Symplastic intercellular transport in plants is achieved by plasmodesmata (PD). These cytoplasmic channels are well known
to interconnect plant cells to facilitate intercellular movement of water, nutrients, and signaling molecules including
hormones. However, it is not known whether Al may affect this cell-to-cell transport process, which is a critical feature for
roots as organs of nutrient/water uptake. We have microinjected the dye lucifer yellow carbohydrazide into peripheral root
cells of an Al-sensitive wheat (Triticum aestivum cv Scout 66) either before or after Al treatment and followed the cell-to-cell
dye-coupling through PD. Here we show that the Al-induced root growth inhibition is closely associated with the
Al-induced blockage of cell-to-cell dye coupling. Immunofluorescence combined with immuno-electron microscopic techniques
using monoclonal antibodies against 133b-d-glucan
(callose) revealed circumstantial evidence that Al-induced
callose deposition at PD may responsible for this blockage of symplastic transport. Use of 2-deoxy-d-glucose, a callose
synthesis inhibitor, allowed us to demonstrate that a reduction in callose particles correlated well with the improved
dye-coupling and reduced root growth inhibition. While assessing the tissue specificity of this Al effect, comparable
responses were obtained from the dye-coupling pattern in tobacco (Nicotiana tabacum) mesophyll cells. Analyses of the
Al-induced expression of PD-associated proteins, such as calreticulin and unconventional myosin VIII, showed enhanced
fluorescence and co-localizations with callose deposits. These results suggest that Al-signal mediated localized alterations
to calcium homeostasis may drive callose formation and PD closure. Our data demonstrate that extracellular Al-induced
callose deposition at PD could effectively block symplastic transport and communication in higher plants.