Genomic Selection in Rubber Tree Breeding: A Comparison of Models and Methods for Managing G×E Interactions

Several genomic prediction models combining genotype × environment (G×E) interactions have recently been developed and used for genomic selection (GS) in plant breeding programs. G×E interactions reduce selection accuracy and limit genetic gains in plant breeding. Two data sets were used to compare the prediction abilities of multienvironment G×E genomic models and two kernel methods. Specifically, a linear kernel, or GB (genomic best linear unbiased predictor [GBLUP]), and a nonlinear kernel, or Gaussian kernel (GK), were used to compare the prediction accuracies (PAs) of four genomic prediction models: 1) a single-environment, main genotypic effect model (SM); 2) a multienvironment, main genotypic effect model (MM); 3) a multienvironment, single-variance G×E deviation model (MDs); and 4) a multienvironment, environment-specific variance G×E deviation model (MDe). We evaluated the utility of genomic selection (GS) for 435 individual rubber trees at two sites and genotyped the individuals via genotyping-by-sequencing (GBS) of single-nucleotide polymorphisms (SNPs). Prediction models were used to estimate stem circumference (SC) during the first 4 years of tree development in conjunction with a broad-sense heritability (H ²) of 0.60. Applying the model (SM, MM, MDs, and MDe) and kernel method (GB and GK) combinations to the rubber tree data revealed that the multienvironment models were superior to the single-environment genomic models, regardless of the kernel (GB or GK) used, suggesting that introducing interactions between markers and environmental conditions increases the proportion of variance explained by the model and, more importantly, the PA. Compared with the classic breeding method (CBM), methods in which GS is incorporated resulted in a 5-fold increase in response to selection for SC with multienvironment GS (MM, MDe, or MDs). Furthermore, GS resulted in a more balanced selection response for SC and contributed to a reduction in selection time when used in conjunction with traditional genetic breeding programs. Given the rapid advances in genotyping methods and their declining costs and given the overall costs of large-scale progeny testing and shortened breeding cycles, we expect GS to be implemented in rubber tree breeding programs.

Biosynthesis of Natural Rubber: Current State and Perspectives

Natural rubber is a kind of indispensable biopolymers with great use and strategic importance in human society. However, its production relies almost exclusively on rubber-producing plants Hevea brasiliensis, which have high requirements for growth conditions, and the mechanism of natural rubber biosynthesis remains largely unknown. In the past two decades, details of the rubber chain polymerization and proteins involved in natural rubber biosynthesis have been investigated intensively. Meanwhile, omics and other advanced biotechnologies bring new insight into rubber production and development of new rubber-producing plants. This review summarizes the achievements of the past two decades in understanding the biosynthesis of natural rubber, especially the massive information obtained from the omics analyses. Possibilities of natural rubber biosynthesis in vitro or in genetically engineered microorganisms are also discussed.

The relationship between latex metabolism gene expression with rubber yield and related traits in Hevea brasiliensis

Background

Expression patterns of many laticifer-specific gens are closely correlative with rubber yield of Hevea brasiliensis (para rubber tree). To unveil the mechanisms underlying the rubber yield, transcript levels of nine major latex metabolism-related genes, i.e., HMG-CoA synthase (HMGS), HMG-CoA reductase (HMGR), diphosphomevalonate decarboxylase (PMD), farnesyl diphosphate synthase (FPS), cis-prenyltransferase (CPT), rubber elongation factor (REF), small rubber particle protein (SRPP), dihydroxyacid dehydratase (DHAD) and actin depolymerizing factor (ADF), were dertermined, and the relationship between rubber yield with their expression levels was analysed.

Results

Except HbHMGR1, HbPMD and HbDHAD, most of these genes were predominantly expressed in latex, and bark tapping markedly elevated the transcript abundance of the analyzed genes, with the 7th tapping producing the greatest expression levels. Both ethephon (ETH) and methyl jasmonate (MeJA) stimulation greatly induced the expression levels of the examined genes, at least at one time point, except HbDHAD, which was unresponsive to MeJA. The genes’ expression levels, as well as the rubber yields and two yield characteristics differed significantly among the different genotypes examined. Additionally, the latex and dry rubber yields increased gradually but the dry rubber content did not. Rubber yields and/or yield characteristics were significantly positively correlated with HbCPT, HbFPS, HbHMGS, HbHMGR1 and HbDHAD expression levels, negatively correlated with that of HbREF, but not significantly correlated with HbPMD, HbSRPP and HbADF expression levels. In addition, during rubber production, significantly positive correlations existed between the expression level of HbPMD and the levels of HbREF and HbHMGR1, between HbSRPP and the levels of HbHMGS and HbHMGR1, and between HbADF and HbFPS.

