Several lanthanides activate malate efflux from roots of aluminium-tolerant wheat

Many elements of the lanthanide series exist as trivalent cations in solution below pH 6. The present study was carried out to investigate whether lanthanides could stimulate malate efflux from wheat (Triticum aestivum L.) roots, as has been found for trivalent aluminium (Al) cations. Excised root apices treated with 100 mum of each of seven different lanthanide elements (lanthanum, praseodymium, europium, gadolinium, terbium, erbium, and ytterbium) stimulated malate efflux, with five- to fifty-fold more malate being released from an Al-tolerant wheat line than from a near-isogenic Al-sensitive line. As erbium stimulated the greatest malate efflux of the lanthanides tested, this response was characterized further. The characteristics of the erbium-activated efflux were similar to the Al-activated efflux described previously suggesting that both of these ions activate the same transport mechanism. The capacity for erbium-activated malate efflux cosegregated with Al tolerance in wheat seedlings derived from a cross between Al-sensitive and Al-tolerant near-isogenic lines. This is the first study to identify cations, other than Al, which can activate malate release from wheat roots. It also provides additional evidence that malate efflux from root apices is the primary mechanism for Al tolerance in wheat.