MULTIPLE ALUMINUM-RESISTANCE MECHANISMS IN WHEAT - ROLES OF ROOT APICAL PHOSPHATE AND MALATE EXUDATION

Although it is well known that aluminum (Al) resistance in wheat (Trit icom aestivum) is multigenic, physiological evidence for multiple mech anisms of Al resistance has not yet been documented. The role of root apical phosphate and malate exudation in Al resistance was investigate d in two wheat cultivars (Al-resistant Atlas and Al-sensitive Scout) a nd two near-isogenic lines (Al-resistant ET3 and Al-sensitive ES3). In Atlas Al resistance is multigenic, whereas in ET3 resistance is condi tioned by the single Alt1 locus. Based on root-growth experiments, Atl as was found to be 3-fold more resistant in 20 mu M Al than ET3. Root- exudation experiments were conducted under sterile conditions; a large malate efflux localized to the root apex was observed only in Atlas a nd in ET3 and only in the presence of Al (5 and 20 mu M). Furthermore, the more Al-resistant Atlas exhibited a constitutive phosphate releas e localized to the root apex. As predicted from the formation constant s for the Al-malate and Al-phosphate complexes, the addition of either ligand to the root bathing solution alleviated Al inhibition of root growth in Al-sensitive Scout. These results provide physiological evid ence that Al resistance in Atlas is conditioned by at least two genes. In addition to the alt locus that controls Al-induced malate release from the root apex, other genetic loci appear to control constitutive phosphate release from the apex. We suggest that both exudation proces ses act in concert to enhance Al exclusion and Al resistance in Atlas.