[Wolfgang Schmidt] Scopoletin 8-hydroxylase-mediated fraxetin production is a decisive factor for the acquisition of immobilized iron

As a key edaphic factor, soil pH has a strong impact on the availability of mineral nutrients and the distribution of species in natural plant communities. Iron solubility decreases dramatically with increasing pH, excluding so-called calcifuge (‘chalk-fleeing’) species from carbonate-rich, alkaline soils due to their inability to acquire sufficient Fe under such conditions. The question as to the traits that allow calcicole plants to thrive on neutral or alkaline soil has puzzled plant biologists for more than two centuries. In collaboration with the metabolomics core facility at IPMB, Wolfgang Schmidt’s group showed that scopoletin 8-hydroxylase (S8H; At3g12900) participates in Fe acquisition by mediating an as yet undocumented step in the biosynthesis of fraxetin (7,8-dihydroxy-6-methoxycoumarin), a coumarin derived from the scopoletin pathway. Fraxetin exhibited Fe-reducing properties in vitro with higher rates being observed at neutral relative to acidic pH. In natural Arabidopsis accessions differing in their performance on media containing immobilized Fe, the amount of secreted fraxetin was highly correlated with growth, Fe and chlorophyll content, indicating that fraxetin secretion is a decisive factor for calcicole-calcifuge behavior of plants.