A role for cyclic hydroxamates in aluminium resistance in maize?

Hydroxamate siderophores have been found to alleviate Al toxicity in bacteria. In Poaceae plants cyclic hydroxamates, like DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) and its derivatives have mostly been studied in relation to either defence against insects or allelopathy. In this study the influence of Al on concentrations of these benzoxazinoids (Bx) in root tips, whole roots and root xylem exudates of Zea mays L. varieties differing in Al resistance was analyzed by HPLC-MS. Aluminium resistant maize variety Sikuani maintained considerably higher Bx levels in root tips than the Al sensitive variety Bakero. In vitro binding of Al to DIMBOA was shown by fluorescence quenching. Addition of DIMBOA to Al-containing nutrient solution protected the sensitive maize against Al toxicity as shown by bioassays using callose and haematoxylin staining of root tips as stress indicators. This is the first study showing that Bx can detoxify Al in solution. Tissue analysis data provide first, circumstantial, support for a role of Bx in defence against Al toxicity also in planta.