End-Poles of Root Cells as Auxin Transporting Plant Synapses
 
Markus Schlicht1, Boris Voigt1, Alina Schick1 Miroslav Strnad2, Klaus Palme3, Dieter Volkmann1, Diedrik Menzel1, František Baluška1,*
1 Institute of Molecular and Cellular Biology, University of Bonn , Bonn , Germany
2 Laboratory of Growth Regulators, Palacký University & Institute of Experimental Botany AS CR, CZ-78371 Olomouc, Czech Republic
3 Institut für Biologie II, Universität Freiburg, Freiburg, Germany
*email: baluska@uni-bonn.de
 

Putative auxin influx and efflux carriers (PIN/AUX proteins) show polar localizations in root cells which is in a good agreement with predictions of the chemiosmotic theory as related of the polar auxin transport (PAT). In this classical model, the role of these carriers have been interpreted to act as plasma membrane transporters. However several published data indicate that the exocytosis inhibitor Brefeldin A stops the PAT within few minutes, while the putative transport proteins of the PIN and AUX families are still polarly localised at the plasma membrane of the end-poles. Furthermore, it became clear that PAT inhibitors like TIBA and NPA are, in fact, general inhibitors of the endocytosis in plant cells (1). These data support an alternative model proposing that auxin is secreted in a neurotransmitter-like mode via BFA-sensitive and F-actin dependent vesicular exocytosis (2). This new view of the PAT is in a good accordance with the preferential localization of polarized secretion machinery at the end-poles of maize root cells transporting auxin (3). These end-poles have several other properties allowing to define them as plant synapses (4). Besides abundant F-actin, myosin VIII, dynamins, Rho-GTPases (5); the auxin secreting end-poles are equipped also with plant homologues of mammalian neuronal molecules synaptotagmins. Taken together, all these data enable us to propose that the end-poles of root cells, especially those of the root transition zone which are particularly active in the PAT, resemble neuronal synapses.

  1. Geldner N, Friml J, Stierhof YD, Jurgens G, Palme K (2001) Auxin transport inhibitors block PIN1 cycling and vesicle trafficking. Nature 413: 425-428
  2. Baluška F, Šamaj J, Menzel D (2003) Polar transport of auxin: carrier-mediated flux across the plasma membrane or neurotransmitter-like secretion? Trends Cell Biol 13: 282-285
  3. Baluška F, Hlavačka A, Šamaj J, Palme K, Robinson DG, Matoh T, McCurdy DW, Menzel D Volkmann D (2002) F-actin-dependent endocytosis of cell wall pectins in meristematic root cells: insights from BFA-induced compartments. Plant Physiol 130: 422-431
  4. Baluška F, Volkmann D, Menzel D (2005) Plant synapses: actin-based domains for cell-to-cell communication. Trends Plant Sci 10: 106-111
  5. Baluška F, Wojtaszek P, Volkmann D, Barlow PW (2003) The architecture of polarized cell growth: the unique status of elongating plant cells. BioEssays 25: 569-576
 
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