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. Springer- Verlag, New York, NY, USA. Bolton, H., J.K. Fredrickson, and L.E. Elliot. 1992. Microbial ecology of the rhizosphere. p. 27- 63. 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. _________________________________________________________ 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 ________________________________________________________ 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 2- alanyl-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.