The Protective Role of a Unique Tissue Detachment on Root Cap Cells in a Strong Aluminum-Resistant Tree Acacia mangium
Acacia mangium Willd. is a highly Al-resistant leguminous tree with modest organic anion exudation from roots. We have recently found the root apices of A. mangium to be surrounded by detached tissues with a unique structure under hydroponic conditions. To understand high Al tolerance mechanisms, we examined the roles of a cap-like structure on the Al-resistant root growth in A. mangium. The structures were autonomously sloughed off the root, and redeveloped within a week. Al only slightly inhibited the elongation of roots both with or without a cap-like structure. However, the removal of a root caplike structure facilitated Al-inducible root bending that only occurs immediately after the exposure. This facilitated root curving appears to be a major factor in slight enhancement in Al-induced root elongation inhibition in roots without a cap-like structure. The anchor position of a whole root cap-like structure to the root was limited to the columella cap region, where their connection became more rigid with longer Al-exposure periods. These results suggest that cap-like structures around the root may play a role in protecting root cap cells from the immediate impact of Al.