Quantification des flux métaboliques: un outil pour appréhender la régulation du métabolisme chez le fruit de tomate – tomatoflux

Objectifs Fruits and vegetables are important components of a healthy diet and could help in the prevention of major diseases if consumed daily in sufficient amounts. The recent Joint FAO/WHO Expert Consultation (http://www.who.int/dietphysicalactivity/en/ ) recommended the intake of a minimum of 400g of fruit and vegetables per day for the prevention of chronic diseases such as heart disease, cancer, diabetes and obesity, as well as for the prevention and alleviation of several micronutrient deficiencies, especially in less developed countries. To boost fruit consumption, increasing fruit quality is needed. Fruit quality depends on very heterogeneous traits, encompassing organoleptic quality, nutritional and health quality, processing, and conservation. Since tomato is the most consumed fruit in the world, and constitutes the model for fleshy fruit, it became an obvious priority to study the biology of tomato at INRA (the French institute devoted to plant research and agriculture) especially in the UMR Plant Physiology and Biotechnology in Bordeaux (UMRPBV). Tomato fruit development is strictly dependent upon carbon import, mainly as sucrose, and the composition of ripe fruit depends largely on carbon fate in metabolic networks (primary and secondary metabolism) during the early stages of development. Major metabolites, such as sugars, organic acids and amino acids, are involved in the osmotic regulation of cell enlargement and therefore in fruit size, and also determine fruit sweetness and acidity. Numerous secondary metabolites determine fruit flesh and skin colour (isoprenoids), and have health benefits for humans. For instance, this is the case of vitamin C (L-Ascorbic acid or AsA) a highly abundant metabolite that plays, as a key antioxidant, significant roles in photoprotection, stress response and plant development, but which is also crucial for humans, who have lost the capacity to synthesize it. Thanks to its high consumption in our countries, tomato is one of the major sources of vitamin C in our diet. Surprisingly, given its widespread roles, AsA biosynthesis is not completely elucidated, and its regulation remains largely unknown in the plants. In preliminary experiments, the study of transgenic lines in which two key enzymes activities involved in vitamin C biosynthesis were reduced, revealed a profound effect on central metabolism, with strong changes in some TCA cycle intermediates, sugars and aminoacids (metabolome analysis, coll. A. Fernie, MPI Gölm), which could explain the overall effect on plant and fruit growth. These observations reveal the complex interconnection between secondary and primary metabolism: changes in fluxes of the secondary metabolism modify the demand for precursors and/or energy (ATP) and lead to a reorganization of the fluxes of the primary metabolism. Thus, a better understanding of interactions between central and secondary metabolism is a prerequisite to control and manipulate plant development and fruit quality. The emergence of 'omics' technologies (transcriptomics, proteomics and metabolomics), providing rich information on levels of mRNA, proteins and metabolites in cells or tissues, has opened the way to the investigation of biological complexity, making possible the overall behaviour of living systems to be examined properly. Taken together, these data help to formulate hypotheses about system regulation that have to be validated or not experimentally. However, all these approaches give an overview of the cell at a precise time. Metabolite profiling provides clues about the metabolic activity of a tissue. But the majority of the metabolites determined so on are intermediates of metabolic pathway, and not end-products. There are examples in plants and other organisms that the flux in a pathway can be modified, without variations in the levels of metabolic intermediates or even of the end product. As a consequence, these approaches are incapable to resolve issue of metabo..