Aluminum-induced 1 -> 3-beta-D-glucan inhibits cell-to-cell trafficking ofmolecules through plasmodesmata. A new mechanism of aluminum toxicity in plants

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 mol ecules including hormones. However, it is not known whether Al may affect t his cell-to-cell transport process, which is a critical feature for roots a s organs of nutrient/water uptake. We have microinjected the dye lucifer ye llow carbohydrazide into peripheral root cells of an Al-sensitive wheat (Tr iticum aestivum cv Scout 66) either before or after Al treatment and follow ed the cell-to-cell dye-coupling through PD. Here we show that the Al-induc ed root growth inhibition is closely associated with the Al-induced blockag e of cell-to-cell dye coupling. Immunofluorescence combined with immune-ele ctron microscopic techniques using monoclonal antibodies against 1-->3-beta -D-glucan (callose) revealed circumstantial evidence that Al-induced callo se deposition at PD may responsible for this blockage of symplastic transpo rt. 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 t he tissue specificity of this Al effect, comparable responses were obtained From the dye-coupling pattern in tobacco (Nicotiana tabacum) mesophyll cel ls. Analyses of the Al-induced expression of PD-associated proteins, such a s 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 call ose formation and PD closure. Our data demonstrate that extracellular Al-in duced callose deposition at PD could effectively block symplastic transport and communication in higher plants.