Interactions with plant roots

In the rhizosphere, the region of the soil under the influence of plant roots, many types of interactions occur. Because it is the region of the soil which provides the plant with its soluble nutrients, other soil organisms must affect the plant through this area. In the same area any effect of the root 

Soil microorganisms can influence plant roots in a number of ways. One of them is the secretion of membrane-active antibiotics which stimulate further leakage of plant cell constituents into the soil. Microorganisms can also affect the availability of nutrients, compete with the plant for water and nutrients, etc. 

Positive effects on the plant are produced by microorganisms that enter more intimate relationships with the plant roots. Such effects are known in the case of symbiotic nitrogen-flxing bacteria, actinomycetes and cyanobacteria as well as of symbiotic mycorrhizal fungi. 

Bacteria of the genus Rhizobium occur in nodules on the roots of many legumes, but have also been found in one non-leguminous plant, Trema aspera. 

Rhizobium occurs free-living, but cannot fix nitrogen when it is existing as a natural member of the soil population. The sequence of events in the infection of roots and formation of an active, nitrogen-flxing nodule reveals a complex series of interactions between symbiont and host. 

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In conclusion, it appears that the uptake enhancement of macro- and micronutrients due to HS is a synergic sum of various effects exerted by these molecules at a rhizosphere level. Besides a direct source of nutrient subsequent their decomposition and apart from their chelating properties, HS interaction with plant root plasma membrane has been demonstrated in relation to its solubility, its surfactant-... 

The indirect pathway by which air pollutants interact with plants is through the root system. Deposition of air pollutants on soils and surface waters can cause alteration of the nutrient content of the soil in the vicinity of the plant. This change in soil condition can lead to indirect or secondary effects of air pollutants on vegetation and plants. 

Figure 4.3. Plant roots with adhering soil illustrating the interaction between plant roots and soil.

An important feature of the biogeochemistry of trace elements in the rhizosphere is the interaction between plant root surfaces and the ions in the soil solution. These ions may accumulate in the aqueous phases of cell surfaces external to the plasma membranes (PMs). In addition, ions may bind to cell wall (CW) components or to the PM surface with variable strength. In this chapter, we shall describe the distribution of ions among the extracellular phases using electrostatic models (i.e. Gouy-Chapman-Stem and Donnan-plus-binding models) for which parameters are now available. Many plant responses to ions correlate well with computed PM-surface activities, but only poorly with activities in the soil solution. These responses include ion uptake, ion-induced intoxication, and the alleviation of intoxication by other ions. We illustrate our technique for the quantitative resolution of multiple ion effects by inserting cell-surface activities into nonlinear equations. 

Phenolic acids in soils occur either in a free state in the soil solution, reversibly sorbed to soil particles, fixed (irreversibly sorbed) very tightly to soil particles (e.g., recalcitrant organic matter, and clays), and/or on and in living and dead plant tissues/residues ( free , reversibly sorbed, and fixed). Of general interest to plant-plant allelopathic interactions are the free and reversibly sorbed states frequently referred to as the available fraction. Of particular interest is the active fraction of available phenolic acids, the fraction of available phenolic acids that actually interact with seeds, roots and microbes. Unfortunately we presently do not have a means of quantifying the active fraction, thus the focus on the available fraction. 

Bais, H.P., Weir, T.L., Petty, L.G., Gilroy, S., and Vivanco, J.M. (2006) The role of root exudates in rhizosphere interactions with plants and other organisms. Annu Rev Plant Biol 57 2i3-266. 

R. Scott Russell, Plant-Root System Their Functions and Interactions with Soil. McGraw-Hill, London, 1977. 

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