"The diversity of forms in nature raises questions on the mechanisms by which these forms originate. In many plant species, variations in leaf shape and size between juvenile and adult leaves accompany the acquisition of reproductive competence. This is termed heteroblasty. The canonical molecular pathway controlling heteroblasty has been recently described. It is topped by miR156, a miRNA which acts as a master switch to prevent the adult behaviour through the repression of transcription factors. However, the way how these genes control the growth patterns by which heteroblastic forms arise remains unknown. In this project, I propose to study the dynamics of heteroblasty establishment in Arabidopsis, by combining the knowledge acquired in molecular biology on the control of heteroblasty with the expertise on modelling of leaf growth in the host group (Prof. Enrico Coen and Prof. Andrew Bangham).Preliminary results suggest that the heteroblastic changes in leaf shape are ruled by the miR156 pathway until a certain leaf developmental stage, while the changes in leaf size are conditional on plant’s capacity to exploit its environmental resources. To test this hypothesis, I will i) use a modelling platform developed in the host group to compare quantitatively the early growth pattern of leaf 6 (adult) with the one of leaf 1 (juvenile), ii) develop an inducible line to analyze and model the spatiotemporal effects of the miR156 pathway in these early growth patterns, iii) modify the environment during leaf 6 development to analyze the plasticity of this leaf, and iv) analyze and disturb the expression of miR156 during leaf 6 development to question the involvement of the miR156 pathway in the control of final shape and size of this leaf.Through a combination of developmental, physiological and modelling approaches, this integrative analysis of heteroblasty will generate fundamental knowledge on leaf growth, a key target for breeding robust crops within a changing world."