Does Iron Deficiency in Pisum sativum Enhance the Activity of the Root Plasmalemma Iron Transport Protein?

Roots of Fe-sufficient and Fe-Deficient pea (Pisum sativum L.) were studied to determine the effect of Fe-deficiency on the activity of the root-cell plasmalemma Fe(2+) transport protein. Rates of Fe(III) reduction and short-term Fe(2+) influx were sequentially determined in excised primary lateral roots using Fe(III)-ethylene-diaminetetraacetic acid (Fe[III]-EDTA). Since the extracellular Fe(2+) for membrane transport was generated by root Fe(III) reduction, rates of Fe(2+) influx for each root system were normalized on the basis of Fe(III) reducing activity. Ratios of Fe(2+) influx to Fe(III) reduction (micromole Fe(2+) absorbed/micromole Fe[III] reduced) revealed no enhanced Fe(2+) transport capacity in roots of Fe-deficient peas (from the parental genotype, Sparkle) or the functional Fe-deficiency pea mutant, E107 (derived from Sparkle), relative to roots of Fe-sufficient Sparkle plants. Data from studies using 30 to 100 micromolar Fe(III)-EDTA indicated a linear relationship between Fe(2+) influx and Fe(III) reduction (Fe(2+) generation), while Fe(2+) influx saturated at higher concentrations of Fe(III)-EDTA. Estimations based on current data suggest the Fe(2+) transport protein may saturate in the range of 10(-4.8) to 10(-4) molar Fe(2+). These results imply that for peas, the physiological rate limitation to Fe acquisition in most well-aerated soils would be the root system's ability to reduce soluble Fe(III)-compounds.