Natural rubber (cis-1,4-polyisoprene, NR) is an important raw material utilized widely in the manufacturing of medical, agricultural, and industrial products. Rubber tree (Hevea brasiliensis) and several alternative rubber-producing plants (Taraxacum kok-saghyz, Lactuca sativa, and Parthenium argentatum) have the capability to produce high-quality NR. With the progress of genome sequencing, similar rubber biosynthesis pathways have been discovered among different rubber-producing plant species. NR is synthesized and stored in rubber particles, which are specialized organelles comprising a hydrophobic NR core surrounded by a lipid monolayer and membrane-bound proteins. The rubber transferase complex is considered to be the pivotal enzyme involved in catalyzing NR biosynthesis. However, the exact compositions of the RT complex in rubber-producing plants remain elusive and poorly understood. Here, we review the progress of genome sequencing, natural rubber biosynthesis, and the components of the RT complex in rubber-producing plants. We emphasize that identifying the detailed components of the RT complex holds great significance for exploring the mechanism of NR biosynthesis and accelerating molecular breeding in rubber-producing plants.

Summary: Rubber particles (RPs) are specialized organelles for the biosynthesis and storage of natural rubber in rubber‐producing plants. However, the mechanisms underlying the biogenesis and development of RPs remain unclear. In this study, two latex‐specific cis‐prenyltransferases (CPTs), TkCPT1 and TkCPT2, were identified in Taraxacum kok‐saghyz, with almost identical orthologues retained across other Taraxacum species. For the first time, Tkcpt1 single and Tkcpt1/2 double mutants were successfully generated using the CRISPR/Cas9 system. Rubber biosynthesis was significantly depressed in Tkcpt1 mutants and completely blocked in Tkcpt1/2 mutants. The absence of RPs in the Tkcpt1/2 was confirmed using oil red O and Nile red staining, high‐speed centrifugal stratification, cryo‐SEM and TEM on fresh latex or laticifer cells. Transcriptomic and proteomic analyses revealed that, in the latex of Tkcpt1/2, rubber biosynthesis was blocked at the protein level, while metabolomic profiling indicated an enrichment of lipids and terpenoids. Furthermore, knockout of TkCPTL1, a latex‐specific CPT‐like gene that encodes a rubber transferase activator, resulted in outright disruption of rubber biosynthesis and RP ontogeny, a phenotype similar to that of Tkcpt1/2 mutants. These findings indicate that rubber biosynthesis is a driving force for the biogenesis and development of RPs, providing new insights into rubber production mechanisms.

Transgenic technology is a crucial tool for gene functional analysis and targeted genetic modification in the para rubber tree (Hevea brasiliensis). However, low efficiency of plant regeneration via somatic embryogenesis remains a bottleneck of successful genetic transformation in H. brasiliensis. Enhancing expression of GROWTH-REGULATING FACTOR 4 (GRF4)-GRF-INTERACTING FACTOR 1 (GIF1) has been reported to significantly improve shoot and embryo regeneration in multiple crops. Here, we identified endogenous HbGRF4 and HbGIF1 from the rubber clone Reyan7-33-97, the expressions of which dramatically increased along with somatic embryo (SE) production. Intriguingly, overexpression of HbGRF4 or HbGRF4-HbGIF1 markedly enhanced the efficiency of embryogenesis in two H. brasiliensis callus lines with contrasting rates of SE production. Transcriptional profiling revealed that the genes involved in jasmonic acid response were up-regulated, whereas those in ethylene biosynthesis and response as well as the S-adenosylmethionine-dependent methyltransferase activity were down-regulated in HbGRF4- and HbGRF4-HbGIF1-overexpressing H. brasiliensis embryos. These findings open up a new avenue for improving SE production in rubber tree, and help to unravel the underlying mechanisms of HbGRF4-enhanced somatic embryogenesis.

Rubber tree (Hevea brasiliensis) provides the sole commercial source of natural rubber in the world. Over the past three decades, a number of latex physiological parameters has been applied to construct tapping systems, enhance natural rubber production, monitor tapping panel dryness, and evaluate rubber clone performance. Inorganic phosphorus and sucrose content (Pi and Suc) are two well-interpreted parameters to describe the latex metabolic status of rubber trees, whereas the candidate genes responsible for regulating Pi and Suc have never been reported. Here, a natural population consisting of 218 rubber tree individuals was exploited to identify candidate genes associated with Pi and Suc. The phenotype of this population was measured over a period of two years. Genome-wide association study (GWAS) was conducted using mixed linear models and 7,835,736 single-nucleotide polymorphisms (SNP). A total of 26 and 11 SNPs were significantly associated with Pi and Suc, respectively. By combining the GWAS results with genomic annotation data, 57 and 29 candidate genes within 50 kb of the significant SNPs were identified for Pi and Suc, respectively. The expression patterns of these candidates were further investigated, and five for Pi and three for Suc were identified to be specifically expressed in the latex. A DOF transcription factor and a histone H2A gene have been interpreted, while the annotations for the other candidate genes remain limited. These findings contribute to understanding the genetic foundation of Pi and Suc in the latex as well as the breeding of high-yielding rubber clones by employing latex physiological parameters.

MicroRNAs (miRNAs) are widely involved in various aspects of plant growth and development. However, how miRNAs and their targets regulate natural rubber metabolism remains unclear in the rubber-producing dandelions, which are being developed as alternative commercial sources of natural rubber. Here, we combined small RNA sequencing, degradome sequencing, target gene prediction, and mRNA sequencing to identify miRNAs and their targets in two dandelion species, the high rubber-yielding Taraxacum kok-saghyz (Tk) and the low rubber-yielding T. spadiceum (Ts). A total of 142 miRNAs, including 108 known and 34 novel ones, were discovered, with 53 identified as differentially expressed (DE) between the latex of Tk and Ts. Degradome sequencing identified 145 targets corresponding to 74 miRNAs. TAPIR and psRNATarget, respectively, predicted 165 and 164 non-redundant targets for the 53 aforementioned DE miRNAs. Gene ontology (GO) enrichment analysis indicated the DE miRNAs and their targets might affect natural rubber production via regulating macromolecular biosynthesis and metabolism in latex. Four critical types of regulatory modules, including miR172-AP2/ERF, miR164-NAC, miR160-ARF, and miRN19-protein kinase, were identified and their interaction networks were constructed, indicating a potential involvement in natural rubber production. The findings and the large miRNA dataset presented here are beneficial to further deciphering the roles of miRNAs in the biosynthesis of natural rubber and medicinal metabolites in dandelion.