Identification of Putative Fruit Growth Regulatory Genes in Apple

Apple fruit growth is achieved through the combination of cell production and expansion. Molecular mechanisms regulating apple fruit growth are still poorly understood. Multiple genes regulating cell production or cell expansion, and organ growth have been identified in other species such as Arabidopsis thaliana and tomato (Solanum lycopersicum). The main objective of this study was to identify such genes from apple and to evaluate their roles in fruit growth regulation. Five gene families potentially associated with cell production or cell expansion were investigated: CELL NUMBER REGULATOR (CNR), KLUH/CYP78A5 (KLU), GROWTH-REGULATING FACTOR (GRF), GRF-INTERACTING FACTOR (GIF), and AUXIN-REGULATED GENE INVOLVED IN ORGAN SIZE (ARGOS). Multiple candidate genes from these families were identified in apple based on their sequence similarity with genes from other species, and 33 were selected for further analysis. The expression levels of these candidate genes were analyzed across different stages of apple fruit development using quantitative RT-PCR. The effects of shading, pollination, and crop load reduction on the expression of these candidate genes were also investigated. Genes within the KLUH/CYP78A5 family displayed low levels of expression in the fruit. However, several genes from the other families, especially GIF, CNR and ARGOS, displayed expression patterns consistent with roles in regulating cell production or cell expansion, and apple fruit growth. Further investigation of these genes may lead to a better understanding of mechanisms of fruit growth in apple.