The Wildfire Evolutionary Response

Meet the Karrikins

Karrikins are a family of small organic compounds — made only of carbon, hydrogen, and oxygen — produced whenever plant material burns. They were unknown to science until the early 2000s, when researchers in Western Australia noticed that seeds of many species germinated explosively after bushfires, not because of the heat, but because of chemicals in the smoke. After years of painstaking work — separating smoke water into thousands of components and testing each fraction for biological activity — the active compound was isolated. Its structure, confirmed through chemical synthesis, was published in Science in 2004 by Flematti and colleagues: a butenolide ring fused to a pyran ring, produced when polysaccharides in plant cell walls are burned.

The name “karrikin” honours the Aboriginal Noongar people of Western Australia, derived from karrik, their word for smoke. Several related compounds were subsequently identified and numbered (KAR₁ through KAR₄). KAR₁, known as karrikinolide, is typically the most abundant and the most biologically active in isolation. Seeds of some fire-specialist plants respond to KAR₁ at concentrations as low as one part in ten billion — a sensitivity comparable to that of plant hormones (Flematti et al., 2015).

The Fire-Follower Strategy

The most striking examples of karrikin biology are the “fire-followers” — plants whose entire life strategy is organised around wildfire. Their seeds remain dormant in the soil for decades, surviving repeated cycles of wetting and drying without germinating. When fire passes through, smoke carries karrikins and dozens of co-occurring bioactive compounds that bind to soil particles. The first rains then wash those compounds into the root zone, and seeds that have waited years in the dark germinate en masse within days.

The ecological logic is elegant. Fire eliminates competing vegetation, releases nutrients locked in plant biomass, and creates open habitat where seedlings can establish before being shaded out. Plants that can germinate and complete their life cycle in this window gain exclusive access to temporarily abundant resources. Fire-followers are so precisely adapted to this opening that they typically flower, set seed, and die within one or two years — leaving a fresh dormant seed bank ready for the next fire cycle. One remarkable feature of this strategy: even repeated rainfall without a preceding fire does not trigger germination. The seed “knows” the difference (Flematti et al., 2015).

An Ancient Molecular Switch

What makes the karrikin story scientifically remarkable is what researchers found when they traced the molecular machinery back through evolutionary time. The karrikin receptor — a protein called KAI2 (KARRIKIN-INSENSITIVE2) — is not new. It can be traced all the way back to single-celled algae, long before plants colonised land, before seeds existed, and hundreds of millions of years before flowering plants appeared. This means the ancestral function of KAI2 was not to detect fire at all.

KAI2 originally evolved to respond to an endogenous karrikin-like signal produced by the plant itself, controlling aspects of seed development and seedling growth. When fire became a recurrent feature of terrestrial ecosystems during the Cretaceous period — when angiosperms were rapidly diversifying — some plant lineages evolved KAI2 so that it could also recognise fire-derived karrikins, co-opting an existing developmental switch for a new ecological purpose. Evidence for this ancient function comes from experiments showing that a KAI2 protein from Selaginella — a non-seed plant that diverged from our ancestors before seeds existed — can rescue normal seedling development in karrikin-insensitive Arabidopsis mutants, despite not itself responding to karrikins or strigolactones (Flematti et al., 2015).

A related gene duplication event created a second receptor protein, DWARF14, which became the sensor for strigolactones — the hormones plants use to signal to mycorrhizal fungi in the soil. KAI2 and DWARF14 are molecular cousins. Both relay their signals through the same downstream scaffold protein, MAX2, before diverging to control different processes. The deep evolutionary relationship between fire-response chemistry and mycorrhizal-symbiosis chemistry suggests that these two apparently unrelated ecological functions share a common molecular origin — a single ancient signalling pathway that evolution has bent to multiple purposes.

The Signalling Cascade

When KAI2 detects a karrikin, it triggers a cascade through the MAX2 protein that degrades a transcriptional repressor called SMAX1. With SMAX1 removed, genes involved in germination, seedling development, and early growth are switched on. The parallel strigolactone pathway works through the same MAX2 scaffold, but targets a different repressor (DWARF53 in rice), producing different growth outcomes. This shared-scaffold architecture explains how two chemically related signals — karrikins from fire, strigolactones from mycorrhizal communication — can activate the same molecular relay while producing very different effects on the plant.

The results are measurable and rapid. In Arabidopsis thaliana, karrikin treatment causes dormant seeds to germinate readily and seedlings to develop larger cotyledons while keeping the hypocotyl compact — a morphology precisely suited to rapid establishment in an open, sun-exposed, post-fire environment.

Smoke Is More Than Karrikin

Here the science takes a turn that is both humbling and important. Karrikinolide was isolated as the single most active compound in smoke water — but subsequent transcriptomic research has revealed that purified KAR₁ and whole smoke water, while producing similar germination outcomes, activate strikingly different gene expression programmes in the plant.

