Arbuscular Mycorrhizal Fungi: A Potential Candidate for Nitrogen Fixation

Arbuscular mycorrhizal fungi (AMF) and rhizobia are the two most important plant symbionts. The function of rhizobia in nitrogen fixation is well known. Apart from bacteria, AMF also assist plants and provide benefits in many ways. AMF are one of the types of mycorrhiza having specialized branched hyphae structure called arbuscules. They show association with about 80% of land plant species, play an important role in nutrient uptake, and improve soil texture and quality. AMF also provide tolerance to abiotic stress and protection against pathogens. They also enhance nutritional value in fruits and accumulate secondary metabolites in plants. The main role of AMF is the uptake of immobile nutrients like phosphate and zinc. However, nitrogen uptake by AMF is still controversial. Various studies have demonstrated a pathway of nitrogen transport by AMF to the host plant, during which ERM (Extraradical mycelium) of AMF takes up nitrogen in the form of NH4+ and NO3−. These forms are converted to arginine and transferred to IRM (Intraradical mycelium), and finally, they are transported to the host plant. Genes like AAP and RIPTR2 are also discovered, which facilitate the uptake of different organic nitrogen forms. AMF and rhizobia also show similarities in their signaling pathways known as "common symbiotic pathways," which are also helpful in a better understanding of AMF function. In a few studies, it is indicated that AMF enhance Ca2+ uptake, which boosts up the process of nodulation and nitrogen fixation indirectly in leguminous plants. On the other hand, some studies suggest that nodulation suppresses AMF colonization due to competition between rhizobia and AMF for carbohydrates. Further research is required to understand the complete nitrogen-fixing pathway by AMF. In this chapter, various roles of AMF as a symbiont, their similarities with bacterial symbionts, and their current knowledge with nitrogen uptake are summarized.