Background Several mechanisms regulating gene expression contribute to restore and reestablish cellular homeostasis so that plants can adapt and survive in adverse situations. MicroRNAs (miRNAs) play roles important in the transcriptional and post-transcriptional regulation of gene expression, emerging as a regulatory molecule key in the responses to plant stress, such as cold, heat, drought, and salt. This work is a comprehensive and large-scale miRNA analysis performed to characterize the miRNA population present in oil palm (Elaeis guineensis Jacq.) exposed to a high level of salt stress, to identify miRNA-putative target genes in the oil palm genome, and to perform an in silico comparison of the expression profile of the miRNAs and their putative target genes. Results A group of 79 miRNAs was found in oil palm, been 52 known miRNAs and 27 new ones. The known miRNAs found belonged to 28 families. Those miRNAs led to 229 distinct miRNA-putative target genes identified in the genome of oil palm. miRNAs and putative target genes differentially expressed under salinity stress were then selected for functional annotation analysis. The regulation of transcription, DNA-templated, and the oxidation-reduction process were the biological processes with the highest number of hits to the putative target genes, while protein binding and DNA binding were the molecular functions with the highest number of hits. Finally, the nucleus was the cellular component with the highest number of hits. The functional annotation of the putative target genes differentially expressed under salinity stress showed several ones coding for transcription factors which have already proven able to result in tolerance to salinity stress by overexpression or knockout in other plant species. Conclusions Our findings provide new insights into the early response of young oil palm plants to salinity stress and confirm an expected preponderant role of transcription factors - such as NF-YA3, HOX32, and GRF1 - in this response. Besides, it points out potential salt-responsive miRNAs and miRNA-putative target genes that one can utilize to develop oil palm plants tolerant to salinity stress.

Oil palm (Elaeis guineensis Jacq.) is the number one source of consumed vegetable oil nowadays. It is cultivated in areas of tropical rainforest, where it meets its natural condition of high rainfall throughout the year. The palm oil industry faces criticism due to a series of practices that was considered not environmentally sustainable, and it finds itself under pressure to adopt new and innovative procedures to reverse this negative public perception. Cultivating this oilseed crop outside the rainforest zone is only possible using artificial irrigation. Close to 30% of the world’s irrigated agricultural lands also face problems due to salinity stress. Consequently, the research community must consider drought and salinity together when studying to empower breeding programs in order to develop superior genotypes adapted to those potential new areas for oil palm cultivation. Multi-Omics Integration (MOI) offers a new window of opportunity for the non-trivial challenge of unraveling the mechanisms behind multigenic traits, such as drought and salinity tolerance. The current study carried out a comprehensive, large-scale, single-omics analysis (SOA), and MOI study on the leaves of young oil palm plants submitted to very high salinity stress. Taken together, a total of 1239 proteins were positively regulated, and 1660 were negatively regulated in transcriptomics and proteomics analyses. Meanwhile, the metabolomics analysis revealed 37 metabolites that were upregulated and 92 that were downregulated. After performing SOA, 436 differentially expressed (DE) full-length transcripts, 74 DE proteins, and 19 DE metabolites underwent MOI analysis, revealing several pathways affected by this stress, with at least one DE molecule in all three omics platforms used. The Cysteine and methionine metabolism (map00270) and Glycolysis/Gluconeogenesis (map00010) pathways were the most affected ones, each one with 20 DE molecules.

The American oil palm [Elaeis oleifera (Knuth) Cortés] has pronounced importance in oil palm breeding programs. Here, a germplasm bank (GB) of E. oleifera plants collected in the Amazon rainforest in Brazil was submitted to single nucleotide polymorphism (SNP) marker identification, selection, and use, aiming to characterize genetic diversity and population structure and to design a core collection (CC). Five hundred and fifty‐three plants from 206 subsamples, collected at 19 localities spread throughout six geographic regions, were submitted to genotyping‐by‐sequencing analysis. A set of 1,827 high‐quality SNP markers was then selected and used to run the genetic diversity and population structure analysis. The genetic diversity found is of moderate degree, and probably only a small portion of the species diversity is represented in the collection. The possible reason for that is the collecting strategy used, which collected subsamples only around the most prominent watercourses in the region. The average degree of genetic differentiation among subsamples is very high, indicating the presence of high interpopulation differentiation. The collection showed a low level of endogamy. The low average gene flow found indicates that genetic isolation caused by drift is occurring, and there is a need to review the conservation strategy. A set of 245 SNPs distributed throughout all 16 chromosomes was used to design CC based on maximizing the strategy of diversity. The optimal adjustment of the validated parameters, maintained while taking fewest subsamples, led to the choice of a model containing 20% of the entire collection as the ideal to form the CC.

Aiming at generating a comprehensive genomic database on Elaeis spp., our group is leading several R&D initiatives with Elaeis guineensis (African oil palm) and Elaeis oleifera (American oil palm), including the whole-genome sequencing of the last. Genome size estimates currently available for this genus are controversial, as they indicate that American oil palm genome is about half the size of the African oil palm genome and that the genome of the interspecific hybrid is bigger than both the parental species genomes. We estimated the genome size of three E. guineensis genotypes, five E. oleifera genotypes, and two interspecific hybrids genotypes. On average, the genome size of E. guineensis is 4.32 ± 0.173 pg, while that of E. oleifera is 4.43 ± 0.018 pg. This indicates that both genomes are similar in size, even though E. oleifera is in fact bigger. As expected, the hybrid genome size is around the average of the two genomes, 4.40 ± 0.016 pg. Additionally, we demonstrate that both species present around 38% of GC content. As our results contradict the currently available data on Elaeis spp. genome sizes, we propose that the actual genome size of the Elaeis species is around 4 pg and that American oil palm possesses a larger genome than African oil palm.