Conclusions

The up-regulation of these genes might be related to the latex production of rubber trees under the stress of bark tapping and latex metabolism. The various correlations among the genes implied that there are differences in their synergic interactions. Thus, these nine genes might be related to rubber yield and yield-related traits in H. brasiliensis, and this work increases our understanding of their complex functions and how they are expressed in both high-and medium-yield rubber tree varieties and low-yield wild rubber tree germplasm.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5242-4) contains supplementary material, which is available to authorized users.

Coryneumheveanum sp. nov. (Coryneaceae, Diaporthales) on twigs of Para rubber in Thailand

During studies of microfungi on para rubber in Thailand, we collected a new Coryneum species on twigs which we introduce herein as C.heveanum with support from phylogenetic analyses of LSU, ITS and TEF1 sequence data and morphological characters. Coryneumheveanum is distinct from other known taxa by its conidial measurements, number of pseudosepta and lack of a hyaline tip to the apical cell.

Somatic Embryogenesis as Key Technology for Shaping the Rubber Tree of the Future

Worldwide, Hevea producers face the need to replant large surfaces in the coming years. The rubber yield per ha, produced by trees grafted on heterogeneous illegitimate seedling rootstocks, has reached its maximum. For long-standing Hevea clones, as for a lot of other tree species, one of the consequences of physiological aging is reduced in vitro growth and the lack of a proper geotropic (tap) root system. Somatic embryogenesis on young inner seed integument or stamen filaments provides a mean to regain ontogenetic juvenility. The process is limited by irregular germination of the somatic embryos. Nevertheless, with the obtained in vitro plants, juvenile lines have been established of the most important profitable rubber tree clones. Currently they are micropropagated on a commercial scale. Moreover, the produced plants can serve as mother plants for propagation by means of macro-cutting. Somatic embryogenesis enables the production of transgenic Hevea brasiliensis as well. Genes conferring plant disease resistance, abiotic stress tolerance and production of foreign proteins in the lactiferous vessels will further shape the rubber tree of the future.

Virome analyses of Hevea brasiliensis using small RNA deep sequencing and PCR techniques reveal the presence of a potential new virus

Background

Hevea brasiliensis is an important commercial crop due to the high quality of the latex it produces; however, little is known about viral infections in this plant. The only virus described to infect H. brasiliensis until now is a Carlavirus, which was described more than 30 years ago. Virus-derived small interfering RNA (vsiRNAs) are the product of the plant’s antiviral defense triggered by dsRNA viral intermediates generated, during the replication cycle. These vsiRNAs are complementar to viral genomes and have been widely used to identify and characterize viruses in plants.

Methods

In the present study, we investigated the virome of leaf and sapwood samples from native H. brasiliensis trees collected in two geographic areas in the Brazilian Amazon. Small RNA (sRNA) deep sequencing and bioinformatic tools were used to assembly, identify and characterize viral contigs. Subsequently, PCR amplification techniques were performed to experimentally verify the presence of the viral sequences. Finally, the phylogenetic relationship of the putative new virus with related viral genomes was analyzed.

Results

Our strategy allowed the identification of 32 contigs with high similarity to viral reference genomes, from which 23 exhibited homology to viruses of the Tymoviridae family. The reads showed a predominant size distribution at 21 nt derived from both strands, which was consistent with the vsiRNAs profile. The presence and genome position of the viral contigs were experimentally confirmed using droplet digital PCR amplifications. A 1913 aa long fragment was obtained and used to infer the phylogenetic relationship of the putative new virus, which indicated that it is taxonomically related to the Grapevine fleck virus, genus Maculavirus. The putative new virus was named Hevea brasiliensis virus (HBrV) in reference to its host.

Conclusion

The methodological strategy applied here proved to be efficient in detecting and confirming the presence of new viral sequences on a ‘very difficult to manage’ sample. This is the second time that viral sequences, that could be ascribed as a putative novel virus, associated to the rubber tree has been identified.

Electronic supplementary material

The online version of this article (10.1186/s12985-018-1095-3) contains supplementary material, which is available to authorized users.

QTL detection for growth and latex production in a full-sib rubber tree population cultivated under suboptimal climate conditions

BACKGROUND

Rubber tree is cultivated in mainly Southeast Asia and is by far the most significant source of natural rubber production worldwide. However, the genetic architecture underlying the primary agronomic traits of this crop has not been widely characterized. This study aimed to identify quantitative trait loci (QTLs) associated with growth and latex production using a biparental population established in suboptimal growth conditions in Brazil.

