Angiosperm seed development requires co-ordinated development of the embryo and a second zygotic tissue, the endosperm. In Arabidopsis thaliana, the endosperm is ephemeral and is largely consumed by the embryo during seed development. In addition to a role in embryo nutrition, it is also likely that the endosperm may play a more direct role in signalling to the embryo to regulate development. Despite their importance for embryo development, these processes are very poorly understood. The ZHOUPI (ZOU) gene provides an important tool to address these problems. Firstly, ZOU likely regulates endosperm breakdown. Whereas wild-type seed have a single layer of endosperm at maturity, zou seed has a large persistent endosperm and a correspondingly small embryo. The small zou embryo does not fill the seed so that the seed shrivels as it desiccates during maturation. Secondly, zou embryos have defects in their cuticle, so that the endosperm adheres to the embryo throughout seed development. After seed germination, zou cotyledons develop holes in their epidermis as they expand, probably due to the defects in the cuticle. ZHOUPI (ZOU) encodes a bHLH transcription factor and is expressed in the embryo surrounding region (ESR) of endosperm but not in the embryo itself. The role of ZOU in cuticle development is partly mediated by the ABNORMAL LEAF SHAPE1 (ALE1) gene. Thus, ale1 mutants also show defects in embryonic cuticle development and ALE1 is specifically expressed in ESR in a ZOU-dependent fashion. It was unclear whether the effects of ZOU upon embryo development are an indirect consequence of the persistent endosperm mechanically impeding embryo expansion, or rather reflect a more direct role of the ESR in signalling to the embryo. The main aims of this thesis were 1) to provide evidence that ZOU regulates endosperm cell death and 2) to test whether ZOU function in controlling endosperm cell death could be separated from that in embryonic epidermal cuticle development. To achieve this goal, 1) TUNEL assays were performed in the seeds to confirm the zou endosperm cell death phenotype, 2) ALE1 expression in the ESR in zou mutants was rescued using the ZOU-independent AtSUC5 promoter to investigate whether one or both of zou phenotypes were complemented, 3) Candidate ZOU target genes were validated and characterized to determine their functions in endosperm cell death and/or embryonic epidermal cuticle development. The TUNEL assays revealed that zou mutants display less DNA fragmentation in the ESR than that of the wild-type, but that zou did not have defects in cell death outside the seeds suggesting ZOU specifically regulated endosperm cell death. The AtSUC5::ALE1 transgene partially rescued zou defects in epidermal cuticle but not in endosperm cell death. This shows that the defects in the zou cuticle are not caused by the defective endosperm, rather zou has distinct, separable functions. Lastly, I characterised several novel ZOU targets and showed that RGP3 may be a direct ZOU target as it is expressed in ESR in ZOU dependent fashion, whereas RGP4 is likely indirect as it is expressed in the testa and up-regulated in zou mutants. In conclusion, ZOU has independent roles in endosperm cell death and embryonic epidermal cuticle development. Because ALE1, which largely mediates the role in cuticle development, is less widely conserved than is ZOU, the role in promoting endosperm cell death may be the ancestral function of ZOU.

Le grain de maïs est composé de 3 compartiments : l’embryon et l’albumen issus de la double fécondation et l’enveloppe d’origine maternelle. Le développement du grain et l’accumulation de réserves demande l’établissement d’une communication étroite entre l’embryon et l’albumen pour coordonner leur développement respectif. Si, des régulateurs majeurs impliqués dans le développement de la graine d’Arabidopsis ont été décrits, ces connaissances restent parcellaires chez les céréales. Les objectifs de ma thèse consistaient d’une part à étudier le contrôle de la communication entre l’embryon et l’albumen et d’autre part la régulation du remplissage du grain de maïs. Par l’analyse de lignées transgéniques sous exprimant ZmZHOUPI (ZmZOU-RNAi), nous avons établi que ce facteur de transcription à domaine bHLH, bien que s’exprimant exclusivement dans l’albumen, affecte significativement le développement de l’embryon (taille de l’embryon, persistance du suspenseur). L’analyse de données RNAseq (grains sauvages versus grains ZmZOU-RNAi) a permis d’identifier des gènes cibles potentiels de ZmZOU. De plus, nous avons montré que 3 facteurs de transcription de type bHLH homologues d’INDUCER OF CBP EXPRESSION (ICE) forment un partenariat avec ZmZOU.D’autre part, nous avons étudié les homologues d'ABA INSENSITIVE3, FUSCA3 et LEAFY COTYLEDON2 (AFL) qui forment un réseau de facteurs de transcription, à domaine B3, régulant l’accumulation d’huile et de protéines de réserves dans l’embryon d’Arabidopsis. Grâce à des analyses phylogénétiques et d’expression, nous avons établi que chez le maïs le réseau AFL, constitué de 5 membres (ZmAFLs), est partiellement conservé. Par dosages et analyse d’expression, nous avons montré que ZmAFL4, en particulier, est impliqué dans le contrôle de la biosynthèse de l’amidon dans l’albumen. / Maize kernel is composed of three major compartments: an embryo and an endosperm both produced by double fertilization and the maternally derived seed coat. Seed development and reserves accumulation demands coordination and thus communication between embryo and endosperm allowing specific growth of each compartment. While major regulators involved in seed development have been already described in Arabidopsis, knowledge in cereals remains limited. My thesis purposes were to study on one hand the control of communication between embryo and endosperm and on the other hand regulation of maize kernel filling.By analysis of transgenic lines knock down ZmZHOUPI (ZmZou-RNAi), we showed that this bHLH domain transcription factor, exclusively expressed in endosperm, affect significantly embryo development, size of embryo proper and suspensor persistence. RNAseq data analyses let find putative direct targets of ZmZOU. Additionally, we identified ZmZOU partners, 3 bHLH domain transcription factor homologs of INDUCER OF CBP EXPRESSION (ICE).Furthermore, we studied homologs of three B3 domain transcription factors named ABA INSENSITIVE3, FUSCA3 et LEAFY COTYLEDON2 (AFL) which form a regulatory network governing oil and seed storage proteins accumulation in Arabidopsis embryo. By phylogenetic and expression analysis, we established that 5 genes (ZmAFLs) constitute in maize a partially conserved AFL network. Through dosage and expression analysis, we established that particularly ZmAFL4 is involved in starch biosynthesis regulation.