Al-sensitive (als) mutants of Arabidopsis were isolated and characteri
zed with the aim of defining mechanisms of Al toxicity and resistance.
Most als mutants selected on the basis of root growth sensitivity to
Al were recessive, and together the mutants constituted eight compleme
ntation groups. Also, in most als mutants, Al sensitivity appeared to
be specific for Al relative to La (another trivalent cation), except a
ls2, which was more sensitive to La than wild type. The tendency of ro
ots on mutant seedlings to accumulate Al was examined by staining with
morin and hematoxylin, dyes used to indicate the presence of Al. A si
gnificant increase in morin staining was observed in als5, consistent
with its increased sensitivity to Al. Unexpectedly, als7 and als4 show
ed less morin staining, suggesting that the roots on these mutants acc
umulate less Al than wild type after exposure to Al-containing solutio
ns. Roots of wildtype seedlings produce callose in response to AlCl3 c
oncentrations that inhibit root growth. Only als5 accumulated more cal
lose than wild type in response to low levels (25 mu M) of AlCl3. Howe
ver, als4 and als7 did not accumulate callose at this AlCl3 concentrat
ion even though root growth was significantly inhibited. The lack of c
allose accumulation in als4 and als7 suggests that there is not an obl
igatory relationship between callose deposition and Al-induced inhibit
ion of root growth.