The role of molybdenum in agricultural plant production

BACKGROUND: The importance of molybdenum for plant growth is disproportionate with respect to the absolute amounts required by most plants. Apart from Cu, Mo is the least abundant essential micronutrient found in most plant tissues and is often set as the base from which all other nutrients are compared and measured. Molybdenum is utilized by selected enzymes to carry out redox reactions. Enzymes that require molybdenum for activity include nitrate reductase, xanthine dehydrogenase, aldehyde oxidase and sulfite oxidase. SCOPE: Loss of Mo-dependent enzyme activity (directly or indirectly through low internal molybdenum levels) impacts upon plant development, in particular, those processes involving nitrogen metabolism and the synthesis of the phytohormones abscisic acid and indole-3 butyric acid. Currently, there is little information on how plants access molybdate from the soil solution and redistribute it within the plant. In this review, the role of molybdenum in plants is discussed, focusing on its current constraints in some agricultural situations and where increased molybdenum nutrition may aid in agricultural plant development and yields. CONCLUSIONS: Molybdenum deficiencies are considered rare in most agricultural cropping areas; however, the phenotype is often misdiagnosed and attributed to other downstream effects associated with its role in various enzymatic redox reactions. Molybdenum fertilization through foliar sprays can effectively supplement internal molybdenum deficiencies and rescue the activity of molybdoenzymes. The current understanding on how plants access molybdate from the soil solution or later redistribute it once in the plant is still unclear; however, plants have similar physiological molybdenum transport phenotypes to those found in prokaryotic systems. Thus, careful analysis of existing prokaryotic molybdate transport mechanisms, as well as a re-examination of know anion transport mechanisms present in plants, will help to resolve how this important trace element is accumulated.     

Biochemical and Spectroscopic Characterization of the Human Mitochondrial Amidoxime Reducing Components hmARC-1 and hmARC-2 Suggests the Existence of a New Molybdenum Enzyme Family in Eukaryotes

The mitochondrial amidoxime reducing component mARC is a newly discovered molybdenum enzyme that is presumed to form the catalytical part of a three-component enzyme system, consisting of mARC, heme/cytochrome b₅, and NADH/FAD-dependent cytochrome b₅ reductase. mARC proteins share a significant degree of homology to the molybdenum cofactor-binding domain of eukaryotic molybdenum cofactor sulfurase proteins, the latter catalyzing the post-translational activation of aldehyde oxidase and xanthine oxidoreductase. The human genome harbors two mARC genes, referred to as hmARC-1/MOSC-1 and hmARC-2/MOSC-2, which are organized in a tandem arrangement on chromosome 1. Recombinant expression of hmARC-1 and hmARC-2 proteins in Escherichia coli reveals that both proteins are monomeric in their active forms, which is in contrast to all other eukaryotic molybdenum enzymes that act as homo- or heterodimers. Both hmARC-1 and hmARC-2 catalyze the N-reduction of a variety of N-hydroxylated substrates such as N-hydroxy-cytosine, albeit with different specificities. Reconstitution of active molybdenum cofactor onto recombinant hmARC-1 and hmARC-2 proteins in the absence of sulfur indicates that mARC proteins do not belong to the xanthine oxidase family of molybdenum enzymes. Moreover, they also appear to be different from the sulfite oxidase family, because no cysteine residue could be identified as a putative ligand of the molybdenum atom. This suggests that the hmARC proteins and sulfurase represent members of a new family of molybdenum enzymes.     

The identification of a novel protein involved in molybdenum cofactor biosynthesis in Escherichia coli

In the second step of the molybdenum cofactor (Moco) biosynthesis in Escherichia coli, the l-cysteine desulfurase IscS was identified as the primary sulfur donor for the formation of the thiocarboxylate on the small subunit (MoaD) of MPT synthase, which catalyzes the conversion of cyclic pyranopterin monophosphate to molybdopterin (MPT). Although in Moco biosynthesis in humans, the thiocarboxylation of the corresponding MoaD homolog involves two sulfurtransferases, an l-cysteine desulfurase, and a rhodanese-like protein, the rhodanese-like protein in E. coli remained enigmatic so far. Using a reverse approach, we identified a so far unknown sulfurtransferase for the MoeB-MoaD complex by protein-protein interactions. We show that YnjE, a three-domain rhodanese-like protein from E. coli, interacts with MoeB possibly for sulfur transfer to MoaD. The E. coli IscS protein was shown to specifically interact with YnjE for the formation of the persulfide group on YnjE. In a defined in vitro system consisting of MPT synthase, MoeB, Mg-ATP, IscS, and l-cysteine, YnjE was shown to enhance the rate of the conversion of added cyclic pyranopterin monophosphate to MPT. However, YnjE was not an enhancer of the cysteine desulfurase activity of IscS. This is the first report identifying the rhodanese-like protein YnjE as being involved in Moco biosynthesis in E. coli. We believe that the role of YnjE is to make the sulfur transfer from IscS for Moco biosynthesis more specific because IscS is involved in a variety of different sulfur transfer reactions in the cell.     