RESULTS

A full-sib population composed of 251 individuals was developed from crossing two high-producing Asiatic rubber tree cultivars, PR 255 and PB 217. This mapping population was genotyped with microsatellite markers from enriched genomic libraries or transcriptome datasets and single-nucleotide polymorphism (SNP) markers, leading to construction of a saturated multipoint integrated genetic map containing 354 microsatellite and 151 SNP markers. Height and circumference measurements repeated over a six-year period and registration of cumulative latex production during six consecutive months on the same individuals allowed in-depth characterization of the genetic values of several growth traits and precocious latex production. Growth traits, circumference and height, were overall positively correlated, whereas latex production was not correlated or even negatively correlated with growth traits. A total of 86 distinct QTLs were identified, most of which were detected for only one trait. Among these QTLs, 15 were linked to more than one phenotypic trait (up to 4 traits simultaneously). Latex production and circumference increments during the last wintering period were associated with the highest numbers of identified QTLs (eleven and nine, respectively), jointly explaining the most significantly observed phenotypic variances (44.1% and 44.4%, respectively). The most important QTL for latex production, located on linkage group 16, had an additive effect of the male parent PB 217 and corresponded to a QTL at the same position detected in a previous study carried out in Thailand for the biparental population RRIM 600 x PB 217.

CONCLUSIONS

Our results identified a set of significant QTLs for rubber tree, showing that the performance of modern Asiatic cultivars can still be improved and paving the way for further marker-assisted selection, which could accelerate breeding programs.

Heterografting induced DNA methylation polymorphisms in Hevea brasiliensis

Heterografting induced intraclonal epigenetic variations were detected among rubber plants. Interaction between genetically divergent root stock and scion tissues might have triggered these epigenetic changes which may eventually lead to intraclonal variability in rubber. DNA methylation in response to stress may be associated with the alteration in gene transcription leading to morphological changes in plants. Rubber tree is commercially propagated by bud grafting where the scion of a high yielding variety is grafted on to a genetically divergent root stock. Still, significant levels of intraclonal variations exist among them. Epigenetic changes associated with heterografting may be partly responsible for this conundrum. In the present study, an attempt was made to identify the impact of divergent root stock on the epigenome of scion in grafted rubber plants. Heterografts were developed by grafting eye buds from a single polyembryony derived seedling on to genetically divergent root stocks of unknown parentage. The plants were uniformly maintained and their DNA was subjected to MSAP analysis. Polymorphic DNA methylation bands corresponding to CG as well as the plant-specific CHG types of methylation were observed. Cloning of selected polymorphic regions and bisulfite sequencing confirmed the presence of methylation in the promoter and coding region of important genes including an LRR receptor kinase gene. Since divergent root stock is the major factor differentiating the grafted plants, the changes in DNA methylation patterns might have been triggered by the interaction between the two genetically different tissues of stock and scion. The study assumes importance in Hevea, because accumulation and maintenance of epigenetic changes in functional genes and promoters during subsequent cycles of vegetative propagation may contribute towards intraclonal variability eventually leading to altered phenotypes.

Unrestrictive identification of post-translational modifications in Hevea brasiliensis latex

The natural rubber latex extracted from the bark of Hevea brasiliensis plays various important roles in modern society. Post-translational modifications (PTMs) of the latex proteins are important for the stability and functionality of the proteins. In this study, latex proteins were acquired from the C-serum, lutoids, and rubber particle layers of latex without using prior enrichment steps; they were fragmented using collision-induced dissociation (CID), higher-energy collisional dissociation (HCD), and electron-transfer dissociation (ETD) activation methods. PEAKS 7 were used to search for unspecified PTMs, followed by analysis through PTM prediction tools to crosscheck both results. There were 73 peptides in 47 proteins from H. brasiliensis protein sequences derived from UniProtKB were identified and predicted to be post-translationally modified. The peptides with PTMs identified include phosphorylation, lysine acetylation, N-terminal acetylation, hydroxylation, and ubiquitination. Most of the PTMs discovered have yet to be reported in UniProt, which would provide great assistance in the research of the functional properties of H. brasiliensis latex proteins, as well as being useful biomarkers. The data are available via the MassIVE repository with identifier MSV000082419.

Over-expression of 3-hydroxy-3- methylglutaryl-coenzyme A reductase 1 (hmgr1) gene under super-promoter for enhanced latex biosynthesis in rubber tree (Hevea brasiliensis Muell. Arg.)

