Recent progress in strigolactone biosynthesis and transport in Arabidopsis and rice

Strigolactones (SLs) are a class of plant hormones that regulate diverse developmental processes and environmental responses. SLs also play important roles as allelochemicals in interactions with arbuscular mycorrhizal fungi (AMF) and root parasitic plants in the rhizosphere. Since their discovery as plant hormones nearly 20 years ago, SL biosynthesis, transport, and signaling have been extensively studied, primarily by characterizing mutants with increased shoot branching and by utilizing reverse genetic approaches in various plant species. Emerging evidence has revealed a series of new components of SL biology, expanding our knowledge of how a single plant species produces various types of SLs with diverse chemical structures and how SLs are released from roots into the soil. However, the bioactive forms of SLs that function as plant hormones and the mechanisms underlying their root-to-shoot transport have not yet been clearly elucidated. In this review, we summarize the current understanding of SL biosynthesis and transport in Arabidopsis thaliana and Oryza sativa. In addition, we discuss the physiological functions of different SL species as plant hormones and rhizosphere signaling molecules, which largely remain unresolved.