Analysis of the mechanism of iron assimilation Chlamydomonas reinhardtii: A model system for strategy I plants

The molecular mechanism of iron (Fe) assimilation in strategy I plants is o nly beginning to be understood. In an attempt to establish a model system f or crop plants, the mechanism of Fe assimilation was investigated in Chlamy domonas reinhardtii. As in strategy I plants, Chlamydomonas responded to Fe deficiency through the induction of a cell surface Fe3+-chelate reductase. Within 24 h following Fe removal, the Fe3+-chelate reductase was increased by greater than or equal to 15-fold compared to Fe-sufficient cells. The r eductase activity was negatively correlated with the Fe concentration in th e media, and resupply of Fe to Fe-deficient cells resulted in a decrease in activity to control levels within 4 - 5 h. The Fe3+-chelate reductase acti vity in Fe-starved cells was saturable and was inhibited by uncouplers of t he transmembrane proton gradient but not by SH-specific reagents. The analysis of uptake using radiolabelled Fe demonstrated that Fe uptake w as only observed in cells grown in Fe-deficient media. When Fe2+ was offere d as the sole substrate, uptake increased slightly. However, Fe uptake rate s were several orders of magnitude less than those for Fe3+-chelate reducti on, and it appeared that reduction was not the rate limiting step in Fe ass imilation. Thus, both Fe3+ reduction and uptake in C. reinhardtii were regu lated by the nutritional status; characteristics that were reminiscent of t he mechanism of Fe uptake in strategy I higher plants.