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=Taraxacum kok-saghyz=

General introduction
''Taraxacum kok-saghyz, often abbreviated as TKS and commonly referred to as the Kazakh dandelion, rubber root, or Russian dandelion,[1][2] is a species of dandelion native to Kazakhstan, Kyrgyzstan and Uzbekistan, that is notable for its production of highquality rubber.[3] T. kok-saghyz was discovered in Kazakhstan in 1932 by scientists serving the Soviet Union in an effort to find a domestic source of rubber.[2][4] ''

Etimology
''Kok-saghyz is derived from the Kazakh kök-sağız (көк-сағыз), with kök meaning root and saghyz meaning rubber or gum. Its latex was traditionally used as a kind of chewing gum.[5]''

History
TKS was cultivated on a large scale in the Soviet Union during World War II. ''The Soviet Union cultivated Taraxacum kok-saghyz, together with Taraxacum hybernum and Scorzonera tau-saghyz, on a large scale between 1931 and 1950 [] as an emergency source of rubber when supplies of rubber from Hevea brasiliensis in Southeast Asia were threatened. The United States, the UK, Germany, Sweden and Spain also cultivated the plant for the same reason. During this time period, the highest yields achieved by the U.S. reached 110 kg of rubber per hectare, while the USSR achieved yields of 200 kg of rubber per hectare ((DRIVE4EU: Dandelion Rubber and Inulin Valorization and Exploitation for Europe | DRIVE4EU Project | Fact Sheet | FP7 | CORDIS | European Commission, s.d.)). -The Raisko sub-camp of Auschwitz was a production factory for the plant. Some of the women who been deported on the Convoi des 31000 worked on its production there.[9]- With the conclusion of World War II and the return of affordable Hevea brasiliensis rubber (which has 8 to 10 times the yield[8]), the majority of T. kok-saghyz programs ceased. ''

Plant description
''Taraxacum kok-saghyz is a perennial plant with a yellow composite flower characteristic of the genus Taraxacum. Each flower head may be approximately one inch in diameter and be made up for 50 to 90 florets.[6] Plants may contain 25 to 50 leaves arranged in one or more rosettes at the upper end of the root. Taraxacum kok-saghyz can be differentiated from the common dandelion (Taraxacum officinale) by its generally smaller, grayish green leaves and hornlike structures on the bracts surrounding the bud.[6] Flowers are hermaphrodite, insect-pollinated (entomophilous) and are on stalks that reach about a foot in height. Taraxacum kok-saghyz is usually in flower from May through June, with seeds ripening from June through July.[7] It is a diploid species that reproduces sexually,[2] and produces good pollen.[1] Another species, a triploid apomict that also has horned bracts and produces rubber, T. brevicorniculatum Korol. has frequently been misidentified as T. kok-saghyz.[1][2] ''

Growing conditions (soil and climate requirements)
Taraxacum kok-saghyz does best in loose, well-drained soils with high moisture retention and a pH between 5.5 and 8.5, in full or nearly-full sun.[7] The plant grows well in temperate climates (Keener et al., 2018). The greatest growth is recorded in soils containing 2–8% organic matter (Whaley et al., 1947).

Seeding
TK produces about 1 million seeds per pound (Keener et al., 2018) (Keener et al., 2018). The seeds readily germinate but grow very slowly despite the early development of a strong root system. Because of their slow growth, the seedlings are vulnerable to being outcompeted by native weeds. Slow growth and weed control remain a major challenge in successfully growing TK today (Krotkov, 1945). The direct seeding of TK has been identified by researchers as the preferred method for establishing a TK crop (Keener et al., 2018). European field studies showed that TK germinates in May (Seasonal pattern of biomass and rubber and inulin of wild Russian dandelion (Taraxacum koksaghyz L. Rodin) under experimental field conditions - ScienceDirect, s.d.). The vulnerability during the early life stages of TKS is the main reason why the pest risk assessment, conducted by the Julius Kühn institute, revealed little invasive potential for TKS in Germany [12].

Challenges
Currently, the main challenges of growing TK include plant emergence, seedling vigor, growth rate, and weed control. Weed control remains a major challenge, since no herbicides are currently available for TK production. Uncontrolled weed pressure reduces crop survival and plant growth rate (Keener et al., 2018). In addition, the cultivation of TK planted in rows is impractical due to the morphology of the plant. Current studies of plant genome aim to identify genetic markers to improve TK germplasm. Moreover, the development of weed resistance through traditional plant breeding techniques is being investigated (Keener et al., 2018).

Pests and disease
In 2022, scientists have reported the first rust disease on TKS, caused by Puccinia hieracii. It resulted in moderate to high yield losses and poses a potential threat for large scale production of TKS [13].

Harvest
TK can be harvested after a single growing season. Rubber accumulation in TK begins at germination, peaks at around one year, and then levels off or decreases due to older root tissues being shed and destroyed by microorganisms (The physiology and biochemistry of rubber formation in plants | SpringerLink, s.d.).

