Team:CU-Boulder/Description

Project Description

All plants need nitrogen to survive but they cannot use nitrogen (N2) directly from the atmosphere. They must receive their nitrogen as ammonia (NH3) that has been fixated by other sources such as symbiotic bacteria, decomposing plants, or synthetic fertilizers. Today, the world population is only sustainable if ammonia-rich fertilizers are used to increase crop production. However, fertilizer is an environmental hazard due to the large amount of carbon dioxide released during the production of ammonia. Also, fertilizer runoff causes algae populations to grow faster than the ecosystem can handle, creating oxygen-depleted dead zones within the ocean. Fertilizer can also pose a safety risk as well, for example, the 2020 Beirut explosion was caused by a decomposing fertilizer supply.

When looking for ammonia sources other than synthetic fertilizer, we noticed that certain plants get their ammonia from specific strains of bacteria. The most common strain being rhizobium, which shares a symbiotic relationship with plants such as legumes. The bacteria will live on the plant roots to receive sugar and in turn, bacteria provide the plant ammonia. These ammonia producing bacteria produce ammonia using an enzyme called nitrogenase. The nitrogenase enzyme can reduce N2 to ammonia by using an active site composed of iron and sulfur clusters. The overall structure of nitrogenase derived from rhizobium is not overly complicated, so we can engineer this enzyme into a plant directly; therefore creating a self-fertilizing plant. However, we were not alone in this thought. For many years researchers have been trying to figure out how to create self-fertilizing plants with this enzyme. The biggest obstacle being that radical oxygen is found in plants and exposure to radical oxygen at the active site inhibits and destroys the nitrogenase complex, rendering it useless. And it is to our knowledge that no one has been able to overcome this obstacle yet.

To overcome the obstacle of radical oxygen, we propose attaching a superoxide dismutase (SOD) enzyme to the nitrogenase. SOD will convert radical oxygen into hydrogen peroxide. By doing this, we hope to convert radical oxygen before it can attack the active site of the nitrogenase. From literature, we know that Azorhizobium caulinodans (azotobacter) utilize nitrogenase and SOD to control radical oxygen within the cell. The main difference between SOD containing bacteria and our design is that the bacteria have SOD freely moving inside the cell whereas we plan to attach it directly to nitrogenase. By directly attaching it, only the radical oxygen that comes close enough to the nitrogenase active site will be converted while other radical oxygen (that is important to plant cell function) will not be harmed. Also, given that we will only convert the radical oxygen that comes in close contact, we will likely not produce enough hydrogen peroxide to harm the cell. Therefore, we hypothesize that adding a superoxide dismutase complex to a nitrogenase structure will create an effective nitrogen fixating complex compatible with a plant environment.