Control of Vein Network Formation by Auxin Transport in Arabidopsis Leaves

• Author / Creator

  De Agostini Verna, Carla

• Vascular networks transport water, signals and nutrients in both plants and animals; what controls the formation of these networks is thus a central question in biology. In animals, vascular network formation 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 vascular networks, such as the vein networks of leaves, by a different mechanism. Furthermore, many animal vascular networks are stereotyped; by contrast, the vein networks of plant leaves are both reproducible and variable. Consider, for example, the vein network of an Arabidopsis leaf: lateral veins branch from a single midvein and connect to distal veins to form loops; minor veins branch from midvein and loops and connect to other veins to form a mesh; and loops and minor veins curve near the leaf margin to lend a scalloped outline to the vein network. Features of the vein network such as these are reproducible from leaf to leaf—so much so that they are used to define species. By contrast, the number of veins differs from leaf to leaf, and whether a vein will connect to another vein on both ends or one end will terminate free of contact with other veins is unpredictable; this is always so for minor veins, but even loops can occasionally fail to connect to other veins at one end. This coexistence of reproducibility and variability argues against a rigid specification of leaf vein networks and instead suggest a self-organizing mechanism that functionally integrates vein network formation with leaf growth.
  Varied evidence implicates the plant signal auxin and its polar transport through plant tissues in the control of vein network formation. (i) 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 vascular 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 vascular 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. (ii) Auxin application to developing leaves induces 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 the pre-existing vasculature. (iii) Both restriction of PIN1 expression and polarization of PIN1 localization initiate and proceed away from pre-existing, narrow PIN1 expression domains and are delayed by chemical inhibition of auxin transport. (iv) 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 of possibly the other seven PIN genes in Arabidopsis.
  Here I tested these hypotheses. My results suggest that: (i) PIN-mediated auxin transport controls both reproducible and variable features of leaf vein networks. (ii) PIN1 is the only known gene to be nonredundantly required for both the reproducible features of leaf vein networks and their variable ones. (iii) The expression of PIN1 that is required for vein network patterning depends on a 205-bp region of the PIN1 promoter that contains conserved putative binding-sites for transcription factors of the MYELOBLASTOMA and DNA-BINDING WITH ONE FINGER families. (iv) Auxin-induced polar-vein-formation occurs in the absence of the function of PIN proteins or of any known intercellular auxin transporter. (v) The auxin-transport-independent vein-patterning activity relies on auxin signaling. (vi) A polarizing signal that depends on the function of the GNOM guanine-nucleotide exchange factor for ADP-rybosilation-factor GTPases, which regulates vesicle formation in membrane trafficking, acts upstream of both auxin transport and auxin signaling in leaf vein formation. My results define new inputs of auxin in the control of vein network formation.

• Subjects / Keywords

  • vein formation
  • Arabidopsis
  • cell polarity
  • vein network
  • vein patterning
  • auxin

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3MP4W40D

• License

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