Genotypes adapted to contrasting environments are expected to behave differently when placed in common controlled conditions, if their sensitivity to environmental cues or intrinsic growth behaviour are set to different thresholds, or are limited at distinct levels. This allows natural variation to be exploited as an unlimited source of new alleles or genes for the study of the genetic basis of quantitative trait variation. My doctoral work focuses on analysing natural variation for shoot growth and response to the environment in A. thaliana. Natural variation analyses aim at understanding how molecular genetic or epigenetic diversity controls phenotypic variation at different scales and times of plant development and under different environmental conditions, and how selection or demographic processes influence the frequency of those molecular variants in populations for them to get adapted to their local environment. As such, the analysis of A. thaliana natural variation can be addressed using a variety of approaches, from genetics and molecular methods to ecology and evolutionary questions. During my PhD, I got the chance to tackle several of those aspects through my contributions to three independent projects which have in common to exploit A. thaliana natural variation. The first one is the analysis of the pattern of polymorphism from a set of 102 A. thaliana accessions at the MOT1 gene coding for a molybdate transporter (an essential micronutrient) and responsible for contrasted growth and fitness among accessions in response to Mo availability in the soil. I showed at different geographical scales that MOT1 pattern of polymorphisms is not consistent with neutral evolution and shows signs of diversifying selection. This work helped reinforce the hypothesis that in some populations, mutations in MOT1 have been selected to face soils rich in Mo and potentially deleterious despite their negative effect on Mo-limiting soils. The second project consists in the characterisation and functional analysis of two putative receptor-like kinases (RLKs) identified from their effect on shoot growth specifically under mannitol-supplemented media and not in response to other osmotic constraints. The function of such RLKs in A. thaliana, which is not known to synthesize mannitol was intriguing at first but, through different experiments, we built the hypothesis that those RLKs could be activated by the mannitol produced by some pathogens such as fungi and participate to plant defensive response. The third project, in collaboration with Michel Vincentz's team from CBMEG (Brasil) and Vincent Colot (IBENS, Paris), consists in the analysis of the occurrence of natural epigenetic variants of the QQS gene in different populations from Central Asia and their possible phenotypic and adaptive consequences. Overall, these analyses of the genetic and epigenetic molecular variation leading to the biomass phenotype(s) in interaction with the environment provide clues as to how and where in the pathways adaptation is shaping natural variation.

La déficience des sols en éléments minéraux, particulièrement le phosphore (P) est une limitation majeure pour la croissance et le développement des légumineuses fixatrices d’azote. L’application des fertilisants phosphatés est une pratique traditionnelles pour satisfaire les besoins des plantes en P. Ainsi, pour tester l’efficacité d'utilisation du P pour la fixation symbiotique de l’azote (FSN) sous déficience en P, plusieurs lignées recombinantes (RILs) de haricot contrastantes dans leurs tolérance au déficit en P ont été utilisées. L’objectif principal de cette étude est d’évaluer l’aptitude de ces RILs pour l’amélioration de la fertilité phosphatée des sols déficients en P et sa relation avec la croissance et la nodulation de la légumineuse. Pour atteindre ces objectifs, des expériences ont été réalisées sous serre et en champs d'agriculteurs. Sous conditions contrôlées (serre) et non contrôlées (champs), les résultats ont montré une diminution du pH des sols associée à une augmentation du P assimilable. Une telle augmentation de la disponibilité du P a eu un effet positif sur la nodulation et la croissance de certaines des RILs testées. Aussi, l’amélioration de l’aptitude de ces RILs à fixer l’azote atmosphérique et l’élévation de la libération des protons H+ par les racines nodulées ont été quantifiés en milieu contrôlé. Ainsi la diminution du pH du sol a permis de réduire l’indisponibilité du P dans la solution du sol en le transformant en une forme directement biodisponible pour ces plantes. En outre, les résultats ont montré des différences significatives entre les différentes RILs en termes de la biomasse aérienne et nodulaire selon les sites d'observation. Nous concluons que, outre leur aptitude de fixation d’azote, l’utilisation efficace du phosphore pour sauver des engrais minéraux et de réduire les risques de pollution et pourrait améliorer la disponibilité des sols P. / The deficiency of soil minerals, particularly phosphorus (P) is a major limitation for growth and development of nitrogen-fixing by legumes. The application of phosphate fertilizers is a traditional practice to meet the needs of plant P. Thus, to test the effectiveness of use of P for symbiotic nitrogen fixation (NSF) under P deficiency, several recombinant inbred lines (RILs) of beans in their contrasting tolerance to P deficiency have been used. The main objective of this study is to assess the ability of these RILs to improve the fertility of soils deficient in phosphorus (P) and its relationship with growth and nodulation of the legume. To achieve these objectives, some experiments were conducted in greenhouses and fields. Under controlled conditions (greenhouse) and uncontrolled conditions (field), the results showed decreasing in soil pH associated with Proton release H+ and P acquisition. The increasing in available P had a positive effect on nodulation and growth of some RILs tested. Also, improving the ability of these RILs to fix atmospheric nitrogen and release H+ by nodulated roots were quantified in a controlled environment. Thus the decreasing in soil pH has reduced the un-availability of P in soil solution by transforming it into a bio-available form to the plants. In addition, the results showed significant differences between RILs in biomass and nodulation as observation in sites. We conclude that, in addition to their ability to fix nitrogen, effective use of phosphorus to save mineral fertilizer and reduce the pollution risks and could improve the availability of soil P.