Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/1996
Publication Date: N/A
Interpretive Summary: Large areas of land within the U.S. and over 40% of the world's arable lands are acidic. In these acid soils, the aluminum (Al) ion is toxic to root growth and function. Thus, Al toxicity is the primary factor limiting crop production on these acid soils. Understanding the cellular mechanisms that some plant species use to tolerate Al are essential in order to develop crop species that can be cultivated on these acid soils. In this paper, we studied the role of the root exudation of Al-binding compounds in Al tolerance. We found that in moderately Al tolerant types of wheat, a single gene controls the release of an Al-binding organic acid. In a different, very Al resistant wheat genotype where multiple genes are involved in Al resistance, there is a constant release of phosphate from the root tip in addition to the Al triggered release of the organic acid seen in less Al resistant wheat lines. Both of these compounds can bind Al in the soil and prevent it from entering the root. These results identify two separate cellular mechanisms of Al exclusion from the root that confer Al tolerance in wheat. Understanding of the genetic and physiological control of these processes will assist scientists in the production of new Al tolerant crop varieties.
Technical Abstract: Although it is well known that Al resistance in wheat is multigenic, physiological evidence for multiple mechanisms of Al resistance has not yet been documented. The role of root apical phosphate and malate exudation in aluminum (Al) resistance was investigated in two wheat cultivars (Al- resistant Atlas and Al-sensitive Scout) and two near-isogenic lines (Al- resistant ET3 and sensitive ES3). In Atlas, Al resistance is multigenic while in ET3, resistance is conditioned by the single alt locus. Based on root growth experiments, Atlas was found to be 3-fold more resistant in 20 uM 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 and ET3 and only in the presence of Al (5 and 20 uM). Furthermore, the more Al-resistant Atlas also exhibited a constitutive phosphate release localized to the root apex. As predicted from the formation constants for 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 evidence 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 processes act in concert to enhance Al exclusion and thus Al resistance in Atlas.