Rhizosphere
The rhizosphere is the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Soil pores in the rhizosphere can contain many bacteria and other microorganisms that feed on sloughed-off plant cells, termed rhizodeposition, and the proteins and sugars released by roots, termed root exudates. This symbiosis leads to more complex interactions, influencing plant growth and competition for resources. Much of the nutrient cycling and disease suppression by antibiotics required by plants occurs immediately adjacent to roots due to root exudates and metabolic products of symbiotic and pathogenic communities of microorganisms. The rhizosphere also provides space to produce allelochemicals to control neighbours and relatives.
The rhizoplane refers to the root surface including its associated soil particles which closely interact with each other. The plant-soil feedback loop and other physical factors occurring at the plant-root soil interface are important selective pressures in communities and growth in the rhizosphere and rhizoplane. Root respiration and exudation can generate Anoxic microsites in soil adjacent to roots, shaping microbial community structure.
Background
The term "rhizosphere" was used first in 1904 by the German plant physiologist Lorenz Hiltner to describe how plant roots interface with the surrounding soil. The prefix rhiza- comes from the Greek, meaning "root". Hiltner postulated the rhizosphere was a region surrounding the plant roots and populated with microorganisms under some degree of control by chemicals released from the plant roots.Chemical interactions
Chemical availability
Plant roots may exude 20–40% of the sugars and organic acids—photosynthetically fixed carbon. Plant root exudates, such as organic acids, change the chemical structure and the biological communities of the rhizosphere in comparison with the bulk soil or parent soil. Concentrations of organic acids and saccharides affect the ability of the biological communities to shuttle phosphorus, nitrogen, potassium, and water to the root cap, and the total availability of iron to the plant and to its neighbors. The ability of the plant's root and its associated soil microorganisms to provide specific transport proteins affects the availability of iron and other minerals for it and its neighbors. This can affect the composition of the community and its fitness.Root exudates come in the form of chemicals released into the rhizosphere by cells in the roots and cell waste referred to as rhizodeposition. This rhizodeposition comes in various forms of organic carbon and nitrogen that provide for the communities around plant roots and dramatically affect the chemistry surrounding the roots. Exopolysaccharides, such as polyglycolide, affect the ability of roots to uptake water by maintaining the physical stability of the soil carbon sponge and controlling the flow of water. For example, a tomato field study showed that exopolysaccharides extracted from the rhizosphere were different depending on the tomato varieties grown, and that under water deficit conditions, the increase in exopolysaccharide production and microbial activity affected water retention in the soil and field performance of tomato. In potato cultivar root exudates, phenols and lignins comprise the greatest number of ion influencing compounds regardless of growing location; however, the intensity of different compounds was found to be influenced by soils and environmental conditions, resulting in variation amongst nitrogen compounds, lignins, phenols, carbohydrates, and amines.
Allelochemicals
Although it goes beyond the rhizosphere area, it is notable that some plants secrete allelochemicals from their roots, which inhibits the growth of other organisms. For example, garlic mustard produces a chemical that is believed to prevent mutualisms forming between the surrounding trees and mycorrhiza in mesic North American temperate forests where it is an invasive species.Ecology of the rhizosphere
Rhizodeposition allows for the growth of communities of microorganisms directly surrounding and inside plant roots. This leads to complex interactions between species, including mutualism, predation/parasitism, and competition.Predation
Predation is considered to be top-down because these interactions decrease the population. Still, the closeness of species interactions directly affects the availability of resources, causing the population to be affected by bottom-up controls. Without soil fauna, microbes that directly prey upon competitors of plants, and plant mutualists, interactions within the rhizosphere would be antagonistic toward the plants. Soil fauna provides the rhizosphere's top-down component while allowing for the bottom-up increase in nutrients from rhizodeposition and inorganic nitrogen. The complexity of these interactions has also been shown through experiments with common soil fauna, such as nematodes and protists. Predation by bacterial-feeding nematodes was shown to influence nitrogen availability and plant growth. There was also an increase in the populations of bacteria to which nematodes were added. Predation upon Pseudomonas by amoeba shows predators can upregulate toxins produced by prey without direct interaction using supernatant. The ability of predators to control the expression and production of biocontrol agents in prey without direct contact is related to the evolution of prey species to signals of high predator density and nutrient availability.The food web in the rhizosphere can be considered as three different channels with two distinct sources of energy: the detritus-dependent channels are fungi and bacterial species, and the root energy-dependent channel consists of nematodes, symbiotic species, and some arthropods. This food web is constantly in flux since the amount of detritus available and the rate of root sloughing changes as roots grow and age. This bacterial channel is considered to be a faster channel because of the ability of species to focus on more accessible resources in the rhizosphere and have faster regeneration times compared with the fungal channel. All three of these channels are also interrelated to the roots that form the base of the rhizosphere ecosystem and the predators, such as the nematodes and protists, that prey upon many of the same species of microflora.
