ALUMINUM INTERACTIONS WITH VOLTAGE-DEPENDENT CALCIUM-TRANSPORT IN PLASMA-MEMBRANE VESICLES ISOLATED FROM ROOTS OF ALUMINUM-SENSITIVE AND ALUMINUM-RESISTANT WHEAT CULTIVARS

The role of Al interactions with root-cell plasma membrane (PM) Ca2+ c hannels in Al toxicity and resistance was studied. The experimental ap proach involved the imposition of a transmembrane electrical potential (via K+ diffusion) in right-side-out PM vesicles derived from roots o f two wheat (Triticum aestivum L.) cultivars (Al-sensitive Scout 66 an d Al-resistant Atlas 66). We previously used this technique to charact erize a voltage-dependent Ca2+ channel in the wheat root PM (J.W. Huan g, D.L. Grunes, L.V. Kochian [1994] Proc Natl Acad Sci USA 91: 3473-34 77). We found that Al3+ effectively blocked this PM Ca2+ channel; howe ver, Al3+ blocked this Ca2+ channel equally well in both the Al-sensit ive and -resistant cultivars. It was found that the differential genot ypic sensitivity of this Ca2+ transport system to Al in intact roots v ersus isolated PM vesicles was due to Al-induced malate exudation loca lized to the root apex in Al-resistant Atlas but not in Al-sensitive S cout. Because malate can effectively chelate Al3+ in the rhizosphere a nd exclude it from the root apex, the differential sensitivity of Ca2 influx to Al in intact roots of Al-resistant Versus Al-sensitive whea t cultivars is probably due to the maintenance of lower Al3+ activitie s in the root apical rhizosphere of the resistant cultivar.