Stereoselective coupling of hemigossypol to form (+)-gossypol in moco cotton is mediated by a dirigent protein

The terpenoid gossypol, a secondary metabolite found in the cotton plant, is synthesized by a free radical dimerization of hemigossypol. Gossypol exists as an atropisomeric mixture because of restricted rotation around the central binaphthyl bond. The dimerization of hemigossypol is regiospecific in cotton. In the case of some moco cotton, the dimerization also exhibits a high level of stereoselectivity. The mechanism that controls this stereoselective dimerization is poorly understood. In this paper, we demonstrate that a dirigent protein controls this stereoselective dimerization process. A partially purified protein preparation from cotton flower petals, which by itself is unable to convert hemigossypol to gossypol, converts hemigossypol with a 30% atropisomeric excess into (+)-gossypol when combined with an exogenous laccase, which by itself produces racemic gossypol.     

Molecular mapping of the cnx2 locus involved in molybdenum cofactor biosynthesis in rice (Oryza sativa L.)

Molybdenum cofactor (Moco) is essential for nitrate reductase (NR), xanthine dehydrogenase (XDH), and aldehyde oxidase to perform their catalytic functions in plants. Moco biosynthesis is a complex process involving many genes. Little is known about the genetics and molecular aspects of Moco biosynthesis in plants and other eukaryotes. In rice, we previously isolated a Moco mutant C25 with a mutation in the CNX2 gene from a mutagenized indica cultivar IR30 and characterized its biochemical properties. This mutant was crossed with a japonica cultivar, Norin 8, to investigate the linkage of cnx2 to restriction fragment length polymorphism (RFLP) and cleaved amplified polymorphic sequence (CAPS) markers. Chlorate resistance was used to trace the cnx2 mutation because of its cosegregation with the loss of NR and XDH activities observed earlier. RFLP and CAPS analyses show the location of the cnx2 locus on the long arm of chromosome 4. It is mapped between RFLP markers C513 and C377 with a distance of 9.5 and 13.1 cM, respectively. It is also linked with CAPS marker RA0738 at a distance of 30.3 cM. Received: 25 June 2000 / Accepted: 31 August 2000     

Abscisic aldehyde oxidase in leaves of Arabidopsis thaliana

Abscisic acid (ABA) is a plant hormone involved in seed development and responses to various environmental stresses. Oxidation of abscisic aldehyde is the last step of ABA biosynthesis and is catalysed by aldehyde oxidase (EC 1.2.3.1). We have reported the occurrence of three isoforms of aldehyde oxidase, AOalpha, AObeta and AOgamma, in Arabidopsis thaliana seedlings, but none oxidized abscisic aldehyde. Here we report a new isoform, AOdelta, found in rosette leaf extracts, which efficiently oxidizes abscisic aldehyde. AO delta was specifically recognized by antibodies raised against a recombinant peptide encoded by AAO3, one of four Arabidopsis aldehyde oxidase genes (AAO1, AAO2, AAO3 and AAO4). Functionally expressed AAO3 protein in the yeast Pichia pastoris showed a substrate preference very similar to that of rosette AOdelta. These results indicate that AOdelta is encoded by AAO3. AOdelta produced in P. pastoris exhibited a very low Km value for abscisic aldehyde (0.51 microM), and the oxidation product was determined by gas chromatography-mass spectrometry to be ABA. Northern analysis showed that AAO3 mRNA is highly expressed in rosette leaves. When the rosette leaves were detached and exposed to dehydration, AAO3 mRNA expression increased rapidly within 3 h of the treatment. These results suggest that AOdelta, the AAO3 gene product, acts as an abscisic aldehyde oxidase in Arabidopsis rosette leaves.     

The role of xanthine oxidoreductase and uric acid in metabolic syndrome

Xanthine oxidoreductase (XOR) could contribute to the pathogenesis of metabolic syndrome through the oxidative stress and the inflammatory response induced by XOR-derived reactive oxygen species and uric acid. Hyperuricemia is strongly linked to hypertension, insulin resistance, obesity and hypertriglyceridemia. The serum level of XOR is correlated to triglyceride/high density lipoprotein cholesterol ratio, fasting glycemia, fasting insulinemia and insulin resistance index. Increased activity of endothelium-linked XOR may promote hypertension. In addition, XOR is implicated in pre-adipocyte differentiation and adipogenesis. XOR and uric acid play a role in cell transformation and proliferation as well as in the progression and metastatic process. Collected evidences confirm the contribution of XOR and uric acid in metabolic syndrome. However, in some circumstances XOR and uric acid may have anti-oxidant protective outcomes. The dual-face role of both XOR and uric acid explains the contradictory results obtained with XOR inhibitors and suggests caution in their therapeutic use.