Conserved Genetic Modules Controlling Lateral Organ Development: Polycomb Repressive Complex 2 and ASYMMETRIC LEAVES1 Homologs in the Lower Eudicot Aquilegia (Columbine).

Abstract

Development in multicellular organisms relies on establishing and maintaining gene expression profiles that give cells identity. Transcription factors establish gene expression profiles by integrating positional, temporal, and environmental cues to regulate genes essential for a cell's identity. These signals are often short lived while the differentiated state may persist for a long time. Epigenetic factors maintain these gene expression profiles by making heritable chemical alterations to target gene chromatin to stabilize transcriptional patterns. Here we explore the evolution and function of an epigenetic regulator, the Polycomb Repressive Complex 2 (PRC2), and a transcription factor, ASYMMETRIC LEAVES 1 (AS1) , in the lower eudicot Aquilegia. PRC2 is an important and deeply conserved epigenetic regulator, which is critical to many plant developmental processes, including the regulation of major developmental transitions and lateral organ development. We find that Aquilegia has a relatively simple complement of PRC2 genes that are expressed throughout development. Contrary to findings in other plant species, two members of the Aquilegia PRC2, AqSWN and AqCLF, are not imprinted in Aquilegia endosperm. Using virusinduced gene silencing (VIGS), we determined that Aquilegia PRC2 regulates aspects of lateral organ development, including branching within the leaf and lamina expansion, along with caroteinoid production in floral organs. PRC2 targeting of several floral MADS box genes may be conserved in Aquilegia, but other known targets such as the class I KNOX gene are not. AS1 is a transcription factor that plays a conserved role in controlling differentiation and polarity of lateral organs. In species with simple leaves, AS1 promotes cell determination by suppressing the expression of the class I KNOX genes in leaf primordia and regulates abaxial-adaxial polarity in the developing leaf. However, in species with compound leaves, KNOX genes and AS1 often work together to control leaflet initiation and arrangement. In Aquilegia, AqAS1 appears to primarily contribute to proper regulation of class I KNOX genes with a more minor role in leaflet polarity and positioning. Most interestingly, these combined datasets suggest that contrary to the widely held model, class I KNOX genes are neither necessary nor sufficient for leaf complexity in Aquilegia.