The Physcomitrium (Physcomitrella) patens PpKAI2L receptors for strigolactones and related compounds highlight MAX2 dependent and independent pathways

ABSTRACTIn 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. However, perception of these still unknown compounds does not require the PpMAX2 homolog. Putative 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 karrikins and a still elusive endogenous KAI2-Ligand (KL). Based on germination assays using seeds of the parasitic plant Phelipanche ramosa, we propose that PpCCD8-derived compounds are non-canonical SLs. We show that all tested PpKAI2L proteins can bind and cleave SL analogs, some with similar affinities to AtKAI2. The PpKAI2L-H protein shows a strong hydrolytic activity not found for other PpKAI2L. Moss mutants for all PpKAI2L gene subclades were obtained and tested for their response to SL analogs. We show that PpKAI2L-A to -E genes encode redundant proteins that are not involved in PpCCD8-derived compound perception, but rather act in a PpMAX2-dependant pathway. In contrast, mutations in PpKAI2L-G, and -J genes abolish the response to the SL analog (+)-GR24, suggesting that both these encoded proteins are receptors for PpCCD8-derived molecules.