Modélisation Spatio-Temporelle des Réseaux de Gènes Impliqués dans la Régulation de l'Absorption Racinaire d’Azote chez Arabidopsis thaliana – ModeIN

Nitrogen is the mineral nutrient required in the highest amount by plants and is most frequently limiting for growth and yield, leading to massive use of N fertilizer in agriculture. Only part of the applied fertilizers is actually taken up by plants in agro-systems, which results in pollution problems (contamination of fresh or marine water by nitrate) and low energy balance of crop production (N fertilizer are synthesized at a high energy cost). Thus, improving N use efficiency of crops is an absolute requirement for modern agriculture, to face the challenge of enhancing yield for food and bio-fuel production in a sustainable, environment friendly way. A classical approach to improve N use efficiency of crops is to exploit the ability of the plants to develop adaptive responses to low N supply, which result in a strong stimulation of root uptake efficiency. The elucidation of the N signalling pathways responsible for these responses has thus a strategic importance to design biotechnological or breeding strategies for better N fertilizer use. In a typical aerobic agricultural soil, nitrate (NO3-) is the main N source taken up from the soil solution by root cells. During the past decade the molecular basis of root NO3- acquisition has been the matter of intensive studies. One main outcome is the identification of NRT2.1 as a major transporter for root NO3- uptake in Arabidopsis thaliana. Furthermore, NRT2.1 not only plays a role in NO3- transport, but also acts as a sensor or transducer governing root growth responses to NO3-. At the transcript level, NRT2.1 has been shown to be the target of all the regulations affecting NO3- uptake activity. It is induced upon initial NO3- supply, and repressed by downstream products of NO3- assimilation. These regulations account for the modulation of N use efficiency by both changes in external NO3- availability, and variations of the organic N demand of the plant for growth. Most importantly, many recent studies have shown that NRT2.1 is dramatically induced by light and sugars. This is expected to ensure the necessary integration of both N acquisition by roots and C acquisition by shoots, and indicates that the regulation of N use efficiency cannot be investigated independently from C metabolism. This consideration has far-reaching consequences, because it suggests that global climate changes (i.e., increase in atmospheric CO2 concentration) will have a profound impact on the use of N fertilizers in agriculture. This is not really taken into account in the current strategies for improved N use efficiency by crops, due to the lack of basic knowledge on how a modified CO2 assimilation by plants and thus a modified C/N signalling network will affect the overall N budget in future agro-systems. The molecular mechanisms involved in N and C signalling pathways are mostly unknown but a large body of physiological evidence suggests that multiple interactions occur between these pathways, giving rise to the concept of 'C/N regulatory web' to describe them. To address this complex problem, we propose to use a pioneer and innovative systems biology approach to understand and model the regulatory gene networks involved in the control of NRT2.1 and determine how it impacts on root NO3- acquisition and plant growth traits such as biomass production. We will combine skills in physics, mathematics and computer science with genetics, plant molecular biology and plant physiology to (1) integrate genomic data and build predictive dynamic models in specific root tissue, (2) generate testable hypothesis concerning the genes networks involved in the regulation of NRT2.1 and (3) use experimental approaches to test and validate these hypothesis for the impact of atmospheric CO2 level on the regulation of root N uptake and the consequences on plant growth. We believe that this partnership created at the interface of math, computer science, theoretical physics and plant biology will provide powerful tools to find emergent properties and reach an unprecedented cell-specific detail in modelling the mechanisms involved in the control of root nutrient acquisition in plants. This is the first step towards developing strategies that intervene in molecular networks for biotechnological purposes such as improving N use efficiency in plants.