Rhizosphere

[7] 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.

Concentrations of organic acids and saccharides affect the ability of the biological communities to shuttle phosphorus, nitrogen,[12][13] potassium, and water to the root cap,[4] and the total availability of iron to the plant and to its neighbors.

[14] 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.

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.

Soil fauna provides the rhizosphere's top-down component while allowing for the bottom-up increase in nutrients from rhizodeposition and inorganic nitrogen.

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 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.

[30] 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.

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.

[38] This description has been used to explain the complex interactions that plants, their fungal mutualists, and the bacterial species that live in the rhizosphere have entered into throughout their evolution.

[36][6] Although various studies have shown that single microorganisms can benefit plants, it is increasingly evident that when a microbial consortium—two or more interacting microorganisms—is involved, additive or synergistic results can be expected.

Beneficial mechanisms of plant growth stimulation include enhanced nutrient availability, phytohormone modulation, biocontrol, and biotic and abiotic stress tolerance) exerted by different microbial players within the rhizosphere, such as plant-growth-promoting bacteria (PGPB) and fungi such as Trichoderma and mycorrhizae.

Recent advances in plant-microbe interactions research have shown that communication, both inter-kingdom and intra-kingdom, is shaped by a broad spectrum of factors.

In this context, the rhizosphere (i.e., the soil close to the root surface) provides a specific microhabitat where complex interactions occur.

The relationship established by rhizobia with other rhizospheric organisms and the influence of environmental factors results in their beneficial role on host plant health.

Thus, this environment is a hot spot for numerous inter-kingdom signal exchanges involving plant-associated microbial communities (rhizobiome).

The microbial community's composition is mainly shaped and recruited by hundreds of metabolites released in the soil by plant roots, which normally facilitate interactions with the biotic and abiotic environment.

[46][51][52][41] The most known plant-microbe dialogue on the rhizosphere scene, determining direct and indirect advantages to the partners, was properly addressed as early as 1904 when Hiltner described the symbiotic interaction among legumes and rhizobia.

In this mutualistic interaction, rhizobia positively influences the host's growth thanks to the nitrogen fixation process and, at the same time, can benefit from the nutrients provided by the plant.

[53][54] However, the knowledge about the earlier steps of rhizosphere colonization, namely the opening line at the root surface, remains poorly characterized.

Increasing data have shown the importance of intraspecies and multispecies communications among rhizospheric biotic components for improving rhizobia–legumes interaction.

Some rhizosphere processes in the soil
(A) Root system architecture is concerned with structural features of the root and responds to with environmental stimuli. (B) The rhizosphere produces photosynthetically fixed carbon that exudes into the soil and influences soil physicochemical gradients. (C) Free-living or parasitic nematodes interact with the rhizosphere via signaling interactions. (D) Mycorrhizal fungi create intimate relationships with the roots and engage in nutrient exchange. (E) Bacterial composition is distinct upon different parts, age, type of the roots. [ 1 ]
Sunlight and carbon dioxide from the atmosphere are absorbed by the leaves in the plant and converted to fixed carbon. This carbon travels down into the plant's roots, where some travels back up to the leaves. The fixed carbon traveling to the root is radiated outward into the surrounding soil, where microbes use it as food for growth. In return, microbes attach to the plant root, which improves the root's access to nutrients and its resistance to environmental stress and pathogens. In specific plant/root symbiotic relationships, the plant root secretes flavonoids into the soil, which is sensed by microbes, which release nod factors to the plant root, which promotes the infection of the plant root. These unique microbes carry out nitrogen fixation in root nodules, which supplies nutrients to the plant.
Predicted effects of elevated carbon dioxide on soil carbon reserves [ 21 ]
In the short term, plant growth is stimulated by elevated carbon dioxide, resulting in increased rhizodeposition, priming microbes to mineralize soil organic carbon (SOC) and adding CO2 to the atmosphere through respiration. But the net impact on greenhouse gas emissions will be reduced by the increased uptake of CO2 from the atmosphere by increased plant growth. However, over the long term, soil reserves of easily decomposed carbon will be depleted by the increase in microbial activity, resulting in increased catabolism of SOC reservoirs, thus increasing atmospheric CO2 concentrations beyond what is taken up by plants. This is predicted to be a particular problem in thawing permafrost that contains large reserves of SOC that are becoming increasingly susceptible to microbial degradation as the permafrost thaws. [ 22 ] [ 23 ]
Rhizosphere microbial consortia [ 40 ]
Formation of N-fixing nodules induced by rhizobia [ 40 ]
Plant responses to bacteria in the rhizosphere [ 40 ]
Communication in the rhizosphere [ 41 ]
Actors and interactions in the rhizosphere: Inter-kingdom and intra-kingdom communication involving plants and microbes in the rhizosphere, including the consistent role of rhizobia.
VOCs = volatile organic compounds; PGP = plant growth promoting; AMF = arbuscular mycorrhizal fungi
Root nodules, each containing billions of Rhizobiaceae bacteria
Illustration of the rhizosphere [ 58 ]
A = amoeba consuming bacteria; BL = energy limited bacteria ; BU = non-energy limited bacteria; RC = root derived carbon; SR = sloughed root hair cells; F = fungal hyphae ; N = nematode worm. [ 56 ]
Growth chamber designs for studying rhizosphere interactions
(A) Rhizotron/Rhizobox set up, (B) Rhizobox with side-compartment, (C) vertical root mat chambers; a modular option is show where the plant can be pre-grown in a separate compartment and transplanted afterward onto the main examination chamber, inset shows a modular set up option, (D) horizontal root mat in rhizobox, (E) Mycorrhizal compartments, (F) split-root systems shown here in a rhizobox set up; (G) Nylon bag to separate root and root-free soil; roots may be restricted in the bag or the soil may be protected from root penetration by the bag. [ 1 ]