Intégration de l'écophysiologie et de la génétique quantitative pour comprendre la plasticité de la floraison, la réponse à la sélection et l'adaptation chez le maïs

As sessile organisms, plants have evolved mechanisms to respond to environmental variations, a phenomenon known as phenotypic plasticity. It results from genotype (G), environment (E), and their interaction (G×E), such that P=G+E+G×E. This interaction can lead to adaptive plasticity, improving crop fitness, or negative plasticity, detrimental to fitness. In the context of climate change, breeding for adaptation relies on identifying genotypes with beneficial plasticity. As G×E is central to crop adaptation, numerous molecular and quantitative genetic studies have advanced our understanding of underlying genetic pathways. Reaction norms have been developed to quantify plasticity across environmental gradients, yet the links between environmental mechanisms and genetic control remain unclear.This thesis investigates these mechanisms using ecophysiological and genetic approaches in maize (Zea mays ssp. mays), with flowering time as a model trait. In maize, G×E for flowering time arises from historical selection for adaptation to temperate latitudes. While tropical maize maintains plasticity in response to photoperiod and temperature, temperate maize has lost photoperiod sensitivity through selection. This work addresses three main objectives: (i) modeling a physiological reaction norm for flowering time plasticity (PRN-FTP) to identify environmental drivers and their role inG×E; (ii) linking PRN-FTP components to genetic pathways by characterizing haplotypic diversity associated with selection for temperate adaptation; and (iii) assessing how temperature-photoperiod interactions could support tropical maize adaptation to temperate environments under climate change.PRN-FTP analysis revealed distinct temperature and photoperiod response components, each with independent selection responses that structured flowering time plasticity in trait space and contributed toG×E. The genetic analysis uncovered a complex architecture, with haplotypes and allelic variants at candidate genes linked to temperature and photoperiod response, exhibiting genetic background dependency. Finally, simulations of flowering diversity under future warming scenarios suggested that by 2050, nearly 50% of tropical diversity could be accessible in temperate environments without prior selection for adaptation.Overall, this thesis advances our understanding of the environmental and genetic controls of flowering time plasticity in maize, offering new perspectives for breeding adaptation strategies. These findings could be extended to other crops.