Download XI. BIOCHEMISTRY OF THE PLANT ROOT RHIZOSPHERE

XI. BIOCHEMISTRY OF THE PLANT ROOT
RHIZOSPHERE
Required Readings:
Wasaki, J., A. Rothe, A. Kania, G. Neumann, V. Romheld, T. Shinano, M. Osaki and E.
Kandeler 2005. Root exudation, phosphorus acquisition, and microbial diversity in the
rhizosphere of white lupine as affected by phosphorus supply and atmospheric carbon dioxide
concentration. Journal of Environmental Quality 34:2157–2166.
Cattelan, A.J., P.G. Hartel and J.J. Fuhrmann. 1998. Bacterial composition in the rhizosphere of
nodulating and non-nodulating soybean. Soil Science Society of America Journal 62:1549-1555.
Herman, D.J., K.K. Johnson, C.H. Jaeger III, E. Schwartz, and M.K. Firestone. 2006. Root
influence on nitrogen mineralization and nitrification in Avena barbata rhizosphere soil. Soil
Science Society of America Journal 70:1504–1511.
Suggested Readings:
Curl, E.A. and B. Truelove. 1986. The Rhizosphere. p. 1-8, Introduction; p. 140-166, Microbial
Interactions; p. 167-190, Rhizosphere in Relation to Plant Nutrition and Growth. SpringerVerlag, New York, NY, USA.
Bolton, H., J.K. Fredrickson, and L.E. Elliot. 1992. Microbial ecology of the rhizosphere. p. 2763. In F. Blaine Metting, Jr. (Ed.), Soil Microbial Ecology. Marcel Dekker, New York, NY,
USA.
Interaction between the soil and plant root systems are intensely studied by soil scientists,
microbiologist, and plant pathologists. Plant roots, can affect soil microorganisms, and soil
microorganisms can, in turn, effect plant growth. Plant root activity of one species can also effect
the health of another plant species.
In 1904, Lorenz Hiltner, a professor of Agronomy at the Technical College of Munich, Germany
defined rhizosphere as the specific region of soil affected by plant roots. The word, "rhizosphere"
comes from rhizo or rhiza which is a Greek word for root, and sphere which denotes an
environment or area of influence. Rhizoplane is more narrowly defined and describes the surface
of the plant root itself along with the tightly adhering soil particles.
The practical definition of rhizosphere soil is that soil which adheres to or is influenced by the
root but which can be removed from the root by gentle shaking in sterile water. Rhizoplane soil
is that which is obtained when the roots are transferred to a fresh sterile solution and shaken
vigorously. A control or bulk soil is soil which does not adhere to the plant root and is not
influenced by the root.
Although the rhizosphere, obviously extends into the soil for some distance, the total volume of
rhizosphere soil is difficult to assess. The rhizosphere volume can be altered by plant species,
soil type, soil moisture, portion of the root being evaluated, and the method used to determine the
rhizosphere volume. Eelworms have been used to measure the extent of the rhizosphere as they
are highly specific in responding to stimulants produced by plants. In a wet sand, eelworm cysts
were stimulated to hatch at a distance of 3 cm from the plant roots and larvae could be attracted
from as far away as 4.5 cm. However, in a finer textured soil where movement of water is
impeded and where large number of highly reactive surfaces are available to bind organic
compound, the rhizosphere effect would not extend as far. Using electron microbeam analysis
and scanning electron microscopy of soil-root surfaces, the soil rhizosphere associated with
peanut or soybean roots has been estimated as being approximately 0.2 mm thick.
Several interesting calculations have been made which indicate the influence of the rhizosphere
in field soils may be quite extensive. For example, 25% of the total volume of the top 15 cm of a
soil under an oat crop at the dough stage lies within 0.l mm of a root. For an Italian ryegrass
sward, the mean distance between roots, in a horizontal plane 2 cm beneath the surface was
calculated as 3 mm.
To quantitate the rhizosphere effect, an R/S ratio has been used. The R/S ratio is determined by
dividing the number of microorganisms (or the rate of a biochemical process) per gram of
rhizosphere soil by the number of microorganisms in a g of the control soil. Since the
rhizosphere effect greatly decreases as we move away from the root, it is not surprising to find
that R/S ratios can be made to change simply by varying the amount of soil removed from the
root during preparation of the rhizosphere soil sample. For this reason, comparison of
rhizosphere effects, as determined by R/S ratios, from different laboratories must be made with
care.
Factors Responsible for the Rhizosphere Effect. Several factors play a role in developing the
rhizosphere effect (Table 11.1). The three most important factors which alter the biochemical
activity in the vicinity of the plant root are the soluble organic materials that are secreted or
exuded from the plant root cells, the debris derived from the root-cap cell, dying root hairs and
cortical cells, and the lysis of plant root cells. The increased availability of organic carbon in the
rhizosphere provides a habitat that is highly favorable for the proliferation of microorganisms.
This microbial community brings about further change by altering various chemical and
biological properties of the rhizosphere.
Table 11.1 - Factors Responsible for the Development of the Soil-Plant
Root Rhizosphere.
