THyM · Why is Transparent Hypocotyl Mutant showed reduced phototropic response?

Plants can reorient their growth towards a favorable light environment to optimize photosynthesis in a process called phototropism. This process is initiated by the phototropin blue light receptors, perceiving light gradients to trigger downstream signaling leading to the asymmetric distribution of auxin across the photo-stimulated stem. The Fankhauser lab showed that AtABC, an ABC (ATP-binding cassette) transporter family protein is important for phototropism. AtABC shares homology with Drosophila transporters, which are involved in eye pigment precursor transport and play a vital role in insect vision. In addition to reduced phototropism, Atabc mutants have transparent hypocotyls. The primary objective of this project is to understand the function of AtABC in phototropism and to test whether this transporter is required for light gradient establishment across the hypocotyl. First, I will characterize the Atabc mutant at the tissue, cellular and subcellular levels (e.g. staining for cell wall components) to determine what defect underlies the transparent hypocotyl phenotype. Using several approaches including confocal microscopy to visualize light-activated proteins and fiber-optic techniques, I will measure the light gradient across the hypocotyl of wild type and Atabc mutants. I will characterize phototropin signaling using biochemical and microscopic approaches to determine at which signaling step AtABC is required. To characterize AtABC, I will determine its expression pattern and subcellular localization using GFP-tagged AtABC. Together with the phenotypic characterization of the mutant, this will provide testable hypotheses regarding the substance(s) transported by AtABC. Finally, to determine the functional conservation of AtABC in other plants, I will characterize Brasicca rapa mutants defective in the orthologous gene. The functional characterization of AtABC may provide key insights into light gradient establishment in a plant photosensory organ.