Mapping-by-sequencing using NGS-based 3'-MACE-Seq reveals a new mutant allele of the essential nodulation gene Sym33 (IPD3) in pea (Pisum sativum L.)

The Effects of Rhizophagus irregularis Inoculation on Transcriptome of Medicago lupulina Leaves at Early Vegetative and Flowering Stages of Plant Development

The study is aimed at revealing the effects of Rhizophagus irregularis inoculation on the transcriptome of Medicago lupulina leaves at the early (second leaf formation) and later (flowering) stages of plant development. A pot experiment was conducted under conditions of low phosphorus (P) level in the substrate. M. lupulina plants were characterized by high mycorrhizal growth response and mycorrhization parameters. Library sequencing was performed on the Illumina HiseqXTen platform. Significant changes in the expression of 4863 (padj < 0.01) genes from 34049 functionally annotated genes were shown by Massive Analysis of cDNA Ends (MACE-Seq). GO enrichment analysis using the Kolmogorov-Smirnov test was performed, and 244 functional GO groups were identified, including genes contributing to the development of effective AM symbiosis. The Mercator online tool was used to assign functional classes of differentially expressed genes (DEGs). The early stage was characterized by the presence of six functional classes that included only upregulated GO groups, such as genes of carbohydrate metabolism, cellular respiration, nutrient uptake, photosynthesis, protein biosynthesis, and solute transport. At the later stage (flowering), the number of stimulated GO groups was reduced to photosynthesis and protein biosynthesis. All DEGs of the GO:0016036 group were downregulated because AM plants had higher resistance to phosphate starvation. For the first time, the upregulation of genes encoding thioredoxin in AM plant leaves was shown. It was supposed to reduce ROS level and thus, consequently, enhance the mechanisms of antioxidant protection in M. lupulina plants under conditions of low phosphorus level. Taken together, the obtained results indicate genes that are the most important for the effective symbiosis with M. lupulina and might be engaged in other plant species.

Bulked segregant transcriptome analysis in pea identifies key expression markers for resistance to Peyronellaea pinodes

Peyronellaea pinodes is a devastating pathogen of pea crop. Quantitative trait loci (QTL) associated with resistance have been identified, as well as genes differentially expressed between resistant and susceptible pea lines. The key question is which of these many genes located into these QTLs, or differentially expressed, are the key genes that distinguish resistant from susceptible plants and could be used as markers. To identify these key genes, in the present study we applied MACE (Massive Analysis of cDNA Ends) -Seq to a whole Recombinant Inbred Line population segregating for resistance to this disease and their parental lines and identified those genes which expression was more correlated with the level of resistance. We also compared gene expression profiles between the most resistant and the most susceptible families of the RIL population. A total of 6780 transcripts were differentially expressed between the parental lines after inoculation. Of them, 803 showed the same expression pattern in the bulks formed by the most resistant and most susceptible RIL families. These genes, showing a consistent expression pattern, could be used as expression markers to distinguish resistant from susceptible plants. The analysis of these genes also discovered the crucial mechanisms acting against P. pinodes.

A variable gene family encoding nodule-specific cysteine-rich peptides in pea (Pisum sativum L.)

Various legume plants form root nodules in which symbiotic bacteria (rhizobia) fix atmospheric nitrogen after differentiation into a symbiotic form named bacteroids. In some legume species, bacteroid differentiation is promoted by defensin-like nodule-specific cysteine-rich (NCR) peptides. NCR peptides have best been studied in the model legume Medicago truncatula Gaertn., while in many other legumes relevant information is still fragmentary. Here, we characterize the NCR gene family in pea (Pisum sativum L.) using genomic and transcriptomic data. We found 360 genes encoding NCR peptides that are expressed in nodules. The sequences of pea NCR genes and putative peptides are highly variable and differ significantly from NCR sequences of M. truncatula. Indeed, only one pair of orthologs (PsNCR47-MtNCR312) has been identified. The NCR genes in the pea genome are located in clusters, and the expression patterns of NCR genes from one cluster tend to be similar. These data support the idea of independent evolution of NCR genes by duplication and diversification in related legume species. We also described spatiotemporal expression profiles of NCRs and identified specific transcription factor (TF) binding sites in promoters of "early" and "late" NCR genes. Further, we studied the expression of NCR genes in nodules of Fix^- mutants and predicted potential regulators of NCR gene expression, one among them being the TF ERN1 involved in the early steps of nodule organogenesis. In general, this study contributes to understanding the functions of NCRs in legume nodules and contributes to understanding the diversity and potential antibiotic properties of pea nodule-specific antimicrobial molecules.

