Copyright 2006 Accademia dei Georgofili
The rhizosphere is a biologically active area where microbes develop, interact with each other, and compete for space, nutrients and root exudates. Mycorrhizal fungi, and in particular those forming arbuscular mycorrhizas (AMs) are a significant rhizosphere component. This short review is focused on the events which occur in the rhizosphere and at the root surface, the rhizoplane. The signals which allow the establishment of AM symbiosis are still largely unknown; however, root exudates as chemotropic molecules and fungal soluble molecules, are expected to play an active role in the pre-symbiosis steps. Colonization success is the further requisite for nutrient exchanges. Early events are located in the rhizoplane, involving epidermal cells and extraradical hyphae. In order to identify genetically defined steps in the development of the symbiotic interaction, plant mutants impaired in AM symbiosis were used. The results show relevant changes in the cell responses of epidermis, in the cytoskeleton organisation and in the expression of fungal genes related to the oxidative burst. Once the symbiosis is established, the symbiotic contribution of AM fungi derives principally from the active network of soil hyphae extending from the root surface into the rhizosphere. Molecular techniques recently offered insights on the mechanisms developed by AM fungi for improving access to limiting soil resources. Taken in their whole the results show that the main checkpoints, which trigger the symbiosis with the release of bioactive molecules, control the success of plant-AM fungi interactions and lastly improve plant nutrition, are located in the rhizosphere-rhizoplane continuum.
