ThePhyscomitrium (Physcomitrella) patensPpKAI2L receptors for strigolactones and related compounds highlight MAX2 dependent and independent pathways

In flowering plants, the α/β hydrolase DWARF14 (D14) perceives strigolactone (SL) hormones and interacts with the F-box protein MORE AXILLARY GROWTH2 (MAX2) to regulate developmental processes. The key SL biosynthetic enzyme, CAROTENOID CLEAVAGE DEOXYGENASE8 (CCD8), is present in the mossPhyscomitrium (Physcomitrella) patens,and PpCCD8-derived compounds regulate plant extension. Based on germination assays using seeds of the parasitic plantPhelipanche ramosa,we propose that these compounds are non-canonical SLs. Perception of PpCCD8-derived compounds does not require the PpMAX2 homolog. Candidate receptors are among the 13PpKAI2LIKE-A to -Lgenes, homologous to the ancestralD14paralogKARRIKIN INSENSITIVE2 (KAI2).In Arabidopsis, AtKAI2 is the receptor for a still elusive endogenous KAI2-Ligand (KL). We show that inP. patens,among SL analogs, the (+)-GR24 enantiomer is a good mimic for PpCCD8-derived compounds, while the effects of the (-)-GR24 enantiomer, a KL mimic in flowering plants, are opposite. Interaction and binding assays of seven PpKAI2L proteins using pure enantiomers pinpoint at stereoselectivity towards (-)-GR24 for the (A-E) clade. Enzyme assays highlight strong hydrolytic activity of the PpKAI2L-H protein. Moss mutants for allPpKAI2Lgene subclades were obtained and tested for their response to both enantiomers. We show thatPpKAI2L-Ato-Egenes are not involved in PpCCD8-derived compound perception, but act in a PpMAX2-dependant pathway. In contrast, mutations inPpKAI2L-G, and-Jgenes abolish the response to (+)-GR24, suggesting that encoded proteins are receptors for PpCCD8-derived SLs.