A slight level up with the terminology here. The phytomicrobiome or Plant Microbiome (PM) is the microbial community associated with a plant. These microbes can be bacteria, archaea, fungi, or protists.
As mentioned previously, these little friends are all over the place, so the phytomicrobiome comprises 3 main regions: the rhizophere, the phyllosophere, and the endomicrobiome. In simple words, these refer to the soil, the leaves, and the inner plant environment respectively.
What we haven't mentioned yet though, is that these microorganisms do much more than just fix nutrients. Their main functions include influencing plant root exudation, degrading toxic materials, conceding its host with more tolerance against biotic and abiotic stresses, promoting growth, as well as protecting it against pathogens of different kinds.
As their name suggests, endophytes ****are microorganisms that live inside plants, in the intracellular spaces. This totally makes the difference. Not only do they have a competitive advantage over other microbes that are fighting for growth and survival, but they can also have a more direct communication with their host.
Not only that, but endophytes are the only type of microorganism in the phytomicrobiome that can be transmitted vertically. In other words, the embryo will receive these types of bugs too, which represents a huge leverage point for fertilizers.
Going back to communication, we know that living things talk chem. So what happens here is that plants can send chemical signals that have been shown to activate gene expression in endophytes. That is mind-blowing! 🤯
We had already bragged already about the many different functions that the plant microbiome has 🙌. At this point though, we know that the most essential one clearly is nitrogen fixation. Microbes that do this are called diazotrophes. Thus, endophytic diazotrophes are microbes that live inside plants and fix nitrogen for them.
Nitrogenases are enzymes produced (usually) by cyanobacteria. They are responsible for the cutting of nitrogen (N2) to ammonia (NH3). Nitrogen is used by all living things to build proteins and nucleic acids. Nitrogen gas (N2) however is very stable and difficult to break apart. The amount of nitrogen in the soil surrounding the plant equates to the plants growth. Currently nitrogen gas is being formed through:
MoFE Cluster + Stabilizing Cluster - homocitrate molecule (pink + white) = Bottom, P-Cluster = Middle & Iron-Sulfur = At the Top
Currently, industrialized ammonia requires a lot of effort. It requires high temperatures and pressure (300 atmospheres). Bottom line: it is unsustainable. Not only do people disregard and work to avoid fertilized food, but it isn't economically sustainable to continue producing food that costs more to produce than your house!
On the other hand, in nitrogen-fixing bacteria Nitrogenase drives the reaction with a large quantity of ATP (possible future problem/opportunity). It also uses a collection of metal ions (molybdenum ions).
Nitrogenase is composed of 2 things: the MoFe (Molybdenum + Iron) protein shown in blue and purple above contains all of the machinery necessary to perform the reaction, but it requires a steady source of electrons. The reaction requires an additional 6 electrons for each nitrogen molecule that is split into 2 ammonia molecules. The Fe protein (shown in green) uses the breakage of the ATP to pump these electrons into the MoFe protein. In the typical reaction, 2 molecules of ATP are consumed for each transferred electron. Nitrogenase also converts hydrogen ions to hydrogen ions at the same time, therefore consuming even more ATP in the process.
So, now, you might be asking, who is keeping count of all the used up ATP? Let's go into why in the end we still need this reaction! Overall summary: