A Study of Aluminum Dependent Root Growth Inhibition in Arabidopsis thaliana
Aluminum (Al) toxicity is a global agricultural problem and is one of the major factors that limit crop productivity on acidic soils. Plant Al toxicity is involved in many hypothesized mechanisms since it can interact with many intracellular and extracellular structures of the root. Mechanisms of Al exclusion, which confers Al-tolerance and resistance, have been studied in depth while internalized Al-tolerance mechanisms have not been fully described due to the complexity of the predicted cellular targets of Al.
Several Al hypersensitive mutants have been identified suggesting that they represent mutations in genes that are required for Al-tolerance or resistance. One of these mutants, als7 was identified as a mutation in Slow Walker2. Molecular analysis revealed that Al hypersensitivity in als7-1 is correlated with loss of expression of a factor required for methionine recycling, which leads to reduced levels of endogenous polyamines. Further analysis shows that Al-dependent root growth inhibition is reversed by addition of exogenous spermine. Spermine likely functions to compete with Al3+ for binding to extra- and intracellular anionic sites, which is shown by the severe reduction in Al accumulation in spermine-treated roots.
Several suppressor mutations were identified that masked the Al hypersensitivity of als3-1. One of these mutations, alt2-1 was isolated and represents a mutation in a WD40 protein that contains a DWD motif. Loss of alt2-1 causes the plant to fail to detect DNA damage caused by Al stress. This prevents a halt in the cell cycle for DNA repair, which in turn maintains the quiescent center and allows the plant to tolerate high levels of Al. Furthermore, alt2-1 is hypersensitive to the interstrand and intrastrand cross-linking agents. From in vitro studies, Al has been shown to be a DNA cross-linker, suggesting that alt2-1 is required to detect DNA cross-links. These results taken together suggest that Al is acting as a DNA cross-link mimic or a weak DNA cross-linker, which has minor immediate consequence on the plant if the cross-link is not detected.