The rhizosphere is an exceptionally nutrient-dense region compared to bulk soil, with energy derived from root exu-dates, sloughed root cells, dead and decaying root tissue, cellular leakage derived from herbivory and pathogen attack, proteinaceous secretions (mucilage), and symbiotic relationships between plants and microorganisms which shuttle carbon from the root to surrounding soil. This rich nutrient source supports a dense and diverse population of primary consumers and an elaborate trophic web with the root as the primary carbon source. The length of a trophic chain depends on the amount of input from the primary producer: the more nutrient input, the longer the theoretical upper limit of chain length. In the rhizosphere, plant roots are the primary source ofcarbon, with roots serving as a sink for aboveground photosynthetically fixed atmospheric carbon. Detrital food webs are able to support anywhere from three to eight trophic nodes, and the rhizosphere food web is predicted to support a web ofsimilar length and complexity.
The primary consumers of plant exudates are microbes including bacteria and fungi. The nutrient source for these organisms includes small metabolites, which may be either actively or passively released by the plant into the surrounding soil. Root exudates include metabolites such as amino acids, organic acids such as citric and malic acids, and secondary plant compounds such as flavonoids and terpenoids. The microbial community is able to utilize these various compounds with some specificity, with individual microbial species more effectively utilizing a given carbon substrate than another species. Further, the root exudates from two different varieties of the same plant can select for different genotypes of the same bacterial species, in part through a differing transcriptional (gene expression) response by the bacteria to the exudates. Plant secondary metabolites, which often contain phenolic ring structures, are particularly resistant to general microbial degradation. However, certain microbial species are capable of utilizing these compounds, despite the fact that these compounds often display general antimicrobial activity. Larger metabolites and biopolymers such as polysaccharides and polypep-tides are also actively secreted from the root, which can be degraded and utilized by rhizosphere microbes. Additionally, soil microbes can be pathogenic to the plant, invading and killing the root or root system. Death of a root will rapidly generate a large detrital nutrient pool, containing both intracellular contents and cell wall and membrane components, which can then be utilized by saprophytic fungi or bacteria.
Invertebrates including insects, non-insect arthropods, and nematodes may also be primary consumers, feeding on live, dead, or decaying plant material. The most common root herbivores are soil nematodes, which feed on living root tissue via piercing/sucking mouthparts. The digestive system of herbivorous nematodes serves as a conduit for plant nutrient to be passed from the root to the soil via defecation. Likewise, nematode feeding can induce the death of the root in that region, feeding the saprophytic food web. Alternatively, arthropods such as collembola and soil mites may feed on dead or decaying vegetation. These arthropods will also commonly feed on rhizosphere microbes, thus as a class they may be considered either primary or secondary consumers. Predatory amoeba will feed on soil bacteria and can be consumed by nematodes. Likewise, certain nematode species feed on fungi, bacteria, or even other nematodes, so can be considered primary, secondary, or even higher-order consumers. However, feeding preference will generally be species-specific, with a given species either herbivorous or predatory. Many insect species spend part of their life cycle beneath the soil, and many are specifically adapted for root herbivory. Consumption of the root by an insect (commonly the immature larval stages of beetles) will result in wounds that expose plant cellular contents to the microbial community. Often the larval stage will complete development below ground, and much of the material consumed by the insect passes through the digestive system, again increasing nutrient availability for microbes. Predatory arthropods including insects and mites may also be tightly associated with the rhizosphere, feeding on nematodes, collembola, and mites. So a theoretical food chain might proceed from plant root exudate through bacteria, amoeba, nematode, predatory nematode, predatory mite, predatory insect larvae. These insect larvae can then serve as a food source for larger vertebrates such as mice or birds - extending the food chain above ground.
Rhizosphere trophic cascades are documented in which predatory nematodes cause a decrease in root damage inflicted by an herbivorous moth species. When the predatory nematodes are present, the host plant demonstrates increased growth and seed set over the course of a single growing season. The logical extension of the observed effects (increased growth of the host plant, in this case a member of the leguminous Lupinus genus) is that with increased growth and seed production may come increased Lupinus biomass and hence increased levels of symbiotic nitrogen fixation by rhizobia.
