2018 ASHS Annual Conference

Apple fruit consists of multiple tissues of distinct origins, primarily: the cortex derived from the floral cup, and the pith derived at least in part from the ovary. The cortex and the core tissues display differences in growth characteristics during fruit development. Temporally, fruit growth is achieved through an early phase of cell production followed by post-mitotic cell expansion. Regulation of growth across the fruit tissues and during different stages of development is not well understood. To determine the dynamic gene expression networks regulating apple fruit growth, we performed transcriptome profiling of the cortex and pith tissues at different stages of development and in response to crop load reduction (thinning) in ‘Golden Delicious Smoothee’. Thinning was performed at 11 days after full bloom, and resulted in increased fruit size at harvest, primarily due to differences in the size of the cortex. The pith and cortex tissues were sampled for RNA-Seq analyses at 8 days after thinning treatment (8 DAFT), 19 DAFT, and 47 DAFT, corresponding to the period of high cell production, transition from cell production to expansion, and high cell expansion, respectively. The average sequencing depth of all samples was around 20 million reads. Multidimensional scaling analyses indicated that the largest transcriptomic differences were between the earliest stage and the two later stages, suggesting distinct regulatory programs between early and later stages of fruit development. Within each stage, the cortex and pith tissues displayed major differences in gene expression, indicating tissue specific regulatory networks facilitating fruit growth and development. Relatively, lesser differential gene expression was noted in response to thinning. This was especially apparent at 47 DAFT where only a few genes were differentially expressed between these samples, suggesting that the transcriptome changes induced by thinning during early fruit development (19 DAFT) had diminished by later stages. A weighted gene co-expression network analysis (WGCNA) uncovered a network module significantly correlated with the pith/cortex area, fructose and glucose concertation. Several hub genes within the module were discovered that may contribute to differences in growth patterns within the cortex and pith tissues. Data from this study provide insights into the spatial and temporal mechanisms regulating fruit growth in apple.