User:Akziq/Karrikin

Karrikins help stimulate seed germination and plant development because they mimic a signaling hormone known as the Strigolactone hormone. 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.

The evolution of flower plants during the fire-prone times in the Cretaceous period lead to the discovery of Karrikins. Karrikins are organic compounds that are released into the air upon burning of plants (Figure 2), Karrikins then get deposited on the soil surface (Figure 3) and stimulate seed germination after a rainfall (Figure 3). Since Karrikins are released from the smoke, they are released in huge quantities, which stimulate seed germination and thus plant growth in a massive amount. Interestingly, some plants known as "fire-followers" are unable to germinate without Karrikins. Fire-followers need rain after a massive fire occurs in order to be able to germinate; this means that they remain dormant and viable for decades until that happens. It was long known that compounds released from smoke stimulate seed germination. A group of researchers decided to identify the active compounds that contribute to seed germination activity. The 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'. The six different Karrikins that were identified are KAR1, KAR2, KAR3, KAR4, KAR5, and KAR6. KAR1 is also known as Karrikinolode and was the first Karrikin to be discovered.

Properties of Karrikin;

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, and the other is a five membered ring known as a butenolide. Karrikins easily dissolve in water, they are transparent, and have a melting point of 118–119 °C. 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.

Note that karrikins can only travel as far as the smoke does.

Karrikin signaling:

Studies have shown that Arabidopsis responds to the two signals; KAR1, and KAR2. 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. 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. Karrikins could be used as agricultures, considering the environmental challenges that are occurring nowadays.

Response to Karrikins:

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. The difference between fire-followers and plants that respond to karrikins is their dependence on karrikins. 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.