Molybdenum Metabolism in Plants

Abstract

Among the micronutrients essential for plant growth and formicrosymbionts, Mo is required in minute amounts. However, since Mo isoften sequestered by Fe- or Al-oxihydroxides, especially in acidicsoils, the concentration of the water-soluble molybdate anion availablefor uptake by plants may be limiting for the plant, even when the totalMo content of the soil is sufficient. In contrast to bacteria, nospecific molybdenum uptake system is known for plants, but sincemolybdate and sulfate behave similarly and have similar structure,uptake of molybdate could be mediated unspecifically by one of thesulfate transporters. Transport into the different plant organs proceedsvia xylem and phloem. A pterin-bound molybdenum is the cofactor ofimportant plant enzymes involved in redox processes: nitrate reductase,xanthine dehydrogenase, aldehyde oxidase, and probably sulfite oxidase.Biosynthesis of the molybdenum cofactor (Moco) starts with aguanosine-X-phosphate. Subsequently, a sulfur-free pterin issynthesized, sulfur is added, and finally molybdenum is incorporated. Inaddition to the molybdopterin enzymes, small molybdopterin bindingproteins without catalytic function are known and are probably involvedin the storage of Moco. In symbiotic systems the nitrogen supply of thehost plant is strongly influenced by the availability of Mo in soil,since both bacterial nitrogenase and NADPH-dependent nitrate reductaseof mycorrhizal fungi are Mo enzymes.