Strigolactones in Rhizosphere Communication: Multiple Molecules With Diverse Functions

Strigolactones (SLs) are root-secreted small molecules that influence organisms living in the rhizosphere. While SLs are known as germination stimulants for root parasitic plants and as hyphal branching factors for arbuscular mycorrhizal fungi, recent studies have also identified them as chemoattractants for parasitic plants, sensors of neighboring plants and key players in shaping the microbiome community. Furthermore, the discovery of structurally diverged SLs, including so-called canonical and non-canonical SLs in various plant species, raises the question of whether the same SLs are responsible for their diverse functions 'in planta' and the rhizosphere or whether different molecules play different roles. Emerging evidence supports the latter, with each SL exhibiting different activities as rhizosphere signals and plant hormones. The evolution of D14/KAI2 receptors has enabled the perception of various SLs or SL-like compounds to control downstream signaling, highlighting the complex interplay between plants and their rhizosphere environment. This review summarizes the recent advances in our understanding of the diverse functions of SLs in the rhizosphere.