Competition
The competition between plants due to released exudates is dependent upon geometrical properties, which determine the capacity of interception of exudates from any point on the plant's roots, and physicochemical properties, which determine the capacity of each root to take up exudates in the area. Geometrical properties are the density of roots, root diameter, and distribution of the roots. Physicochemical properties are exudation rate, decay rate of exudates, and the properties of the environment that affect diffusion. These properties define the rhizosphere of roots and the likelihood that plants can directly compete with neighbors.Plants and soil microflora indirectly compete against one another by tying up limiting resources, such as carbon and nitrogen, into their biomass. This competition can occur at varying rates due to the ratio of carbon to nitrogen in detritus and the ongoing mineralization of nitrogen in the soil. Mycorrhizae and heterotrophic soil microorganisms compete for both carbon and nitrogen, depending upon which is limiting at the time, which heavily depends on the species, scavenging abilities, and the environmental conditions affecting nitrogen input. Plants are less successful at the uptake of organic nitrogen, such as amino acids than the soil microflora that exists in the rhizosphere. This informs other mutualistic relationships formed by plants around nitrogen uptake.
Competition over other resources, such as oxygen in limited environments, is directly affected by the spatial and temporal locations of species and the rhizosphere. In methanotrophs, proximity to higher-density roots and the surface is important and helps determine where they dominate over heterotrophs in rice paddies.
The weak connection between the various energy channels is essential in regulating predator and prey populations and the availability of resources to the biome. Strong connections between resource-consumer and consumer-consumer create coupled systems of oscillators, which are then determined by the nature of the available resources. These systems can then be considered cyclical, quasi-periodic, or chaotic.
Mutualism
Plants secrete many compounds through their roots to serve symbiotic functions in the rhizosphere. Strigolactones, secreted and detected by mycorrhizal fungi, stimulate the germination of spores and initiate changes in the mycorrhiza that allow it to colonize the root. The parasitic plant, Striga, also detects the presence of strigolactones and will germinate when it detects them; they will then move into the root, feeding off the nutrients present.Symbiotic nitrogen-fixing bacteria, such as Rhizobium species, detect compounds like flavonoids secreted by the roots of leguminous plants and then produce nod factors that signal to the plant that they are present and will lead to the formation of root nodules. Bacteria are housed in symbiosomes in these nodules, where they are sustained by nutrients from the plant and convert nitrogen gas to a form that the plant can use. Non-symbiotic nitrogen-fixing bacteria may reside in the rhizosphere just outside the roots of certain plants and similarly "fix" nitrogen gas in the nutrient-rich plant rhizosphere. Even though these organisms are thought to be only loosely associated with the plants they inhabit, they may respond very strongly to the status of the plants. For example, nitrogen-fixing bacteria in the rhizosphere of the rice plant exhibit diurnal cycles that mimic plant behavior and tend to supply more fixed nitrogen during growth stages when the plant exhibits a high demand for nitrogen.
In exchange for the resources and shelter plants and roots provide, fungi and bacteria control pathogenic microbes. The fungi that perform such activities also serve close relationships with species of plants in the form of mycorrhizal fungi, which are diverse in how they relate to plants. Arbuscular mycorrhizal fungi and the bacteria that make the rhizosphere their home also form close relationships to be more competitive. which plays into the bigger cycles of nutrients that impact the ecosystem, such as biogeochemical pathways.