Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/12/1995
Publication Date: N/A
Interpretive Summary: Approximately 40% of the world s cultivated lands are acidic, and in many acid soils, aluminum (Al) toxicity is a major growth limiting factor for crop plants. Understanding the physiological causes of Al toxicity and the cellular mechanisms that allow some plant species and varieties to tolerate high levels of soil Al are essential to the development of crop species that can be successfully cultivated on Al toxic soils. In this study, we continued to investigate Al disruption of calcium uptake into cells of the root tip as a mechanism of Al toxicity. We developed new methods that let us study the uptake of calcium into plants at the cellular level. Using this new technique, we showed that Al directly blocked the calcium uptake system in cells from roots of both Al tolerant and sensitive wheat cultivars. We found that the major difference between the Al tolerant and resistant cultivars was that Al triggered a release of the organic acid, malate, from the root tip of the Al tolerant cultivar. Malate very effectively binds Al outside of the root and renders it non-toxic. These results are significant, for they identify an important new trait that confers Al tolerance in crop plants. These results could prove useful in the production of new Al tolerant crop varieties.
Technical Abstract: Calcium channels in the plant cell plasma membrane (PM) are presumed to play important roles in signal transduction processes, as well as in the provision of Ca2+ for plant mineral nutrition. Recent research on aluminum (Al)/Ca2+ transport interactions in plants indicates that Al may block root-cell PM Ca2+ channels, and this may play an important role in the mechanisms of Al phytotoxicity. In this study, we investigated Al interac- tions with voltage-dependent Ca2+ transport across the root-cell PM. The experimental approach involved the imposition of a transmembrane electrical potential (via K+ diffusion ) in populations of purified, right-side-out PM vesicles derived from roots of two wheat (Triticum aestivum L.) cultivars (Al-sensitive Scout 66 & Al-resistant Atlas 66), using aqueous two-phase partitioning. At pH 4.5, Al rapidly & effec- tively blocked the voltage- dependent Ca2+ influx into PM vesicles. However, Al blocked this Ca2+ channel equally well in PM vesicles derived from Al-sensitive & resistant cultivars. The difference in responses of Ca2+ transport systems to Al in intact roots versus PM vesicles isolated from these roots suggests that in roots, a cellular Al exclusion mechanism such as release of Al-chelating compounds is occurring. It was found that Al exposure triggered malate exudation in Al-resistant Atlas, but not in Al-sensitive Scout. Malate exudation is thought to confer Al resistance by chelating & thus lowering the activity of Al3+ in the solution surrounding the root apex. Thus, the differential sensitivity of Ca2+ influx to Al in intact roots of Al- resistant versus Al-sensitive wheat cultivars is probably due to the maintenance of lower Al3+ activities in the root apical rhizosphere of the resistant cultivar.