Molecular Physiology of Aluminum Toxicity and Tolerance in Plants

Aluminum being the third most abundant metal in the earth's crust poses a serious threat to crop productivity in acid soils, which comprise almost half of the arable land. This review travels across time and updates research done on aluminum stress in plants. In its phytotoxic forms, aluminum affects root growth by acting in the root apical zone, resulting in growth inhibition in a very short time at micromolar concentrations. The mechanisms of aluminum toxicity in plants may proceed by growth inhibition, callose accumulation, cytoskeletal distortion, disturbance of plasma membrane surface charge, and H+-ATPase activity, lipid peroxidation of membranes, production of reactive oxygen species in cytosol and mitochondria, respiratory dysfunction, opening of mitochondrial permeability transition pores, collapsing of inner mitochondrial membrane potential, activation of mitochondrial protease, and induction of nuclear apoptosis, resulting ultimately in programmed cell death. In contrast, the mechanism of tolerance involves the exudation of organic acid anions, complexation of aluminum with organic acids, and subsequent detoxification. Many oxidative stress genes and other metabolically important genes have also been found to be induced under aluminum stress, and overexpression analyses have also shown some plants to develop some degree of tolerance. In the future, researchers in the area of aluminum research should investigate more basic mechanisms of aluminum toxicity and discover and study more aluminum-responsive genes that confer resistance against this toxic metal, to ensure food security for ever-increasing human populations in the future.