While some nitrifying bacteria are content to live in the soil and pass off the fruits of their labor to their fellow plants, some want to take this step further and will actually perform nitrogen fixation inside plant roots. Mostly, this occurs in plants of the legume family. Legumes are flowering plants with the Latin name Fabaceae, and they carry these rhizobia bacteria in what are called nodules in their roots. Now, rhizobia are gram-negative soil bacteria that perform nitrogen fixation, and they do so in the roots of these legumes. Technically, that makes them endophytes, or organisms that live inside other plants. Usually, they are fungi or bacteria. Nodules are swollen nodes or lumps in the roots where these bacteria have infected the plants. And make no mistake, even though this relationship is beneficial for both organisms, this is technically a bacterial infection in the plant, and that's why the swelling occurs. I mean, if you've ever seen a swollen lump in a tree stump or something like that, that's also a site of infection. That's in part how plants will respond to infections, and that is why these swollen lumps look like infections because they are infections. But it's not, you know, an unwanted infection. In fact, plants are inviting these bacteria in by releasing these chemicals into the soil called flavonoids. And these actually signal the rhizobia, who in turn release their own signal, which are called nod factors, and these nod factors will actually come into contact with the root hairs.

So, I'm going to jump out of the image here, and just draw right here, so let's pretend, here we have rhizobia bacteria. The flavonoids will be released by the root hair, and they're going to make their way to the rhizobia over here. And in turn, she is going to release her nod factors, and that is going to stimulate the root hair and cause a morphological change that will actually allow the bacteria to enter into the cortex, the inner part of the plant, through what's called an infection thread. So basically, the rhizobia here, who I'm just going to put 'rr' for now, she is going to take advantage of that morphological change, enter the root hair, go through the root hair cell, and make her way into the cortex. So this is supposed to be a cortex cell here, and this of course is a root hair. So, once inside the actual plant cortex, these rhizobia are going to set up shop. You can see an actual image of what this looks like here. All of these dark lumps are bacteria living in the plant cell, and they're going to get along swimmingly actually. They have a mutualistic relationship because the plant provides carbohydrates and protection to the bacteria. Right? And the bacteria are going to provide usable nitrogen. And remember earlier I said that the energy demands of performing nitrogen fixation are very great, but these plants, right, in this mutualistic relationship, they're helping the bacteria out with those energy demands. They're saying, hey, look, you're giving us nitrogen, here's some carbohydrate that you'll be able to convert into ATP, and then you can use that ATP to make the nitrogen usable for us. So it's like, you know, you scratch my back I'll scratch yours.

Now, in addition, plants will also produce this molecule called leghemoglobin. And hopefully, you can pick out that word, hemoglobin, in there. Remember, that is the oxygen-binding molecule found in red blood cells. Well, this is like a special kind of hemoglobin in a sense, this leg hemoglobin. It still binds oxygen, but the reason it's binding this oxygen is actually to protect nitrogenase, that enzyme that's going to carry out the chemical reaction that turns gaseous nitrogen into ammonia; it's going to protect that enzyme complex from oxygen poisoning. So it's just one more way in which plants are actually helping out these rhizobia bacteria.

Now, I also mentioned that these bacteria weren't the only way that plants can obtain nitrogen. And I mentioned detritivores, and of course, when you think detritivores, hopefully, you're thinking fungi. Obviously, bacteria play a role in the breakdown of organic matter as well. Most certainly not going to discount how important bacteria are, but these fungi will form, remember, mycorrhizae, those fungus roots, those associations between the fungal hyphae and the plant roots. And in this association, they will actually be able to provide nutrients for the plants, like nitrogen and phosphorus—both super important. Right? Both going to wind up components of nucleic acids, for example. Now the hyphae increase the surface area for absorption, so they're going to help plants absorb nutrients better. They also, you know, the fungus also will break down organic matter, which is going to free up these elements for absorption. Right? They're going to help break down the structures that they're part of, making them easier for plants to absorb. And we're actually going to see two types of mycorrhizae. There's ectomycorrhizae, which I have pictures of here. And basically, this is mycorrhizae where the hyphae wrap around plant cells but don't actually penetrate into plant cells. So these are not endophytes. Our arbuscular mycorrhizae, on the other hand, are endophytes. They're going to actually penetrate their hyphae into the cortical cells of plant roots. And I don't have a picture of that here, but, you know, you can imagine, looking at this image here for example, that one of these purple hyphae would extend into a plant cell in arbuscular mycorrhizae. Here, what I actually have pictures of are ectomycorrhizae. So this is a more zoomed-out image. These brown twiggy-looking things are plant roots, and all this white gunk all over it, that kind of looks like mold, is the fungal part of the mycorrhizae. So you can see, you know, with the naked eye, basically, that close association of plant and fungus. Here we have some more zoomed-in pictures like right here, and especially right here. Here we're looking at a fungal sheath around the root, and here in this image that's behind my head, you can actually see the hyphae wrapped around plant cells. These dark spots are going to be the actual cells of the plant, and all this white gunk in-between them is the hyphae, so that's the fungus part of it. So this close association not only helps plants absorb nutrients but can also help provide some nitrogen and phosphorus for plants, and also don't forget that there's just a bunch of other fungi in the soil, free-living, that's also breaking down organic matter and contributing to the amount of free nitrogen available in the soil. That's all I have for this lesson. See you guys next time.