Karrikins

Karrikins are a class of plant growth regulators found in the smoke produced by burning vegetation. They promote seed germination and seedling development across a wide range of plant species and ecological contexts. First identified in smoke during the early 2000s, the karrikin family includes several related compounds, with KAR1 among the best studied.

Chemically, karrikins are small, oxygen-containing molecules that contain a butenolide ring, a feature shared by the active members of the family.

Mode of action: Karrikins are perceived by the plant receptor KAI2, which interacts with the F-box protein MAX2 to initiate a signaling cascade. This leads to changes in gene expression that promote germination and early seedling growth. The pathway overlaps with, but is distinct from, the signaling pathway used by strigolactones via the receptor D14; both pathways involve MAX2, indicating convergence at some points. Additionally, plants may respond to an endogenous ligand (KL) that is perceived by KAI2, suggesting that the karrikin pathway intersects with normal plant development beyond smoke responses.

Ecology and applications: Karrikins occur naturally in smoke and ash after fires and are thought to contribute to post-fire plant recruitment in ecosystems. They have been used experimentally to enhance germination in horticulture and restoration practices, especially for fire-adapted ecosystems and species with dormant seeds. However, responses are species-specific, and not all seeds germinate in response to karrikins.