Marion Gauthier

La plasticité phénotypique, la capacité d’un génotype à adapter son phénotype à ses conditions de croissance, est un aspect essentiel des interactions plante-environnement. Les modèles sont des outils permettant d’intégrer ces interactions, en particulier les modèles structure-fonction (FSPM) car ils proposent une description explicite de l'architecture de la plante. À ce jour, les FSPM traitant du couplage entre la disponibilité des ressources et la croissance sont limités car ils reposent largement sur des formalismes empiriques. L'objectif est de construire un FSPM de morphogenèse aérienne de Poacées qui considère le fonctionnement de la plante entière et qui simule l'acquisition de traits comme propriétés résultant des interactions entre la morphogenèse, les facteurs environnementaux et les activités source-puits au sein de la plante. Le modèle est piloté par le métabolisme du carbone et de l’azote dans chaque organe et par des règles de coordinations entre feuilles successives. Parallèlement au développement du modèle, une expérimentation a été menée pour recueillir des données nécessaires à la calibration et à l’évaluation du modèle. Le modèle a été calibré pour le blé et a simulé des traits foliaires et de plante réalistes, et la réponse de ces traits aux conditions de croissance était cohérente avec la bibliographie. En particulier, le modèle a simulé, comme propriétés émergentes, des caractéristiques clés qui sont fixes ou simulées empiriquement dans la plupart des modèles : profil de dimensions finales des feuilles, stabilité du phyllochrone, rapport aérien/souterrain, dilution du N, etc.

Net nitrogen (N) uptake results from active and water-mediated N inflows, partly offset by N exudation. These processes exhibit significant variations along root axes, suggesting the existence of specific zones of higher N exchanges. However, the precise location, drivers, and significance of such active root zones in root N budget remain unclear. Here, we identified and characterized active root zones using Root-CyNAPS, a new functional-structural plant model that simulates the space-time variations in net N uptake, based on the interactions between nitrogen, carbon, and water flows, and root anatomy in each segment of a 3D root system architecture. Our simulations on wheat for various plant ages and external nitrate concentrations revealed two zones of preferential net N uptake: one near the root apices and the other one coinciding with the lateral root initiation zone, both characterized by a net N uptake activity about 10 times higher than adjacent segments. Higher water uptake and N exudation rates were also located next to apices. The contribution of most active root zones (defined here by the top 10% of root length) varied between 20% and 80\% of plant net N uptake, depending on root environment and age. Our simulations also showed that water-mediated N uptake could represent up to two thirds of gross N uptake, while N losses could offset more than half of it. Root-CyNAPS offers coupling opportunities with other functional-structural models to simulate nitrogen, carbon, and water multiscale interactions across the soil-plant-atmosphere continuum.

Background and Aims: Rhizodeposition, i.e. the release of organic matters by roots, constitutes a significant fraction of the plant carbon (C) budget and plays a key role in plant-soil interactions. However, its spatial and temporal dynamics remain poorly understood. Methods: We developed RhizoDep, a new functional-structural root model that simulates 3D root growth, respiration, and rhizodeposition based on C balance and root morphology at the individual root segment level. Results: Our model successfully reproduced the dynamics of belowground C flows observed in a previous pulse-labelling field experiment on spring wheat. Our simulations revealed that root C exudation largely dominated over mucilage secretion and cap cells sloughing in terms of C release. The spatial distribution of exudation rate along the roots was driven by the preferential unloading of sugars to support root elongation and emergence, and was modulated by the formation of apoplastic barriers. Furthermore, our results demonstrated that, for a given C allocation flow to roots, variations in root hairs or lateral root number had minimal effects on rhizodeposition, whereas changes in root tissue density had a significant impact. Conclusion: RhizoDep offers a new opportunity to explore the dynamics of C exchange at the plant-soil interface and to identify traits and environmental conditions that favor rhizodeposition.

Rhizodeposition, i.e. the release of organic matters by roots, constitutes a significant fraction of the plant carbon (C) budget and plays a key role in soil-plant interactions. However, its spatial and temporal dynamics remain poorly understood. We developed RhizoDep, a new functional-structural root model that simulates 3D root growth, respiration, and rhizodeposition based on C balance and root morphology at the individual root segment level. Our model successfully reproduced the dynamics of belowground C flows observed in a previous pulse-labelling field experiment on spring wheat. Our simulations revealed that root C exudation largely dominated over mucilage secretion and cap cells sloughing in terms of C release. The spatial distribution of exudation rate along the roots was driven by the preferential unloading of sugars to support root elongation and emergence, and was modulated by the formation of apoplastic barriers. Furthermore, our results demonstrated that, for a given C allocation flow to roots, variations in root hairs or lateral root number had minimal effects on rhizodeposition, whereas changes in root tissue density had a significant impact. RhizoDep offers a new opportunity to explore the dynamics of C exchange at the plant-soil interface and to identify traits and environmental conditions that favor rhizodeposition. Highlight: Using the new model RhizoDep , we simulated distinct spatial and temporal patterns of rhizodeposition along the roots of spring wheat over its complete growth cycle, and identified their main drivers. Graphical

Phenotypic plasticity—the ability of one genotype to produce different phenotypes depending on growth conditions—is a core aspect of the interactions between plants and the environment. The model CN-Wheat simulates the functioning of a grass culm and the construction of traits as properties emerging from the feedback loops between morphogenesis, the environmental factors and source–sink activities. The plant is seen as a self-regulated system where leaf growth is driven by carbon and nitrogen metabolism within each leaf and by coordination rules between successive leaves. Here, we investigated the ability of this approach to simulate realistic grass phenotypic plasticity and explored plant behaviour in a wide range of growth conditions. The growth of grass monoculms, with traits similar to a wheat stem, was simulated for highly contrasting conditions of soil nitrogen concentration, incident light and planting density. The monoculms were kept vegetative and produced ~15 mature leaves at the end of the simulations. The model simulated highly contrasting phenotypes. Overall, the simulated trends and the magnitude of responses of leaf and plant traits to growth conditions were consistent with the literature on grass species. These results demonstrate that integrating plant functioning at organ scale can simulate, as an emergent property, the phenotypic plasticity of plants in contrasting light and nitrogen conditions. Besides, simulations of the internal variables of plants gave access to plant trophic status across plant ontogeny and plant environments. In conclusion, this framework is a significant step towards better integration of the genotype–environment interactions.

Shoot architecture is a key component of the interactions between plants and their environment. We present a novel model of grass, which fully integrates shoot morphogenesis and the metabolism of carbon (C) and nitrogen (N) at organ scale, within a three-dimensional representation of plant architecture. Plant morphogenesis is seen as a self-regulated system driven by two main mechanisms. First, the rate of organ extension and the establishment of architectural traits are regulated by concentrations of C and N metabolites in the growth zones and the temperature. Second, the timing of extension is regulated by rules coordinating successive phytomers instead of a thermal time schedule. Local concentrations are calculated from a model of C and N metabolism at organ scale. The three-dimensional representation allows the accurate calculation of light and temperature distribution within the architecture. The model was calibrated for wheat (Triticum aestivum) and evaluated for early vegetative stages. This approach allowed the simulation of realistic patterns of leaf dimensions, extension dynamics, and organ mass and composition. The model simulated, as emergent properties, plant and agronomic traits. Metabolic activities of growing leaves were investigated in relation to whole-plant functioning and environmental conditions. The current model is an important step towards a better understanding of the plasticity of plant phenotype in different environments.