"Arbuscular mycorrhizal (AM) symbioses are mutualistic associations between soil fungi and most vascular plants. During this symbiosis AM fungi help on plant fitness by providing plants with water and essential nutrients allowing them to survive under stressful conditions, specially under phosphate-limited conditions. On the other hand, AM fungi depend on the carbon provided by their plant host to complete their developmental cycle. In the AM symbiosis a fine-tune communication between the host plant and fungal partners is required for a good establishment. This underground signaling is very complex and signals produced by the plants to attract a symbiotic partner can be used or abused by other organisms in the rhizosphere. In addition, plants can modulate their requirement to establish a symbiotic relationship depending on the soil nutrient conditions by controlling the production of the signalling molecules. In this symbiotic interaction, the apocarotenoids including the hormone abscisic acid, strigolactones, mycorradicin and cyclohexenone derivatives have been recently shown to play an important role as signaling molecules in this interaction. These secondary metabolites form part of the large group of carotenoid cleavage products found in nature. However, although it is clear that apocarotenoids are important molecules involved in the AM symbiosis, their precise role as signaling molecules in this symbiosis is still unclear. The aim of this research project is to elucidate the signaling mechanisms involved in the regulation of the AM symbiosis establishment, studying specifically the role of the apocarotenoids in it. The final practical goal of this project is the reduction of agrochemical inputs by the rational exploitation of natural resources such as beneficial organisms."