When plants work alone, they can only achieve that much. But when plants work side by side with some type of fungus or bacteria, they can go much further! With the aid of both organism, the plants may survive living in the harshest condition possible. Let us dwell into the synergy of plant with both microorganism to learn how do this cooperation happen.

Knowing fungus and bacteria

Fungus and bacteria both are types of microorganism, that is very fine to the naked eye. However, fungus is easier to be seen compared to bacteria. You might remember the mushroom you eat. It is actually the ‘fruit’ of fungus! Some of the time, we need to use microscope to see the actual body of fungus, but normally we can see them as ‘cottony strands’ much like what you see growing when you left a piece of bread on the counter top. This is quite different from bacteria, where it can only be seen under microscope with some chemical treatment, and with an isolation technique to identify them. This article will provide an insights on the two major players of symbionts in legume nodule, both fungi and bacteria.

Mycorrhizae-root symbionts

First thing is, we need to understand what is symbiosis is all about. ‘Symbiosis’ is a term where two unrelated organisms work with each other, each is giving something and receive benefit from one another. To most of the people, fungus is a bad thing to mess with. Our parents must in some point in our lives had reminded us not to touch that stale bread or those wild mushroom around your garden. Rather for the plants, some of it is very precious where they aid the plant in taking in food material, that it is believed that 80% of the trees around us depend on them for nutrient uptake. This fungus, forms a very close symbiotic process and are called ‘mycorrhizae,’ or literally translated; ‘root fungus.’ How does mycorrhizal fungi form symbiotic relationship with plants? It begins with nodule formation, much like swelling on the root surface that houses the fungi. Nodulation process on roots with mycorrhizae begins with the fungal spores on the soil interacts with chemical released by the root. It later germinated, forms threads of hyphae which later penetrated the root, and from there the fungi establishes network with the plant cell, and forms extensive network of hyphae. Did you know that nutrients in the soil can be classified as mobile and immobile? It is quite different from mobile and immobile nutrients on the plant, where they are divided in different capability to move inside the plant. For immobile nutrient in soil such as phosphorus they are very scarce in the soil, and only available to plants via diffusion. In other words the root itself needs to be literally beside every immobile nutrients in order to be taken by the root. However, this is not possible. Roots of plants can only absorb nutrient in a small range, called as ‘zone of absorption,’ in which the area roots are able to take in available nutrients. Beyond this zone, immobile nutrients are not available to the plants. This is where the beneficial symbiotic relationship in mycorrhizae comes along. Their extensive network of filamentous hyphae extends the zone of absorption of the root, enabling uptake of nutrients to be brought back to the plant, in exchange for the carbohydrate obtained from them.

Bacteria and root nodule

When bacteria forms symbiotic relationship with roots, it also form root nodules similar like mycorrhizae, however they are ‘picky’ on who would be their host. This relationship has its own name; ‘rhizobia’. The most famous example of bacteria and root relationship in legumes are the nitrogen-fixation process, and nitrogen is one of the most important nutrient a plant need. Now, take some time to breath. As you breathe in, almost 78% of them are actually nitrogen; with only 21% of them is oxygen that we use on our body. However, these nitrogen are not able to be used by plants on its own, much like we cannot eat flour on its own. It needs to baked to make bread before we can eat it. This is similar to nitrogen, where it needs to be converted into usable form to be taken by plants. Now, the role of rhizobia comes into play. A group of bacteria called Rhizobium and Azobacter are essential for nitrogen-fixation with the leguminous plant. These bacteria, when are able to infect the roots through wounds on root hairs, quickly multiply themselves inside the roots. They induces the root to form a ‘shephard’s crook’ curl, and later form nodules. Within the nodule, the rhizobia turns into bacteriod, which are a ‘friendly bacteria’ to the plant. It utilises nitrogenase enzyme, where it converts the atmospheric nitrogen (N2) to ammonia (NH3), a usable form of nitrogen to the plants. This is the ‘bread’ that plant could benefit from. One problem is that nitrogenase are very sensitive to oxygen, and with its presence nitrogenase would not work. To solve this problem, the answer lays on the most striking active interface showing the activity of nitrogen fixation in plant. It is called leghemoglobin. Ever heard of it before? You might recognise it when learning about the composition of human blood, hemoglobin, or red blood cell. They are having similar structure with each other, and even functions the same; to bind itself with oxygen and removes excess carbon dioxide. The difference is they are relatively pink in colour. Leghemoglobin performs a very tricky work in the nodules however, it needs to keep just enough oxygen to allow rhizobia to respire, however low enough to prevent nitrogenase action from being disabled. There might be other things happen just right below our feet, yet it may very crucial in our lives. All we need to do is understand them, and appreciate all of the gifts Nature had given us.

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