Rubber
Natural rubber (cis-1,4-polyisoprene) is a polymer produced by plants, with compared significance to water or oil. Natural rubber is obtained from coagulating and refining latex from plant species (Keener et al., 2018). In many of its most significant applications, cannot be replaced by synthetic rubber alternatives (van Beilen and Poirier 2007). Some of the unique and unreplaceable properties are abrasion resistance, elasticity, tear and impact resistance, malleability at cold temperatures, and efficient heat dispersion (Salehi et al. 2022). The production of this essential material is concentrated in the tropical areas of Asian-Pacific countries and it comes from the Para rubber tree (Hevea brasiliensis) (Molinu et al. 2019). The natural rubber market is coordinated by the Association of Natural Rubber Producing Countries (ANRPC). This intergovernmental association represents 92% of the world's natural rubber production (Student & Liubov, 2019). There are a number of factors driving the search for alternatives to natural rubber production.

''One threat to Hevea brasiliensis rubber production is the South American Leaf Blight (SALB) caused by Pseudocercospora ulei fungi, which has afflicted conventional rubber production in South America since 1934. This blight may spread to the Hevea brasiliensis trees in Southeast Asia, which are genetically very similar to each other and to those of South America. Furthermore, land used for rubber production is being converted[by whom?] to palm-oil plantations in order to produce biofuel, and labor costs reduce the profitability of Hevea brasiliensis plantations, as each tree must be manually tapped in order to harvest its latex. Rising oil-prices limit the economic viability of synthetic rubbers, and synthetic equivalents often cannot pragmatically replace natural rubber.[4][2] In May 2019 German tire-maker Continental AG announced it was about to begin production of the "first bicycle tyre made with sustainable rubber from dandelions", which it intended to grow on the grounds of its own manufacturing plants, avoiding several of the traditional issues with H. brasiliensis latex—from the long lead-time between planting and cultivating (only six months for the dandelion, rather than seven years for the rubber tree) and volatile prices of the product due to the long transport-distances between places where the rubber can be grown and the company's factories.[10] '' Additionally, there are increasing evidence of allergenic reactions to Hevea rubber used in medical devices (gloves, condoms, catheters, and other medical products), potential shortages of supply due to increasing demand (The global market of natural rubber increases annually by 1- 3% (Student & Liubov, 2019)) and changes in land use, and a general trend towards the replacement of petroleum-derived chemicals with renewables, which are pushing the search for new sources of natural rubber ([1], van Beilen and Poirier 2007).

One of the alternative option is the Taraxacum kok-saghyz. TKS contains on average 12% natural rubber and has an average yield of 225-750 kg/ha (Student & Liubov, 2019). Compared to Hevea brasiliensis, the yield of TKS is not enough to cover the cost of both collection and processing (Student & Liubov, 2019). Researchers have started to develop Taraxacum kok-saghyz cultivars which are easier to cultivate and which produce more and better rubber as part of a large research project at many institutions.[8][11][2]

Processing of TK for rubber
After harvesting, the roots are washed from dirt and soil. If roots are not directly processed, because for example of transportation reason, they have to be dried and then can be stored for several months under appropriate conditions (van Beilen & Poirier, 2007).

The process of rubber-extraction follow his pattern (van Beilen & Poirier, 2007):


 * 1) leach the roots in hot water: softens the tissue and coagulates the latex in fine filament. Also, this removes the soluble carbohydrates (mainly inulin), that can be further used for the ethanol production.
 * 2) Pebble grinding: loosen the plant tissue and rolls together the rubber filaments, which can be separated from the root solids by passing over a vibrating screen and flotation
 * 3) Separation through vibrating screen and flotation: for the separation of the rubber filament from the root solids.

This process recovered well over 90% of the total rubber in the roots. Another process option would be the extraction of rubber as latex.

Dandelion by-products
Inulin produced by T. kok-saghyz is a sugar that could be used in non-food applications or be turned into bioethanol through fermentation. The remaining plant biomass could be used to produce biogas.[4]

Breeding goals
The growing interest in natural rubber increases the interest in genetically modified organisms. This need is explained by the decrease of natural rubber world resources, the indispensability of natural rubber for industry, and the strategic necessity of a national industry to be independent of the import (Student & Liubov, 2019). The main breeding goals are both increasing the percentage of rubber content in the plant and the vegetative mass of the plant (Student & Liubov, 2019).

Breeding tools
Three genes are mainly responsible for the biosynthesis of rubber: REF (rubber elongation factor), SRPP (small rubber particle protein) and CPT (cis-prenyltransferase). The role of these genes was also proved in 2016 by the hevea genome sequence analysis. With the introduction of these genes into the nuclear DNA of TKS, it is possible to augment the content and the quality of rubber in the plant (Student & Liubov, 2019).

Genetic engineering allows to increase rubber content in the kok-saghyz plant using two ways: inulin degradation by gene 1-FEN expression or knockdown of gene fructan 1-fructosyl transferase (1-FFT), coding for fructose involved in inulin synthesis (Student & Liubov, 2019).