Karrikins are a group of plant growth regulators found in the smoke of burning plant material. [1] [2] Karrikins help stimulate seed germination and plant development because they mimic a signaling hormone known as strigolactone. Strigolactones are hormones that help increase growth of symbiotic arbuscular mycorrhizal fungi in the soil, which enhances plant growth and leads to an increase in plant branching. [3] [4]
Smoke from wildfires or bushfires has been known for a long time to stimulate the germination of seeds. [5] [6] In 2004, the butenolide karrikinolide (KAR1) was shown to be responsible for this effect. [7] Later, several closely related compounds were discovered in smoke, and are collectively known as karrikins. [2]
Karrikins are formed by the heating or combustion of carbohydrates, including sugars and polysaccharides, mainly cellulose. [8] When plant material burns, these carbohydrates convert to karrikins. Burning plant products, such as straw, filter paper, cigarettes, and some sugars, can also produce karrikins. Seed germination activity can be generated within 30 minutes of heating plant material at 180 °C (356 °F). [1] The pyran moiety of karrikins is probably directly derived from a pyranose sugar. There is no evidence that karrikins occur naturally in plants, but it has been postulated that karrikin-like molecules do. [9]
It has long been known that compounds released from smoke stimulate seed germination. To identify the active compounds that contribute to seed germination activity, smoke compounds were separated by liquid fractionation and were each tested for their effects on seed germination activity. Bioassays identified several related compounds that were named karrikins. [1]
Six karrikins have so far been discovered in smoke, and they are designated KAR1, KAR2, KAR3, KAR4, KAR5 and KAR6. KAR1 to KAR4 are the most active karrikins. [10] KAR1 is also known as karrikinolode and was the first karrikin to be discovered. [1]
Karrikins are released into the air upon the burning of plants. Subsequently, karrikins then get deposited on the soil surface and stimulate seed germination after rainfall. Since karrikins are released from smoke, they are released in huge quantities. [7] Some plants which are known as "fire-followers" are unable to germinate without karrikins. Fire-followers need rain after massive fires in order to germinate; this means that they may remain dormant and viable for decades until the right combination of fire occur in proper succession. [1]
The first karrikin discovered, abbreviated as KAR1, was initially named gavinone in reference to its discovery by chemist Gavin Flematti. After consulting with an etymologist, Flematti proposed changing the name of the molecule and its related compounds to karrikin. One of the first recorded Western Australian Noongar words for 'smoke' from the Perth area in the 1830s, is 'karrik'. [2] [11] [12] [13]
Karrikins produced by bushfires occur largely in the ash at the site of the fire. Rains occurring after the fire wash the karrikins into the soil where dormant seeds reside. The karrikins and water can provide a 'wake-up call' for such seeds, triggering germination of the soil seed bank. The plants that depend on karrikins to grow are known as "fire-followers", [1] they emerge grow quickly, flower and produce new seeds, which fall to the ground. These seeds can remain in the soil for decades, until the next fire produces fresh karrikins. Plants with this lifestyle are known as fire ephemerals. They thrive because the fire removes competing vegetation and provides nutrients and light for the emerging seedlings. Plants in many families respond to smoke and karrikins, suggesting that this response has evolved independently in different groups. [10]
Fire-followers are not the only plants that respond to karrikins. Seeds from a number of different flowering families like tomatoes, lettuce, and trees respond to karrikin signaling. [1] Other studies have found that seed of ostensibly fire-adapted species do not display a sensitivity to karrikins. [14] The difference between fire-followers and plants that respond to karrikins is their dependence on karrikins. [1] Plants' response to karrikins is fundamental because karrikins mimic the strigolactone hormones which are originally required for growth in plants. Fire-followers, on the other hand, have fine-tuned their responses according to the availability of karrikins. [1]
