The green revolution of the mid-twentieth century saw the development of high-yielding varieties of rice and wheat for use in agriculture. But to produce high yields, these green-revolution varieties require a large supply of nitrogen. Developing green-revolution varieties that use nitrogen more efficiently is an important goal for sustainable crop breeding. In a paper in Nature, Li et al.1 report a previously unknown function for the rice transcription factor OsGRF4 in nitrogen use. By modulating the OsGRF4 gene, the researchers produced plants that use nitrogen efficiently and have a high yield.
Proteins of the DELLA family inhibit plant growth, whereas hormones called gibberellins promote plant growth by triggering the destruction of DELLA proteins. Green-revolution varieties of rice and wheat harbour genetic mutations that lead to the accumulation of DELLA proteins. As a result, these plants are shorter than are normal varieties, and so are resistant to lodging2,3 — the process by which plants are flattened by wind and rain. This lodging resistance is a fundamental mechanism for achieving increased crop yield in green-revolution varieties.
DELLA accumulation also inhibits nitrogen uptake and nitrogen-related growth responses — traits that are associated with the inefficient use of nitrogen4. Consequently, farmers have to apply large amounts of environmentally damaging nitrogen-based fertilizer to their crops to achieve high yields in green-revolution varieties. Although DELLA accumulation increases the yield, it therefore also has a negative impact in terms of sustainable agriculture.
Li et al. set out to overcome the negative impact of DELLA accumulation. They crossed varieties of the rice subspecies Oryza sativa indica that showed differing rates of nitrogen uptake. They then performed genetic analyses on the resulting plants, which had a range of yields. In doing so, they found that OsGRF4 is associated with nitrogen uptake. OsGRF4 has previously been found to regulate the size of rice grains5–7 and the levels of growth molecules called cytokinins8, both of which affect crop yield. But no relationship between OsGRF4 and nitrogen-use efficiency has previously been described.
The researchers genetically engineered green-revolution varieties of rice to lack OsGRF4. Compared with control plants carrying the wild-type gene, mutants showed less nitrogen-dependent growth and reduced nitrogen uptake and assimilation (the process by which inorganic nitrogen from fertilizers is converted into useful organic compounds such as amino acids). By contrast, plants that were selectively bred to express OsGRF4 at higher than normal levels showed an increased rate of nitrogen uptake. Thus, OsGRF4 promotes various nitrogen-related events.
Li et al. then demonstrated that OsGRF4 acts in opposition to the DELLA protein SLR1 in rice (Fig. 1). Transcriptional activation by OsGRF4 is known to be promoted by physical interactions between OsGRF4 and another protein, OsGIF15,6. The authors found that OsGRF4, promoted by OsGIF1, binds to a specific DNA sequence (the core motif GCGG) to drive the expression of genes that encode a range of proteins involved in nitrogen metabolism, uptake and assimilation. However, the accumulation of SLR1, as occurs in green-revolution varieties of rice, inhibits the interaction between OsGRF4 and OsGIF1, thereby suppressing the expression of the genes involved in nitrogen uptake and metabolism. This SLR1-mediated inhibition was relieved by the presence of gibberellin. Li and colleagues also showed that the expression of OsGRF4 itself is activated by the OsGRF4–OsGIF1 complex. Therefore, OsGRF4 transcription is suppressed by SLR1.
Next, the group discovered that OsGRF4 and SLR1 have the same antagonistic relationship in another key process in plant metabolism, carbon assimilation. Products of carbon and nitrogen assimilation act together to form the building blocks needed for metabolic processes in plants, and a balance between the two is therefore essential for optimal growth and yield. The authors showed that OsGRF4 promotes, and that SLR1 inhibits, the expression of various genes that are involved in three carbon-related processes: photosynthesis, sucrose transport and sucrose metabolism. Furthermore, the same relationship governs the expression of several genes involved in cell-cycle progression. Li and colleagues therefore propose that the antagonistic relationship between OsGRF4 and SLR1 provides a regulatory link that coordinates plant growth, nitrogen metabolism and carbon assimilation.
Finally, the authors used their findings to improve the yields of green-revolution varieties. They applied breeding strategies to generate rice plants that produce high levels of OsGRF4 but retain the short stature of green-revolution varieties. The resulting plants had wider leaves and stems, and showed an increased nitrogen uptake compared with normal plants. Consequently, crop yield was increased, even at low levels of nitrogen; an optimal carbon–nitrogen balance was attained; and the plants maintained their beneficial stature. Li et al. achieved a similar effect by increasing OsGRF4 expression in the rice subspecies Oryza sativa japonica, as well as in green-revolution varieties of wheat. They have therefore successfully disconnected gibberellin-mediated control of plant height from regulation of nitrogen metabolism, producing plants that grow better without an increased risk of lodging.
Li and colleagues’ study raises the question of how increased levels of OsGRF4 can increase plant growth horizontally (by increasing leaf and stem width) but not vertically (through stem elongation). Answering this question will involve in-depth studies of nitrogen-related genes that are under the regulatory control of OsGRF4.
More importantly, the authors’ work not only reminds us of the disadvantages of green-revolution varieties, but also demonstrates that they can be overcome by implementing breeding strategies to increase levels of OsGRF4. By improving the efficiency of nitrogen use by green-revolution varieties, the amount of nitrogen-based fertilizers that are needed for agriculture could be reduced, which would improve our ability to grow crops sustainably. Li and colleagues’ research should also stimulate the discovery of other genes and molecules with roles in nitrogen use that are independent of gibberellin-regulated plant growth. Identifying fresh targets for breeding strategies in this way could usher in a new green revolution.