Soós et al. (2010) exposed germinating maize kernels to either smoke water or purified KAR₁ at equivalent biologically active concentrations and profiled gene expression across the first 24 hours of germination. The physiological response — improved germination rate and seedling vigour — was similar in both treatments. But the molecular signatures were almost entirely distinct. Smoke water strongly activated protein ubiquitination and protein-degradation pathways — a broad cellular remodelling response consistent with the dismantling of germination inhibitors and the mobilisation of stored reserves. This ubiquitination response was completely absent in kernels treated with purified KAR₁. Instead, KAR₁ specifically and distinctly upregulated a single aquaporin gene, facilitating water uptake into the embryo. The authors concluded that the array of bioactive compounds present in smoke water forms a composite environmental signal that plants may have evolved to read as a whole — and that karrikinolide is one entry point into that programme, not the programme itself.

This is not a finding that diminishes karrikin biology. It contextualises it. What plants evolved to respond to was not a single molecule but an entire chemical vocabulary — the complex mixture produced when plant biomass smoulders incompletely. A hot, complete-combustion fire produces primarily carbon dioxide and water. The chemically rich smoke that carries biological signals is generated during the smouldering, low-temperature phase of combustion, where biomass is thermally decomposed rather than fully oxidised. It is this incomplete thermal conversion — not clean combustion — that produces the phenolics, organic acids, furans, butenolides, and carbonyl compounds that constitute the smoke signal plants evolved to read.

Not Just for Fire Country

One of the most striking findings from karrikin research is how broadly the response is distributed across the plant kingdom. Seeds from many botanical families — trees, shrubs, herbs, annuals, grasses, and conifers — respond to smoke-derived compounds. Most of these species are not fire-followers and would rarely encounter wildfire smoke in nature. Even crop plants with no particular association with fire respond: smoke water improves germination vigour in tomato, promotes more rapid seedling growth in maize, and enhances germination in lettuce.

This wide distribution is explained by the ancient, conserved nature of KAI2 and of the broader smoke-response machinery. Because these pathways predate fire-responsive seeds, they were present in the common ancestor of virtually all seed plants. Fire-followers intensified and specialised a response that was already latent. For agriculture, this means the molecular machinery for responding to smoke chemistry is almost certainly present in every crop species — not as a dormant relic, but as an active developmental pathway that normally responds to the plant’s own endogenous karrikin-like signal.

PyGrow and the Wildfire Signal

Pyroligneous acid — the liquid condensate produced when wood and plant biomass are subjected to low-temperature thermal decomposition — contains the same classes of organic compounds that wildfires release into smoke during their smouldering phase. The process produces phenols, organic acids, carbonyl compounds, furans, and butenolide lactones — including karrikin-class compounds — from the thermal conversion of polysaccharides, hemicellulose, and lignin in the biomass. Karrikins themselves are produced when polysaccharides undergo thermal degradation, with the pyran ring derived directly from pyranose sugars present in plant cell walls.

The chemical resemblance to wildfire smoke is not incidental — it reflects a shared process. In both a smouldering fire and a low-temperature pyrolysis reactor, plant biomass is undergoing the same incomplete thermal conversion, releasing the same classes of condensable organic compounds into a vapour phase that subsequently condenses into liquid. PyGrow is that condensate, collected and refined.

When PyGrow is applied at appropriate dilutions, it delivers this chemical vocabulary to plant roots and soil. The response it elicits is not mediated by a single compound through a single receptor. It is, as the transcriptomic evidence suggests, a composite signal activating multiple pathways simultaneously — protein homeostasis and turnover, aquaporin-mediated water uptake, and the deep developmental programme associated with post-fire recovery. This is not forcing growth by loading the plant with external nutrients. It is presenting the plant with a signal it has been reading, in one form or another, for 400 million years.

Agricultural Implications

The broad, conserved nature of smoke-response biology makes these signals relevant across virtually all crop systems. Smoke water — made by passing smoke through water, capturing karrikins and the broader complex of bioactive compounds — has been used commercially for years to improve germination of horticultural and garden seeds. And the compounds show promise for ecological restoration, stimulating rapid establishment of native plant communities on degraded or burned land.

For production agriculture, the most practical implication is this: if the smoke-response programme promotes root expansion, seedling vigour, and early establishment in nearly all plant species, then delivering the chemistry of natural smoke through a refined pyrolysis product like PyGrow represents a biologically sound and low-cost approach to eliciting those benefits in any crop. The mechanism is not agrochemical novelty. It is the 400-million-year relationship between plants and fire, presented in the form that plants evolved to receive it.

Key Findings from Smoke-Response Research

• Seeds respond to smoke-derived compounds at concentrations as low as 10⁻¹⁰ M — comparable in potency to plant hormones
• The karrikin receptor KAI2 is conserved from algae to flowering plants, confirming an ancient endogenous function
• Karrikin and strigolactone signalling share the MAX2 molecular scaffold, linking fire-response to mycorrhizal biology
• Whole smoke water and purified karrikinolide produce similar germination outcomes but activate distinct molecular programmes — suggesting a composite signal rather than a single active compound
• Smoke water activates protein ubiquitination and degradation pathways; purified KAR₁ does not — indicating that the full smoke chemistry is required to replicate the natural plant response
• Crop plants including maize, tomato, and lettuce respond to smoke-derived compounds even with no fire history
• Pyrolysis of plant biomass at low temperatures produces the same condensable organic chemistry as smouldering biomass in wildfire, making pyroligneous acid a natural analogue of smoke water