Molecular characterization and functional analysis of a novel WRKY transcription factor HbWRKY83 possibly involved in rubber production of Hevea brasiliensis.

WRKY transcription factors play important roles in plant growth and developmental processes and various stress responses, and are also associated with jasmonic acid (JA) signaling in the regulation of secondary metabolite biosynthesis in plants. The regulatory networks mediated by WRKY proteins in the latex production of Hevea brasiliensis (the Pará rubber tree) are poorly understood. In this study, one novel WRKY gene (designated HbWRKY83) was identified from the latex of H. brasiliensis, and its functions were characterized via gene expression analysis in both the latex and HbWRKY83-overexpressing transgenic Arabidopsis. HbWRKY83 gene contains an open reading frame (ORF) of 921 bp encoding a 306-amino-acid protein which is clustered with group IIc WRKY TF. HbWRKY83 is a nuclear-localized protein with transcriptional activity. Real-time quantitative PCR analysis demonstrated that the transcription level of HbWRKY83 was up-regulated by exogenous methyl jasmonate, Ethrel (ethylene releaser) stimulation, and bark tapping (mechanical wounding). Compared with the wild-type plants, overexpression of HbWRKY83 improved the tolerance of transgenic Arabidopsis lines to drought and salt stresses by enhancing the expression levels of ethylene-insensitive3 transcription factors (EIN3s) and several stress-responsive genes, including Cu/Zn superoxide dismutases CSD1 (Cu/Zn-SOD1) and CSD2 (Cu/Zn-SOD2), related to reactive oxygen species scavenging. Additionally, these genes were also significantly up-regulated by bark tapping. In combination, these results suggest that HbWRKY83 might act as a positive regulator of rubber production by activating the expression of JA-, ethylene-, and wound-responsive genes in the laticiferous cells of rubber trees.

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.

Molecular mechanism of ethylene stimulation of latex yield in rubber tree (Hevea brasiliensis) revealed by de novo sequencing and transcriptome analysis.

BACKGROUND: Rubber tree (Hevea brasiliensis) is an important industrial crop cultivated in tropical areas for natural rubber production. Treatment of the bark of rubber trees with ehephon (an ethylene releaser) has been a routine measure to increase latex yield, but the molecular mechanism behind the stimulation of rubber production by ethylene still remains a puzzle. Deciphering the enigma is of great importance for improvement of rubber tree for high yield. RESULTS: De novo sequencing and assembly of the bark transciptomes of Hevea brasiliensis induced with ethephon for 8 h (E8) and 24 h (E24) were performed. 51,965,770, 52,303,714 and 53,177,976 high-quality clean reads from E8, E24 and C (control) samples were assembled into 81,335, 80,048 and 80,800 unigenes respectively, with a total of 84,425 unigenes and an average length of 1,101 bp generated. 10,216 and 9,374 differentially expressed genes (DEGs) in E8 and E24 compared with C were respectively detected. The expression of several enzymes in crucial points of regulation in glycolysis were up-regulated and DEGs were not significantly enriched in isopentenyl diphosphate (IPP) biosynthesis pathway. In addition, up-regulated genes of great regulatory importance in carbon fixation (Calvin cycle) were identified. CONCLUSIONS: The rapid acceleration of glycolytic pathway supplying precursors for the biosynthesis of IPP and natural rubber, instead of rubber biosynthesis per se, may be responsible for ethylene stimulation of latex yield in rubber tree. The elevated rate of flux throughout the Calvin cycle may account for some durability of ethylene-induced stimulation. Our finding lays the foundations for molecular diagnostic and genetic engineering for high-yielding improvement of rubber tree.