(70f) Evolution of Inter-Organismal Strigolactone Biosynthesis in Seed Plants

Strigolactones (SLs) are a class of methylbutenolide molecules in plants that are derived from β-carotene via an intermediate molecule, carlactonoic acid (CLA). Canonical SLs, which have a tricyclic molecular scaffold, are signals to soil microbes and other plants, while non-canonical SLs, which have variable scaffolds, are primarily plant hormones. The cytochrome P450 CYP722C catalyzes a critical step in the conversion of CLA to canonical strigolactones in most angiosperms. Here we developed an efficient SL-producing Escherichia coli-Saccharomyces cerevisiae biosynthetic platform for investigating the function of CYP722A, an evolutionary predecessor of CYP722C. We discovered that CYP722A converts CLA into a non-canonical SL, 16-hydroxy-CLA (16-OH-CLA). 16-OH-CLA is only detected in the shoots, but not roots, of various flowering plants, disappearing when the branching phase completes. 16-OH-CLA application restores shoot branching control to SL-deficient mutants in Arabidopsis thaliana and is perceived by the SL signaling pathway. Furthermore, 16-OH-CLA is converted by CARLACTONOIC ACID METHYLTRANSFERASE (CLAMT) and LATERAL BRANCHING OXIDOREDUCTASE (LBO) into other SLs that may be more bioactive forms of the hormone. We conclude that biosynthesis of 16-OH-CLA by CYP722A was a metabolic stepping stone in the evolution of canonical SLs that mediate rhizospheric signaling in seed plants. Our work reveals a missing link in the transition between non-canonical strigolactone and canonical strigolactone biosynthesis.