The physioiogy of aiuminum tolerance in higher fiants

Abstract

Hypotheses regarding mechanisms of aluminum (Al) tolerance in higher plants are grouped into two categories. With exclusion tolerance mechanisms, Al is prevented from entering the symplasm and reaching sensitive metabolic sites. With internal tolerance mechanisms, Al enters the symplasm and tolerance is achieved by detoxification, immobilization, or changes in metabolism. Aluminum is clearly immobilized at the root‐soil interface, however, the extent to which exclusion plays a role in the physiology of Al tolerance is not known. Exclusion mechanisms such as immobilization of Al at the cell wall, selective permeability of the plasma membrane, plant‐induced pH changes in the rhizosphere, and exudation of chelate ligands have been suggested, but experimental support is meagre. Chelation of Al in the cytosol by organic acids or proteins, compartmentation of Al in the vacuole and evolution of Al‐tolerant enzymes have been suggested as possible internal tolerance mechanisms. Despite the volume of literature on this subject, a workable hypothesis to explain how plants flourish on Al‐rich substrates is lacking.

Key words:

• aluminum
• internal tolerance
• exclusion
• mechanisms detoxification
• immobilization
• compartmentation
• chelation