Small molecules below‐ground: the role of specialized metabolites in the rhizosphere

Summary Soil communities are diverse taxonomically and functionally. This ecosystem experiences highly complex networks of interactions, but may also present functionally independent entities. Plant roots, a metabolically active hotspot in the soil, take an essential part in below‐ground interactions. While plants are known to release an extremely high portion of the fixated carbon to the soil, less information is known about the composition and role of C‐containing compounds in the rhizosphere, in particular those involved in chemical communication. Specialized metabolites (or secondary metabolites) produced by plants and their associated microbes have a critical role in various biological activities that modulate the behavior of neighboring organisms. Thus, elucidating the chemical composition and function of specialized metabolites in the rhizosphere is a key element in understanding interactions in this below‐ground environment. Here, we review key classes of specialized metabolites that occur as mostly non‐volatile compounds in root exudates or are emitted as volatile organic compounds (VOCs). The role of these metabolites in below‐ground interactions and response to nutrient deficiency, as well as their tissue and cell type‐specific biosynthesis and release are discussed in detail. Significance Statement The below‐ground environment of plants is a dynamic setting involving complex interactions. Specialized metabolites exuded from roots to the rhizosphere play a key role in the crosstalk between roots and other neighboring soil organisms. Here, we provide an overview of the role of non‐volatile as well as emitted compounds in below‐ground interactions. Consequently, this review will likely inspire future research to develop solutions for combating soil pathogens and bio‐stimulate growth and productivity of crops.