Biotic and Abiotic Influences on Rhizosphere Properties
The rhizosphere is a highly dynamic region, the properties of which are directly influenced by abiotic factors such as mineral composition and physical properties of the soil. Physical properties such as water permeability, soil texture, abrasiveness, and mineral composition and distribution can determine which plant species survive. Mineral concentrations in even a small region of soil can vary in concentration by 100-10 000-fold. Additionally, minerals can bind to plant-derived organic compounds (a process known as chelating), potentially altering their availability to soil microorganisms. These and similar soil characteristics can affect both plant growth and the micro-bial community. In this way, abiotic factors can influence root growth and can dictate the biotic effects on the rhizosphere physical and chemical properties (Figure 2).
After a given plant is established at a site, the soil characteristics will influence rhizosphere properties through effects on plant growth and physiology. Roots function in the uptake of nutrients, and the relative immobility of certain nutrients (particularly phosphate, potassium, and ammonium) in the soil results in local depletion of essential nutrients, which demands new root growth to probe for nutrient-rich regions. Individual plant roots are relatively short-lived. For many agicultural crops, approximately 20-50% of the individual roots die within
Plant growth and physiology
Rhizosphere soil characteristics
Microbial growth and physiology
Figure 2 Simple model of the biotic interactions influencing the rhizosphere.
a week, an indicator of the dynamic nature of the root system. This implies that any abiotic factor that a plant can detect and respond to may affect the rhizosphere through root death or new growth.
Soil physical characteristics will directly impact plant growth and physiology through a variety of mechanisms. For example, physical abrasion in sandy soil may result in elevated rates of carbon transfer from root to rhizosphere, likely through increased sloughing of cells and secretion of polysaccharide mucilage to prevent root damage. Likewise, a highly compacted soil becomes difficult for plant roots to penetrate, and thereby restricts root mass and surface area, and hence rhizosphere volume.
Plant root architecture is also influenced by nutrient availability. Soils that are low in phosphate, for example, may induce increased production of fine root hairs with a decrease in secondary roots. Likewise, nutrient depletion often results in an increased root:shoot ratio, often with decreased absolute biomass of the root system. Nutrient deficiency or mineral toxicity (such as aluminum) will often result in an increased secretion of organic materials into the rhizosphere. These compounds may be organic acids, which regulate rhizosphere pH and thus reduce the solubility of aluminum in soil water, or higher molecular weight proteins, which are thought to bind and sequester aluminum. Likewise, plants can alter exudation in response to phosphorus deficiency. These secretions may increase availability of sparse minerals and decrease the toxicity of overly abundant minerals. Though a plant may secrete such compounds into the soil for the purpose of detoxification of minerals, such secretion will simultaneously increase carbon availability in the rhizosphere, as many microbes can metabolize these compounds.
The highly dynamic nature of the rhizosphere is governed in part simply by plant growth and death. However, this view is simplistic in that the inhabitants of the rhizo-sphere impact soil nutrient status and plant physiology. The plant response to these stresses will further affect rhizosphere characteristics. The rhizosphere is a continuously evolving habitat, the characteristics of which are impacted by interacting biotic components.
Both bacterial and fungal species produce secondary metabolites that are of biological and ecological importance to rhizosphere dynamics. Both clades are capable of producing metabolites that mimic plant hormones such as auxin and giberellins. As plant growth is governed, at least in part, by hormone signaling, these metabolic products can directly impact root growth and therefore rhizosphere dynamics. Additionally, expression of the genes responsible for biosynthesis of these secondary metabolites can be regulated by environmental factors such as carbon status of the plant, nitrogen status of the soil, and rhizosphere pH - all of which are impacted by plant physiological mechanisms. Further, some of the secondary metabolites of fungi have antimicrobial properties, and bacterial products can inhibit growth of other bacteria or fungi. Thus microbial competition in the rhizosphere is partially mediated by secondary metabolites, in addition to direct competition for organic and inorganic resources. In addition, plant secondary metabolites can influence this competition, favoring those microbes that can metabolize the specific molecules in the root exudates. Further, plant exudates have been demonstrated to regulate the virulence of the soil microbes - that is, whether a particular microbial species is pathogenic (virulent) to the plant or is simply a rhizosphere inhabitant (avirulent).
