Transcriptional and Translational Regulation of Leaf Polarity (Paperback)

Normal biological functions of leaves such as intercepting light and exchanging gases during photosynthesis rely on proper differentiation of adaxial (dorsal)-abaxial (ventral) identity. Although several families of transcriptional and translational regulators have been identified in leaf polarity, their targets and the molecular basis for the regulatory circuitry are largely unknown. KANADI1 (KAN1), a member of the GARP family of transcription factors, is a key regulator of adaxial identity in leaf morphogenesis. The goal of my thesis study is to discover novel players and mechanisms associated with KAN to better elucidate the establishment of leaf polarity. My dissertation investigated the DNA binding specificity of KAN1 both in vitro and in vivo. In the in vitro assay, I identified the 6 base pair motif GNATA (A/T) that the Myb-like domain in KAN1 recognizes. I also found that KAN1 acts as a transcriptional repressor in vivo and directly regulates several genes implicated in auxin responses and one in gibberellin (GA) metabolism. In addition, I studied in detail a specific target ASYMMETRIC LEAVES2 (AS2 ), a key promoter of adaxial leaf fate. I demonstrated that KAN1 directly interacts with AS2 and represses its transcription in abaxial cells. Mutation of a single nucleotide in a KAN1 binding motif in the AS2 promoter abolishes KAN1 targeting leading to ectopic expression of AS2 in abaxial cells and conferring a dominant, adaxialized phenotype. These results suggest the significant role of KAN1 in determining abaxial fate and provide novel insights in dissecting the transcriptional network of leaf morphogenesis. In addition to the transcriptional regulation, I also gave my focus to the translational regulation of leaf polarity by characterizing an enhancer of KAN, ARROW1 (ARO1). aro1-1 mutant shows pleotropic defects in development with significantly reduced overall growth rate that is due to the impaired proliferation of division competent cells in both leaves and roots. It also interacts with mutants of important leaf polarity genes probably by altering their expression timing, which suggests that ARO1 plays significant roles in dorsiventral patterning. I cloned ARO1 and it encodes a protein with a Pumilio/PUF RNA-binding domain. PUF domains have been shown to function in sequence-specific RNA binding and translational inhibition in various organisms. I also found that ARO1 functions specifically in 18S rRNA biosynthesis, a critical step for translational regulation in eukaryotes. These results indicate the importance of the translational network in controlling leaf polarity and provide a novel view for understanding the relationship of growth and leaf patterning, two essential factors responsible for leaf morphogenesis.