# Proteomic Landscape of the Mature Roots in a Rubber-Producing Grass _Taraxacum Kok-saghyz_
## 1 Introduction  
Natural rubber (NR, cis-1,4-polisoprene) is a biopolymer with high economic value and it is widely used as a strategic raw material in more than 40,000 products [1; 2]. More than 2500 plant species can biosynthesize NR [3; 4]. However, high-quality natural rubber in viable quantities is only observedin a few plant species, such as the _Para_ rubber tree _Hevea brasiliensis_[5; 6], Russian dandelion _Taraxacum kok-saghyz_ Rodin (TKS, also called the rubber grass) or its close species _Taraxacum brevicorniculatum_[7], _[PERSON] ulmoides Oliver_ and a guayule shrub _Paruthenium argentatum_[8; 9]. Currently, the _Para_ rubber tree is nearly the only commercial plant to cultivate the exclusive source of NR [5; 6]. However, rubber production of the _Para_ rubber tree has reached the limit due to the strict climatic requirements for its planting areas [10], little genetic variability, labor cost, and threats of fatal fungal plant diseases [1]. Therefore, it is critical to find an alternative source and a model plant for NR production.  
TKS has drawn special attention since the 1940s. It belongs to the Compositae family and originates from the Tekes River Basin near Tianshan Mountain border between Kazakhstan and China [11]. TKS root contains NR ranging from 3% to 28% of total dry weight [1] and its rubber property and molecular characteristics are similar with NR in the _Para_ rubber tree [12; 13]. TKS grows widely in temperate and cold areas, and its root also contains about 28% of inulin, which is an important material for bioethanol and the food industry [14; 15]. With its advantages of high rubber content and quality, wide planting area, a relatively short life cycle, ease of transformation and harvesting method [14; 15], TKS is becoming a promising crop for natural rubber production. In addition, TKS is a perennial herb with a relatively simple genome (1.29 Gb), containing 46,731 protein-coding genes [16]. It is easily manipulated for genetic transformation and can be used as an ideal model plant for studying the rubber biosynthesis mechanism [17].  
NR is biosynthesized by two pathways: the mevalonate acid (MVA) pathway [18; 19] and the methylerythnitol phosphate (MEP) pathway [20]. Recently, to determine the protein-based regulation mechanism of NRB, several proteomics have been conducted on the total latex [21; 22; 23; 24; 25] and rubber particles [26; 27; 28; 29; 30; 31] of the rubber tree _H. brasiliensis_. But no proteomic analysis has been performed on TKS, and only one study on determining proteins from the rubber phase and pellet phase of the latex from _T. brevicorniculatum_ (TBR) was reported and 278 unique proteins from the one- and two-dimensional gel electrophoresis (1-DE and 2-DE) gels were identified [32]. In order to gain further insight into the protein-based regulation mechanism of rubber biosynthesis, we performed a comprehensive proteomics analysis of the mature roots of TKS at six months (6M) by 2-DE and mass spectrometry (MS), and then identified thousands of proteins by a large-scale shotgun method. These results may deepen our understanding of the root proteome and provide valuable gene candidates for genetic improvement of TKS breeding as a commercial alternative crop.