The origins and mechanisms of karrikin signalling

• Karrikins are seed germination stimulants structurally similar to strigolactones.
• The α/β-hydrolases KAI2 and D14 play a central role in plant responses to each.
• KAI2 and D14 have both receptor-like and enzyme-like characteristics.
• Evolutionary specialisation of KAI2 potentially allowed responses to karrikins.

Karrikins are butenolides in smoke and char that stimulate seed germination. Karrikin action in Arabidopsis requires the F-box protein MAX2 and the α/β-hydrolase KAI2, a paralogue of D14 that is required for perception of strigolactones (SL). SL response involves hydrolysis by D14, whereas karrikins bind to KAI2 without apparent hydrolysis. We discuss the current understanding of the mechanisms of karrikin perception and response. The usual function of KAI2 is unclear, but we hypothesise that the similarity between karrikins and the endogenous ligand for KAI2 made adaptation of some plants to karrikins possible.