Making the "Green Revolution" Truly Green: improving crop nitrogen use efficiency.

Traditional breeding for high-yielding crops has mainly relied on the widespread cultivation of gibberellin (GA)-deficient semi-dwarf varieties, as dwarfism increases lodging resistance and allows for high nitrogen use, resulting in high grain yield. Although the adoption of semi-dwarf varieties in rice and wheat breeding brought big success to the "Green Revolution" in the 20th century, it consequently increased the demand for nitrogen-based fertilizer, which causes severe threat to ecosystems and sustainable agriculture. In order to make the "Green Revolution" truly green, it is necessary to develop new varieties with high nitrogen-use efficiency (NUE). Under this demand, research on NUE, mainly for rice, has made great strides in the last decade. This mini-review focuses on three aspects of recent epoch-making findings on rice breeding for high NUE. The first one on "NUE genes related to GA signaling" shows how promising it is to improve NUE in semi-dwarf Green Revolution Varieties. The second aspect centers around the nitrate transporter1.1B, NRT1.1B; studies have revealed a nutrient signaling pathway through the discovery of the nitrate-NRT1.1B-SPX4-NLP3 cascade. The last one is based on the recent finding that the Teosinte branched1, Cycloidea, Proliferating cell factor (TCP)-domain protein 19 underlies the genomic basis of geographical adaptation to soil nitrogen; OsTCP19 regulates the expression of a key transacting factor, DLT/SMOS2, which participates in the signaling of four different phytohormones, GA, auxin, brassinosteroid and strigolactone. Collectively, these breakthrough findings represent a significant step towards breeding high NUE rice in the future.