A comparative structural analysis of direct and indirect shoot ...

fluorometric analyses of Al sorption to derived cell walls
does not necessarily need to reflect the situation in intact
roots of Arabidopsis exposed to morin.
Impact of aluminium on NO production and polar
transport processes
The data reveal that internalization of aluminium into plant
endosomes alters their behaviour as they fail to form the
BFA-induced compartments. Moreover, this endosomal
aluminium might also influence nitric oxide (NO) production,
which showed its maximum in the cells of DTZ in
control root apices but was suppressed after aluminium
treatment. In animal cells, NO is active in endosomes involved
in the processing of internalized heparan sulphate
(Cheng et al., 2002). In addition, NO regulates endocytosis
and vesicle recycling especially at neuronal synapses
(Meffert et al., 1996; Huang et al., 2005; Kakegawa and
Yuzaki, 2005; Wang et al., 2006). In this respect it is most
interesting that plant synapses (Baluškaet al., 2005b), which
are very active in both endocytosis and vesicle recycling
(Baluška et al., 2003, 2005b), are located exactly in the
aluminium-sensitive distal portion of the transition zone
(Sivaguru and Horst, 1998; Sivaguru et al., 1999).
Finally, there are obvious links between the aluminium
toxicity and the inhibition of the basipetal polar transport
of auxin in the epidermis and outer cortex cells. Hasenstein
and Evans (1988) were the first to discover that aluminium
inhibits the basipetal flow of auxin. Later, this result was
fully confirmed by Kollmeier et al. (2000) who also showed
that the distal portion of the transition zone is the most relevant
in the aluminium-based inhibition of basipetal auxin
transport. Interestingly, the cells in this zone are unique with
respect to auxin and its role in cell growth regulation
(Ishikawa and Evans, 1993; Baluška et al., 1994, 1996,
2004). Doncheva et al. (2005) documented that, in an aluminium-sensitive
maize line, aluminium treatment mimics
auxin transport inhibitors in their morphogenic effects on
cell division planes in the PTZ (see also Dhonukshe
et al., 2005). Polar auxin transport is insensitive to aluminium
in aluminium-tolerant mutant AlRes4 of tobacco (Ahad
and Nick, 2006).
The DTZ cells are not only the most sensitive towards
aluminium toxicity (Sivaguru and Horst, 1998; Sivaguru
et al., 1999), but are also the most active ones in the
cell-to-cell transport of auxin (Mancuso et al., 2005;
Santelia et al., 2005). Recently published data revealed
that polar auxin transport is linked to active vesicle trafficking
and that auxin is secreted out of cells via vesicle recycling
(Schlicht et al., 2006). It is shown here that internalized
aluminium affects the behaviour of endosomes as well as
the production of NO. This indicates that the extraordinary
sensitivity of the DTZ cells towards aluminium could be a
consequence of an extremely active vesicle recycling driving
extensive polar auxin transport in this particular root
apex zone.
Aluminium sensitivity of root cells related to aluminium internalization 4211
Acknowledgements
The authors thank Milada Čiamporova for critical comments on the
manuscript. This work was supported in part by the Grant Agency
VEGA (Grants nos 2/5085/25, 2/5086/25, and 2/3051/23). MO was
supported by a Marie Curie European Reintegration Grant No.
MERG-CT-2005–031168 within the 6th European Community
Framework Programme. Financial support by grants from the
Deutsches Zentrum für Luft- und Raumfahrt (DLR, Cologne,
Germany; project 50WB 0434), from the European Space Agency
(ESA-ESTEC Noordwijk, The Netherlands; MAP project AO-
99–098), and from the Ente Cassa di Risparmio di Firenze (Italy) is
gratefully acknowledged too.
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