Natural rubber (cis-1, 4-polyisoprene) is being produced from bark laticifer cells of Hevea brasiliensis and the popular high latex yielding Indian rubber clones are easily prone to onset of tapping panel dryness syndrome (TPD) which is considered as a physiological syndrome affecting latex production either partially or completely. This report describes an efficient protocol for development of transgenic rubber plants by over-expression of 3-hydroxy 3-methylglutaryl Co-enzyme A reductase 1 (hmgr1) gene which is considered as rate limiting factor for latex biosynthesis via Agrobacterium-mediated transformation. The pBIB plasmid vector containing hmgr1 gene cloned under the control of a super-promoter was used for genetic transformation using embryogenic callus. Putatively transgenic cell lines were obtained on selection medium and produced plantlets with 44% regeneration efficiency. Transgene integration was confirmed by PCR amplification of 1.8 kb hmgr1 and 0.6 kb hpt genes from all putatively transformed callus lines as well as transgenic plants. Southern blot analysis showed the stable integration and presence of transgene in the transgenic plants. Over expression of hmgr1 transgene was determined by Northern blot hybridization, semi-quantitative PCR and real-time PCR (qRT-PCR) analysis. Accumulation of hmgr1 mRNA transcripts was more abundant in transgenic plants than control. Increased level of photosynthetic pigments, protein contents and HMGR enzyme activity was also noticed in transgenic plants over control. Interestingly, the latex yield was significantly enhanced in all transgenic plants compared to the control. The qRT-PCR results exhibit that the hmgr1 mRNA transcript levels was 160-fold more abundance in transgenic plants over untransformed control. These results altogether suggest that there is a positive correlation between latex yield and accumulation of mRNA transcripts level as well as HMGR enzyme activity in transgenic rubber plants. It is presumed that there is a possibility for enhanced level of latex biosynthesis in transgenic plants as the level of mRNA transcripts and HMGR enzyme activity is directly correlated with latex yield in rubber tree. Further, the present results clearly suggest that the quantification of HMGR enzyme activity in young seedlings will be highly beneficial for early selection of high latex yielding plants in rubber breeding programs.

Physiological and Proteomic Analyses of Molybdenum- and Ethylene-Responsive Mechanisms in Rubber Latex

Molybdenum (Mo) is an essential micronutrient in many plants. In the rubber tree Hevea brasiliensis, Mo application can reduce the shrinkage of the tapping line, decrease tapping panel dryness, and finally increase rubber latex yield. After combined Mo with ethylene (Eth), these effects become more obvious. However, the molecular mechanism remains unclear. Here, we compared the changed patterns of physiological parameters and protein accumulation in rubber latex after treated with Mo and/or Eth. Our results demonstrated that both Eth and Mo can improve the contents of thiol, sucrose, and dry yield in rubber latex. However, lutoid bursting is significantly inhibited by Mo. Comparative proteomics identified 169 differentially expressed proteins, including 114 unique proteins, which are mainly involved in posttranslational modification, carbohydrate metabolism, and energy production. The abundances of several proteins involved in rubber particle aggregation are decreased upon Mo stimulation, while many enzymes related to natural rubber biosynthesis are increased. Comparison of the accumulation patterns of 25 proteins revealed that a large portion of proteins have different changed patterns with their gene expression levels. Activity assays of six enzymes revealed that Mo stimulation can increase latex yield by improving the activity of some Mo-responsive enzymes. These results not only deepen our understanding of the rubber latex proteome but also provide new insights into the molecular mechanism of Mo-stimulated rubber latex yield.

Microbial communities in natural rubber coagula during maturation: impacts on technological properties of dry natural rubber

AIM

To characterize microbial communities present in natural rubber (NR) coagula from Hevea brasiliensis latex during maturation and identify microbial taxa (bacteria and fungi) having an impact on dry NR properties.

METHODS AND RESULTS

Microbial community dynamics in NR coagula maturated under controlled conditions were compared and related with the evolution of dry NR properties. The pyrosequencing of 16S (119 837 effective reads) and 18S (131 879 effective reads) rRNA gene regions was performed on 21 samples covering different maturation times and two aeration conditions. Results showed a relatively high bacterial richness (Chao1 estimates of 200-1000) associated with significant bacterial dynamics. Lactic acid bacteria (LAB) were dominant in the first days of maturation. Then, in aerobic conditions, development of Actinobacteria represented by the family Microbacteriaceae was associated with alkalinization of the samples and a higher sensitivity of NR to thermo-oxidation as evaluated by its plasticity retention index (PRI). In anaerobiosis, the reduced development of bacteria, mostly LAB present, was associated with improved NR properties (higher initial plasticity P₀ and PRI).

CONCLUSIONS

The involvement of micro-organisms in the evolution of dry NR properties during the maturation of NR coagula was confirmed. The importance of the structure and dynamics of microbial communities is specifically highlighted.

SIGNIFICANCE AND IMPACT OF THE STUDY

Natural rubber is a key elastomer for the tyre industry and for a variety of other applications. The majority of raw NR is obtained by natural coagulation of H. brasiliensis latex under the activity of micro-organisms. An improved understanding of the microbial communities involved in the maturation of NR coagula may lead to an improvement in the production process of raw NR to provide a better consistency in NR quality.

Latex Allergy: Where Are We Now and How Did We Get There?

