Regulation of Fruit Growth and Fruit Size in Apple

Abstract

Fruit development in apple cultivars varying in their ultimate fruit size was analyzed using cytology, flow cytometry (FCM), and semi-quantitative RT-PCR. Fruit size variation across cultivars was largely explained by variation in cell number. The cell division phase lasted for less than 30 days in all varieties, less than previously believed. A distinct overlap between the cell division and cell expansion phases was present. Analysis of the relative cell production rate (rCPR) showed a major peak about 10 days after full bloom (DAFB) after which it declined. Comparison of the rCPR across varieties suggested distinct patterns of cell production with `Gala' having a low but sustained rCPR, `Pixy Crunch' a short but high rCPR, and `Golden Delicious' having a high and sustained rCPR. FCM analysis also showed similar patterns with a peak in the proportion of dividing cells about 10 DAFB followed by a decline. To further understand regulation of cell number, four cell cycle related genes were cloned from `Gala'. Cyclin Dependent Kinase B (CDK B) and Cyclin B were found to be highly cell division phase specific in their expression. Analysis of gene expression by semi-quantitative RT-PCR indicated peak expression of these two genes at 5-10 DAFB, consistent with the peaks in rCPR and proportion of dividing cells. Comparison of gene expression across the varieties showed higher peak expression of the above genes in the larger-fruited `Golden Delicious' than in the smaller-fruited `Gala.' This study provides novel insight into the regulation of fruit development in apple and also suggests a role for the cell cycle genes in fruit size regulation.