Ethephon, an ethylene releaser, is used to stimulate latex production in Hevea brasiliensis. Ethylene induces many functions in latex cells including the production of reactive oxygen species (ROS). The accumulation of ROS is responsible for the coagulation of rubber particles in latex cells, resulting in the partial or complete stoppage of latex flow. This study set out to assess biochemical and histological changes as well as changes in gene expression in latex and phloem tissues from trees grown under various harvesting systems. The Tapping Panel Dryness (TPD) susceptibility of Hevea clones was found to be related to some biochemical parameters, such as low sucrose and high inorganic phosphorus contents. A high tapping frequency and ethephon stimulation induced early TPD occurrence in a high latex metabolism clone and late occurrence in a low latex metabolism clone. TPD-affected trees had smaller number of laticifer vessels compared to healthy trees, suggesting a modification of cambial activity. The differential transcript abundance was observed for twenty-seven candidate genes related to TPD occurrence in latex and phloem tissues for ROS-scavenging, ethylene biosynthesis and signalling genes. The predicted function for some Ethylene Response Factor genes suggested that these candidate genes should play an important role in regulating susceptibility to TPD.

Background: Ethylene production and signalling play an important role in somatic embryogenesis, especially for species that are recalcitrant in in vitro culture. The AP2/ERF superfamily has been identified and classified in Hevea brasiliensis. This superfamily includes the ERFs involved in response to ethylene. The relative transcript abundance of ethylene biosynthesis genes and of AP2/ERF genes was analysed during somatic embryogenesis for callus lines with different regeneration potential, in order to identify genes regulated during that process. Results: The analysis of relative transcript abundance was carried out by real-time RT-PCR for 142 genes. The transcripts of ERFs from group I, VII and VIII were abundant at all stages of the somatic embryogenesis process. Forty genetic expression markers for callus regeneration capacity were identified. Fourteen markers were found for proliferating calli and 35 markers for calli at the end of the embryogenesis induction phase. Sixteen markers discriminated between normal and abnormal embryos and, lastly, there were 36 markers of conversion into plantlets. A phylogenetic analysis comparing the sequences of the AP2 domains of Hevea and Arabidopsis genes enabled us to predict the function of 13 expression marker genes. Conclusions: This first characterization of the AP2/ERF superfamily in Hevea revealed dramatic regulation of the expression of AP2/ERF genes during the somatic embryogenesis process. The gene expression markers of proliferating callus capacity to regenerate plants by somatic embryogenesis should make it possible to predict callus lines suitable to be used for multiplication. Further functional characterization of these markers opens up prospects for discovering specific AP2/ERF functions in the Hevea species for which somatic embryogenesis is difficult.

Tolerance of recurrent mechanical wounding and exogenous ethylene is a feature of the rubber tree. Latex harvesting involves tapping of the tree bark and ethephon is applied to increase latex flow. Ethylene is an essential element in controlling latex production. The ethylene signalling pathway leads to the activation of Ethylene Response Factor (ERF) transcription factors. This family has been identified in Hevea brasiliensis. This study set out to understand the regulation of ERF genes during latex harvesting in relation to abiotic stress and hormonal treatments. Analyses of the relative transcript abundance were carried out for 35 HbERF genes in latex, in bark from mature trees and in leaves from juvenile plants under multiple abiotic stresses. Twenty-one HbERF genes were regulated by harvesting stress in laticifers, revealing an overrepresentation of genes in group IX. Transcripts of three HbERF-IX genes from HbERF-IXc4, HbERF-IXc5 and HbERF-IXc6 were dramatically accumulated by combining wounding, methyl jasmonate and ethylene treatments. When an ethylene inhibitor was used, the transcript accumulation for these three genes was halted, showing ethylene-dependent induction. Subcellular localization and transactivation experiments confirmed that several members of HbERF-IX are activator-type transcription factors. This study suggested that latex harvesting induces mechanisms developed for the response to abiotic stress. These mechanisms probably depend on various hormonal signalling pathways. Several members of HbERF-IX could be essential integrators of complex hormonal signalling pathways in Hevea.

