Abstract :

In the context of a second Green Revolution, which, unlike the first, aims to increase crop yields in a context of low fertility, the strategies implemented by plants for optimal assimilation of soil nutrients are at the heart of the problem. In order to solve this problem and to identify the ideal varieties among the genetic diversity of cultivated plants, the root systems, their development and architecture, are called upon to play the primary role. The work presented in this thesis focuses on the lateral roots of maize (Zea mays L.) in an effort to reveal the processes underlying the intrinsic variations in root development. In particular, it relies on phenotyping lateral roots on an unprecedented scale, tracking the daily growth of thousands of them at high spatial resolution, to precisely characterise the spatio-temporal variations between and within root individuals. Individual growth rate profiles were analysed using a statistical model that identified three main temporal trends in growth rates leading to the definition of three classes of lateral roots with distinct growth rate and duration. Differences in diameter at the emergence of these roots (which originate in the primordium stage) probably condition the subsequent growth trend but are not sufficient to determine the fate of the root. Finally, these root classes are randomly distributed along the primary root, suggesting that no local stimulation or inhibition exists between neighbouring roots.To explain the origin of the observed variations in growth, this work was complemented by a multi-scale characterisation of groups of lateral roots showing distinct growth, at a cellular, anatomical and molecular level. A particular effort was directed to the analysis of cell length profiles in root apices for which we introduced a segmentation model to identify developmental zones. Using this method, a strong modulation in the length of the division and elongation zones was revealed, linked to variations in lateral root growth. The regulatory role of auxin on the balance between cell proliferation and elongation processes was shown with the use of mutant lines. Ultimately, growth variations between lateral roots were traced back to carbon assimilate allocation and root transport capacity, suggesting the existence of a feedback mechanism that could play a key role in setting up contrasting trends in lateral root growth.