Tomato fruit growth : integrating cell division, cell growth and endoreduplication by experimentation and modelling

Keywords: cell division, cell growth, cell endoreduplication, fruit growth, genotype, G×E interaction, model, tomato. Fruit size is a major component of fruit yield and quality of many crops. Variations in fruit size can be tremendous due to genotypic and environmental factors. The mechanisms by which genotype and environment interact to determine fruit size are complex and poorly understood. Genotype-by-environment interactions emerge from cellular and molecular processes underlying fruit growth. In this thesis the basis for variations in tomato fruit size was analysed through the development of a dynamic fruit growth model integrating three fundamental fruit cellular processes: cell division, cell growth and cell endoreduplication. Experiments were carried out to understand the link between cellular processes and fruit growth and their responses to genotypic factors, contrasting fruit loads and temperature conditions. Experimental data showed that the contribution of cell number and cell size to the genotypic variation in final fruit size depends on the timing of assimilate supply to the fruit. Genotypic variation in fruit fresh weight, pericarp volume and cell volume was linked to pericarp glucose and fructose content. Genotypic variation in cell number was positively correlated with variation in pericarp fructose content. Reduction in final fruit size of early-heated fruit was mainly associated with reduced final cell volume in the pericarp. Early heating increased the number of cell layers in the pericarp, but did not affect the total number of pericarp cells significantly. Continuously heating of a fruit reduced anticlinal (direction perpendicular to fruit skin) cell expansion more than periclinal (direction parallel to fruit skin) cell expansion. Information derived from the experiments was incorporated into a dynamic model of fruit growth. The model describes fruit growth from anthesis until maturation and covers the stages of cell division, endoreduplication and cell growth. Model development relied on understanding and integrating biological interactions between processes at the cell, tissue and fruit scales. The model was parameterized and calibrated for low fruit load conditions and was validated for high fruit load and various temperature conditions. The model was able to accurately predict final cell number, cell mass and pericarp mass under contrasting fruit load and most of the temperature conditions. Model sensitivity analysis showed that variations in final fruit size are mainly associated with variations in parameters involved in the dynamics of cell division. Among these parameters, cell division duration had the strongest influence on final cell number and pericarp mass. The model can be used to carry out virtual experiments with treatments that are difficult or impossible to test experimentally and allows for predicting and analysing fruit growth responses to genotype-by-environment interactions. This thesis has contributed to closing the gap between genotype and phenotype related to tomato fruit growth. An integral and coherent development of models at relevant levels of plant organization can further help to close this gap. </p

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

Celvorm van tomatenvrucht reageert sterk op temperatuur (promovendus Julienne Fanwoua en haar begeleiders)

Wageningen UR onderwerpt de verschillende plantdelen aan specifieke onderzoeken om zo een beter beeld te krijgen van de verschillende processen die spelen in een gewas. En, heel belangrijk, welke invloed hebben de omgevingsfactoren op deze processen? Onlangs werd de vruchttemperatuur van tomaat onderzocht. Hoe verhouden eigenschappen als groei, celgrootte en celaantal zich tot de temperatuur

Elements for a model of carbon transport from sources to sinks within the carrier branch of apple

National audienceStudying transport of sugar and water using ecophysiological experimentation in connection with modeling is a very useful tool for a deeper understanding of the role of the different branch organs and their geometrical and topological arrangement in the branch for sugar production and partitioning. In a schematic way, transport of sugars takes place from sugar synthesizing organs called sources to sugar consuming (or storing) organs called sinks. These generalities have been known in principle. However, many aspects of sugar assimilation and partitioning between source and sink organs are still poorly understood in several plant species including apple. This can partly be explained by the complex architecture and composition of the apple branch, and the diversity of branch organs involved in carbon transport. Additionally, rules governing the physiological mechanisms of transport depend on tree developmental stages, which make transport and the role of the different organs dynamic in time

A dynamic model of tomato fruit growth integrating cell division, cell growth and endoreduplication