Carbon, hydrogen, and oxygen make up the two ring structures found in karrikins, one of which is a six-membered, heterocyclic ring with a molecular formula of C5H6O known as pyran,[ citation needed ] and the other is a five-membered lactone ring known as a butenolide. [1]
Karrikins easily dissolve in water, they are transparent, and have a melting point of 118–119 °C. [1] However, they are unstable at very high temperatures and during common daylight, which means that they decay more rapidly than common active compounds which are not sensitive to sunlight. [15] [1]
The mode of action of karrikins has been largely determined using the genetic resources of Arabidopsis thaliana . Perception of karrikins by Arabidopsis requires an alpha/beta-fold hydrolase named KARRIKIN-INSENSITIVE-2 (KAI2). [16] The KAI2 protein has a catalytic triad of amino acids which is essential for activity, consistent with the hypothesis that KAI2 hydrolyses its ligand. [17] [18] This model is consistent with the perception of the chemically related strigolactone hormones which involves hydrolysis by their receptor protein DWARF14, an alpha/beta hydrolase related to KAI2. [16] [19] The question of whether karrikins act directly in plants is controversial. While some studies suggest that karrikins can bind directly to KAI2 protein, [20] others do not support this. [18] It is possible that karrikins produced by wildfires are converted to a different compound by the plant, before interaction with KAI2. The ability of different plants to carry out this conversion could partly explain differences in their ability to respond to karrikins and to smoke.
The activity of karrikins requires an F-box protein named MORE AXILLARY GROWTH-2 (MAX2) in Arabidopsis. [21] This protein is also required for strigolactone signaling in Arabidopsis. Homologs of MAX2 are also required for strigolactone signaling in rice (known as DWARF3) petunia (DAD2) and pea (RMS4). Karrikin signaling also requires a protein named SUPPRESSOR OF MORE AXILARY GROWTH2-1 (SMAX1) [22] which is a homolog of the DWARF53 protein required for strigolactone signaling in rice. [23] [24] SMAX1 and DWARF53 proteins could be involved in the control of cellular functions such as transport or transcription. [1] The present model for karrikin and strigolactone signaling involves interaction of KAI2 or DWARF14 with SMAX1 or DWARF53 proteins respectively, which targets those proteins for ubiquitination and destruction. [25]
Arabidopsis has been shown to respond to the two signals; KAR1, and KAR2.[ citation needed ] The two genes, MORE AXILLARY GROWTH2 (MAX2) and KARRIKIN-INSENSITIVE2 (KAI2) are essential for understanding the actions of karrikins and were discovered in Arabidopsis mutants which failed to respond to karrikins. In rice, strigolactones interact with the F-box protein knowns as DWARF3 upon their hydrolysis by the DWARF14 (also known as D14-type proteins). This interaction targets the ubiquitination and destruction of proteins which are responsible for different aspect of plant growth, like the outgrowth of lateral shoots. This means that strigolactones, upon their interaction with D3 and D14; ubiquinate, and destroy proteins like DWARF53, which are responsible for the outgrowth of lateral shoots, and for the inhibition of stem thickening and root branching. [26] In Arabidopsis, karrikins work in a similar way to strigolactones; they require homologous proteins known as KARRIKIN-INSENSITIVE1 (KAI1 or MAX2) in order to be able to interact with KARRIKIN-INSENSITIVE2 which is responsible for hypocotyl elongation and the inhibition of seed germination. The ubiquination of KAI2, therefore stimulate seed germination and inhibits hypocotyl elongation. [1] [27] Karrikins could be used as agricultures[ clarification needed ], considering the environmental challenges that are occurring nowadays. [28]
Karrikins not only stimulate seed germination, but are reported to increase seedling vigour. [29] In Arabidopsis, karrikins influence seedling photomorphogenesis, resulting in shorter hypocotyls and larger cotyledons. Such responses could provide seedlings with an advantage as they emerge into the post-fire landscape. The KAI2 protein is also required for leaf development, implying that karrikins could influence other aspects of plant growth.
