Ethrel-stimulated prolongation of latex flow in the rubber tree (Hevea brasiliensis Muell. Arg.): an Hev b 7-like protein acts as a universal antagonist of rubber particle aggregating factors from lutoids and C-serum

Genomic insight into domestication of rubber tree

Understanding the genetic basis of rubber tree (Hevea brasiliensis) domestication is crucial for further improving natural rubber production to meet its increasing demand worldwide. Here we provide a high-quality H. brasiliensis genome assembly (1.58 Gb, contig N50 of 11.21 megabases), present a map of genome variations by resequencing 335 accessions and reveal domestication-related molecular signals and a major domestication trait, the higher number of laticifer rings. We further show that HbPSK5, encoding the small-peptide hormone phytosulfokine (PSK), is a key domestication gene and closely correlated with the major domestication trait. The transcriptional activation of HbPSK5 by myelocytomatosis (MYC) members links PSK signaling to jasmonates in regulating the laticifer differentiation in rubber tree. Heterologous overexpression of HbPSK5 in Russian dandelion (Taraxacum kok-saghyz) can increase rubber content by promoting laticifer formation. Our results provide an insight into target genes for improving rubber tree and accelerating the domestication of other rubber-producing plants.

Identification of the targets of HbEIN3/EILs in genomic wide in Hevea brasiliensis

EIN3/EILs are key regulators in ET signaling pathway. In this work, 4 members of EIN3/EILs of Hevea brasiliensis (HbEIN3/EILs) showed interaction with two F box proteins, HbEBF1 and HbEBF2. HbEIN3 located in nucleus and exhibited strong transcriptional activity. HbEIN3 was induced by ET treatment in C-serum, but not in B-serum of latex. HbEIN3/EILs bound to G-box cis-element. To globally search the potential targets of HbEIN3/EILs, genomic sequences of H. brasiliensis was re-annotated and an HCES (Hevea Cis-Elements Scanning) program was developed ( www.h-brasiliensis.com ). HCES scanning results showed that ET- and JA- responsive cis-elements distribute overlapping in gene promoters. 3146 genes containing G-box in promoters are potential targets of HbEIN3, including 41 genes involved in biosynthesis and drainage of latex, of which 7 rate-limiting genes of latex production were regulated by both ET and JA, suggesting that ET and JA signaling pathways coordinated the latex biosynthesis and drainage in H. brasiliensis. Abbreviations: ABRE: ABA responsive elements; bHLH: basic helix-loop-helix; COG: Orthologous Groups; DRE: dehydration response element; ERE: ethylene responsive element; ET: Ethylene; GO: Gene Ontology; HCES: Hevea Cis-Elements Scanning; JA: jasmonates; JRE: Jasmonate-responsive element; KEGG: Kyoto Encyclopedia of Genes and Genomes; NR: non-redundant database; PLACE: Plant Cis-acting Regulatory DNA Elements; qRT-PCR: quantitative real-time RT-PCR.

The formation and accumulation of protein-networks by physical interactions in the rapid occlusion of laticifer cells in rubber tree undergoing successive mechanical wounding

BACKGROUND

Although the wound response of plants has been extensively studied, little is known of the rapid occlusion of wounded cell itself. The laticifer in rubber tree is a specific type of tissue for natural rubber biosynthesis and storage. In natural rubber production, tapping is used to harvest the latex which flows out from the severed laticifer in the bark. Therefore, study of the rapid wound-occlusion of severed laticifer cells is important for understanding the rubber tree being protected from the continuously mechanical wounding.