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Release of soluble organic compounds by plant roots
Sloughed off root cell debris and dying root hairs
Plant root cell lysis
Higher concentration of carbon dioxide
Lower concentration of oxygen
Lower concentration of nutrient ions
Partial desiccation of soil due to absorption of water by roots
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The proportion of total photosynthate transferred to roots ranges from 30% for cereals to nearly
60% for some trees. Not all of the carbon input to the roots will be introduced to the soil as root
secretions or exudates. However, at different stages of root development, growth, and decay
much of the fixed carbon that is transferred to the root is eventually cycled through the
rhizosphere. Measurements of the amount of carbon translocated to roots of wheat grown in
Australia between seeding and flowering is approximately 1500 kg/ha. Approximately 1000
kg/ha is not retained in the root biomass but is released into the soil rhizosphere.
A large percentage of the carbon in the root rhizosphere is a result of cuticle of the root being
lysed or ruptured by mechanical abrasion. Histochemical tests have shown that under field
conditions, the primary wall of the plant root is initially bounded externally by a thin cuticle.
Mechanical action of roots forcing a passage through the soil cause mineral particles to rupture
the cuticle. Lytic action of microorganisms can also cause the cuticle to rupture. The breaking of
the cuticle allows the mucilage from the cells on the surface of the root to enter the soil matrix
and enclose nearby soil colloids to form mucigel.
A second important source of carbon in the rhizosphere is the organic material introduced as root
exudate or secretion. There is a subtle difference between root exudation and secretion
processes. Root exudates are low molecular weight compounds that leak from all cells either into
the intracellular spaces and then into the soil or directly through the epidermal cell walls into the
soil. The release of these compounds is not metabolically mediated. Secretions are compounds of
both low and high molecular weight that are released from the plant root as a result of metabolic
processes.
Many specific compounds have been identified as being derived from plant roots, some of which
(e.g. amino acids) are commonly found in most plants. Other compounds are specific to certain
plant species. For example, in root-tip exudates of pea seedlings a -D-glutamyl-D-alanine and 2alanyl-isoxozolin-5-one were detected but were not found in other legumes investigated. Specific
compounds in root exudates or secretions play an important role in the infection of roots by
beneficial or pathogenic microorganisms. Fungal spores will germinate near a root or, in many
cases, when treated with root exudate or a specific compound in an exudate. The germination
process in the rhizosphere may be very specific and occurs only in the presence of a specific
host. Often, however, a nonspecific stimulation occurs.
Plant roots may also produce compounds that exhibit bactericidal, fungicidal, or herbicidal
activity. The term, allelopathy, was coined by Molisch in 1937 to describe this type of
interaction. Allelopathy, in its broadest sense is a biochemical interaction between all types of
plants, including microorganisms. Chapter IX provides a more thorough discussion of
allelopathic chemicals in soil.
Some of the carbon translocated to the root will be introduced to the rhizosphere as sloughed off
root-cap cells. The root cell debris provides nutrients to soil microorganisms near the root tip and
the zone of elongation. The amount of root cell debris contributed to the rhizosphere is difficult
to assess but is considered less than the amounts introduced through lysis of plant root cells or
through active secretion.
Use of 14C in Rhizosphere Studies. The individual compounds within the rhizosphere which are
derived from the root are present in very small quantities. This causes difficulties in their
identification and assay. However, plants may be easily labelled with 14C by allowing
photosynthesis to proceed in an atmosphere containing the label. The soil surrounding the root
can then be extracted and the total amount of labelled carbon can be determined. Alternatively,
individual compounds can be separated and determined using 14C as the tracer.
Use of 14C in studies of root rhizospheres ensures (1) that all the major components in the
exudate are detected, whereas in normal chemical and colorimetric tests the reagents determine
the type of compound detected and (2) is sufficiently sensitive for the study of exudate from
individual plants collected over short periods of time.
Modification of the Rhizosphere. Rhizosphere microorganisms may be either detrimental or
beneficial. Supposedly, certain beneficial microorganisms can promote plant growth by means of
aggressive colonization of roots causing displacement of the deleterious microorganisms from
the root environment. A second hypothesis is that there is a direct attack by the beneficial
microorganisms on the deleterious ones causing their death. Beneficial microorganisms may also
bring about growth promotion through the production of growth hormones such as auxins and
kinetins.
Manipulation of the root environment to favor a particular microorganism could have a dramatic
impact on overall plant health and growth. One approach is to inoculate the seed or root of a
plant. For this approach to be successful, the inoculated microorganism must be able to
effectively compete against the native microbial population for sites on the root. The inoculated
microorganism must also be able to spread or grow with the root to bring about its positive effect
over a time span sufficient for the plant to have a chance to favorably respond.
Demonstrations that foliar spraying of urea, antibiotics, and exogenous plant-growth regulators
can modify the root exudate and, sometimes, the balance of microorganisms in the rhizosphere
indicate that this approach modifying the microbial composition of rhizosphere. For this
approach to be successful, however, an understanding of the fundamental processes in foliar
absorption, transports to the root, exudation into the root rhizosphere, and metabolism of the
applied compound must be obtained.
Conclusion. Although our knowledge of the rhizosphere is still limited in many areas, its role in
terms of plant nutrition and health are clearly evident. When assessing the significance of the
rhizosphere to soil biochemical processes, it must be remembered that a large percentage of the
soil under a grass sod or cereal crop that is approaching maturity must be considered as
rhizosphere.