The gene for KAI2 protein is present in lower plants including algae and mosses, whereas the DWARF14 protein evolved with seed plants, probably as a result of duplication of KAI2 followed by functional specialisation. Karrikin signaling could have evolved with seed plants as a result of the divergence of KAI2 and DWARF14 functions, possibly during the Cretaceous period when fires were common on Earth. [30]
Karrikins are produced by wildfires but all seed plants contain KAI2 proteins, raising the question of the usual function of this protein. There is compelling evidence that plants contain an endogenous compound that is perceived by KAI2 to control seed germination and plant development, but this compound is neither a karrikin nor a strigolactone. [1]
Vernalization is the induction of a plant's flowering process by exposure to the prolonged cold of winter, or by an artificial equivalent. After vernalization, plants have acquired the ability to flower, but they may require additional seasonal cues or weeks of growth before they will actually do so. The term is sometimes used to refer to the need of herbal (non-woody) plants for a period of cold dormancy in order to produce new shoots and leaves, but this usage is discouraged.
Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, ferns, bacteria, and the growth of the pollen tube from the pollen grain of a seed plant.
The meristem is a type of tissue found in plants. It consists of undifferentiated cells capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells continue to divide until a time when they get differentiated and then lose the ability to divide.
Plant hormones are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, including embryogenesis, the regulation of organ size, pathogen defense, stress tolerance and reproductive development. Unlike in animals each plant cell is capable of producing hormones. Went and Thimann coined the term "phytohormone" and used it in the title of their 1937 book.
Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essential for plant body development. The Dutch biologist Frits Warmolt Went first described auxins and their role in plant growth in the 1920s. Kenneth V. Thimann became the first to isolate one of these phytohormones and to determine its chemical structure as indole-3-acetic acid (IAA). Went and Thimann co-authored a book on plant hormones, Phytohormones, in 1937.
Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence.
Phytochromes are a class of photoreceptor proteins found in plants, bacteria and fungi. They respond to light in the red and far-red regions of the visible spectrum and can be classed as either Type I, which are activated by far-red light, or Type II that are activated by red light. Recent advances have suggested that phytochromes also act as temperature sensors, as warmer temperatures enhance their de-activation. All of these factors contribute to the plant's ability to germinate.
Jasmonate (JA) and its derivatives are lipid-based plant hormones that regulate a wide range of processes in plants, ranging from growth and photosynthesis to reproductive development. In particular, JAs are critical for plant defense against herbivory and plant responses to poor environmental conditions and other kinds of abiotic and biotic challenges. Some JAs can also be released as volatile organic compounds (VOCs) to permit communication between plants in anticipation of mutual dangers.
Plant senescence is the process of aging in plants. Plants have both stress-induced and age-related developmental aging. Chlorophyll degradation during leaf senescence reveals the carotenoids, such as anthocyanin and xanthophylls, which are the cause of autumn leaf color in deciduous trees. Leaf senescence has the important function of recycling nutrients, mostly nitrogen, to growing and storage organs of the plant. Unlike animals, plants continually form new organs and older organs undergo a highly regulated senescence program to maximize nutrient export.
In developmental biology, photomorphogenesis is light-mediated development, where plant growth patterns respond to the light spectrum. This is a completely separate process from photosynthesis where light is used as a source of energy. Phytochromes, cryptochromes, and phototropins are photochromic sensory receptors that restrict the photomorphogenic effect of light to the UV-A, UV-B, blue, and red portions of the electromagnetic spectrum.
Shade avoidance is a set of responses that plants display when they are subjected to the shade of another plant. It often includes elongation, altered flowering time, increased apical dominance and altered partitioning of resources. This set of responses is collectively called the shade-avoidance syndrome (SAS).