Microbes may form symbiotic relationships with plants which benefit the plant through increased growth and/or seed production. This relationship can be formed by certain taxonomic groups of fungi and bacteria. In the case of mycorrhizal fungi, the fungus transfers primarily mineral nutrients and water to the plant in exchange for photosynthetically derived carbon. The carbon supports extensive growth of the fungus outside of the rhizosphere, which increases the area available for nutrient uptake by the fungus. This relationship then benefits both partners. Fungal hyphae can extend well beyond the reach of the root system, and this network of fungal influence is called the mycorhizosphere. Dinitrogen-fixing bacteria (diazo-trophs) can either be contained within specialized root organs called nodules or living in the soil matrix surrounding the root. Diazotrophs such as Rhizobia spp. and Frankia spp. are contained within root nodules, and thus contribute little to the rhizosphere nitrogen pool directly. They directly transfer fixed nitrogen to host plants. Nitrogen is a limiting nutrient in many ecosystems, and plants that are able to form symbioses with nitrogen-fixing bacteria often display increased growth following nodulation, which indirectly increases the size and complexity of the rhizosphere.
Plant-growth-promoting rhizobacteria constitute another functionally (not necessarily taxonomically) related group that influences plant growth. However, this class does so without developing an endosymbiosis. In this class, the presence ofspecific bacterial species in the rhizo-sphere promotes the growth of the plant through associative dinitrogen fixation, nutrient mineralization and chelation, and protection from pathogens. Many dia-zotrophs are rhizobacteria and they can directly contribute available nitrogen to the rhizosphere. One study revealed that a possible mode of communication between rhizobac-teria and plants is through volatile metabolites produced by the rhizobacteria. When the plant was exposed to these compounds, it responded with increased growth rate.
The properties of the rhizosphere are also dependent on the properties of the aboveground portion of the plant. Herbivory on leaf tissue can alter gene expression in the root system, often resulting in altered susceptibility to soil pathogens. Aboveground wounding can increase the rates of symbiosis between plants and arbuscular mycorrhizal colonization. This wounding has also been demonstrated to result in increased quantities of bacterial-feeding nematodes in the rhizosphere. Conversely, root herbivory will effect aboveground physiology and can reduce fitness parameters, such as seed production. Nutrient status can also affect the ability of a plant to mount an induced defense upon exposure to aboveground herbivory. The major wound hormone of plants, jasmonic acid, can be transported from shoot to root, allowing leaf herbivory to elicit defense responses in roots.
Soil nutrient status will also affect the interactions between rhizosphere inhabitants. Symbiosis between dia-zotrophs and leguminous or actinorhizal plants is more likely to be established in soil of low nitrogen availability than soil with abundant nitrogen. Under low-nitrogen conditions, leguminous plants will increase production of flavonoid secondary metabolites which, when released into the soil, serve a communicative role to nodule-forming rhizobia. Likewise, when the rhizobia recognize the presence of a legume root (via detection of the flavonoid signal), they release lipooligosaccharides into the soil which the plant recognizes to initiate nodule formation. These compounds clearly serve a role in communication between two highly coevolved species. Following the establishment of nodules, plants respond with increased growth and the rhizosphere microbial community becomes more active, presumably due to increased carbon availability in the rhizosphere.
Plant roots have the ability to not only contribute carbon to the rhizosphere, but to take organic metabolites from the rhizosphere. The sum of the rate of efflux and influx provides a measure ofnet contribution to the rhizo-sphere. Microbial secondary metabolites have been demonstrated to increase the efflux of plant-derived amino acids from the root into the rhizosphere by 200-2000%. Bacterial Pseudomonas spp. produce a metabolite, 2,4-diacetylphloroglucinol, which was found to block amino acid uptake by the plant, while fungal Fusarium spp. produce a metabolite, zearalenone, which increases amino acid efflux from the roots of alfalfa. In this way microbes are thought to play an active role in the plants' ability to modify the rhizosphere.
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