In this study, we developed a model of tomato (Solanum lycopersicum L.) fruit growth integrating cell division, cell growth and endoreduplication. The fruit was considered as a population of cells grouped in cell classes differing in their initial cell age and cell mass. The model describes fruit growth from anthesis until maturation and covers the stages of cell division, endoreduplication and cell growth. The transition from one stage to the next was determined by predefined cell ages expressed in thermal time. Cell growth is the consequence of sugar import from acommon pool of assimilates according to the source–sink concept. During most parts of fruit growth, potential cell growth rate increases with increasing cell ploidy and follows the Richards growth function. Cell division or endoreduplication occurs when cells exceed a critical threshold cell mass : ploidy ratio. The model was parameterised and calibrated for low fruit load conditions and was validated for high fruit load and various temperature conditions. Model sensitivity analysis showed that variations in final fruit size are associated with variations in parameters involved in the dynamics of cell growth and cell division. The model was able to accurately predict final cell number, cell mass and pericarp mass under various contrasting fruit load and most of the temperature conditions. The framework developed in this model opens the perspective to integrate information on molecular control of fruit cellular processes into the fruit model and to analyse gene-by-environment interaction effects on fruit growth

A dynamic model of tomato fruit growth integrating cell division, cell growth and endoreduplication

In this study, we developed a model of tomato (Solanum lycopersicum L.) fruit growth integrating cell division, cell growth and endoreduplication. The fruit was considered as a population of cells grouped in cell classes differing in their initial cell age and cell mass. The model describes fruit growth from anthesis until maturation and covers the stages of cell division, endoreduplication and cell growth. The transition from one stage to the next was determined by predefined cell ages expressed in thermal time. Cell growth is the consequence of sugar import from acommon pool of assimilates according to the source–sink concept. During most parts of fruit growth, potential cell growth rate increases with increasing cell ploidy and follows the Richards growth function. Cell division or endoreduplication occurs when cells exceed a critical threshold cell mass : ploidy ratio. The model was parameterised and calibrated for low fruit load conditions and was validated for high fruit load and various temperature conditions. Model sensitivity analysis showed that variations in final fruit size are associated with variations in parameters involved in the dynamics of cell growth and cell division. The model was able to accurately predict final cell number, cell mass and pericarp mass under various contrasting fruit load and most of the temperature conditions. The framework developed in this model opens the perspective to integrate information on molecular control of fruit cellular processes into the fruit model and to analyse gene-by-environment interaction effects on fruit growth

Response of Cell Division and Cell Expansion to Local Fruit Heating in Tomato Fruit

To improve our understanding of fruit growth responses to temperature, it is important to analyze temperature effects on underlying fruit cellular processes. This study aimed at analyzing the response of tomato (Solanum lycopersicum) fruit size to heating as affected by changes in cell number and cell expansion in different directions. Individual trusses were enclosed into cuvettes and heating was applied either only during the first 7 days after anthesis (DAA), from 7 DAA until fruit maturity (breaker stage), or both. Fruit size and histological characteristics in the pericarp were measured. Heating fruit shortened fruit growth period and reduced final fruit size. Reduction in final fruit size of early-heated fruit was mainly associated with reduction in final pericarp cell volume. Early heating increased the number of cell layers in the pericarp but did not affect the total number of pericarp cells. These results indicate that in the tomato pericarp, periclinal cell divisions respond differently to temperature than anticlinal or randomly oriented cell divisions. Late heating only decreased pericarp thickness significantly. Continuously heating fruit reduced anticlinal cell expansion (direction perpendicular to fruit skin) more than periclinal cell expansion (direction parallel to fruit skin). This study emphasizes the need to measure cell expansion in more than one dimension in histological studies of frui

Histological and molecular investigation of the basis for variation in tomato fruit size in response to fruit load and genotype