A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All parasitic plants develop a specialized organ called the haustorium, which penetrates the host plant, connecting them to the host vasculature – either the xylem, phloem, or both. For example, plants like Striga or Rhinanthus connect only to the xylem, via xylem bridges (xylem-feeding). Alternately, plants like Cuscuta and some members of Orobanche connect to both the xylem and phloem of the host. This provides them with the ability to extract resources from the host. These resources can include water, nitrogen, carbon and/or sugars. Parasitic plants are classified depending on the location where the parasitic plant latches onto the host, the amount of nutrients it requires, and their photosynthetic capability. Some parasitic plants can locate their host plants by detecting volatile chemicals in the air or soil given off by host shoots or roots, respectively. About 4,500 species of parasitic plants in approximately 20 families of flowering plants are known.
Butenolides are a class of lactones with a four-carbon heterocyclic ring structure. They are sometimes considered oxidized derivatives of furan. The simplest butenolide is 2-furanone, which is a common component of larger natural products and is sometimes referred to as simply "butenolide". A common biochemically important butenolide is ascorbic acid. Butenolide derivatives known as karrikins are produced by some plants on exposure to high temperatures due to brush fires. In particular, 3-methyl-2H-furo[2,3-c]pyran-2-one was found to trigger seed germination in plants whose reproduction is fire-dependent.
Brassica tournefortii is a species of plant known by the common names Asian mustard, pale cabbage, African mustard, and Sahara mustard, and is well known as an invasive species, especially in California.
GAI or Gibberellic-Acid Insensitive is a gene in Arabidopsis thaliana which is involved in regulation of plant growth. GAI represses the pathway of gibberellin-sensitive plant growth. It does this by way of its conserved DELLA motif.
Arabinogalactan-proteins (AGPs) are highly glycosylated proteins (glycoproteins) found in the cell walls of plants. Each one consists of a protein with sugar molecules attached. They are members of the wider class of hydroxyproline (Hyp)-rich cell wall glycoproteins, a large and diverse group of glycosylated wall proteins.
Steven M. Smith is Emeritus Professor of Plant Genetics and Biochemistry at the University of Tasmania in Australia and Chief Investigator in the Australian Research Council Centre of Excellence for Plant Success in Nature and Agriculture.
Strigolactones are a group of chemical compounds produced by roots of plants. Due to their mechanism of action, these molecules have been classified as plant hormones or phytohormones. So far, strigolactones have been identified to be responsible for three different physiological processes: First, they promote the germination of parasitic organisms that grow in the host plant's roots, such as Strigalutea and other plants of the genus Striga. Second, strigolactones are fundamental for the recognition of the plant by symbiotic fungi, especially arbuscular mycorrhizal fungi, because they establish a mutualistic association with these plants, and provide phosphate and other soil nutrients. Third, strigolactones have been identified as branching inhibition hormones in plants; when present, these compounds prevent excess bud growing in stem terminals, stopping the branching mechanism in plants.
Dmitri Nusinow is an American chronobiologist who studies plant circadian rhythms. He was born on November 7, 1976, in Inglewood, California. He currently resides in St. Louis, and his research focus includes a combination of molecular, biochemical, genetic, genomic, and proteomic tools to discover the molecular connections between signaling networks, circadian oscillators, and specific outputs. By combining these methods, he hopes to apply the knowledge elucidated from the Arabidopsis model to other plant species.
Ethylene (CH
2=CH
2) is an unsaturated hydrocarbon gas (alkene) acting as a naturally occurring plant hormone. It is the simplest alkene gas and is the first gas known to act as hormone. It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, the abscission (or shedding) of leaves and, in aquatic and semi-aquatic species, promoting the 'escape' from submergence by means of rapid elongation of stems or leaves. This escape response is particularly important in rice farming. Commercial fruit-ripening rooms use "catalytic generators" to make ethylene gas from a liquid supply of ethanol. Typically, a gassing level of 500 to 2,000 ppm is used, for 24 to 48 hours. Care must be taken to control carbon dioxide levels in ripening rooms when gassing, as high temperature ripening (20 °C; 68 °F) has been seen to produce CO2 levels of 10% in 24 hours.