PT - JOURNAL ARTICLE AU - Yueming Kelly Sun AU - Adrian Scaffidi AU - Jiaren Yao AU - Kim Melville AU - Steven M Smith AU - Gavin R Flematti AU - Mark T Waters TI - Divergent receptor proteins confer responses to different karrikins in two ephemeral weeds AID - 10.1101/376939 DP - 2018 Jan 01 TA - bioRxiv PG - 376939 4099 - http://biorxiv.org/content/early/2018/07/25/376939.short 4100 - http://biorxiv.org/content/early/2018/07/25/376939.full AB - Seed germination is responsive to environmental chemical cues, such as karrikins, which are germination stimulants produced from wildfires1,2. Seeds of Brassica tournefortii – a widespread problematic weed associated with post-fire and disturbance sites – are highly sensitive to KAR1, the most abundant karrikin, whereas Arabidopsis thaliana responds more sensitively to KAR2, a less abundant karrikin3,4. The mechanism for these differential responses is unclear. KARRIKIN INSENSITIVE2 (KAI2), a paralogue of the strigolactone receptor DWARF14 (D14), is the putative karrikin receptor identified from Arabidopsis4,5. Here we show that B. tournefortii exhibits greater sensitivity for KAR1 relative to KAR2 in multiple biological assays. We demonstrate that B. tournefortii differentially expresses three KA12 homologues, two of which rescue the Arabidopsis kai2-2 null mutant. Notably, the most abundantly expressed homologue in B. tournefortii is sufficient to confer enhanced responses to KAR1 relative to KAR2 when expressed in Arabidopsis. We further identify two variant amino acid residues that support differential ligand preferences between the two B. tournefortii homologues. Our findings provide clear evidence that KAI2 is the receptor for karrikins, since specific amino acid substitutions near the active site change ligand specificity. Our results also suggest that duplication and diversification of KAI2 proteins could provide weedy ephemerals with the ability to respond sensitively to different karrikins or related compounds produced by environmental disturbance.