Understanding the molecular mechanisms and cellular dynamics that cause variation in fruit size is critical for the control of fruit growth. The aim of this study was to investigate how both genotypic factors and carbohydrate limitation cause variation in fruit size. We grew a parental line (Solanum lycopersicum L.) and two inbred lines from Solanum chmielewskii (C.M.Rick et al.; D.M.Spooner et al.) producing small or large fruits under three fruit loads (FL): continuously two fruits/truss (2&2F) or five fruits/truss (5&5F) and a switch from five to two fruits/truss (5&2F) 7 days after anthesis (DAA). Final fruit size, sugar content and cell phenotypes were measured. The expression of major cell cycle genes 7 DAA was investigated using quantitative PCR. The 5&5F treatment resulted in significantly smaller fruits than the 5&2F and 2&2F treatments. In the 5&5F treatment, cell number and cell volume contributed equally to the genotypic variation in final fruit size. In the 5&2F and 2&2F treatment, cell number contributed twice as much to the genotypic variation in final fruit size than cell volume did. FL treatments resulted in only subtle variations in gene expression. Genotypic differences were detected in transcript levels of CycD3 (cyclin) and CDKB1 (cyclin-dependent-kinase), but not CycB2. Genotypic variation in fruit FW, pericarp volume and cell volume was linked to pericarp glucose and fructose content (R2 = 0.41, R2 = 0.48, R2 = 0.11 respectively). Genotypic variation in cell number was positively correlated with pericarp fructose content (R2 = 0.28). These results emphasise the role of sugar content and of the timing of assimilate supply in the variation of cell and fruit phenotype

Response of Cell Division and Cell Expansion to Local Fruit Heating in Tomato Fruit

To improve our understanding of fruit growth responses to temperature, it is important to analyze temperature effects on underlying fruit cellular processes. This study aimed at analyzing the response of tomato (Solanum lycopersicum) fruit size to heating as affected by changes in cell number and cell expansion in different directions. Individual trusses were enclosed into cuvettes and heating was applied either only during the first 7 days after anthesis (DAA), from 7 DAA until fruit maturity (breaker stage), or both. Fruit size and histological characteristics in the pericarp were measured. Heating fruit shortened fruit growth period and reduced final fruit size. Reduction in final fruit size of early-heated fruit was mainly associated with reduction in final pericarp cell volume. Early heating increased the number of cell layers in the pericarp but did not affect the total number of pericarp cells. These results indicate that in the tomato pericarp, periclinal cell divisions respond differently to temperature than anticlinal or randomly oriented cell divisions. Late heating only decreased pericarp thickness significantly. Continuously heating fruit reduced anticlinal cell expansion (direction perpendicular to fruit skin) more than periclinal cell expansion (direction parallel to fruit skin). This study emphasizes the need to measure cell expansion in more than one dimension in histological studies of frui

Histological and molecular investigation of the basis for variation in tomato fruit size in response to fruit load and genotype

Understanding the molecular mechanisms and cellular dynamics that cause variation in fruit size is critical for the control of fruit growth. The aim of this study was to investigate how both genotypic factors and carbohydrate limitation cause variation in fruit size. We grew a parental line (Solanum lycopersicum L.) and two inbred lines from Solanum chmielewskii (C.M.Rick et al.; D.M.Spooner et al.) producing small or large fruits under three fruit loads (FL): continuously two fruits/truss (2&2F) or five fruits/truss (5&5F) and a switch from five to two fruits/truss (5&2F) 7 days after anthesis (DAA). Final fruit size, sugar content and cell phenotypes were measured. The expression of major cell cycle genes 7 DAA was investigated using quantitative PCR. The 5&5F treatment resulted in significantly smaller fruits than the 5&2F and 2&2F treatments. In the 5&5F treatment, cell number and cell volume contributed equally to the genotypic variation in final fruit size. In the 5&2F and 2&2F treatment, cell number contributed twice as much to the genotypic variation in final fruit size than cell volume did. FL treatments resulted in only subtle variations in gene expression. Genotypic differences were detected in transcript levels of CycD3 (cyclin) and CDKB1 (cyclin-dependent-kinase), but not CycB2. Genotypic variation in fruit FW, pericarp volume and cell volume was linked to pericarp glucose and fructose content (R2 = 0.41, R2 = 0.48, R2 = 0.11 respectively). Genotypic variation in cell number was positively correlated with pericarp fructose content (R2 = 0.28). These results emphasise the role of sugar content and of the timing of assimilate supply in the variation of cell and fruit phenotype