The Physcomitrium (Physcomitrella) patens PpKAI2L 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 moss Physcomitrium (Physcomitrella) patens, and PpCCD8-derived compounds regulate plant extension. Based on germination assays using seeds of the parasitic plant Phelipanche 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 13 PpKAI2LIKE-A to -L genes, homologous to the ancestral D14 paralog KARRIKIN INSENSITIVE2 (KAI2). In Arabidopsis, AtKAI2 is the receptor for a still elusive endogenous KAI2-Ligand (KL). We show that in P. 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 all PpKAI2L gene subclades were obtained and tested for their response to both enantiomers. We show that PpKAI2L-A to -E genes are not involved in PpCCD8-derived compound perception, but act in a PpMAX2-dependant pathway. In contrast, mutations in PpKAI2L-G , and -J genes abolish the response to (+)-GR24, suggesting that encoded proteins are receptors for PpCCD8-derived SLs.