Rhizosphere (ecology)
Encyclopedia
The rhizosphere is the narrow region of soil
that is directly influenced by root
secretions and associated soil microorganism
s. Soil which is not part of the rhizosphere is known as bulk soil
. The rhizosphere contains many bacteria
that feed on sloughed-off plant cells, termed rhizodeposition, and the proteins and sugars released by roots. Protozoa
and nematode
s that graze on bacteria are also more abundant in the rhizosphere. Thus, much of the nutrient cycling and disease suppression needed by plants occurs immediately adjacent to roots.
s, secreted and detected by mycorhizal fungi, stimulate the germination
of spores and initiate changes in the mycorhiza that allow it to colonise 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 the Rhizobium
species, detect an unknown compound secreted by the roots of leguminous plants and then produce nod factor
s which signal to the plant that they are present and will lead to the formation of root nodule
s, in which the bacterium, sustained by nutrients from the plant, converts nitrogen gas to a form that can be used by the plant. Non-symbiotic (or “free-living”) nitrogen-fixing bacteria may reside in the rhizosphere just outside the roots of certain plants (including many grasses), and similarly “fix” nitrogen gas in the nutrient-rich plant rhizosphere. Even though these organisms are thought to be only loosely associated with 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.
Some plants secrete allelochemicals
from their roots which inhibit the growth of other organisms. For example garlic mustard
produces a chemical which is believed to prevent mutualisms forming between the tree
s and mycorhiza in mesic North America
n temperate forests.
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
that is directly influenced by root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
secretions and associated soil microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
s. Soil which is not part of the rhizosphere is known as bulk soil
Bulk soil
Bulk soil is soil outside the rhizosphere. Bulk soil is not penetrated by plant roots. Natural organic compounds are much lower in bulk soil than in the rhizosphere. Microbial populations are typically lower in bulk soil than in rhizospheric soil. Furthermore, bulk soil inhabitants are...
. The rhizosphere contains many bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
that feed on sloughed-off plant cells, termed rhizodeposition, and the proteins and sugars released by roots. Protozoa
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
and nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
s that graze on bacteria are also more abundant in the rhizosphere. Thus, much of the nutrient cycling and disease suppression needed by plants occurs immediately adjacent to roots.
Secretions
Plants secrete many compounds into the rhizosphere which serve different functions. StrigolactoneStrigolactone
Strigolactones are plant hormones that have been implicated in inhibition of shoot branching. Strigolactones are carotenoid-derived and trigger germination of parasitic plant seeds and stimulate symbiotic mycorrhizal fungi...
s, secreted and detected by mycorhizal fungi, stimulate the germination
Germination
Germination is the process in which a plant or fungus emerges from a seed or spore, respectively, and begins growth. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However the growth of a sporeling from a spore, for example the...
of spores and initiate changes in the mycorhiza that allow it to colonise the root. The parasitic plant, Striga
Striga (plant)
Striga, commonly known as witches weed, is a genus of 28 species of parasitic plants that occur naturally in parts of Africa, Asia, and Australia. Witchweeds are characterized by bright-green stems and leaves and small, brightly colored flowers. Furthermore, they are obligate parasites of roots...
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
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
bacteria, such as the Rhizobium
Rhizobium
Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium forms an endosymbiotic nitrogen fixing association with roots of legumes and Parasponia....
species, detect an unknown compound secreted by the roots of leguminous plants and then produce nod factor
Nod factor
Nodulation factors are signaling molecules produced by bacteria known as rhizobia during the initiation of nodules on the root of legumes. A symbiosis is formed when legumes take up the bacteria...
s which signal to the plant that they are present and will lead to the formation of root nodule
Root nodule
Root nodules occur on the roots of plants that associate with symbiotic nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia...
s, in which the bacterium, sustained by nutrients from the plant, converts nitrogen gas to a form that can be used by the plant. Non-symbiotic (or “free-living”) nitrogen-fixing bacteria may reside in the rhizosphere just outside the roots of certain plants (including many grasses), and similarly “fix” nitrogen gas in the nutrient-rich plant rhizosphere. Even though these organisms are thought to be only loosely associated with 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.
Some plants secrete allelochemicals
Allelopathy
Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial or detrimental effects on the target organisms...
from their roots which inhibit the growth of other organisms. For example garlic mustard
Garlic Mustard
Garlic mustard is a biennial flowering plant in the Mustard family, Brassicaceae. It is native to Europe, western and central Asia, and northwestern Africa, from Morocco, Iberia and the British Isles, north to northern Scandinavia, and east to northern India and western China...
produces a chemical which is believed to prevent mutualisms forming between the tree
Tree
A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
s and mycorhiza in mesic North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
n temperate forests.
Further reading
- Digging in the Dirt: Is the Study of the Rhizosphere Ripe for a Systems Biology Approach? - A review from the Science Creative Quarterly (retrieved 4 December 2006)