Latex allergy emerged as an epidemic of anaphylaxis, occupational asthma, and clinical dilemmas in the 1980s. A systematic recognition, investigation, discovery, epidemiology, and prevention strategy followed. International attention and collaborations of investigators, government agencies, manufacturing, and health policy resulted in near elimination of a global epidemic. This article summarizes nearly 4 decades of work in control of this epidemic and focuses attention on future problems that still require resolution.

Subcellular proteome profiles of different latex fractions revealed washed solutions from rubber particles contain crucial enzymes for natural rubber biosynthesis

Rubber particle (RP) is a specific organelle for natural rubber biosynthesis (NRB) and storage in rubber tree Hevea brasiliensis. NRB is processed by RP membrane-localized proteins, which were traditionally purified by repeated washing. However, we noticed many proteins in the discarded washing solutions (WS) from RP. Here, we compared the proteome profiles of WS, C-serum (CS) and RP by 2-DE, and identified 233 abundant proteins from WS by mass spectrometry. Many spots on 2-DE gels were identified as different protein species. We further performed shotgun analysis of CS, WS and RP and identified 1837, 1799 and 1020 unique proteins, respectively. Together with 2-DE, we finally identified 1825 proteins from WS, 246 were WS-specific. These WS-specific proteins were annotated in Gene Ontology, indicating most abundant pathways are organic substance metabolic process, protein degradation, primary metabolic process, and energy metabolism. Protein-protein interaction analysis revealed these WS-specific proteins are mainly involved in ribosomal metabolism, proteasome system, vacuolar protein sorting and endocytosis. Label free and Western blotting revealed many WS-specific proteins and protein complexes are crucial for NRB initiation. These findings not only deepen our understanding of WS proteome, but also provide new evidences on the roles of RP membrane proteins in NRB.

SIGNIFICANCE

Natural rubber is stored in rubber particle from the rubber tree. Rubber particles were traditionally purified by repeated washing, but many proteins were identified from the washing solutions (WS). We obtained the first visualization proteome profiles with 1825 proteins from WS, including 246 WS-specific ones. These WS proteins contain almost all enzymes for polyisoprene initiation and may play important roles in rubber biosynthesis.

Chilling-induced DNA Demethylation is associated with the cold tolerance of Hevea brasiliensis

BACKGROUND

Low temperature influences the development and latex production of rubber trees (Hevea brasiliensis) when extension to suboptimal high-latitude areas. The successful extension of Hevea brasiliensis cultivation to high-latitude areas has long believed to benefit from the breeding of cold-tolerant cultivars. A puzzling incongruity is the variation in cold tolerance among the cultivated clones despite their similar genetic make-up.

RESULTS

To investigate this, we first transferred cultivar Reyan 7-33-97 to short-term cold treatment, and showed that cold-related genes (such as HbICE1 and HbCBF2), cold-responsive (COR) genes, and DNA-methylation related genes (such as HbMET1) were induced by cold treatment. Furthermore, long-term cold treatment not only elevated the transcriptional activities of the HbICE1, HbCBF2, and HbMET, but also induced DNA demethylation of their promoters. Cold treatment increased the transcriptional activities of demethylation-related genes such as the HbDME, HbROS, and HbDML genes, but did not alter the promoter methylation status. Furthermore, the HbICE1 and HbMET promoters showed hypomethylation status in samples collected at the end of winter from 12 different cultivars grown in four geographical locations, but switched to hypermethylation status at the end of summer. Expression of COR was correlated with the low temperature. Given that little genetic diversity exists in the HbICE1 and HbMET promoters among different cultivars, the DNA demethylation induced by cold was highly correlated with low temperature, but not with the genetic backgrounds of cultivars.

CONCLUSION

Cold-induced epigenetic modification might play an important role in cold tolerance of H. brasiliensis.

Effects of lipid extraction on nutritive composition of winged bean (Psophocarpus tetragonolobus), rubber seed (Hevea brasiliensis), and tropical almond (Terminalia catappa)

AIM

This experiment aimed to evaluate the nutritive composition and in vitro rumen fermentability and digestibility of intact and lipid-extracted winged bean, rubber seed, and tropical almond.

MATERIALS AND METHODS

Soybean, winged bean, rubber seed, and tropical almond were subjected to lipid extraction and chemical composition determination. Lipid extraction was performed through solvent extraction by Soxhlet procedure. Non-extracted and extracted samples of these materials were evaluated for in vitro rumen fermentation and digestibility assay using rumen: Buffer mixture. Parameters measured were gas production kinetics, total volatile fatty acid (VFA) concentration, ammonia, in vitro dry matter (IVDMD) and in vitro organic matter digestibility (IVOMD). Data were analyzed by analysis of variance and Duncan's multiple range test.

