Explaining tomato fruit growth by a multi-scale model on regulation of cell division, cell growth and carbohydrate dynamics

A multi-scale approach to model tomato fruit growth is proposed, in order to account for the interaction between gene functioning and growth conditions, and, ultimately, to explain the fruit phenotype of various genotypes in diverse growth environments. There is particular focus on: (I) cell division regulated by cell cycle genes, (II) cell expansion as influenced by polyploidy resulting from endoreduplication and carbohydrate and water dynamics. The growth processes at gene, cell and tissue, fruit and plant scale have been identified and included in the model. Sub-populations of cells differing in age are considered to act as sinks competing for carbohydrates. The key cell cycle genes of tomato were incorporated into an existing model of the gene regulatory network of the cell cycle. This model was modified to simulate endoreduplication. Moreover, the modelled cell cycle process was made sensitive to temperature and assimilate supply. The multi-scale approach required that a simulation could only proceed if a calculation task at a neighbouring scale had been performed. Preliminary model results indicate that cell number and ploidy level were very important in determining fruit growth. Subsequently, in the cell expansion phase, growth rate was limited by assimilate supply which in the end determined the realized fruit size. Observations at gene, cell and tissue scale are in progress in order to calibrate and validate the model, to enable reliable prediction of cell division and expansion of cells in tomato fruit tissues at contrasting conditions of temperature and carbohydrate supply.