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Control of vein patterning by auxin transport and signaling
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- Author / Creator
- Ravichandran, Sree Janani
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Tissue networks such as the vascular networks of mammalian embryos and the vein network of plant leaves transport water, signals and nutrients; what controls the formation of these networks is thus a key question in biology. In animals, the formation of tissue networks requires direct cell-cell communication and often cell movements, both of which are precluded in plants by a wall that holds cells in place; therefore, plants form tissue networks such as the vein networks in their leaves by a different mechanism.
The details of the mechanism by which plants form leaf vein networks are poorly understood, but available evidence places the plant signal auxin and its polar transport through plant tissues at the core of such mechanism. (1) Expression of the PIN-FORMED1 (PIN1) auxin transporter of Arabidopsis is initiated in broad domains of leaf inner cells that become gradually restricted to files of vein precursor cells in contact with pre-existing, narrow PIN1 expression domains. Within broad expression domains, PIN1 is localized isotropically — or nearly so — to the plasma membrane of leaf inner cells. As expression of PIN1 becomes gradually restricted to files of vein precursor cells, PIN1 localization becomes polarized to the side of the plasma membrane facing the pre-existing, narrow PIN1 expression domains with which the narrowing domains are in contact. (2) Auxin application to developing leaves induces the formation of broad expression domains of isotropically localized PIN1. Such domains become restricted to the sites of auxin-induced vein formation, and PIN1 localization becomes polarized toward pre-existing PIN1 expression domains. (3) Both
restriction of PIN1 expression and polarization of PIN1 localization are delayed by chemical
inhibition of auxin transport. (4) Auxin transport inhibitors induce characteristic vein-pattern defects, similar to — though stronger than — those of pin1 mutants. Therefore, available evidence suggests that auxin induces the polar formation of veins and that such inductive and orienting property of auxin strictly depends on the function of PIN1 and possibly other PIN genes.
How auxin coordinates PIN polarity between auxin-transporting cells to induce the polar formation of veins is unclear, but for the past 20 years the prevailing hypothesis has been that the GNOM (GN) guanine-nucleotide exchange factor for ADP-rybosilation-factor GTPases, which regulates vesicle formation in membrane trafficking, controls the cellular localization of PIN1 and other PIN proteins; the resulting cell-to-cell, polar transport of auxin would coordinate PIN polarity between auxin-transporting cells and control polar developmental processes such as vein formation. Here I tested this hypothesis by a combination of cellular imaging, molecular genetic analysis, and chemical induction and inhibition. Contrary to predictions of the hypothesis, my results suggest that: (1) auxin-induced polar-vein-formation occurs in the absence of PIN proteins or any known intercellular auxin transporter; (2) the residual auxin-transport-independent vein-patterning activity relies on auxin signaling; (3) GN controls both auxin transport and signaling 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 signaling, which takes place in the nucleus and is inherently non-polar. The most parsimonious account is that auxin signaling leads to the production of proteins which control vein patterning and whose localization is controlled by GN. Here I tested this hypothesis by a combination of gene expression screen and molecular genetic analysis, and identified four putative candidates for such proteins.
Finally, to further characterize in the future the function of such putative candidate proteins which are targets of auxin signaling, which control vein patterning, and whose localization is controlled by GN, I have 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 suggest synergism between auxin transport and signaling and their unsuspected control by GN in the formation of plant tissue networks, a control whose logic is unprecedented in multicellular organisms. -
- Subjects / Keywords
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- Graduation date
- Fall 2019
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- Type of Item
- Thesis
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- Degree
- Master of Science
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- License
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