22The embryonic cuticle is necessary for normal seed development and seedling establishment 23 in Arabidopsis. Although mutants with defective embryonic cuticles have been identified, 24 neither the deposition of cuticle material, nor its regulation, has been described during 25 embryogenesis. Here we use electron microscopy, lipid staining and permeability assays to 26 show that cuticle deposition initiates de novo in patches on globular embryos. By combining 27 these techniques with genetics and gene expression analysis, we show that successful patch 28 coalescence to form a continuous cuticle requires a signalling involving the endosperm-29 specific subtilisin protease ALE1 and the receptor kinases GSO1 and GSO2, which are 30 expressed in the developing embryonic epidermis. Transcriptome analysis shows that this 31 pathway regulates stress-related gene expression in seeds. Consistent with these findings we 32 show genetically, and through activity analysis, that the stress-associated MPK6 protein acts 33 downstream of GSO1 and GSO2 in the developing embryo. We propose that a stress-34 related signalling pathway has been hijacked in some angiosperm seeds through the 35 recruitment of endosperm-specific components. Our work reveals the presence of an inter-36 compartmental dialogue between the endosperm and embryo that ensures the formation of 37 an intact and functional cuticle around the developing embryo through an "auto-immune" 38 type interaction. 39 40 41 endosperm breaks down, leaving space for the growing embryo. By the end of seed development, 46 only a single endosperm cell layer envelops the embryonic tissues (reviewed in [1]).47 48 The endosperm is an angiosperm innovation, thought to have arisen through the 49 sexualisation of the central cell of the female gametophyte [2]. The ancestors of angiosperms 50 probably had seeds more similar to those of gymnosperms, in which tissues of the female 51 gametophyte proliferate independently of egg cell fertilization to produce a nutrient rich storage 52 tissue. However, the endosperm plays not only a nutritional role, but also a role in regulating 53 embryo development. For example, the peptide CLAVATA3/EMBRYO SURROUNDING 54 REGION-RELATED8 (CLE8) may act non-cell autonomously to regulate early Arabidopsis 55 embryogenesis [3]. Recently, maternally-expressed peptides present in the central cell pre-56 fertilization, and subsequently in the early EMBRYO SURROUNDING REGION (ESR), were 57 shown to regulate Arabidopsis suspensor development. Genetic analysis suggests that this 58 regulation could be mediated by a pathway involving the Receptor-Like Cytoplasmic Kinase 59 SHORT SUSPENSOR [4,5], although the receptor involved remains unidentified.60 61In previous works we showed genetically that the ESR-specific subtilisin protease 62 Abnormal LEaf-shape1 (ALE1) acts in the same genetic pathway as two embryonically-expressed 63 receptor kinases, GASSHO1 [(GSO1) also known as SCHENGEN3 [6]] and GASSHO2 (GSO2), 64 to control the formation of the embryonic...