Transcriptional Regulation of Cell Cycle Genes During Fruit Development In Apple

Fruit growth in apple is facilitated by the coordinated progression of cell production and cell expansion. Molecular mechanisms involved in the regulation of cell production during early fruit growth are not well understood. Core cell cycle genes control progression through the cell division cycle and may therefore regulate cell production during early fruit development. Fruit growth, cell production, cell expansion and cell cycle gene expression were characterized in ‘Gala’ apples from 11 days before bloom until harvest. The relative fruit growth rate was low before bloom, was rapidly induced after pollination and fertilization, and subsequently decreased at later stages. Analysis of cell production indicated a large increase in the relative cell production rate (RCPR) at 11 days after bloom (DAB) followed by a gradual decline until 32 DAB. Cell production continued during later stages of fruit development, but at a lower rate. Mining publicly available apple EST databases resulted in the identification of 77 core cell cycle genes including cyclins, cyclin-dependent-kinases (CDKs), CDK inhibitors (KRPs), CDK activating kinases, E2F-like transcription factors, retinoblastoma-related genes and a WEE kinase. Expression of 65 of these core cell cycle genes was studied during 15 stages of fruit development using quantitative RT-PCR. Majority of the core cell cycle genes exhibited greater than 3-fold change in expression during apple fruit development. 32 core cell cycle genes including CDKBs, A2-, B-, and D3-type cyclins were expressed at higher levels during early fruit development and at much lower levels during later stages. Among these, 14 genes exhibited expression patterns positively correlated with changes in RCPR. The above genes were induced by 2 to 4-fold at 8-11 DAB, concomitant with an increase in RCPR. These data suggest pollination and fertilization dependent activation of cell cycle gene expression and subsequently, an increase in cell production. Interestingly, these genes also exhibited a decline in expression between 11 days before bloom and 3 DAB indicating active suppression of cell cycle activity, cell production and fruit growth prior to pollination and fertilization. Additionally, 5 genes including a KRP exhibited negative correlation with RCPR. These data suggest dynamic regulation of cell production by the core cell cycle transcriptome during fruit growth. Core cell cycle genes with expression patterns correlated with changes in cell production may have potential roles in regulating early fruit growth and final fruit size.