The karrikin response system of Arabidopsis

Arabidopsis thaliana provides a powerful means to investigate the mode of action of karrikins, compounds produced during wildfires that stimulate germination of seeds of fire-following taxa. These studies have revealed close parallels between karrikin signalling and strigolactone signalling. The two perception systems employ similar mechanisms involving closely related alpha/beta-fold hydrolases (KAI2 and AtD14) and a common F-box protein (MAX2). However, karrikins and strigolactones may be distinguished from each other and elicit different responses. The karrikin response requires a newly discovered protein (SMAX1), a homologue of rice protein D53 that is required for the strigolactone response. Mutants defective in the response to karrikins have seeds with increased dormancy, altered seedling photomorphogenesis and modified leaf shape. As the karrikin and strigolactone response mechanisms are so similar, it is speculated that the endogenous signalling compound for the KAI2 system may be a specific strigolactone. However, new results show that the proposed endogenous signalling compound is not produced by the known strigolactone biosynthesis pathway via carlactone. Structural studies of KAI2 protein and its interaction with karrikins and strigolactone analogues provide some insight into possible protein-ligand interactions, but are hampered by lack of knowledge of the endogenous ligand. The KAI2 system appears to be present throughout angiosperms, implying a fundamentally important function in plant biology.