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

Aluminium sensitivity of root cells related to aluminium internalization 4209
Fig. 8. Effect of aluminium on internalization of endocytic marker FM4-64. Control root after 10 min labelling with FM4-64 dye (A). Formation of
BFA-induced compartments by 35 lM BFA in FM4-64-labelled roots (B). Treatment with 90 lM aluminium for 90 min did not change considerably the
pattern of FM4-64 labelling (C), but it prevented formation of BFA-induced compartments after application of 35 lM BFA (D). Representative of five
seedlings per treatment. Bar¼10 lm.
induced by aluminium were reversible under the experimental
conditions in the recovering cells of both DTZ and PTZ.
Consistent with developmentally dependent differences in
sensing aluminium, the process of plasma membrane recovery
was slower in the cells of the DTZ as compared to those
of PTZ.
The cellular distribution of aluminium
Aluminium either accumulates on the cell surface in the
cell walls (Horst et al., 1999; Marienfeld et al., 2000; Wang
et al., 2004) or it enters the cells (Tice et al., 1992; Lazof
et al., 1994, 1996; Vázquez et al., 1999; Silva et al.,
2000; Jones et al., 2006) during exposure to aluminium.
However, information about the fate of aluminium associated
with cell surfaces during recovery is missing. It is shown
here that root cells can restore membrane functions in recovery
experiments. The restoration of membrane functions
together with the removal of the critical aluminium from
the cell surface via its internalization and sequestering within
the vacuole may contribute to the recovery of the growth.
This scenario has been proposed in the present study. The
vacuolar deposits in aluminium-treated maize roots support
the tentative conclusion that vacuolar localization of
the internalized aluminium might be the mechanism of its
intracellular detoxification (Vázquez et al., 1999).
Interestingly, the high rate of aluminium internalization
was typical only for meristematic cells and for the cells of
the distal portion, but not of the proximal portion of the
transition zone. Extracellular aluminium is mainly associated
with cell wall pectins as was manifested by the
correlation between the pectin content in the cell walls and
the accumulation of aluminium (Horst et al., 1999; Schmohl
and Horst, 2000; Hossain et al., 2006). It is speculated at
this early stage that the internalization of aluminium into
the cells might be closely related to the endocytosis of cell
wall pectins. However, consistent with the pattern of aluminium
internalization, internalization and recycling of cell wall
pectins is also accomplished only in the cells of the meristem
and the distal portion of the transition zone, but not
in the region of rapid cell elongation (Baluška et al., 2002,
2005a; Yuet al., 2002; Paciorek et al., 2005; Dhonukshe
et al., 2006). Indeed, endocytosis was active during the recovery
phase as proved by the internalization of the endocytic
marker FM4-64. Endocytosis proceeded in all cells