Rhizosphere Interactions
2014-08-29

Roots are important not only for water and nutrient supply of the plant, but also to release a wide range of carbon compounds of low molecular weight, such as sugars, amino, and organic acids. These can amount to between 10% and 20% of total net fixed carbon but vary based on species, nitrogen availability, and plant age. In addition, most land plants form symbioses with soil fungi, which in addition cause a considerable drain of photoassimilates. Direct (plant exudates) or indirect (via symbiotic fungi) rhizodeposition of carbon forms the basis for an environment rich in diversified microbiological populations. This was first suggested by Hiltner in 1904. The rhizosphere is now defined as a narrow zone of soil, which is influenced by living roots. It forms a boundary layer between the root and the bulk soil. Here large fluxes of solutes and water, as well as compounds contained in the gas phase, exist. Consequently, physical soil properties can vary considerably. Depending on the demands of the plant, changes in the soil water potential can be high during the day/night cycle. In comparison to bulk soil, the soil water potential can become strongly negative during the day at high transpiration rates and less negative at night because of vertical redistribution by the root system (hydraulic lift). Special conditions also exist with regard to O2 and pH. Following high rates of respiration by both roots and microorganisms, O2 tension can be very low especially in wet soil where water limits diffusion rates. Uptake of solutes is often accompanied by the release of protons and organic acids, which affects the pH at the root surface. Microorganisms of the rhizosphere establish a functional diversity that includes the decomposition of organic matter, nitrogen fixation, conversion of inorganic forms of nitrogen, solubilization of phosphate, transformation of sulfur and iron, production of siderophores (iron-binding compounds), release of plant(phyto)hormones, as well as of compounds, which are used for biotic control. It is obvious that bacteria are an important part of the microorganisms inhabiting this ecological niche. In comparison to bulk soil, the abundance of rhizosphere bacteria is several magnitudes higher (1010-1012 microbes per gram soil versus &amp;lt;108 in bulk soil), but still about 100 times lower than under culture conditions. Bacteria can solubilize nutrients from the mineral soil layer, but will also sequester them. Consumption of bacteria by soil protozoa and nematodes will then liberate nutrients, which in due course will become available for plants. Fungi form another important part of the rhizosphere. Most terrestrial plants develop symbiotic structures (mycorrhiza) with soil-borne fungi, creating another sphere, the mycorrhizosphere. In these interactions, the fungal partner provides the plant with improved access to water and soil nutrients because of more or less complex hyphal structures, which emanate from the root surface and extend far into the soil. The plant, in return, supplies carbohydrates for fungal growth and maintenance. Because of leakage and the turnover of mycorrhizal structures, these are another source for solutes released into the soil where they can be accessed by other microorganisms. In the following interactions of bacteria, fungi and plants, and, finally those of plants with each other are addressed. With regard to soil bacteria, a wide range of bacterial activities exist such as the &amp;quot;good&amp;quot; ones (plant growth promotion, plant disease suppression, nitrogen fixation) and the &amp;quot;bad&amp;quot; ones (plant pathogens), as well as bioactive compounds of bacterial secondary metabolism, which cause the respective effects. Plant-associated bacteria act as opportunistic human pathogens. Fungi form another focus, here especially the bacterial influence on symbiotic and plant pathogenic fungi. Finally, direct (parasitic plants, plant competition) and indirect (by the help of fungi) interactions of plants themselves are described. The examples introduced below show that the contemporary knowledge about organisms and interactions in the rhizosphere has increased recently, mainly because of the many attempts to improve plant growth and fitness. The intense use of metagenomics with soil samples continuously reveals the enormous diversity of microorganisms living and thriving on plant-derived exudates. Interestingly, most of these organisms are not pathogenic in nature. This is understandable because they depend on the continuous delivery of organic compounds by the plant. This could also explain why so many rhizosphere bacteria produce toxins that mainly affect plant pathogenic microorganisms or release compounds that are plant-beneficial. An investigation of the chemical interactions is, however, extremely limited owing to the lack of suitable experimental procedures and of set-ups that represent realistic conditions. Experimentally sound studies can most easily be performed under sterile conditions using cocultures of the respective organisms or exuded compounds. Three partite systems (host plant + two microorganisms (e.g., pathogen + antagonist)) become already very difficult to handle, and the step to field studies is enormous. Many other organisms now interfere, metabolically modifying the &amp;quot;identified bioactive compounds&amp;quot; to an unknown extend. Further, owing to different binding properties of soil particles and other soil chemical factors, the fate of any such compound remains unclear. Thus, there is still a long way to go to broaden our understanding of the processes and molecules involved in rhizosphere interactions.