Coronatine is a next-generation plant growth regulator that is structurally similar to abscisic acid (ABA) and jasmonic acid (JA). Its physiological functions include encouraging cell differentiation, increasing chlorophyll content, regulating plant growth and inhibiting cell aging. However, the high price of plant growth regulators based on abscisic acid and jasmonic acid restricts their large-scale use in agriculture.
With similar properties to abscisic acid and jasmonic acid, coronatine can be produced in large quantities through biological methods and is expected to play a key role in future agricultural production.
1. Development History of Coronatine
In 1977, Ichihara first isolated coronatine (COR) from the culture medium of the deep-red pathogenic variety of pseudomonas syringae, which was initially found to induce yellow blight in Italian ryegrass. Through further research, coronatine was discovered to have a similar structure to jasmonic acid and had comparable physiological functions, such as regulating growth, inhibiting aging, promoting cell differentiation, increasing chlorophyll content and reinforcing plant stress resistance.
Since the discovery of coronatine, producing its pure form has become a hot topic in the field of plant hormone studies. It is very difficult to synthesize coronatine, Ichihara of Hokkaido University, Japan, was only able to produce only 60mg of coronatine from a 250-liter fermentation broth via the fermentation method. Therefore, he predicted that coronatine production would result in low yields, as it is difficult to produce through fermentation, which has restricted the functional study of coronatine for application on plants. Later on, with the confirmation of coronatine’s molecular structure and further functional studies, researchers were able to begin large-scale seed selection and the breeding of coronatine’s production strains, as well as optimize fermentation conditions and develop the appropriate isolation technology.
Multidisciplinary experts from China Agricultural University and other institutions have achieved key breakthroughs in the high-yield cultivation of genetically-engineered bacterium that are resistant to high temperatures. Based on this fact, genetically-engineered strains with high coronatine yields have been produced via transposon induction mutation and gene recombination, resulting in a yield that is five to 10 times higher than the rate reported worldwide. Meanwhile, a standard fermentation process has been established and optimized and has undergone trial production at a quantity of five and 20 tons of liquid fermentation, at a fermentation period of seven to eight days, which is the world’s first coronatine fermentation-based production line established in Chengdu New Sun
’s workshop. At same time, a large-scale concentration and refining process based on membrane filtration and activated carbon adsorption was developed, which involves the removal of fermentation broth thallus and macromolecule protein using an ultrafiltration membrane and nanofiltration membrane, resulting in a coronatine concentrate that can be used for formulation. By adding activated carbon to the concentrate for further adsorption and desorption, a crude product with 50% coronatine and pure coronatine with a content of over 95% can be produced.
Researchers have also developed water-based coronatine formulations and nano controlled-release agents, such as aqueous solution and suspension concentrate. The newly-developed formulations offer stable performance, solving the stability issues affecting biological regulators. Researchers also recently found out that coronatine can improve crops resistance to biological stresses, such as salt, drought, temperature extremes and diseases, and can prolong the growth of cotton and promote defoliation and ripening. Accordingly, key techniques for application on field crops were developed, creating the ideal conditions for field application and industrialized production.
2. Action Mode of Coronatine
Coronatine is a next-generation botanical regulator produced via modern biological fermentation, column separation and purification techniques, and is a novel plant growth regulating substance formed through the amide bonding of coronamic acid and coronafacic acid. It is a type of "allomone" generated by pseudomonas syringae in the process of invasion in plants, which, in high concentrations, can induce the stomata to open quickly and cause damage to the innate immune function of leaves, leading to plant disease. However, when applied at extremely low concentrations, it has a considerable physiological regulation effect on plant growth and promotes differentiation, development, secondary metabolism, drought resistance, cold resistance and resistance to salt and diseases.
As an environmentally-friendly plant growth regulator, coronatine is only effective at very low concentrations and its biological activity is 100 to 10,000 fold of jasmonic acid. Under natural conditions, such as low temperatures, cold damage and drought, coronatine can regulate the growth of plants, induce resistance, reduce plant damage from adverse environmental conditions and prevent damage to plant health caused by changes to the natural environment and disasters.
3. Application Potential of Coronatine
Wheat is an important food crop in China, and its output and quality are directly related to China's food security. Dry hot wind is one of the major negative weather conditions found in wheat growing regions in northern China, which causes 10% to 20% yield reduction in severe cases.
Studies have shown that under the stress of high temperatures, the use of coronatine can help wheat leaves maintain a relatively high water content, to promote the synthetization of soluble protein and improve the osmotic regulation capability of cells, which mitigates, to certain degree, the damage caused by high temperatures to wheat and increase resistance to high temperatures. The root-cap ratio of wheat treated with coronatine increases at low temperatures, indicating that coronatine can promote wheat root growth and increase cold resistance in low temperatures.
The use of coronatine can significantly increase the content of soluble sugar and proline in the leaves of maize seedlings under drought stress, increasing resistant to drought. Drought affects the photosynthesis and respiration of maize seedlings. The stomata of maize seedling treated with coronatine opens wider, which enables increased entry and absorption of carbon dioxide through, thus, maintaining higher photosynthesis and resulting in the synthetization of more dry substances, a larger stem base and longer root length in maize seedlings.
Coronatine soluble concentrate (0.06%) can increase the yield of cotton to a certain degree while being safe on crops and without phytotoxicity. The use of this concentrate enhances the resistance of cotton to disease, promotes the growth of cotton, makes stems stronger and ripens cotton crops earlier, therefore, increasing yield.
Low-concentration coronatine can also increase of plant stress resistance while high-concentration coronatine will cause the degradation of leaf chlorophyll contents and the fall of fruits. High-concentration coronatine can also cause defoliation in cotton, which is advantageous to mechanized harvesting of cotton. Its effectiveness and low cost adds market value to cotton production. High-concentration coronatine can also function as a "systemic herbicide," which is a new option for developing novel herbicides.
Soybean is sensitive to light and temperature, especially during the seedling stage when its sensitivity to low temperature not only restricts production in high latitude regions but also affects the planting of southern spring soybean, which led to the situation in China where smaller areas are being used for planting, resulting in lower soybean yields. Current studies have shown that spraying coronatine in certain concentrations can facilitate the emergence pf soybean seedlings in low temperatures, which may favor China's soybean production and international soybean trade and can be used as a technical reserve for China's food security strategy, to cope with the impact of global trade.
In order to verify the effects of coronatine in terms of regulating growth, promoting cell differentiation, affecting chlorophyll content and improving plant stress resistance under different concentrations, Chengdu New Sun
conducted cold resistance field experiments with coronatine on citrus (soft orange).
The experiments showed that coronatine can stimulate a plant's autoimmune system, induce the generation of intracellular antioxidant enzymes and increase proline and sugar contents, thus, reducing damage. The spraying of coronatine in lower concentrations prior to low temperature conditions can significantly improve the resistance to cold of uncovered citrus fruits and reduce cold injury.
4. Future Expectations
With the continuing effects of climate change and declining ecological conditions, it is important to explore the theory and practice of improving crop stress resistance, to increase crop productivity and promote agricultural development in a timely and healthy manner. The use of crop chemical control processes and the new plant growth regulator, coronatine, which will better enable crops to adapt to adverse environmental conditions, is expected to play a key role in future production practices. After the initial physiological function research on coronatine and its broad industrial and agricultural application prospects, it is important to conduct further studies on its production techniques. Through elaborating the technical process of the biosynthesis of coronatine, a major breakthrough in its research and production is expected. Ongoing coronatine-oriented market exploration, pesticide registration and industrialization will benefit global trade development and global food security.