Ecological relevance of strigolactones in nutrient uptake and other abiotic stresses, and in plant-microbe interactions below-ground

Plants are exposed to ever changing and often unfavourable environmental conditions, which cause both abiotic and biotic stresses. They have evolved sophisticated mechanisms to flexibly adapt themselves to these stress conditions. To achieve such adaptation, they need to control and coordinate physiological, developmental and defence responses. These responses are regulated through a complex network of interconnected signalling pathways, in which plant hormones play a key role. Strigolactones (SLs) are multifunctional molecules recently classified as a new class of phytohormones, playing a key role as modulators of the coordinated plant development in response to nutrient deficient conditions, especially phosphorus shortage. Belowground, besides regulating root architecture, they also act as molecular cues that help plants to communicate with their environment. This review discusses current knowledge on the different roles of SLs below-ground, paying special attention to their involvement in phosphorus uptake by the plant by regulating root architecture and the establishment of mutualistic symbiosis with arbuscular mycorrhizal fungi. Their involvement in plant responses to other abiotic stresses such as drought and salinity, as well as in other plant-(micro)organisms interactions such as nodulation and root parasitic plants are also highlighted. Finally, the agronomical implications of SLs below-ground and their potential use in sustainable agriculture are addressed. Experimental evidence illustrates the biological and ecological importance of SLs in the rhizosphere. Their multifunctional nature opens up a wide range of possibilities for potential applications in agriculture. However, a more in-depth understanding on the SL functioning/signalling mechanisms is required to allow us to exploit their full potential.