NAC regulation of embryo development in conifers

Background In most gymnosperms the cotyledons develop as a crown surrounding the incipient shoot apical meristem (SAM), which maintains the radial symmetry of the plant throughout embryogenesis. This is in contrast to the Arabidopsis embryo in which a symmetry-breaking event (from radial to bilateral symmetry) is associated with the emergence of the SAM and the two cotyledons [1]. We have previously shown that the differentiation of cotyledons and the establishment of the SAM and the root apical meristem (RAM) in somatic embryos of Norway spruce (Picea abies) is dependent on polar auxin transport (PAT) [2]. In Arabidopsis PAT restricts the expression of several genes belonging to the large plant specific NAC family of transcription factors [3-5], which in turn induce developmental processes involved in booth shoot and root formation. Members of the NAC family have a conserved DNA-binding domain in the Nterminal region known as the NAC domain, and a more diverse domain in the C-terminal region, proposed to be responsible for the trans-activation capacity of the protein [6]. Despite the diversity of the NAC gene family, and the importance of NAC proteins in fundamental processes, no NAC genes have to our knowledge been characterized in conifers.