A KARRIKIN INSENSITIVE2 paralog in lettuce mediates highly sensitive germination responses to karrikinolide

Abstract

Karrikins (KARs) are chemicals in smoke that can enhance germination of many plants. Lactuca sativa cv. Grand Rapids (lettuce), germinates in the presence of nanomolar karrikinolide (KAR1). We found that lettuce is much less responsive to KAR2 or a mixture of synthetic strigolactone analogs, rac-GR24. We investigated the molecular basis of selective and sensitive KAR1 perception in lettuce. The lettuce genome contains two copies of KARRIKIN INSENSITIVE2 (KAI2), a receptor that is required for KAR responses in Arabidopsis thaliana. LsKAI2b is more highly expressed than LsKAI2a in dry achenes and during early stages of seed imbibition. Through cross-species complementation assays in Arabidopsis we found that LsKAI2b confers robust responses to KAR1, but LsKAI2a does not. Therefore, LsKAI2b likely mediates KAR1 responses in lettuce. We compared homology models of the ligand-binding pockets of KAI2 proteins from lettuce and a fire follower, Emmenanthe penduliflora. This identified pocket residues 96, 124, 139, and 161 as candidates that influence the ligand-specificity of KAI2. Further support for the significance of these residues was found through a broader comparison of pocket residue conservation among 324 asterid KAI2 proteins. We tested the effects of substitutions at these four positions in Arabidopsis thaliana KAI2 and found that a broad array of responses to KAR1, KAR2, and rac-GR24 could be achieved.