A maize gene encoding an NADPH binding enzyme highly homologous to isoflavone reductases is activated in response to sulfur starvation

Sulfur nutrition plays an important role in the growth and development of higher plants, and glutathione, the main storage form of reduced sulfur, is involved in the response to a variety of stress conditions. The identification of genes activated on sulfur starvation may thus provide insights not only into the mechanisms of adaptation to nutrient limitation but also into the response(s) to stress resulting from glutathione depletion. By applying mRNA differential display analysis to a model system of maize seedlings grown hydroponically under either sulfate-sufficient or sulfate-deprived conditions, we isolated a novel gene that is selectively induced both in roots and shoots in response to sulfur starvation. This gene encodes a cytosolic, monomeric protein of 33 kD that selectively binds NADPH. The predicted polypeptide is highly homologous (>70%) to leguminous isoflavone reductases (IFRs), but the maize protein (IRL for isoflavone reductase-like) belongs to a novel family of proteins present in a variety of plants. Anti-IRL antibodies specifically recognize IFR polypeptides, yet the maize protein is unable to use various isoflavonoids as substrates. IRL expression is correlated closely to glutathione availability: it is persistently induced in seedlings whose glutathione content is about fourfold lower than controls, and it is down-regulated rapidly when control levels of glutathione are restored. This glutathione-dependent regulation indicates that maize IRL may play a crucial role in the establishment of a thiol-independent response to oxidative stress under glutathione shortage conditions.