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 s ulfur, is involved in the response to a variety of stress conditions. The identification of genes activated on sulfur starvation may thus pr ovide insights not only into the mechanisms of adaptation to nutrient limitation but also into the response(s) to stress resulting from glut athione depletion. By applying mRNA differential display analysis to a model system of maize seedlings grown hydroponically under either sul fate-sufficient or sulfate-deprived conditions, we isolated a novel ge ne that is selectively induced both in roots and shoots in response to sulfur starvation. This gene encodes a cytosolic, monomeric protein o f 33 kD that selectively binds NADPH. The predicted polypeptide is hig hly homologous (>70%) to leguminous isoflavone reductases (IFRs), but the maize protein (IRL for isoflavone reductase-like) belongs to a nov el family of proteins present in a variety of plants. Anti-IRL antibod ies specifically recognize IFR polypeptides, yet the maize protein is unable to use various isoflavonoids as substrates. IRL expression is c orrelated closely to glutathione availability: it is persistently indu ced in seedlings whose glutathione content is about fourfold lower tha n controls, and it is down-regulated rapidly when control levels of gl utathione are restored. This glutathione-dependent regulation indicate s that maize IRL may play a crucial role in the establishment of a thi ol-independent response to oxidative stress under glutathione shortage conditions.