The goal of this project is to explore a novel function for the LEAFY floral regulator in the development of plant meristems. Meristems form a niche for the plant stem cells and understanding their biology is a key issue in plant biology. Up to now, most efforts have been devoted to the WUSCHEL and CLAVATA genes as well as the KNOX class of genes. LEAFY (LFY) is a plant transcription factor with a key role in flower formation. A wealth of data in several angiosperms strongly suggests that LFY also plays a role in meristematic formation and growth i) in rice and maize, LFY is essential to promote the meristematic proliferation of the inflorescence. In the absence of LFY activity, the inflorescence terminates very quickly and only a few flowers form. ii) In some legumes, LFY activity is essential for leaflet development through the maintenance of a meristematic fate that is induced by KNOX activity in other species. iii) In some Arabidopsis genetic backgrounds, LFY is required for the formation of meristems in the axils of leaves. These data suggest that LFY possesses an intrinsic capacity to stimulate meristem growth, and that the control of flower formation by LFY might proceed into two steps: first triggering the formation of the meristem itself and then conferring its floral fate. Interestingly, this meristematic function of LFY might actually be its ancestral role. This gene is indeed present in non flowering plants. It is expressed in gymnosperms meristems and in fern leaf tips, and, in the moss Physcomitrella patens, it is required for the first cell division of the zygote. We thus propose a series of experiments in several tissues from a few species to reveal and understand this meristematic function that has remained unnoticed until now and might be of central importance. The project has 3 main axes (Task I is coordination) In the first one (Task II), we will analyze the role of LFY in flower and axillary meristems production in Arabidopsis. For this, we will take advantage of a new LFY allele engineered on structural data by Partner 1 based. This allele uncouples the floral from the meristematic function and will be used as a tool to identify how LFY controls axillary meristem growth. A novel strategy based on a biophysical model describing LFY DNA binding will be tested to help identifying direct LFY targets based on genomic sequences. In the second axis (Task III), we will focus on leaf development both in Medicago (where leaflet formation depends on LFY) and Arabidopsis, which leaves can be made more complex by LFY or KNOX ectopic expression. We will identify and compare the direct targets of these two regulators by combining gene expression analysis on laser assisted dissected tissues, ChIP assays and bioinformatics analysis. Finally, a third axis (Task IV) will be devoted to LFY from the moss Physcomitrella patens. This protein possesses remarkable properties that differs from Arabidopsis LFY and will be studied at the biochemical and structural level. Moreover, we will study its function in vivo to understand how it controls cell division in this organism, hoping to get some hint on LFY ancestral function. This project is based on a range of very diverse expertise such as from laser assisted tissue micro-dissection, ChIP-seq, RNA-seq, structural biology and novel bioinformatics tools. It is based on several promising preliminary results obtained by P1 on LFY meristematic function and on moss LFY properties.