Strigolactones: a cry for help in the rhizosphere

Arbuscular mycorrhizal (AM) symbiosis is a beneficial symbiosis established between fungi of the phylum Glomeromycota and over 80% of terrestrial plants, including most agricultural and horticultural crop species. AM symbiosis improves the nutritional status and fitness of the host plant and enables the plant to perform better under stressful conditions. As a result, when plants are growing under unfavourable conditions, they try to recruit their AM fungal partner in the soil. Symbiosis establishment requires a complex chemical dialogue between the two partners, in which signalling molecules such as the strigolactones play a key role. Under deficient nutrient conditions, the host plant increases the production of strigolactones to promote fungal development and symbiosis establishment (a cry for help). As a clue to host presence in the rhizosphere, strigolactones are also detected by other organisms, particularly root parasitic plants, and therefore promote a parasitic interaction. We review here the role of strigolactones and their interaction with other phytohormones during AM symbiosis, paying special attention to the implications of the chemical communication that takes place in the rhizosphere. Finally, we point out the potential use of this molecular dialogue as a target for developing new biological control strategies against deleterious organisms such as root parasitic weeds.