Soil salinity is one abiotic stress that threatens agriculture in more than 100 countries. Gliricidia [Gliricidia sepium (Jacq.) Kunth] is a multipurpose tree known for its ability to adapt to a wide range of soils; however, its tolerance limits and responses to salt stress are not yet well understood. In this study, after characterizing the morphophysiological responses of young gliricidia plants to salinity stress, leaf metabolic and transcription profiles were generated and submitted to single and integrated analyses. RNA from leaf samples were subjected to RNA sequencing using an Illumina HiSeq platform and the paired‐end strategy. Polar and lipidic fractions from leaf samples were extracted and analyzed on an ultra‐high‐performance liquid chromatography (UHPLC) coupled with electrospray ionization quadrupole time‐of‐flight high‐resolution mass spectrometry (MS) system. Acquired data were analyzed using the OmicsBox, XCMS Online, MetaboAnalyst, and Omics Fusion platforms. The substrate salinization protocol used allowed the identification of two distinct responses to salt stress: tolerance and adaptation. Single analysis on transcriptome and metabolome data sets led to a group of 5,672 transcripts and 107 metabolites differentially expressed in gliricidia leaves under salt stress. The phenylpropanoid biosynthesis was the most affected pathway, with 15 metabolites and three genes differentially expressed. Results showed that the differentially expressed metabolites and genes from this pathway affect mainly short‐term salt stress (STS). The single analysis of the transcriptome identified 12 genes coding for proteins that might play a role in gliricidia response at both STS and long‐term salt stress (LTS). Further studies are needed to reveal the mechanisms behind the adaptation response.

Oil palm (Elaeis guineensis Jacq.) is a oilseed crop of great economic importance drastically affected by abiotic stresses. MicroRNAs (miRNAs) play crucial roles in transcription and post-transcription regulation of gene expression, being essential molecules in the response of plants to abiotic stress. To better understand the molecular mechanisms behind the response of young oil palm plants to drought stress, this study reports on the prediction and characterization of miRNAs and their putative target genes in the apical leaf of plants subjected to 14 days of water deprivation. Then, the data from this study were compared to the data from a similar study that focused on salinity stress. Both, the drought-and salt-responsive miRNAs and their putative target genes underwent correlation analysis to identify similarities and dissimilarities among them. Among the 81 identified miRNAs, 29 are specific for oil palm, including two (egu-miR28ds and egu-miR29ds) new ones – described for the first time. As for the expression profile, 62 miRNAs were significantly differentially expressed under drought stress, being five up-regulated (miR396e, miR159b, miR529b, egu-miR19sds, and egu-miR29ds) and 57 down-regulated. Transcription factors, such as MYBs, HOXs, and NF-Ys, were predicted as putative miRNA-target genes in oil palm under water deprivation; making them the most predominant group of such genes. Finally, the correlation analysis study revealed a group of putative target genes with similar behavior under salt and drought stresses. Those genes that are upregulated by these two abiotic stresses encode lncRNAs and proteins linked to stress tolerance, stress memory, modulation of ROS signaling, and defense response regulation to abiotic and biotic stresses. In summary, this study provides molecular evidence for the possible involvement of miRNAs in the drought stress response in oil palm. Besides, it shows that, at the molecular level, there are many similarities in the response of young oil palm plants to these two abiotic stresses.

RNA-seq is a technique based on the large-scale sequencing of transcript-derived cDNAs using next-generation sequencing platforms mostly used today to characterize an organism’s transcriptome. The analysis of RNA-seq data allows for identifying genes differentially expressed in a given condition, such as salt stress. This study aimed to search and characterize genes from the African oil palm (Elaeis guineensis Jacq.) highly up-regulated during salt stress, with a long-term goal of gene promoter prospection and validation. The apical leaves from the control (electrical conductivity of ~2 dS m-1) and salt-stressed (~40 dS m-1) young oil palm plants, collected at 5 and 12 days after the beginning of the stress, were subjected to extraction of total RNA, with three plants (replicates) per treatment. The complete genome ofE. guineensis, available at the National Center for Biotechnology Information, was used as the reference genome - BioProject PRJNA192219. The differential expression analysis led to the selection for further characterization of seven genes, which had increased expressions of 37-84 times under salt stress. The strategy used in this study enabled the selection of seven salt-responsive genes highly up-regulated during salt stress, and some of them coded for proteins already reported as responsible for salinity tolerance in other plant species through over-expression or knockout.

Transposable elements (TEs) are mobile genetic elements present in almost all eukaryotic genomes. Due to their typical patterns of repetition, discovery, and characterization, they demand analysis by various bioinformatics software. Probably, as a result of the need for a complex analysis, many genomes publicly available do not have these elements annotated yet. In this study, a de novo and homology-based identification of TEs and microsatellites was performed using genomic data from 3 palm species: Elaeis oleifera (American oil palm, v.1, Embrapa, unpublished; v.8, Malaysian Palm Oil Board [MPOB], public), Elaeis guineensis (African oil palm, v.5, MPOB, public), and Phoenix dactylifera (date palm). The estimated total coverage of TEs was 50.96% (523 572 kb) and 42.31% (593 463 kb), 39.41% (605 015 kb), and 33.67% (187 361 kb), respectively. A total of 155 726 microsatellite loci were identified in the genomes of oil and date palms. This is the first detailed description of repeats in the genomes of oil and date palms. A relatively high diversity and abundance of TEs were found in the genomes, opening a range of further opportunities for applied research in these genera. The development of molecular markers (mainly simple sequence repeat), which may be immediately applied in breeding programs of those species to support the selection of superior genotypes and to enhance knowledge of the genetic structure of the breeding and natural populations, is the most notable opportunity.