Mechanisms of Al-induced iron chlorosis in wheat (Triticum aestivum). Al-inhibited biosynthesis and secretion of phytosiderophore

Although Al-induced iron chlorosis has been observed in many plants, the mechanisms responsible for this phenomenon are yet to be understood. We investigated the effect of Al on iron acquisition in a Strategy II plant, wheat (Triticum aestivum L.) using both Al-tolerant (Atlas 66) and -sensitive (Scout 66) cultivars. When iron was supplied as insoluble iron, ferric hydroxide, in the culture solution, both cultivars without Al treatment grew normally, while those with 100 μM AlCl₃ developed chlorosis of the young leaves after 3 days of the treatment. A 21-h treatment with 100 μM AlCl₃ in 0.5 mM CaCl₂ solution (pH 4.5) decreased the amount of 2'-deoxy-mugineic acid (DMA) secreted by Fe-deficient Atlas 66 and Scout 66 plants by 85 and 90%, respectively. The amount of DMA secreted decreased with increasing external Al concentrations. Al treatment during the biosynthesis process caused the inhibition of that of DMA within 3 h. The secretion process was also found to be inhibited by Al, resulting in the biosynthesized DMA remaining in the roots. These results demonstrate the inhibition by Al of both biosynthesis and secretion of DMA attributed to Al-induced iron chlorosis.