RESULTS

Soybean, winged bean, rubber seed, and tropical almond contained high amounts of ether extract, i.e., above 20% DM. Crude protein contents of soybean, winged bean, rubber seed, and tropical almond increased by 17.7, 4.7, 55.2, and 126.5% after lipid extraction, respectively. In vitro gas production of intact winged bean was the highest among other materials at various time point intervals (p<0.05), followed by soybean > rubber seed > tropical almond. Extraction of lipid increased in vitro gas production, total VFA concentration, IVDMD, and IVOMD of soybean, winged bean, rubber seed, and tropical almond (p<0.05). After lipid extraction, all feed materials had similar IVDMD and IVOMD values.

CONCLUSION

Lipid extraction improved the nutritional quality of winged bean, rubber seed, and tropical almond.

Calcium-Dependent Protein Kinase Family Genes Involved in Ethylene-Induced Natural Rubber Production in Different Hevea brasiliensis Cultivars

Natural rubber latex production can be improved by ethylene stimulation in the rubber tree (Hevea brasiliensis). However, the expression levels of most functional proteins for natural rubber biosynthesis are not induced after ethylene application, indicating that post-translational modifications, especially protein phosphorylation, may play important roles in ethylene signaling in Hevea. Here, we performed a comprehensive investigation on evolution, ethylene-induced expression and protein-protein interaction of calcium-dependent protein kinases (CPKs), an important serine/threonine protein kinase family, in Hevea. Nine duplication events were determined in the 30 identified HbCPK genes. Expression profiling of HbCPKs in three rubber tree cultivars with low, medium and high ethylene sensitivity showed that HbCPK6, 17, 20, 22, 24, 28 and 30 are induced by ethylene in at least one cultivar. Evolution rate analysis suggested accelerated evolution rates in two paralogue pairs, HbCPK9/18 and HbCPK19/20. Analysis of proteomic data for rubber latex after ethylene treatment showed that seven HbCPK proteins could be detected, including six ethylene-induced ones. Protein-protein interaction analysis of the 493 different abundant proteins revealed that protein kinases, especially calcium-dependent protein kinases, possess most key nodes of the interaction network, indicating that protein kinase and protein phosphorylation play important roles in ethylene signaling in latex of Hevea. In summary, our data revealed the expression patterns of HbCPK family members and functional divergence of two HbCPK paralogue pairs, as well as the potential important roles of HbCPKs in ethylene-induced rubber production improvement in Hevea.

Genome-Wide Analysis of Corynespora cassiicola Leaf Fall Disease Putative Effectors

Corynespora cassiicola is an Ascomycetes fungus with a broad host range and diverse life styles. Mostly known as a necrotrophic plant pathogen, it has also been associated with rare cases of human infection. In the rubber tree, this fungus causes the Corynespora leaf fall (CLF) disease, which increasingly affects natural rubber production in Asia and Africa. It has also been found as an endophyte in South American rubber plantations where no CLF outbreak has yet occurred. The C. cassiicola species is genetically highly diverse, but no clear relationship has been evidenced between phylogenetic lineage and pathogenicity. Cassiicolin, a small glycosylated secreted protein effector, is thought to be involved in the necrotrophic interaction with the rubber tree but some virulent C. cassiicola isolates do not have a cassiicolin gene. This study set out to identify other putative effectors involved in CLF. The genome of a highly virulent C. cassiicola isolate from the rubber tree (CCP) was sequenced and assembled. In silico prediction revealed 2870 putative effectors, comprising CAZymes, lipases, peptidases, secreted proteins and enzymes associated with secondary metabolism. Comparison with the genomes of 44 other fungal species, focusing on effector content, revealed a striking proximity with phylogenetically unrelated species (Colletotrichum acutatum, Colletotrichum gloesporioides, Fusarium oxysporum, nectria hematococca, and Botrosphaeria dothidea) sharing life style plasticity and broad host range. Candidate effectors involved in the compatible interaction with the rubber tree were identified by transcriptomic analysis. Differentially expressed genes included 92 putative effectors, among which cassiicolin and two other secreted singleton proteins. Finally, the genomes of 35 C. cassiicola isolates representing the genetic diversity of the species were sequenced and assembled, and putative effectors identified. At the intraspecific level, effector-based classification was found to be highly consistent with the phylogenomic trees. Identification of lineage-specific effectors is a key step toward understanding C. cassiicola virulence and host specialization mechanisms.

Construction of Pará rubber tree genome and multi-transcriptome database accelerates rubber researches

BACKGROUND

Natural rubber is an economically important material. Currently the Pará rubber tree, Hevea brasiliensis is the main commercial source. Little is known about rubber biosynthesis at the molecular level. Next-generation sequencing (NGS) technologies brought draft genomes of three rubber cultivars and a variety of RNA sequencing (RNA-seq) data. However, no current genome or transcriptome databases (DB) are organized by gene.

