Real-time [11C]methionine translocation in barley in relation to mugineic acid phytosiderophore biosynthesis

[C-11]Methionine was supplied through barley roots and the C-11 signal was followed for 90 min using a real-time imaging system (PETIS), with subsequent development of autoradiographic images of the whole plant. In all cases, [C-11]methionine was first translocated to the 'discrimination center', the basal part of the shoot, and this part was most strongly labeled. Methionine absorbed by the roots of the plants was subsequently translocated to other parts of the plant. In Fe-deficient barley plants, a drastic reduction in [C-11]methionine translocation from the roots to the shoot was observed, while a greater amount of C-11 was found in the leaves of Fe-sufficient or methionine-pretreated Fe-deficient plants. Treatment of Fe-deficient plants with aminooxyacetic acid, an inhibitor of nicotianamine aminotransferase, increased the translocation of [C-11]methionine to the shoot. The retention of exogenously supplied [C-11]methionine in the roots of Fe-deficient barley indicates that the methionine is used in the biosynthesis of mugineic acid phytosiderophores in barley roots. This and the absence of methionine movement from shoots to the roots suggest that the mugineic acid precursor methionine originates in the roots of plants.