Secretions
Plants secrete many compounds into the rhizosphere which serve different functions. Strigolactones, secreted and detected by mycorhizal fungi, stimulate the germination of spores and initiate changes in the mycorhiza that allow it to colonize the root. The parasitic plant, Striga also detects the presence of strigolactones and will germinate when it detects them; they will then move into the root, feeding off the nutrients present. Symbiotic Nitrogen-fixing bacteria, such as the Rhizobium species, detect an unknown compound secreted by the roots of leguminous plants and then produce nod factors which signal to the plant that they are present and will lead to the formation of root nodules, in which the bacterium, sustained by nutrients from the plant, converts nitrogen gas to a form that can be used by the plant. Non-symbiotic (or "free-living") nitrogen-fixing bacteria may reside in the rhizosphere just outside the roots of certain plants (including many grasses), and similarly "fix" nitrogen gas in the nutrient-rich plant rhizosphere. Even though these organisms are thought to be only loosely associated with plants they inhabit, they may respond very strongly to the status of the plants. For example, nitrogen-fixing bacteria in the rhizosphere of the rice plant exhibit diurnal cycles that mimic plant behavior, and tend to supply more fixed nitrogen during growth stages when the plant exhibits a high demand for nitrogen.
Some plants secrete allelochemicals from their roots which inhibit the growth of other organisms. For example garlic mustard produces a chemical which is believed to prevent mutualisms forming between the trees and mycorhiza in mesic North American temperate forests.
Read more about this topic: Rhizosphere