Ferric-chelate reductase activity is a limiting factor in iron uptake in spinach and kale roots

Abstract

Iron (Fe) is essential for many vital processes in plants, including chlorophyll biosynthesis, DNA synthesis, nitrogen reduction, and photosynthetic electron transfer. However, free Fe ions also participate in the Fenton reaction, which generates reactive oxygen species that induce oxidative stress, leading to lipid peroxidation, protein oxidation, and DNA mutation. Accordingly, plants have developed strategies to prevent roots from absorbing excess Fe. Here, we investigated Fe homeostasis in hydroponically grown spinach and kale under various Fe concentrations (5-800 µM), as well as the resulting changes in bioactive compound levels. Spinach and kale Fe contents did not increase under Fe concentrations in the nutrient solution of up to 200 µM. When spinach plants (grown in nutrient solution containing 2 µM Fe-EDTA) were transferred to 50 µM Fe-EDTA, ferric-chelate reductase activity in roots rapidly decreased within 24 hours. In kale, ferric-chelate reductase activity also significantly decreased with increasing Fe-EDTA concentration. As Fe did not accumulate in spinach plants, no significant differences in plant growth were observed. However, in kale, root growth was slightly suppressed by high Fe (100-200 µM) levels in the rhizosphere. Comparisons to other studies suggest that variations in phenolic and flavonoid contents are dependent on plant species, but overall, Fe treatment did not result in a significant increase in these compounds in spinach or kale. Our results suggest that in spinach and kale roots, the regulation of Fe contents by ferric-chelate reductase is responsible for maintaining Fe homeostasis.