Sustainable crop yields from the coordinated modulation of plant growth and nitrogen metabolism

Over the past 50 years, the Green Revolution based on the adoption of semi-dwarf cereals was able to significantly improve grain yield. The green revolution varieties (GRVs) accumulate more growth-repressing DELLA proteins (DELLAs) owing to mutant alleles at the Rht (wheat) and SD1 (rice) loci, which results in semi-dwarfism and GRV resistance to “lodging” under high levels of nitrogen (N) fertilization. However, GRV DELLAs accumulation confers reduced N-use efficiency (NUE). Achievement of high GRV yield requires excessive use of N fertilizer inputs that unsustainably damage the environment. Therefore, developing new breeding strategies to cut fertilizer use in high-yield cereal crops is an urgent global sustainable agriculture goal. Recently, we showed that the rice GROWTH-REGULATING FACTOR 4 (GRF4) is a positive regulator of N-responsive plant growth and N assimilation, and GRF4 could counteract SLR1-mediated inhibition of N-metabolism. The further experiments demonstrated that homeostatic co-regulation of plant C-N ratio is conferred by balanced activities and physical interaction of GRF4 and the DELLAs. GRF4 promotes and integrates N assimilation, C fixation and plant growth, whilst DELLA inhibits them. Importantly, we showed that increased GRV GRF4 abundance alters the GRF4-DELLA balance to favour increased C and N assimilation without loss of yield-enhancing dwarfism, thus sustainably boosting dwarfism-conferred GRV yield-gains. Therefore, modulating coordination of plant growth and metabolism thus elevates NUE and yield, enabling strategic breeding for sustainably increased global food security, yet reducing environmentally degrading agricultural nitrogen use. This minireview discusses the latest advances in the understanding of molecular mechanisms underlying the improvement of NUE. In addition, N sensing, signaling and responses are complicatedly regulated by various positive and negative components. Thus, identification of new components in the regulation of N metabolism and signaling using systems-wide approaches and the understanding of the cross-talk with other nutrient and phytohormone signaling will provide new strategies for future sustainable food security and a new green revolution.