Creating a New Circular Economy of Nitrogen and Phosphorus Fertilizer in Row Crops
Allocating Nitrogen to the Cob Storage Proteins
Grain crops (e.g. maize, rice, wheat, sorghum) are only about 50% efficient in taking up nitrogen fertilizer. These losses are extremely costly to the farmer and to the environment as they result in water pollution and in the release of nitrous oxide, a potent greenhouse gas, into the atmosphere. We propose altering these annual crops so that instead of allocating nitrogen to grain during plant senescence, nitrogen is allocated to storage proteins in the cob (rachis).
This basic strategy is employed by perennial species (e.g. bark storage proteins) to reuse collected nitrogen year after year with minimal losses, but it could also be applied to annuals. By storing available nitrogen in the cob, the cob becomes a high nitrogen co-product that could be used in a range of applications. The same concept could be applied to phosphorus and to the storage of phytate in a modified cob.
A modified cob has the potential of being tuned as the input of chemical processes, or as a modulated release fertilizer dependent on the properties of the cob’s cellular matrix, which could likely be modified with the use of transgenes. If used as a fertilizer, modified cobs could effectively leave nutrients in production fields, reducing needed supplemental fertilization and the problems associated with it.
Modifying the Cob
We tested whether maize has a natural variation for end-of-season cob nitrogen content, and we have found little evidence for heritable variation. This is unsurprising for annuals as there is a strong selection advantage to partition the seed with as much nitrogen as possible.
Implementing this concept will require developing a strong nitrogen sink within the rachis, while not interfering with the loading of sugars into the kernels. We propose that expressing storage proteins in the cob, while simultaneously reducing kernel storage proteins, would likely result in a substantial reallocation of nitrogen, while requiring the minimum changes in plant physiology and leading to minimal reductions in starch yield. This could be implemented through a combination of transgenes and CRISPR editing.
There are potential market opportunities for crops with this added-value trait, ranging from bioethanol production, to production in sensitive environments where nutrient runoff has a large effect, to contexts where nutrient inputs limit production, such as in the developing world. Crops with this added-value trait would have a low-protein grain product, and as such would likely act synergistically with the production of protein using fermentation technologies.
Click the links below to review additional documents associated with the concept: