Arabidopsis Mutants with lncreased Sensitivity to Aluminum'

AI-sensitive (als) mutants of Arabidopsis were isolated and characterized with the aim of defining mechanisms of AI toxicity and resistance. Most als mutants selected on the basis of root growth sensitivity to AI were recessive, and together the mutants constituted eight complementation groups. Also, in most als mutants, AI sensitivity appeared to be specific for AI relative to La (another trivalent cation), except als2, which was more sensitive to La than wild type. The tendency of roots on mutant seedlings to accumulate AI was examined by staining with morin and hematoxylin, dyes used to indicate the presence of AI. A significant increase in morin staining was observed in als5, consistent with its increased sensitivity to AI. Unexpectedly, als7and als4 showed less morin staining, suggesting that the roots on these mutants accumulate less AI than wild type after exposure to AI-containing solutions. Roots of wildtype seedlings produce callose in response to AICI, concentrations that inhibit root growth. Only als5 accumulated more callose than wild type in response to low levels (25 PM) of AICI,. However, als4 and als7did not accumulate callose at this AICI, concentration even though root growth was significantly inhibited. The lack of callose accumulation in als4 and als7 suggests that there is not an obligatory relationship between callose deposition and AI-induced inhibition of root growth.