RESULTS

A gene-oriented database is a valuable support for rubber research. Based on our original draft genome sequence of H. brasiliensis RRIM600, we constructed a rubber tree genome and transcriptome DB. Our DB provides genome information including gene functional annotations and multi-transcriptome data of RNA-seq, full-length cDNAs including PacBio Isoform sequencing (Iso-Seq), ESTs and genome wide transcription start sites (TSSs) derived from CAGE technology. Using our original and publically available RNA-seq data, we calculated co-expressed genes for identifying functionally related gene sets and/or genes regulated by the same transcription factor (TF). Users can access multi-transcriptome data through both a gene-oriented web page and a genome browser. For the gene searching system, we provide keyword search, sequence homology search and gene expression search; users can also select their expression threshold easily.

CONCLUSION

The rubber genome and transcriptome DB provides rubber tree genome sequence and multi-transcriptomics data. This DB is useful for comprehensive understanding of the rubber transcriptome. This will assist both industrial and academic researchers for rubber and economically important close relatives such as R. communis, M. esculenta and J. curcas. The Rubber Transcriptome DB release 2017.03 is accessible at http://matsui-lab.riken.jp/rubber/ .

Transcriptomic analyses reveal molecular mechanisms underlying growth heterosis and weakness of rubber tree seedlings

BACKGROUND

Breeding rubber tree seedling with growth heterosis is vital for natural rubber production. It is the prerequisites for effectively utilizing growth heterosis to elucidate its molecular mechanisms, but the molecular mechanisms remain poorly understood in rubber tree. To elucidate seedling growth heterosis, we conducted comparative transcriptomic analyses between the two hybrids and their parents.

RESULTS

By identifying and comparing differently expressed genes (DEGs), we found that the hybrids (BT 3410 and WC 11) show significantly differential expression profiles from their parents (PR 107 and RRIM 600). In BT 3410-parent triad, 1092 (49.95%) and 1094 (50.05%) DEGs indicated clear underdominance or overdominance, respectively. Whereas in WC 11-parent triad, most DEGs (78.2%, 721) showed low- or high-parent dominance; 160 (17.35%) exhibited expression patterns that are not statistically distinguishable from additivity, and 8 (0.87%) and 33 (3.58%) DEGs exhibited underdominance and overdominance, respectively. Furthermore, some biological processes are differentially regulated between two hybrids. Interestingly, the pathway in response to stimulus is significantly downregulated and metabolic pathways are more highly regulated in BT 3410.

CONCLUSIONS

Taken together, the genotypes, transcriptomes and biological pathways (especially, carbohydrate metabolism) are highly divergent between two hybrids, which may be associated with growth heterosis and weakness. Analyzing gene action models in hybrid-parent triads, we propose that overdominance may play important roles on growth heterosis, whereas dominance on hybrid weakness in rubber tree seedlings. These findings bring new insights into our understanding of growth heterosis of rubber tree seedling.

Acyl-CoA-binding protein family members in laticifers are possibly involved in lipid and latex metabolism of Hevea brasiliensis (the Para rubber tree)

Background

Acyl-CoA-binding proteins (ACBPs) are mainly involved in acyl-CoA ester binding and trafficking in eukaryotic cells, and their various functions have been characterized in model plants, such as Arabidopsis thaliana (A. thaliana), Oryza sativa (rice), and other plant species. In the present study, genome-wide mining and expression analysis of ACBP genes was performed on Hevea brasiliensis (the para rubber tree), the most important latex-producing crop in the world.

Results

Six members of the H. brasiliensis ACBP family genes, designated HbACBP1-HbACBP6, were identified from the H. brasiliensis genome. They can be categorized into four classes with different amino acid sequences and domain structures based on the categorization of their A. thaliana counterparts. Phylogenetic analysis shows that the HbACBPs were clustered with those of other closely related species, such as Manihot esculenta, Ricinus communis, and Jatropha carcas, but were further from those of A. thaliana, a distantly related species. Expression analysis demonstrated that the HbACBP1 and HbACBP2 genes are more prominently expressed in H. brasiliensis latex, and their expression can be significantly enhanced by bark tapping (a mechanical wound) and jasmonic acid stimulation, whereas HbACBP3-HbACBP6 had almost the same expression patterns with relatively high levels in mature leaves and male flowers, but a markedly low abundance in the latex. HbACBP1 and HbACBP2 may have crucial roles in lipid and latex metabolism in laticifers, so their subcellular location was further investigated and the results indicated that HbACBP1 is a cytosol protein, whereas HbACBP2 is an endoplasmic reticulum-associated ACBP.

Conclusions

In this study, the H. brasiliensis ACBP family genes were identified. Phylogenetic analyses of the HbABCPs indicate that there is a high conservation and evolutionary relationship between ACBPs in land plants. The HbACBPs are organ/tissue-specifically expressed and have different expression patterns in response to stimulation by bark tapping or ethrel/jasmonic acid. HbACBP1 and HbACBP2 are two important latex ACBPs that might be involved in the lipid and latex metabolism. The results may provide valuable information for further investigations into the biological functions of HbACBPs during latex metabolism and stress responses in H. brasiliensis.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4419-6) contains supplementary material, which is available to authorized users.

