Flavin-dependent NmoA catalyzes oxidation of elemental selenium: Regulation by riboflavin synthesis and role in enhancing soil selenium bioavailability

Oxidation of elemental selenium (Se(0)) to Se oxyanions is essential to improve soil Se availability and plant uptake, addressing Se deficiency for one billion people worldwide. However, its molecular mechanism was unclear. Proteomic analysis showed Se(0) induced flavin-dependent nitrate monooxygenase (NmoA) expression in Agrobacterium deltaense T3F4. Analysis of enzyme activity, gene deletion and complementation experiments confirmed that NmoA mediated Se(0) oxidation to Se(IV) by NmoA coupled with flavin reductase (FR). Brassica rapa L. pot experiment using a deletion of nmoA mutant further illustrated the crucial role of NmoA in improving the soil Se bioavailability and enhancing Se uptake by plants. Moreover, proteomic and metabolomic analyses revealed that Se(0) exposure induced riboflavin accumulation and upregulation of expression of the gene encoding riboflavin synthetase. Riboflavin weakly catalyzed Se(0) oxidation in vitro. Heterologous expression of the riboflavin gene cluster ribDEH increased the Se(0) oxidation capacity by more than two times. Deletion of nrdR in strain T3F4 increased Se(0) oxidation by 10%, suggesting that riboflavin production was transcriptionally inhibited by the regulator NrdR. In conclusion, microbe-mediated Se oxidation enhances soil Se bioavailability and plant Se uptake, which is collectively mediated by the enzymatic reaction of flavin monooxygenase (NmoA) and metabolite riboflavin in A. deltaense T3F4. This finding bridges the gap in elucidating the molecular mechanism of Se(0) oxidation in bacteria.