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Rhizosphere
The rhizosphere is the thin area of soil immediately surrounding the root system. It is a densely populated area in which the roots compete with invading root systems of neighboring plant species for space, water, and mineral nutrients as well as form positive and negative relationships with soil-borne microorganisms such as bacteria, fungi and insects. The rhizosphere is a very fruitful area since nearly 5% to 21% of all photosynthetically fixed carbon is transferred from plants to the rhizosphere via root exudates. Root exudates are seen as key mediators in the interaction between plants and soil microbiota.

Root exudates contain a wide variety of molecules released by the plant into the soil. They act as a signaling messenger that allows for communication between soil microbes and plant roots. Exudates influence several factors within the soil such as nutrient availability, soil pH, and recruitment of bacteria and fungi. All of these impact the relationships that plants have with each other as well as soilborne microorganisms. The most notable positive relationship is that of roots and mycorrhizae. It is estimated that 80-90% of plants are colonized by mycorrhizae in nature. Mycorrhizae are known to promote plant growth and increase water use efficiency. Plants establish these mutualistic relationships with bacteria and fungi by modulating the composition of the root exudates. While positive relationships like this do exist, it is worth noting that most microbes have incompatible interactions with plants. One of the main forms of negative relationships in the rhizosphere is allelopathy. This is the act of releasing phytotoxins into the rhizosphere that can influence neighboring plant’s growth, respiration, photosynthesis, metabolism, and water and nutrient uptake. Allelochemicals released by the roots do this by inducing changes in cell structures, inhibiting cell division and elongation, destabilizing the antioxidant system, and increasing membrane permeability.

One study examined the impact of sugar exudation rates from plant roots on bacterial and fungal symbiotic associations in the rhizosphere. They compared four plant families (Asteraceae, Brassicaceae, Fabaceae, and Poaceae) with different bacterial and fungal associations. They found that plant family was the most important source of variation in exudation rates and microbial community structure between plant species. These results allowed them to conclude that root symbiotic associations had great impacts on the rate of sugar exudation in the rhizosphere. Another study found that root exudation impacts microbial activities as well as the diversity of active microbiota involved in root exudate assimilation. While several relationships between root exudates and the rhizosphere have been observed and we see that root exudates play a major role in root-soil contact, the exact purpose of the exudates and the reactions they cause are still poorly understood.