Parthenium argentatum A. Gray, Taraxacum kok-saghyz L.E. Rodin, and Scorzonera tau-saghyz Lipsch. et Bosse as alternative sources of natural rubber: do we really need them?

Natural rubber (NR) is a strategic raw material essential to the manufacture of 50,000 different rubber and latex products. In most cases, e.g., in automobile and aviation industries, it cannot be replaced by synthetic rubber alternatives. There are several important reasons why should we care about alternative sources of NR. Among them are a strong allergic reaction to products made from Hevea latex and a danger of spread of South American late blight (South American Leaf Blight, SALB) in Southeast Asia. The latter would cause irreparable damage to the production of natural polymer. At present, the only commercially significant source of NC is Hevea brasiliensis (Willd. ex A. Juss.) Müll. Arg. - an evergreen tree growing in tropical regions. It is not surprising that the research aimed at finding and creating alternative sources of NR by genetic engineering is intensively developing in Europe and North America. On this issue, there are numerous reviews of leading researchers in this field, in particular, the Dr. K. Cornish’s team. Thus, back in 2000, one of the first detailed reviews devoted to the problem of alternative NC sources was published (H. Mooibroek et al., 2000). A year later, NR biosynthesis in evolutionarily distant rubber plants was described in detail (K. Cornish, 2001). This problem has been further developed in later works of this researcher (K. Cornish, 2017). Detailed reviews of alternative rubber producers have also been published by other leading groups in the field (J. van Beilen et al., 2007; S.C. Gronover et al., 2011; D.T. Ray et al., 2005). We have recently published two review articles describing in detail the biochemical and molecular genetic aspects of NR biosynthesis (A.Y. Amerik et al., 2018; A.Y. Amerik et al., 2021). In this review, we pay special attention to the historical aspects of this problem, which, in our opinion, have not received sufficient consideration in the literature, describe the state of the industry at the present time, and characterize three rubber plants that are promising producers of NR. As alternative sources of NR, two plants are receiving increased attention. These are Mexican guayule shrub ( Parthenium argentatum A. Gray) and kok-saghyz or Russian dandelion ( Taraxacum kok-saghyz L.E. Rodin). We certainly should also mention the undeservedly forgotten, but very promising alternative producer of NR tau-saghyz ( Scorzonera tau-saghyz Lipsch. et Bosse), which, in our opinion, is currently not given enough attention. T. kok-saghys is the most promising alternative rubber plant. For biochemical and molecular genetic studies of the plant, modern molecular biological approaches were used, such as improved transformation protocols, RNA interference (silencing) approaches, and analysis of EST libraries to identify new genes. As a result, the key proteins responsible for NR biosynthesis, cis-prenyltransferase 1-3 (CPT1-3) (T. Schmidt et al., 2010) and CPT activator (RTA) (J. Epping et al., 2015), were identified. It should be noted that the intracellular concentration of CPT regulates NR biosynthesis in cells of Taraxacum brevicorniculatum , the closest relative of T. kok-saghyz . Transgenic lines in which expression of all three CPT genes was suppressed by RNA interference (RNAi) demonstrated almost complete suppression of NR biosynthesis (J. Post et al., 2012). However, more research is needed before T. kok-saghyz NR becomes a commercial alternative to H. brasiliensis NR. Research on P. argentatum is also rapidly developing. In particular, the work carried out in the laboratory of D.K. Ro should be noted. Researchers have identified and characterized a protein complex that includes CPTs and plays a key role in NR biosynthesis (A.M. Lakusta et al., 2019). Unfortunately, research on tau-saghyz ( S. tau-saghyz ) is not so successful. This species was critically undermined during intensive harvesting in the 1940s. Nevertheless, work on the restoration of this unique species, the concentration of NR in roots of which under favorable conditions reaches 40 % (dry weight), is currently being carried out at the Kazakhstan National University (S.K. Turasheva et al., 2016). Thus, there is a need for alternative rubber crops and technologies for processing raw materials into final products. Thermostable derivatives, e.g., epoxidized rubber from alternative crops can enter the market to significantly reduce the carbon footprint.