Auxin-transport-independent control of vein patterning in Arabidopsis thaliana leaves

  • Author / Creator
    Amalraj, Brindhi H
  • Most multicellular organisms solve the problem of long-distance transport of water, signals and nutrients by means of networks of cells and tissues such as the vascular systems of plants and animals. What controls the formation of vascular systems is thus a key question in biology. In animals, where this question has been addressed extensively, the formation of the vascular system requires direct cell-cell interaction and, at least in part, cell migration. Both cell migration and direct cell-cell interaction are precluded in plants by a cell wall that holds cells apart and in place. Therefore, plants form vascular systems differently from animals.
    The mechanism by which plants form their vascular systems is poorly understood, but available evidence places the plant signal auxin and its polar transport through plant tissues at the core of such mechanism. How auxin and its polar transport induce vein formation is unclear, but the prevailing hypothesis has long been that the GNOM (GN) guanine-nucleotide exchange factor for ADP-ribosylation-factor GTPases, which regulates vesicle formation in membrane trafficking, coordinates the cellular localization of auxin transporters of the PIN-FORMED (PIN) family between cells. The resulting cell-to-cell, polar transport of auxin would coordinate the polar localization of PIN proteins between auxin-transporting cells and control polar developmental processes such as vein formation. Contrary to predictions of the hypothesis, however, vein formation occurs in the absence of PIN proteins or any known intercellular auxin transporter; instead, auxin-transport-independent vein patterning relies, at least in part, on auxin signal transduction and GN controls both auxin transport and signalling to induce vein formation.
    Whereas mechanisms by which GN may control PIN polarity and derived polar auxin transport have been suggested, it is unclear how GN could control auxin signalling, which takes place in the nucleus and is inherently non-polar. The most parsimonious account is that auxin signalling leads to the production of proteins which control vein patterning redundantly with auxin transport and whose localization is controlled by GN. Here we tested predictions of this hypothesis by a combination of gene expression screening and molecular genetic analysis and identified a family of putative candidates for such proteins.
    The current hypothesis of vein formation proposes that GN controls both auxin transport and auxin signalling to induce vein formation. However, plants in which both auxin transport and signalling are compromised phenocopy only weak alleles of gn such as fewer roots (fwr), suggesting the presence of additional, yet-to-be-identified GN-dependent pathways that act redundantly to auxin signalling and transport to induce vein formation. To identify such pathways, we screened for mutations that rescued the fragmented vein-pattern phenotype of gnfwr and identified and characterized seven genetic suppressors of gn.
    Finally, for the future characterization of the auxin signalling targets that control vein patterning redundantly with auxin transport and of the genes whose mutation suppresses the phenotype of gn, we identified and characterized GAL4/GFP enhancer-trap lines for the targeted misexpression of genes of interest in specific cells and tissues of developing leaves.
    My results identify GN-dependent auxin-transport-independent pathways of vein formation in plants, a process whose logic is thus far unprecedented in multicellular organisms.

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.