Karrikins: A new family of plant growth regulators in smoke

Karrikins are a chemically defined family of plant growth regulators discovered in smoke from burningplant material. Karrikins are potent in breaking dormancy of seeds of many species adapted toenvironments that regularly experience fire and smoke. The recent discovery that karrikins trigger seedgermination and control seedling growth in taxa that would rarely experience fire indicates that theirsignificance could extend far beyond fire ecology. This is exemplified by new studies showing that seedsof Arabidopsis thaliana respond sensitively and specifically to karrikins in smoke. These excitingdiscoveries might be explained if karrikins are produced in the environment by processes other than fire,such as by chemical or microbial degradation of vegetation in response to disturbance of the soil orremoval of the plant canopy. Another hypothesis is that plants contain endogenous karrikins thatfunction naturally in the control of seed germination and that species from fire-prone habitats haveevolved to respond also to exogenous karrikins. A variant on this hypothesis is that karrikins mimicendogenous plant hormones such as terpenoids that control seed germination. The evidence for thesehypotheses is discussed, but whatever the explanation karrikins are now firmly established as animportant family of naturally occurring plant growth regulators. ©2009 Elsevier Ireland Ltd. All rights reserved.