The AP2/ERF superfamily encodes transcription factors that play a key role in plant development and responses to abiotic and biotic stress. In Hevea brasiliensis, ERF genes have been identified by RNA sequencing. This study set out to validate the number of HbERF genes, and identify ERF genes involved in the regulation of latex cell metabolism. A comprehensive Hevea transcriptome was improved using additional RNA reads from reproductive tissues. Newly assembled contigs were annotated in the Gene Ontology database and were assigned to 3 main categories. The AP2/ERF superfamily is the third most represented compared with other transcription factor families. A comparison with genomic scaffolds led to an estimation of 114 AP2/ERF genes and 1 soloist in Hevea brasiliensis. Based on a phylogenetic analysis, functions were predicted for 26 HbERF genes. A relative transcript abundance analysis was performed by real-time RT-PCR in various tissues. Transcripts of ERFs from group I and VIII were very abundant in all tissues while those of group VII were highly accumulated in latex cells. Seven of the thirty-five ERF expression marker genes were highly expressed in latex. Subcellular localization and transactivation analyses suggested that HbERF-VII candidate genes encoded functional transcription factors.

Oil palm plantation in Indonesia is significantly affected by basal stem rot disease caused by the pathogenic fungus Ganoderma boninense. Tolerant oil palm cultivars toward G. boninense have been developed through a breeding program accelerated by the implementation of the CRISPR/Cas9 technology. This study was conducted to perform a gene knockout (KO) of oil palm that confers a putative defense‐related trait toward G. boninense. A plasmid pCRISPR_EMLP containing modules, i.e., 35S‐CaMV‐promoter‐driven CRISPR/Cas9, U6‐promoter‐driven sgRNA to the target EgEMLP gene (EL695076), and hygromycin resistance gene as the selectable marker, was established for Agrobacterium‐mediated delivery into oil palm calli (OPC). The transformed OPCs were regenerated and screened in DF (de Fossard) media containing hygromycin. The working concentration of hygromycin was successfully optimized for selection at 20 ppm. Through PCR‐based selection using HYG primers, we succeeded in discerning positive transformed OPC clones. The sequenced PCR products of genomic DNA as the template amplified using EMLP1 primers showed a point mutation, causing a frameshift in the edited EgEMLP and premature stop codon. Furthermore, in silico modeling demonstrated that the mutation resulted in a change in the C‐terminal region, affecting the tertiary protein structure. Moreover, electrophoresis analysis of PCR products of cDNA as the template from transformed OPC clones showed several samples with faint or undetected bands. This indicated that the CRISPR/Cas9 module induced a mutation that could destabilize the transcribed mRNA, e.g., premature degradation. Altogether, this study has successfully implemented CRISPR/Cas9 gene editing in oil palm in a model gene that is responsible for putative defense‐related traits toward the pathogenic fungus G. boninense.

Genome editing using CRISPR/Cas9 construct containing sgRNA is the newest technology used to assemble organisms with desired characters through knock out of the target gene. As part of the effort to develop oil palm planting material tolerant to Ganoderma, this research aimed to obtain a technique for introducing CRISPR/Cas9 modules containing sgRNA into oil palm tissue. Based on differential expression studies, two genes encoding isoflavone reductase (IFR) and methallothionine-like protein (MT) were selected as the target for edited genes. The CRISPR-IFR and CRISPR-MT modules each carrying a sequence of gene recognition (sgRNA) were constructed and transformed into oil palm calli using Agrobacterium tumefaciens. The transformed calli were cultured on a modified DF medium for callus propagation, and then sub-cultured into DF media for induction of somatic embryos. PCR analyses using specific primers flanking the DNA targeted region were carried out and the results showed that both constructs had been introduced into the cells. DNA sequence analysis indicated the presence of substitution of several bases in the target area of both genes. The transformed calli grew in a selection medium containing hygromycin. Development of somatic embryos was detected after 3-4 weeks culturing on modified DF media.