Karrikins: Germination Stimulants Produced by Wildfires

Abstract

Karrikins are simple organic chemicals produced by wildfires that stimulate germination of dormant seeds in the soil. They are present in the smoke and char produced by the fire and are washed into the soil by the following rains. Many plant species have evolved such that their seeds remain dormant in the soil until they encounter karrikins, whereupon they can germinate. This strategy has the advantage that emerging seedlings will have plenty of light and a supply of nutrients released by the fire. Such plants are termed ‘fire ephemerals’ because they appear after fires, then flower, produce seed and die. It is believed that karrikins mimic an unidentified endogenous germination hormone. Karrikins are also similar to endogenous hormones called strigolactones that control plant development. Karrikin perception by seeds apparently requires a protein closely related to the strigolactone receptor. Karrikins or smoke can be used to stimulate germination in horticulture and landscape revegetation.

Key Concepts

  • Wildfires stimulate seeds in the soil to germinate which contributes to rapid revegetation.
  • Seeds from many plant species are adapted to respond to chemicals produced by fire.
  • The most active germination stimulant produced by wildfires is a butenolide named ‘karrikin’.
  • Karrikins can influence germination and also seedling development.
  • The receptor protein and molecular components of the response pathway have been identified.
  • Karrikins mimic an ancient endogenous plant hormone that has not yet been identified.
  • Karrikins stimulate germination of agricultural weeds and so might be useful for their control.
  • Karrikins provide tools and knowledge to help us manage our natural landscapes.
  • Karrikins might be useful in seed conservation work and in seed banks.
  • The karrikin signalling system might be applied in crop breeding for environmental resilience.

Keywords: karrikins; wildfire; smoke; pyrolysis; seed germination; soil seed bank; strigolactones; abiotic stress; seed conservation; restoration ecology

Figure 1. Postfire landscape and subsequent growth of a fire ephemeral. Image (a) shows the aftermath of a fire in Banksia woodland in Western Australia. Image (b) shows the growth of the fire ephemeral Anthocercis littorea (Solanaceae) the year following a fire. The seeds of A. littorea remain dormant in the soil until a fire event followed by rains.
Figure 2. Structures of karrikins, strigolactones and related molecules. 1, Karrikin 1 (KAR1), 3‐methyl‐2H‐furo[2,3‐c]pyran‐2‐one; 2, KAR2; 3, Strigol, a naturally‐occurring strigolactone; 4, Pyran; 5, Xylose, a pyranose sugar; 6, glyceronitrile, a cyanohydrin; 7, Synthetic strigolactone analogue GR24 with the same stereochemical configuration as strigol; 8, The enantiomer of (7).
Figure 3. Effects of karrikin signalling on seedling growth. (a) Seeds of wild‐type Arabidopsis thaliana (Landsberg erecta) were germinated on 1% agar medium containing mineral nutrients, and seedlings grown for 7 days in low‐intensity white light (photon flux 50 μmol m−2 s−1) with a 12 h diurnal photoperiod. Seedling on the left is from control medium lacking karrikin, while that on the right was grown with 1 μM KAR1 in the medium. Source: https://bmcbiol.biomedcentral.com/articles/10.1186/s12915‐015‐0219‐0. Licensed under CC by 4.0. (b) Wild‐type A. thaliana (Landsberg erecta) seedlings grown in compost in high light (photon flux 150 μmol m−2 s−1) for 12 days with a 12 h diurnal photoperiod. (c) Seedlings of the karrikin‐insensitive kai2 mutant grown exactly as for wild‐type shown in (b). All experimental details are provided by Waters et al. .
Figure 4. A simplified model for karrikin stimulation of A. thaliana seed germination. Karrikin (KAR1) is taken up by the seed and potentially modified by metabolic activity to create an active butenolide that is recognised by receptor protein KAI2. In the absence of karrikins, the endogenous KAI2 ligand ‘KL’ could activate KAI2. The now‐activated KAI2 recruits MAX2 and SMAX1 into a complex that leads to SMAX1 degradation. Since SMAX1 represses seed germination, its destruction leads to seed germination.
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Further Reading

Flematti GR, Dixon KW and Smith SM (2015) What are karrikins and how were they ‘discovered’ by plants? BMC Biology 13: 108.

Morffy N, Faure L and Nelson DC (2016) Smoke and hormone mirrors: action and evolution of karrikin and strigolactone signaling. Trends in Genetics 32: 176–188.

Smith SM and Li J (2014) Signalling and responses to strigolactones and karrikins. Current Opinion in Plant Biology 21: 23–29.

Smith SM (2014) What are strigolactones and why are they important to plants and soil microbes. BMC Biology 12: 19.

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Smith, Steven M(Jul 2019) Karrikins: Germination Stimulants Produced by Wildfires. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0028348]