The importance of molybdenum (Mo) for plants, animals and microorganisms has been known for a long time (reviewed by Coughlan 1980). In fact, the transition element Mo is essential for (nearly) all organisms and has been found in more than 20 distinct mostly bacterial enzymes catalyzing diverse redox reactions (reviewed by Hille 1996; Stiefel 1996). Molybdenum deficiencies in field-grown plants were first recorded more than 50 years ago (reviewed by Gupta 1997) and could be traced back to a lack of nitrate reductase (NR) activity (Hewitt 1983) which catalyzes the key step in inorganic nitrogen assimilation. Eukaryotic and also prokaryotic NRs contain Mo as catalytic metal in the active center (Hille 1996). In higher plants, further Mo-enzymes have been described: (i) xanthine dehydrogenase is involved in purine catabolism and has importance for certain legumes in which it is crucial for synthesizing allantoic acid as the transport form of nitrogen (reviewed by Nguyen 1986), and (ii) aldehyde oxidase(s) has recently been shown to catalyze the last step in the biosynthesis of the phytohormones indolyl acetic acid (Koshiba et al. 1996) and abscisic acid (Walker-Simmons et al. 1989; Taylor 1991), respectively. Hence a shortage of Mo in the soil or a mutational block of the cellular ability to use Mo leads to the loss of essential metabolic functions