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.

Influence of zinc hyperaccumulation on glucosinolates in Thlaspi caerulescens.

• Previous investigations suggest that in species of the Brassicaceae hyperaccumulation of heavy metals might provide an ecological advantage by protecting the plants against herbivores and/or pathogens while lowering the glucosinolate content. Few analytical data on glucosinolate concentrations in hyperaccumulators are available for supporting this 'trade-off' hypothesis. • This is the first report on the influence of zinc (Zn) hyperaccumulation on the concentrations of individual glucosinolates in Thlaspi caerulescens exposed to different Zn concentrations. • The most abundant glucosinolate within both roots and shoots was p-hydroxybenzyl glucosinolate (sinalbin). Zn hyperaccumulation decreased sinalbin concentrations in shoots, whereas root concentrations increased with Zn accumulation. These changes in sinalbin concentrations were mainly responsible for Zn-induced alterations of total glucosinolate contents. Quantitatively less important was a Zn-induced decrease of indolylglucosinolates observed in both roots and shoots and that of 3-butenylglucosinolate found in roots. • The results presented here support the view of a trade-off between Zn and glucosinolates in shoots but not in roots of Thlaspi caerulescens.