Submitted to: The Federation of European Biochemical Societies
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/1997
Publication Date: N/A
Citation: 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 phytotoxic aluminum (Al) ion, Al3+, is solubilized into soil solution and is toxic to root growth and function. Thus, Al toxicity is the primary factor limiting crop production on these acid soils. There has been a significant research effort by many laboratories directed towards identifying the cellular mechanisms of aluminum toxicity. It is felt that if scientists can better understand how Al is toxic to plants, we will be able to develop appropriate agricultural practices and improved crop species that can be used to cultivate these acidic, Al-toxic soils. In this study, we tested the hypothesis commonly suggested in the literature that Al toxicity involves Al interactions with key enzymes in root cells that damages or inhibits these enzymes. We studied the effects of toxic levels of Al on a range of different enzymes from different locations within the cell and found that Al did not significantly influence the functioning of any of these enzymes, except for a specific enzyme localized in the outer cell membrane of root cells. This enzyme, called phospholipase C, acts on specific lipids (fatty molecules) in the outer membrane and is involved in cell signalling and development. Instead of binding to enzymes, it was found that Al preferentially binds to the membrane lipids that phosphlipase C interacts with, which could explain the specific inhibition of this enzyme by Al. These findings are significant, for they allow us to focus more directly on specific cell components that could be targets of Al toxicity in plants.
Technical Abstract: The trivalent cation aluminum can cause chronic cytotoxicity in plants, animals and microorganisms. It has been suggested that Al interaction with cell membranes and enzyme metal binding sites may be involved in Al cytotoxicity. In this study, the binding of Al to microsomes and liposomes was found to be lipid dependent with the signal transduction element phosphatidylinositol 4,5-bisphosphate having the highest affinity for Al with an Al:lipid stoichiometry of 1:1. Al binding was only reduced in the presence of high concentrations of Ca2+ (> 1 mM). Both citrate, and to a lesser extent malate, were capable of preventing Al lipid binding, which is consistent with the involvement of these organic acids in a recently described Al detoxification mechanism in plants. The effects of AlCl3, Al- citrate and ZnSO4 on metal-dependent enzyme activities (enolase, pyruvate kinase, H+-ATPase, myosin, Calpain, proteinase K, phospholipase A2 and arginase) was assayed in vitro. While Zn2+ was capable of inhibiting all the enzymes except the H+-ATPase, AlCl3 and Al-citrate had minimal effects except for with phospholipase A2 where an interaction with AlCl3 occurred, however, this could be negated by the addition of citrate. The results indicate that contrary to current hypotheses, the toxic mode of Al is not through an interaction with enzymatic catalytic metal binding sites but may be through the interaction with specific membrane lipids.