# Molecular Genetic Research and Genetic Engineering of _Taraxaxacum kok-saghyz_ [PERSON]
## 3 Rubber Biosynthesis Genes  
The most important aspect that TKS breeders should consider is obtaining genotypes with more intensive and stable rubber accumulation, which will help make this plant a more profitable rubber-bearing crop. Determining both classical and biotechnological breeding strategies requires a better understanding of rubber biosynthesis and its control in TKS, as well as the identification of rate-limiting enzymes and competing metabolic pathways for rubber biosynthesis [46]. An early study showed that the small rubber particle protein TkSRPP3 plays an important role in rubber biosynthesis [47]. Transgenic TKS plants with an increased level of expression of the _TKSRPP3_ gene were characterized by an increased accumulation of rubber. At the same time, the TKS lines with an induced RNA interference of the _TKSRPP3_ gene were distinguished not only by a significantly reduced content of rubber but also by a decrease in its molecular weight. Natural rubber is composed of isopentenyl monomers derived from isopentenyl pyrophosphate (IPP) synthesized primarily by the cytosolic mevalonic acid (MVA) pathway as well as the plastid MEP pathway (Figure 2) [6]. New rubber molecules are initiated by the binding of an allyl pyrophosphate, usually cytoplasmic farnesyl pyrophosphate (FPP), and then the rubber is polymerized from a hydrophilic monomeric substrate, IPP, which is produced by both pathways (MVA and MEP) [9]. More than five thousand IPPs make up the chains of high-molecular-weight polymers of NR (_cis_-1,4-polisoprene, >106 kDa) [48]. To date, a total of 90 functional genes have been identified that control the rubber synthesis pathway in TKS, including 32 genes in the MVA pathway, 19 genes in the MEP pathway, 17 genes in rubber synthesis initiation, and 22 genes in rubber chain extension. In addition, many transcription factors have been identified that regulate rubber biosynthesis; however, so far they have not been described in TKS but have been described in other rubber-bearing plants [44]. Isopentenyl pyrophosphate is primarily produced by MVA and is the main building block of not only rubber, but many other secondary metabolites such as monoter-penoids, sesquiterpenoids, triterpenoids, and steroids, which play an important role in plant growth, development, and defense against phytopathogens and herbivores [31]. The MVA pathway generates IPP through six major catalytic steps, using the following enzymes: acetyl-coenzyme A acetyltransferase (ACAT), 3-hydroxy-3-methyl-glutaryl coenzyme A synthase (HMGS), hydroxymethylglutaryl coenzyme A reductase (HMGR), MVK, phosphomevalonate kinase (PMK), and mexolonate diphosphate decarboxylase (MVD). Among these enzymes, HMGR is the key rate-limiting enzyme of the MVA pathway (Figure 2) [49; 50; 51]. Rubber forms on the surface of rubber particles dispersed in the latex of TKS roots [37]. Rubber _cis_-prenyltransferases (CPTs), which catalyze rubber chain extensions, are the main component of the rubber transferase (RTase) complex on the surface of rubber particles [52]. IPP, and its isomer dimethylallyl pyrophosphate (DMAPP), can be catalyzed by short chain prenyltransferases to form prenyl diphosphate intermediates, including geranyl pyrophosphate (GPP), FPP, and geranylgeranyl pyrophosphate (GGPP), which can initiate in vitro rubber biosynthesis. In the biosynthesis of high-molecular-weight rubber, in addition to CPTs, _cis_-prenyltransferase-like proteins (CPTLs), rubber elongation factors (REFs), and SRPPs are also involved (Figure 2) [53]. In TB, a homologue of the human _NgBR_ gene, the _TbRTA_ gene, was also found, the product of which is an important component of the RTase complex, since its knockdown strongly disrupts rubber biosynthesis [54]. More detailed information on all known genes and proteins involved in rubber biosynthesis can be obtained from a review article by [PERSON] et al. [2]. The assembly of the first TKS genome revealed the presence of eight _TKCPT_ genes and two _TKCPTL_ genes [34].