Uptake kinetics of iron-phytosiderophores in two maize genotypes differing in iron efficiency.

Iron inefficiency in the maize (Zea mays L.) mutant ysl is caused by a defect in the uptake system for Fe-phytosiderophores. To characterize this defect further, the uptake kinetics of Fe-phytosiderophores in ysl was compared to the Fe-efficient maize cultivar Alice. Short-term uptake of 59Fe-labeled Fe-deoxymugineic acid (Fe-DMA) was measured over a concentration range of 0.03 to 300 µM. Iron uptake in Fe-deficient plants followed Michaelis-Menten kinetics up to about 30 µM and was linear at higher concentrations, indicating two kinetically distinct components in the uptake of Fe-phytosiderophores. The saturable component had similar Km (∼10 µM) in both genotypes. In contrast, Vmax was 5.5 µmol Fe-DMA g-1 dry weight [30 min]-1 in Alice, but only 0.6 µmol Fe-DMA g-1 dry weight [30 min]-1 in ysl. Uptake experiments with double-labeled 59Fe-[14C]DMA suggest that in both cultivars Fe-DMA was taken up by the roots as the intact chelate. The results indicate the existence of a high-affinity and a low-affinity uptake system mediating Fe-phytosiderophore transport across the root plasma membrane in maize. Apparently, the mutation responsible for Fe inefficiency in ysl affected high-affinity uptake and led to a decrease in activity and/or number of Fe-phytosiderophore transporters. [ABSTRACT FROM AUTHOR]