- Question and Answer
- Open Access
What are karrikins and how were they ‘discovered’ by plants?
© Flematti et al. 2015
- Published: 21 December 2015
Karrikins are a family of compounds produced by wildfires that can stimulate the germination of dormant seeds of plants from numerous families. Seed plants could have ‘discovered’ karrikins during fire-prone times in the Cretaceous period when flowering plants were evolving rapidly. Recent research suggests that karrikins mimic an unidentified endogenous compound that has roles in seed germination and early plant development. The endogenous signalling compound is presumably not only similar to karrikins, but also to the related strigolactone hormones.
- Soil Seed Bank
- Pyran Ring
- Future Fire
- Smoke Water
The formal names of the compounds are too complex for common usage and there are many butenolides in nature, so a more recognisable common name was required. To reflect their discovery in smoke, the family of compounds was named ‘karrikins’ from the word ‘karrik’, which is an Aboriginal term for smoke from the Western Australian Noongar people [4, 5]. It is common practice in biology to add the ‘-in’ suffix to denote a group of related molecules, such as ‘cytokinins’ in plants or ‘endorphins’ in animals. Fire and smoke play important roles in Aboriginal culture in Australia, and the karrikin name acknowledges that fact. The original compound identified is often referred to as ‘karrikinolide’: the ‘-olide’ suffix indicates that it is a lactone. The karrikins are abbreviated to KAR and numbered in order of their identification in smoke (Fig. 2).
Karrikins comprise only C, H and O, and contain two ring structures, one of which is a pyran and the other a lactone comprising a five-membered ring known as a butenolide. The pure compound is a crystalline substance with a melting point of 118–119 °C that readily dissolves in organic solvents and sparingly in water. All the karrikins are closely related in structure. The first of the compounds to be identified, KAR1, is usually the most abundant in smoke and the most active in seed germination. Seeds of some fire-followers can respond to as little as 10−10 M KAR1, which is similar in effectiveness to plant hormones .
It was found that burning many different plant materials, including straw, cellulose filter paper and even sugars, could generate karrikins. Their production from polysaccharides and sugars explains the pyran ring of karrikins, which is proposed to be derived directly from pyranose sugars in plant material (Fig. 2). The precise chemical reaction is unknown but it requires oxygen, and it is even possible to generate seed-germination activity by heating plant material at 180 °C for 30 minutes. So karrikins are probably produced around the home by toasting and roasting certain plant foods. The smoke from cigarettes stimulates seed germination, probably due to the presence of karrikins. Further research has shown that karrikins are unstable at very high temperatures . It is likely, therefore, that they are produced in the less-intense parts of wildfires, vaporise, and collect in the smoke and condensation and become bound to soil particles in the same way that cooled smoke can be deposited onto seeds to stimulate their germination. Karrikins may be ‘carried’ in smoke by a process of steam distillation but are not carried for long distances in smoke, and largely remain close to the source of the fire .
Measurements of karrikins in soil are technically very challenging but seed-germination bioassays can be used to detect activity, one study suggesting that active compound(s) can persist in the soil for over seven years after a fire . Karrikins are unstable in ultraviolet light  so they might be expected to decay rapidly in natural sunlight; however, smoke contains many aromatic compounds that can absorb ultraviolet light and could protect karrikins by acting as organic ‘sunscreens’. On the other hand, karrikins can be washed away by rain and elute through sandy soils relatively quickly, so their concentration will steadily decline.
It might be expected that seeds falling from a new plant in a burnt landscape would immediately encounter karrikins in the soil, and so germinate. While this happens for just a few species, most species need their seeds to be buried in soil for a year or more, a process known as after-ripening, before they become karrikin-responsive. There is also evidence that some seeds may need to undergo a series of wetting and drying cycles in the soil before they become responsive, which means that they depend on a future fire for germination .
