Strigolactone hormones and their stereoisomers signal through two related receptor proteins to induce different physiological responses in Arabidopsis

Two α/β-fold hydrolases, KAI2 and AtD14, are necessary for responses to karrikins and strigolactones in Arabidopsis thaliana. While KAI2 mediates responses to karrikins and some strigolactone analogues, AtD14 mediates strigolactone but not karrikin responses. To further determine the specificity of these proteins, we assessed the ability of naturally occurring deoxystrigolactones (deoxySLs) to inhibit Arabidopsis hypocotyl elongation, regulate seedling gene expression, suppress outgrowth of secondary inflorescences, and promote seed germination. Neither 5-deoxystrigol nor 4-deoxyorobanchol was active in KAI2-dependent seed germination or hypocotyl elongation, but both were active in AtD14-dependent hypocotyl elongation and secondary shoot growth. However, the non-natural enantiomer of 5-deoxystrigol was active through KAI2 in growth and gene expression assays. We found that the four stereoisomers of the strigolactone analogue GR24 had similar activities to their deoxySL counterparts. The results suggest that AtD14 and KAI2 exhibit selectivity towards the butenolide D-ring in the 2′R and 2′S configurations, respectively. However, we found that for CN-debranone, a simple strigolactone analogue, the 2′R configuration is inactive but the 2′S configuration is active through both AtD14 and KAI2. Our results support the conclusion that KAI2-dependent signalling does not respond to canonical strigolactones. Furthermore, racemic mixtures of chemically-synthesised strigolactones and their analogues such as GR24 should be used with caution since they can activate responses that are not specific to naturally occurring strigolactones. In contrast, the use of specific stereoisomers might provide valuable information about the specific perception systems operating in different plant tissues, in parasitic weed seeds and in arbuscular mycorrhizae.