Problems and Prospects for Industrial Cultivation of the Rubber-Producing Plant Taraxacum kok-saghyz in Russia

According to forecasts by the Association of Natural Rubber Producing Countries (ANRPC), total NR consumption is expected to reach 16.7 million tons by 2026, and there is no doubt that demand will continue to increase. At the present time, natural rubber (NR) is produced solely from a single species, the Brazilian rubber tree (Hevea brasiliensis). One of the reasons for the steady growth in demand for NR is the lack of synthetic rubber with physical properties comparable to those of NR, despite all the advances in polymer development. Several rubber-producing species are under development to find a reliable alternative source of NR. These are Rubber dandelion (Taraxacum kok-saghyz), Guayule (Parthenium argentatum), and almost forgotten Tau-saghyz (Scorzonera tau-saghyz). One of them, T. kok-saghyz, fits to obvious requirement to effective NR producer better than others. It has a very short life cycle (6--8 months) relative to other plants, such as H. brasiliensis and P. argentatum, and is amenable to genetic manipulations [1,2]. So, it is not surprising that today the interests of many groups of T. kok-saghyz researchers and NR producers, after a long break, once again are focused on the cultivation of this plant. Currently, scientific and technological developments for the industrial cultivation of T. kok-saghyz are being performed in many regions of the world where H. brasiliensis cannot be cultivated, because the plant easily adapts to the conditions of various climatic zones. Numerous studies on the physical and chemical properties of the dandelion NR clearly demonstrated that it has an outstanding quality and can compete with rubbers from other sources. For instance, tiers, made from the T. kok-saghyz NR are as resilient as those made from H. brasiliensis polymer [3]. Notably, molecular mass of dandelion NR is very high. It was reported that it can be as high as 2,2×106 Da [4]. The only potential problem is a large number of associated proteins, similarly to Hevea NR. The main objectives of improving the characteristics of T. kok-saghyz as a potential alternative source of NR are: accelerating the growth and development of plants, forming a larger root biomass, increasing the content of NR and another important target product inulin, creating and selecting forms resistant to phytopathogens. To date, industrial cultivation of T. kok-saghyz in the field is complicated by the low productivity of plants, even under protected ground conditions, high vulnerability to bacterial and fungal pathogens, and the necessity to create special technologies for collecting and processing rubber-containing roots. The development of a new technology for growing T. kok-saghyz plants under controlled phytotron conditions, created in our laboratory, would speed up the solution of many of these problems [5]. The use of the aeroponic cultivation method with optimization of all factors affecting the rate of plant growth and development may lead to improvement of the biosynthesis of target products. Potentially, this approach has obvious commercial prospects. In our research, we grew T. kok-saghyz in open ground and vegetation containers. We repeatedly observed that plants were affected by bacterial pathogens, this led to vascular bacteriosis (wilting). The plants showed darkening and maceration around the root collar, leading to subsequent death. It was shown that latent infection is present in seeds pre-treated with surface aseptics and is transmitted vertically. Using thermotherapy and chemotherapy protocols, we were able to create a collection of healthy T. kok-saghyz plants. Four species of endogenous bacteria responsible for the pathogenesis were isolated and identified. Molecular genetics and biochemical analysis showed that this species are: Raoultella terrigena, Pseudomonas putida, Stenotrophomonas maltophilia and Paenibacillus xylanexedens. It was shown that only T. kok-saghyz plants cured of phytopathogenic bacteria could withstand high ambient temperatures, up to 35°C. The plants did not enter the defoliation phase, which is typical for them at high summer temperatures in their natural habitat. They actively continued to grow and develop. At the end of the growing season, the average biomass of the cured plants was approximately 3 times higher, and the NR content was 3,7 times higher than that of infected plants. Based on this fact, we proposed a hypothesis that the reason for the limitation of the growing range of T. kok-saghyz plants, exclusively in the intermountain valleys of the Eastern Tien Shan, may have a microbiological (bacterial) nature. Endophytic bacteria that promote or do not hinder the growth of plant species in given conditions most likely limit the vital activity (survival) of plants in other conditions, at first glance, more favorable in terms of soil and climatic parameters. We have shown that plants cured of phytopathogens, including bacterial ones, demonstrate good growth rates, even in uncharacteristic climatic conditions. The practical significance of this work is that the cure of plants from phytopathogens significantly changes the metabolism of plants, which allows them to be successfully grown in various ecological zones, including artificial climatic conditions like phytotrons using the method of substrate-free aeroponics. For commercialization of aeroponic cultivation technology for T. kok-saghyz, a major role will be played by the comprehensive processing of plant biomass, for the simultaneous extraction of all valuable products present in this plant.