Conserved, divergent and heterochronic gene expression duringBrachypodiumandArabidopsisembryo development

Abstract

AbstractEmbryogenesis, transforming the zygote into the mature embryo, represents a fundamental process for all flowering plants. Current knowledge of cell specification and differentiation during plant embryogenesis is largely based on studies of the dicot model plantArabidopsis thaliana. However, the major crops are monocots and the transcriptional programs associated with the differentiation processes during embryogenesis in this clade were largely unknown. Here, we combined analysis of cell division patterns with development of a temporal transcriptomic resource during embryogenesis of the monocot model plantBrachypodium distachyon. We found that early divisions of theBrachypodiumembryo were highly regular, while later stages were marked by less stereotypic patterns. Comparative transcriptomic analysis betweenBrachypodiumandArabidopsisrevealed that the early and late embryogenesis shared a common transcriptional program, whereas mid-embryogenesis was divergent between species. Analysis of orthology groups revealed widespread heterochronic expression of potential developmental regulators between the species. Interestingly,Brachypodiumgenes tend to be expressed at earlier stages thanArabidopsiscounterparts, which suggests that embryo patterning may occur early duringBrachypodiumembryogenesis. Detailed investigation of auxin-related genes shows that the capacity to synthesize, transport, and respond to auxin is established early in the embryo. However, while early PIN1 polarity could be confirmed, it is unclear if an active response is mounted. This study presents a resource for studyingBrachypodiumand grass embryogenesis, and shows that divergent angiosperms share a conserved genetic program that is marked by heterochronic gene expression.Key messageDevelopmental and transcriptomic analysis ofBrachypodiumembryogenesis, and comparison withArabidopsis, identifies conserved and divergent phases of embryogenesis, and reveals widespread heterochrony of developmental gene expression.