Abstract
BackgroundThe processabilities and mechanical properties of natural rubber depend greatly on its molecular weight (MW) and molecular weight distribution (MWD). However, the mechanisms underlying the regulation of molecular weight during rubber biosynthesis remain unclear.ResultsIn the present study, we determined the MW and particle size of latex from 1-year-old virgin trees and 30-year-old regularly tapped trees of the Hevea clones Reyan7-33–97 and RRIM600. The results showed that both the MW and the particle size of latex varied between these two clones and increased with tree age. Latex from RRIM600 trees had a smaller average particle size than that from Reyan7-33–97 trees of the same age. In 1-year-old trees, the Reyan7-33–97 latex displayed a slightly higher MW than that of RRIM600, whereas in 30-year-old trees, the RRIM600 latex had a significantly higher MW than the Reyan7-33–97 latex. Comparative analysis of the transcriptome profiles indicated that the average rubber particle size is negatively correlated with the expression levels of rubber particle associated proteins, and that the high-MW traits of latex are closely correlated with the enhanced expression of isopentenyl pyrophosphate (IPP) monomer-generating pathway genes and downstream allylic diphosphate (APP) initiator-consuming non-rubber pathways. By bioinformatics analysis, we further identified a group of transcription factors that potentially regulate the biosynthesis of IPP.ConclusionsAltogether, our results revealed the potential regulatory mechanisms involving gene expression variations in IPP-generating pathways and the non-rubber isoprenoid pathways, which affect the ratios and contents of IPP and APP initiators, resulting in significant rubber MW variations among same-aged trees of the Hevea clones Reyan7-33–97 and RRIM600. Our findings provide a better understanding of rubber biosynthesis and lay the foundation for genetic improvement of rubber quality in H. brasiliensis.
Highlights
The processabilities and mechanical properties of natural rubber depend greatly on its molecular weight (MW) and molecular weight distribution (MWD)
As the age of Hevea trees increased from 1 to 30 years, the MW of Reyan7-33–97 latex increased from 4.77 × 105 to 6.51 × 105, and the MWD increased from 5.53 to 7.19, while the latex from RRIM600 trees showed that the MW and Number-average molecular weight (MN) dramatically increase from 4.26 × 105 to 8.28 × 105 and 7.18 × 104 to 17.73 × 104, respectively, with a slight decrease in MWD
Two rubber biosynthesis stimulator protein (RBSP) genes and one rubber biosynthesis inhibitor protein (RBIP) gene are indicated. b The top 3 enriched motifs in the promoter regions of upregulated genes in pyruvate metabolism, MVA and MEP pathways in RRIM600y30 latex and the significantly matched transcription factors (TFs) families. c Expression profiles of the differentially expressed TFs belonging to the BASIC PENTACYSTEINE (BPC), WRKY, NAC and AP2 families are shown in the form of a heatmap
Summary
The processabilities and mechanical properties of natural rubber depend greatly on its molecular weight (MW) and molecular weight distribution (MWD). IPP is initially converted by IPP-isomerase to its isomer, dimethylallyl diphosphate (DMAPP), which is subsequently condensed with IPP in the trans-configuration to form geranyl pyrophosphate (GPP), farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP) [4] With these trans-short-chain prenyl pyrophosphates serving as allylic primer substrates, the rubber molecules are biosynthesized by sequential condensation of IPP in cis-configuration via a carbocationic reaction [3, 5]. The raw polymer can be harvested from the Hevea tree by making an incision in the bark and collecting the latex freely flowing out of the vessels [5] In this manner, a large amount of NR is produced in an efficient, sustainable and environmentally friendly manner for large-scale commercial use
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