34.19: The Roles of Bacteria and Fungi in Plant Nutrition
Plants have the impressive ability to create their own food through photosynthesis. However, plants often require assistance from organisms in the soil to acquire the nutrients they need to function correctly. Both bacteria and fungi have evolved symbiotic relationships with plants that help the species to thrive in a wide variety of environments.
The collective bacteria residing in and around plant roots are termed the rhizosphere. These soil-dwelling bacterial species are incredibly diverse. Though some may be pathogenic, most have roles in promoting plant health. In exchange, the bacteria receive nutrition from plants in the form of carbohydrates, amino acids, and nucleic acids.
The bacteria called rhizobacteria can protect plants by producing antibiotics or absorbing toxic metals in the soil. Additionally, bacteria help plants by accessing otherwise unusable stores of nutrients in the soil. For example, plants lack the molecular machinery to utilize nitrogen from the atmosphere directly. Instead, they take up nitrogen in the form of ammonium (NH4+) and nitrate (NO3- ), which is generated by soil-residing bacteria.
During a process called nitrogen fixation, soil-dwelling bacteria convert atmospheric nitrogen to ammonia. Nitrogen-fixation requires large amounts of ATP that bacteria derive from plant-provided carbohydrates. Other groups of bacteria convert ammonia to nitrate during a two-step process called nitrification. These processes provide plants with forms of nitrogen that they can use to synthesize proteins and nucleic acids.
Fungi also participate in symbiotic relationships with plants. Scientists think that the mutualistic relationship between plants and mycorrhizae fungi was a key adaptation in the successful colonization of land, helping early plants acquire essential elements from the soil. Modern mycorrhizae function in water acquisition, growth factor signaling, and plant protection. Ectomycorrhizae cover roots and help absorb water and minerals. At the same time, arbuscular mycorrhizae are embedded within the root tissue, increasing contact between the plant cells and the branching filaments of the fungus, called hyphae.