# Advances in Genome Sequencing and Natural Rubber Biosynthesis in Rubber-Producing Plants
## 3 Natural Rubber Biosynthesis
### Rubber-Synthesizing organelle: -Rubber Particle (RP)  
Natural rubber biosynthesis primarily takes place on the surface of rubber particles. Therefore, analyzing the structure, compositions, and surface proteins of rubber particles constitute the primary focus of NR biosynthesis research [6, 38, 39, 40]. RPs are located within the cytoplasm of laticifer cells, which can be found in the phloem of the rubber tree and TKS, and the parenchyma cells of the guayule [29]. Fresh latex can be divided into three distinct layers through ultracentrifugation: the upper layer consisting of RPs, the middle layer containing cytoplasmic serum, and the bottom layer comprising lutoids [41]. Based on the composition of rubber particles, the ontogenesis of RPs in laticifer cells may originate from either the endoplasmic reticulum or the Golgi apparatus. The hydrophobic core of an RP is primarily composed of cis-polysoprene, which constitutes more than 90% of the RP's weight [6]. Surrounding this core is an outer layer comprised of a lipid monolayer, along with proteins and other components. The protein compositions of rubber particles can be divided into three types: (1) proteins that embed transmembrane domains in the lipid monolayer; (2) proteins without transmembrane domains that are covalently bound to lipid molecules; and (3) proteins that form complexes with the first two types. Typically, these proteins form complexes or act synergistically during natural rubber biosynthesis [41, 42]. RPs exhibit a globular structure, with their diameter varying among different species:0.02-2.0 \(\upmu\)m in _H. brasiliensis_, 0.2-10 \(\upmu\)m in _T. kok-saghyz_, 0.5-2.0 \(\upmu\)m in _P. argentatum_, and 1.6-6.5 \(\upmu\)m in Ficus species [1, 43, 44, 45, 46, 47]. According to the diameter distribution, the RPs from _H. brasiliensis_ can be classified into small rubber particles (SRPs) and large rubber particles (LRPs), with 94% being SRPs and 6% being LRPs in fresh latex [22, 43, 45, 48]. Notably, it is precisely this 6% of LRPs by number that comprise 93% of the rubber by volume in latex. Interestingly, in vitro assays have shown that SRPs exhibit significantly higher rubber biosynthesis activity compared to LRPs [46, 49, 50].  
Figure 1: Natural rubber biosynthesis. ACAT: acetyl-CoA acetyltransferase; HMGS: 3-hydroxy-3-methyl-glutaryl-coenzyme synthase; HMGR: HMG-CoA reductase; MVK: mevalonate kinase; PMK: phosphomevalonate kinase; MVD: mevalonate diphosphate decarboxylase; DXS: 1-deoxy-D-xylulose 5-phosphate synthase; DXP: 3-phosphate into 1-deoxy-D-xylulose 5-phosphate; DXR: 1-deoxy-D-xylulose 5-phosphate reductorisomerase; CMS: 2-C-methyl-D-erythnitrol 4-phosphate cytidyltransferase; CMK: 4-diphosphopcidyl-2-C-methyl-D-erythnitrol kinase; MCS: 2-C-methyl-D-erythnitrol 2,4-cyclodiphosphate synthase; HDS: 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate synthase; HDR: 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate reductase; IPPI: isopentenyl diphosphate isomerase; GPS: geranyl pyrophosphate; FPS: farnesyl diphosphate synthase; GGPS: geranylgeranyl pyrophosphate synthase; IPP: isopentenyl pyrophosphate; CPT: cis-phenyltransferase.