The cell wall (CW) has been recognized as the major target of aluminum (Al) toxicity. However, the components responsible for Al accumulation and the mechanisms of Al-induced CW function disruption are still elusive. The contribution of different CW components (pectin, hemicellulose 1 [HC1], and HC2) to adsorb Al and the effect of Al on xyloglucan endotransglucosylase/ hydrolyase activity were investigated in Arabidopsis (Arabidopsis thaliana) in this study. A fractionation procedure was optimized to effectively extract different CW components, especially to prevent the HC fraction from pectin contamination. When CW materials extracted from Al-treated roots (50 mM Al for 24 h) were fractionated, about 75% of CW Al accumulated in the HC1 fraction. A time-dependent kinetic study showed that only when the HC1 fraction was removed was the amount of Al adsorbed decreased sharply. In vivo localization of xyloglucan endotransglucosylase (XET) activity showed that Al greatly inhibited this enzyme activity within 30 min of exposure, which was concomitant with Al-induced callose deposition in roots. Results from real-time reverse transcription-polymerase chain reaction indicated that three genes may constitute the major contributors to XET activity and that the inhibition of XET activity by Al is caused by transcriptional regulation. These results, to our knowledge for the first time, demonstrate that HC is the major pool for Al accumulation. Furthermore, Al-induced reduction in XET activity could play an important role in Al-induced root growth inhibition.

We demonstrated that aluminum (Al)-induced oxalate secretion and plasma membrane (PM) H(+)-ATPase activity in tomato (Lycopersicon esculentum 'Hezuo903') roots were poorly correlated. In addition, vanadate, an inhibitor of PM H(+)-ATPase, had no effect on Al-induced oxalate secretion, but significantly inhibited enzyme activity. An anion channel inhibitor phenylglyoxal inhibited oxalate secretion, but not PM H(+)-ATPase activity. Exposure of tomato roots to 10 μM LaCl(3) also stimulated PM H(+)-ATPase activity; however, La failed to induce oxalate secretion. Furthermore, Al-induced changes of PM H(+)-ATPase activity were not associated with oxalate secretion in two tomato cultivars differing in the ability to secrete oxalate under Al stress. These results indicate that Al independently regulates oxalate secretion and PM H(+)-ATPase activity in tomato roots. Analysis of expression levels of PM H(+)-ATPase genes by real-time reverse transcription-PCR and protein by Western blot and immunodetection revealed that the regulation of PM H(+)-ATPase in response to Al was subjected to transcriptional and posttranscriptional control. However, since neither transcriptional level of genes nor translational level of proteins directly relate to the enzyme activity, posttranslational modification of PM H(+)-ATPase under Al stress likely contributes to changes in activity of this protein.