Purification and characterization of small and large rubber particles from Hevea brasiliensis

Natural rubber (NR) is synthesized by the rubber transferase (RTase) on rubber particles (RPs) in latex. Due to the heterogeneity of the RPs in latex, it is difficult to precisely characterize the RTase activity. In this study, we separated the RPs of Hevea brasiliensis with different particle size distributions, via stepwise centrifugations. Analyses of protein compositions and size distributions of NR in the RPs suggest that RPs in Hevea latex can be categorized into two distinct subclasses, the larger RPs (termed 1kRP, 2kRP, and 8kRP) and the smaller RPs (termed 20kRP and 50kRP). Precise enzymatic assays using the RPs revealed that 50kRP showed the highest RTase activity, whereas the larger RPs, which had been regarded to have quite low activity, also exhibited a comparable activity to the smaller RPs. Immunological detections of cis-prenyltransferases in the RPs showed that the abundance of these enzymes correlates with the extent of RTase activity.

Transcriptome-Wide Identification and Characterization of MYB Transcription Factor Genes in the Laticifer Cells of Hevea brasiliensis

MYB transcription factors hold vital roles in the regulation of plant secondary metabolic pathways. Laticifers in rubber trees (Hevea brasiliensis) are of primary importance in natural rubber production because natural rubber is formed and stored within these structures. To understand the role of MYB transcription factors in the specialized cells, we identified 44 MYB genes (named HblMYB1 to HblMYB44) by using our previously obtained transcriptome database of rubber tree laticifer cells and the public rubber tree genome database. Expression profiles showed that five MYB genes were highly expressed in the laticifers. HblMYB19 and HblMYB44 were selected for further study. HblMYB19 and HblMYB44 bound the promoters of HbFDPS1, HbSRPP, and HRT1 in yeast. Furthermore, the transient overexpression of HblMYB19 and HblMYB44 in tobacco plants significantly increased the activity of the promoters of HbFDPS1, HbSRPP, and HRT1. Basing on this information, we proposed that HblMYB19 and HblMYB44 are the regulators of HbFDPS1, HbSRPP, and HRT1, which are involved in the biosynthesis pathway of natural rubber.

Depsidones and an α-pyrone derivative from Simplicillium sp. PSU-H41, an endophytic fungus from Hevea brasiliensis leaf [corrected]

Nine previously undescribed depsidones (simplicildones A-I) and one previously undescribed α-pyrone (simplicilopyrone) were isolated from the endophytic fungus Simplicillium sp. PSU-H41 along with 11 known compounds. Their structures were established by extensive spectroscopic analysis. Simplicildone A and known botryorhodine C displayed weak antibacterial against Staphylococcus aureus with equal MIC values of 32 μg/mL. Additionally, botryorhodine C was active against methicillin-resistant S. aureus with the same MIC value. Simplicildone C exhibited weak antifungal activity against Cryptococcus neoformans with an MIC value of 32 μg/mL. In addition, simplicildones A and C and botryorhodine C were noncytotoxic against noncancerous Vero cells.

The SWEET gene family in Hevea brasiliensis - its evolution and expression compared with four other plant species

SWEET proteins play an indispensable role as a sugar efflux transporter in plant development and stress responses. The SWEET genes have previously been characterized in several plants. Here, we present a comprehensive analysis of this gene family in the rubber tree, Hevea brasiliensis. There are 36 members of the SWEET gene family in this species, making it one of the largest families in plant genomes sequenced so far. Structure and phylogeny analyses of these genes in Hevea and in other species demonstrated broad evolutionary conservation. RNA‐seq analyses revealed that SWEET2, 16, and 17 might represent the main evolutionary direction of SWEET genes in plants. Our results in Hevea suggested the involvement of HbSWEET1a, 2e, 2f, and 3b in phloem loading, HbSWEET10a and 16b in laticifer sugar transport, and HbSWEET9a in nectary‐specific sugar transport. Parallel studies of RNA‐seq analyses extended to three other plant species (Manihot esculenta, Populus trichocarpa, and Arabidopsis thaliana) produced findings which implicated MeSWEET10a, 3a, and 15b in M. esculenta storage root development, and the involvement of PtSWEET16b and PtSWEET16d in P. trichocarpa xylem development. RT‐qPCR results further revealed that HbSWEET10a, 16b, and 1a play important roles in phloem sugar transport. The results from this study provide a foundation not only for further investigation into the functionality of the SWEET gene family in Hevea, especially in its sugar transport for latex production, but also for related studies of this gene family in the plant kingdom.