RESULTS

Using cytological and biochemical techniques, we revealed a biochemical mechanism for the rapid occlusion of severed laticifer cells. A protein-network appeared rapidly after tapping and accumulated gradually along with the latex loss at the severed site of laticifer cells. Triple immunofluorescence histochemical localization showed that the primary components of the protein-network were chitinase, β-1,3-glucanase and hevein together with pro-hevein (ProH) and its carboxyl-terminal part. Molecular sieve chromatography showed that the physical interactions among these proteins occurred under the condition of neutral pH. The interaction of β-1,3-glucanase respectively with hevein, chitinase and ProH was testified by surface plasmon resonance (SPR). The interaction between actin and β-1,3-glucanase out of the protein inclusions of lutoids was revealed by pull-down. This interaction was pharmacologically verified by cytochalasin B-caused significant prolongation of the duration of latex flow in the field.

CONCLUSIONS

The formation of protein-network by interactions of the proteins with anti-pathogen activity released from lutoids and accumulation of protein-network by binding to the cytoskeleton are crucial for the rapid occlusion of laticifer cells in rubber tree. The protein-network at the wounded site of laticifer cells provides not only a physical barrier but also a biochemical barrier to protect the wounded laticifer cells from pathogen invasion.

Jasmonate signalling in the regulation of rubber biosynthesis in laticifer cells of rubber tree, Hevea brasiliensis

Rubber trees are the world's major source of natural rubber. Rubber-containing latex is obtained from the laticifer cells of the rubber tree (Hevea brasiliensis) via regular tapping. Rubber biosynthesis is a typical isoprenoid metabolic process in the laticifer cells; however, little is known about the positive feedback regulation caused by the loss of latex that occurs through tapping. In this study, we demonstrate the crucial role of jasmonate signalling in this feedback regulation. The endogenous levels of jasmonate, the expression levels of rubber biosynthesis-related genes, and the efficiency of in vitro rubber biosynthesis were found to be significantly higher in laticifer cells of regularly tapped trees than those of virgin (i.e. untapped) trees. Application of methyl jasmonate had similar effects to latex harvesting in up-regulating the rubber biosynthesis-related genes and enhancing rubber biosynthesis. The specific jasmonate signalling module in laticifer cells was identified as COI1-JAZ3-MYC2. Its activation was associated with enhanced rubber biosynthesis via up-regulation of the expression of a farnesyl pyrophosphate synthase gene and a small rubber particle protein gene. The increase in the corresponding proteins, especially that of farnesyl pyrophosphate synthase, probably contributes to the increased efficiency of rubber biosynthesis. To our knowledge, this is the first study to reveal a jasmonate signalling pathway in the regulation of rubber biosynthesis in laticifer cells. The identification of the specific jasmonate signalling module in the laticifer cells of the rubber tree may provide a basis for genetic improvement of rubber yield potential.

Transcript Profiling of Hevea brasiliensis during Latex Flow

Latex exploitation enhances latex regeneration in rubber trees. The latex exploitation-caused latex flow lasts from 10 min to a few hours, which is convenient for exploring the transcript profiling of latex metabolism-related genes at the different stages of latex flow. In the present study, the expression pattern of 62 latex metabolism-related genes involved in water transportation, carbohydrate metabolism, natural rubber biosynthesis, hormone signaling, ROS generation and scavenging, and latex coagulum across three stages of latex flow between rubber tree clones CATAS7-33-97 and CATAS8-79 were comparatively analyzed by quantitative real-time PCR. The two clones show differences in latex regeneration and have a different duration of latex flow. The results showed that the expression levels of 38 genes were significantly higher in CATAS8-79 latex than in CATAS7-33-97 during latex regeneration, while 45 genes had a notably higher expression level in CATAS8-79 latex during latex flow. Together with the activation of the MEP pathway and jasmonate pathway in CATAS8-79 latex, HbPIP1;3, HbPIP1;4, HbSUT3, HbSus3, HbHMGS1-2, HbMK should contribute to the high latex regeneration ability. The up-regulation of ethylene signaling and Hb44KD and the down-regulation of latex coagulation-related genes in CATAS8-79 latex might contribute to its longer latex flow duration. This study provides some cues for revealing the regulation of latex metabolism in rubber trees.