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Published on December 12, 2008; 10.1104/pp.108.131516

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Received October 21, 2008
Accepted December 5, 2008

Karrikins discovered in smoke trigger Arabidopsis seed germination by a mechanism requiring gibberellic acid synthesis and light

David C. Nelson , Julie-Anne Riseborough , Gavin R. Flematti , Jason Stevens , Emilio L. Ghisalberti , Kingsley W. Dixon , and Steven M. Smith *

Plant Energy Biology; Plant Biology; Biomedical, Biomolecular and Chemical Sciences, The University of Western Australia, Crawley, WA 6009, Australia; Kings Park and Botanic Garden, West Perth, WA 6005, Australia

* Corresponding author; email: ssmith{at}cyllene.uwa.edu.au.

Discovery of the primary seed germination stimulant in smoke, 3-methyl-2H-furo[2,3-c]pyran-2-one (KAR1), has resulted in identification of a family of structurally-related plant growth regulators, karrikins. KAR1 acts as a key germination trigger for many species from fire-prone, mediterranean climates, but a molecular mechanism for this response remains unknown. We demonstrate that Arabidopsis thaliana, an ephemeral of the temperate Northern hemisphere which has never been reported to be responsive to fire or smoke, rapidly and sensitively perceives karrikins. Thus these signaling molecules may have greater significance among angiosperms than previously realized. Karrikins can trigger germination of primary dormant Arabidopsis seed far more effectively than known phytohormones or the structurally-related strigolactone GR-24. Natural variation and depth of seed dormancy affect the degree of KAR1 stimulation. Analysis of phytohormone mutant germination reveals suppression of KAR1 responses by abscisic acid and a requirement for gibberellin (GA) synthesis. The reduced germination of sly1 mutants is partially recovered by KAR1, which suggests germination enhancement by karrikin is only partly DELLA-dependent. While KAR1 has little effect on sensitivity to exogenous GA, it enhances expression of the GA biosynthetic genes GA3ox1 and GA3ox2 during seed imbibition. Neither ABA nor GA levels in seed are appreciably affected by KAR1 treatment prior to radicle emergence, despite marked differences in germination outcome. KAR1 stimulation of Arabidopsis germination is light-dependent and reversible by far-red exposure, although limited induction of GA3ox1 still occurs in the dark. The observed requirements for light and GA biosynthesis provide the first insights into the karrikin mode of action.






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