Biosynthesis and Regulation of Belowground Signaling Molecules

Plants emit signaling molecules to interact with their belowground environment and these interactions are mainly localized in a small area adjacent to the plant root, called the rhizosphere. Signaling molecules so far identified in the rhizosphere mostly belong to the specialized metabolites, such as the terpenoids (including the strigolactones) and phenolics (including the flavonoids). The biological roles of these two classes of molecules in the rhizosphere and their biosynthesis are discussed, as well as the regulation of their formation by biotic stresses, both above- and belowground, such as herbivores, pathogens and beneficial microorganisms. Abiotic stress, especially nutrient deficiency, also affects the production of some of these signaling molecules. Integration of the recently emerging metabolomics, metagenomics, metabolic engineering and systems biology approaches will help to unravel the mechanisms underlying the role of signaling molecules in the interaction between plants and soil biota and will provide the basis for a better exploitation of the hidden, belowground, part of plants.