Aluminum inhibits apoplastic flow of high–molecular weight solutes in root apices of Zea mays L.

Using an aluminum (Al)-sensitive maize cultivar, we investigated the influence of Al on the apoplastic solute bypass flow and its relationship with Al-induced (1 h, 50 μM) callose formation and root growth. We selected the fluorescent probes 8-hydroxypyrene-1,3,6-trisulfonic acid, trisodium salt (MW 524) (HPTS) and dextran-Texas Red (TR) conjugates (MW 3,000, 10,000, and 40,000) to monitor their apoplastic transport. Confocal laser-scanning microscopy (CLSM) analysis and spectrofluorometric quantification showed Al-induced callose formation in peripheral root cells within 1 h. Pretreatment of plants with the callose synthesis inhibitor 2-deoxy-D-glucose (DDG) reduced the callose formation by half. Uptake experiments with both HPTS and dextrans showed uniform dye distribution in control root apices. After Al treatment for 1 or 2 h, which inhibited root growth by 32% or 50%, respectively, the dyes accumulated in the epidermal and outer cortical cell layers, especially in the 1-2 mm apical root zone. Al treatment reduced the export of the dyes out of the apical 1 cm treatment zone. This was due to strong sorption of HPTS but not of dextrans by Al-loaded cell walls. Aluminum treatment reduced loading into the xylem sap particularly of higher-molecular weight dextrans. Pretreatment of roots with DDG and presence of 50 mM mannitol during the Al treatment partially forestalled the inhibitory effect of Al on the dye transport, but only slightly reduced the Al-induced growth inhibition. Exudation experiments revealed that xylem water flow remained unaffected by the Al treatment of the root tips. The results with dextran suggest that Al binding in cell walls of the root apex inhibits apoplastic bypass flow of higher-molecular weight solutes, which might contribute to Al-induced inhibition of root growth.

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