34.19:
The Roles of Bacteria and Fungi in Plant Nutrition
Though plants derive their food through photosynthesis, they also require minerals and nutrients. Some plants rely on soil-dwelling organisms to efficiently acquire resources from the soil.
Rhizobacteria are prokaryotes that live in association with plant roots. This diverse class of organisms receives nutrition from plant roots while providing support for the plant in a variety of ways. They might, for instance, produce antibiotics to protect the plant, absorb unwanted chemicals from the soil, or facilitate the acquisition of essential nutrients.
Nitrogen is a critical component of proteins and nucleic acids. However, plants cannot use atmospheric nitrogen, as they lack the machinery to convert it into biologically useful forms.
During a process called nitrogen fixation, prokaryotes convert atmospheric nitrogen to ammonia, which is modified to ammonium. Other prokaryotes convert ammonia to nitrate. Nitrate and ammonium can be taken up and used by plants.
In legumes, like pea and bean plants, nitrogen fixation occurs at specialized root structures called nodules. In this symbiotic relationship, Rhizobia fix atmospheric nitrogen and make it bioavailable to plants, while the bacteria receive critical nutrients from the legume they are associated with.
Most plant species are also associated with fungi in a mutually beneficial relationship. The fungi, called mycorrhizae, receive nutrition from the plant. In return, mycorrhizae can access soil stores of nutrients that would be otherwise unavailable to plants. For instance, fungi facilitate the acquisition of phosphate, copper, and zinc.
Fungi may also function as a physical barrier against pathogens or produce antibiotics to protect the plant.
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.
Suggested Reading
Kraiser, Tatiana, Diana E. Gras, Alvaro G. Gutiérrez, Bernardo González, and Rodrigo A. Gutiérrez. “A Holistic View of Nitrogen Acquisition in Plants.” Journal of Experimental Botany 62, no. 4 (February 2011): 1455–66. [Source]
Mendes, R, Garbeva, P, Raaijmakers, JM. “The rhizosphere microbiome: significance of plant beneficial, plant pathogenic, and human pathogenic microorganisms.” FEMS Microbiol Rev. 2013 Sep 37(5):634-63. [Source]
van der Heijden, MG, Martin, FM, Selosse, MA, Sanders, IR. “Mycorrhizal ecology and evolution: the past, the present, and the future.” New Phytol. 2015 Mar 205(4):1406-23. [Source]