Seeds from many different families of flowering plants and conifers representing many plant life forms (trees, shrubs, herbs, annuals) will respond to karrikins, and many more respond to smoke, implying that the karrikin response is widespread and may have evolved independently in different groups . Plants with smoke-responsive seed are found in both fire-prone and non-fire-prone environments. Most are dicotyledonous plants but many grasses also respond to smoke. Surprisingly, not only fire-followers respond but also many weedy species, including agricultural weeds . Even seeds of horticultural plants such as lettuce and tomato will have improved germination in response to karrikins under some circumstances. This implies that the ability or potential for a response to karrikins is something fundamental to plants, but the fire-followers have fine-tuned this response to their advantage in post-fire landscapes.
Smoke water is sometimes used to promote germination of garden and horticultural seeds, and can be purchased commercially or easily made. However, many commercial smoke germination products are made from combusting wood that, because of its high lignin content, produces germination inhibitors. Aerosol smoke is also used, and some nurseries or landscape restoration operations use this approach to treat seeds directly or to smoke seedling trays . There has been much interest in the possible use of chemically synthesised karrikins to treat soil to bring about wide-scale and vigorous germination of the resident weed soil seed bank — a process known as ‘suicidal germination’. This could be used for re-vegetation of degraded land, or to promote germination of dormant weed seeds in farmer’s fields so that the weeds can be eliminated. Further research is needed to develop more cost-effective synthesis and improve delivery methods for large-scale commercial application of karrikins.
The realisation that many plant species respond to karrikins led to the discovery that seeds of Arabidopsis thaliana can respond . Arabidopsis is the geneticist’s dream because of the resources and knowledge that are available. Arabidopsis seeds with a small amount of dormancy will respond to KAR1 or KAR2, provided that there is no nitrate present, which causes seeds to germinate regardless of the karrikin. Selection of Arabidopsis mutants that fail to respond to karrikins led to the discovery of two genes that are essential for karrikin action. One gene, named MORE AXILARY GROWTH2 (MAX2), was already known for its role in responses to strigolactone hormones , while the other, KARRIKIN-INSENSITIVE2 (KAI2), was similar to the gene coding for the strigolactone receptor, known as DWARF14 . These discoveries led to the idea that karrikins simply mimic strigolactones, because they both have a butenolide ring (Fig. 2). We now know that this is not the case in Arabidopsis. Karrikins and strigolactones are perceived separately and the plant responds differently to the two classes of compound, but the two systems are obviously very closely related . Formally we still do not know if the mode of action of karrikins in fire-followers is the same as that in Arabidopsis, but all plants apparently contain a KAI2 gene, so it seems likely.
Since evidence for the direct interaction of KAR1 with KAI2 is equivocal, there remains the possibility that the karrikin compounds found in smoke require uptake and conversion by the seed into an active compound before interaction with KAI2. This could explain why some seeds are less responsive than others, if they lack the processes and enzymes for uptake or metabolism. Although there are reports that KAR1 can interact directly with KAI2 protein, other studies do not support this. A further complication is that while strigolactones are destroyed by the hydrolase activity of their receptor protein DWARF14, such evidence is lacking for karrikins and KAI2. It is possible, therefore, that plants take up karrikins and convert them to active compounds that then interact directly with the KAI2 protein.
Treatment of plants with karrikins or analysis of mutants that do not respond to karrikins reveals that signalling though the KAI2 pathway leads to changes in gene expression. A search for mutants that suppress the karrikin-insensitive mutant phenotype led to the discovery of a gene named SUPPRESSOR OF MAX2-1 (SMAX1), which is required for the KAI2 signalling system. At the same time a closely related gene named DWARF53 was found to be required for strigolactone signalling in rice. These genes encode proteins that are repressors of gene transcription, and DWARF53 is degraded when strigolactones are present, leading to activation of target genes. It is likely that karrikin signalling works in the same way, but targets different genes for activation, with different growth responses (Fig. 6) .
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