| Summary: | 博士 === 國立中興大學 === 生物科技學研究所 === 107 === Iron (Fe) is an essential mineral nutrient and an important factor for the composition of natural plant communities. Low Fe availability in aerated soils with neutral or alkaline pH has led to the evolution of elaborate mechanisms that extract Fe from the soil. In Arabidopsis (Arabidopsis thaliana), Fe is acquired by an orchestrated strategy that comprises mobilization and reduction of Fe3+ prior to its uptake. However, the efficiency of this strategy is low at alkaline pH due to compromised reductase activity, suggesting the existence of other mechanisms that support Fe uptake under such conditions. Despite knowing that scopoletin-derived coumarins play an important role in the uptake of Fe in Arabidopsis, the chemical nature and biosynthetic pathway of the active compound have remained enigmatic. The oxygenase At3g12900 participates in Fe acquisition by mediating the biosynthesis of fraxetin (7,8-dihydroxy-6-methoxycoumarin), a coumarin derived from the scopoletin pathway, acting as a scopoletin 8-hydroxylase (S8H). S8H is highly induced in roots of Fe-deficient plants both at the transcript and protein level. Mutants defective in the expression of S8H showed increased sensitivity to growth on pH 7.0 media supplemented with an immobile source of Fe and reduced secretion of fraxetin. Transgenic lines overexpressing S8H displayed an opposite phenotype. Fraxetin exhibited Fe-reducing and mobilization properties in vitro at neutral pH. Supplementing the media containing immobile Fe with fraxetin partially rescued s8h mutants. In natural Arabidopsis accessions differing in their performance on media containing immobilized Fe, the amount of secreted fraxetin was highly correlated with the growth, and Fe and chlorophyll content of the lines, indicating that fraxetin secretion is a decisive factor for the calcicole-calcifuge behavior of plants, i.e. the ability/inability to thrive on alkaline soils.
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