathways. Although CPPU might influence the expression of genes encoding the core complex proteins in photosynthesis, its relationship with terpene synthesis is still unclear. Our results provided resources for the genetic annotation of kiwifruits, and revealed the impact of CPPU on the metabolism of their volatile compounds, laying the theoretical foundation for investigating the use of molecular techniques to inhibit the CPPU-related reduction of fruit quality.