Application of the 3' mRNA-Seq using unique molecular identifiers in highly degraded RNA derived from formalin-fixed, paraffin-embedded tissue

Background

Archival formalin-fixed, paraffin-embedded (FFPE) tissue samples with clinical and histological data are a singularly valuable resource for developing new molecular biomarkers. However, transcriptome analysis remains challenging with standard mRNA-seq methods as FFPE derived-RNA samples are often highly modified and fragmented. The recently developed 3′ mRNA-seq method sequences the 3′ region of mRNA using unique molecular identifiers (UMI), thus generating gene expression data with minimal PCR bias. In this study, we evaluated the performance of 3′ mRNA-Seq using Lexogen QuantSeq 3′ mRNA-Seq Library Prep Kit FWD with UMI, comparing with TruSeq Stranded mRNA-Seq and RNA Exome Capture kit. The fresh-frozen (FF) and FFPE tissues yielded nucleotide sizes range from 13 to > 70% of DV200 values; input amounts ranged from 1 ng to 100 ng for validation.

Results

The total mapped reads of QuantSeq 3′ mRNA-Seq to the reference genome ranged from 99 to 74% across all samples. After PCR bias correction, 3 to 56% of total sequenced reads were retained. QuantSeq 3′ mRNA-Seq data showed highly reproducible data across replicates in Universal Human Reference RNA (UHR, R > 0.94) at input amounts from 1 ng to 100 ng, and FF and FFPE paired samples (R = 0.92) at 10 ng. Severely degraded FFPE RNA with ≤30% of DV200 value showed good concordance (R > 0.87) with 100 ng input. A moderate correlation was observed when directly comparing QuantSeq 3′ mRNA-Seq data with TruSeq Stranded mRNA-Seq (R = 0.78) and RNA Exome Capture data (R > 0.67).

Conclusion

In this study, QuantSeq 3′ mRNA-Seq with PCR bias correction using UMI is shown to be a suitable method for gene quantification in both FF and FFPE RNAs. 3′ mRNA-Seq with UMI may be applied to severely degraded RNA from FFPE tissues generating high-quality sequencing data.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08068-1.

A high coverage reference transcriptome assembly of pea (Pisum sativum L.) mycorrhizal roots

Arbuscular mycorrhiza (AM) is an ancient mutualistic symbiosis formed by 80–90 % of land plant species with the obligatorily biotrophic fungi that belong to the phylum Glomeromycota. This symbiosis is mutually beneficial, as AM fungi feed on plant photosynthesis products, in turn improving the efficiency of nutrient uptake from the environment. The garden pea (Pisum sativum L.), a widely cultivated crop and an important model for genetics, is capable of forming triple symbiotic systems consisting of the plant, AM fungi and nodule bacteria. As transcriptomic and proteomic approaches are being implemented for studying the mutualistic symbioses of pea, a need for a reference transcriptome of genes expressed under these specific conditions for increasing the resolution and the accuracy of other methods arose. Numerous transcriptome assemblies constructed for pea did not include mycorrhizal roots, hence the aim of the study to construct a reference transcriptome assembly of pea mycorrhizal roots. The combined transcriptome of mycorrhizal roots of Pisum sativum cv. Frisson inoculated with Rhizophagus irregularis BEG144 was investigated, and for both the organisms independent transcriptomes were assembled (coverage 177x for pea and 45x for fungus). Genes specific to mycorrhizal roots were found in the assembly, their expression patterns were examined with qPCR on two pea cultivars, Frisson and Finale. The gene expression depended on the inoculation stage and on the pea cultivar. The investigated genes may serve as markers for early stages of inoculation in genetically diverse pea cultivars.

Symbiotic Regulatory Genes Controlling Nodule Development in Pisum sativum L

Analyses of natural variation and the use of mutagenesis and molecular-biological approaches have revealed 50 symbiotic regulatory genes in pea (Pisum sativum L.). Studies of genomic synteny using model legumes, such as Medicago truncatula Gaertn. and Lotus japonicus (Regel) K. Larsen, have identified the sequences of 15 symbiotic regulatory genes in pea. These genes encode receptor kinases, an ion channel, a calcium/calmodulin-dependent protein kinase, transcription factors, a metal transporter, and an enzyme. This review summarizes and describes mutant alleles, their phenotypic manifestations, and the functions of all identified symbiotic regulatory genes in pea. Some examples of gene interactions are also given. In the review, all mutant alleles in genes with identified sequences are designated and still-unidentified symbiotic regulatory genes of great interest are considered. The identification of these genes will help elucidate additional components involved in infection thread growth, nodule primordium development, bacteroid differentiation and maintenance, and the autoregulation of nodulation. The significance of symbiotic mutants of pea as extremely fruitful genetic models for studying nodule development and for comparative cell biology studies of legume nodules is clearly demonstrated. Finally, it is noted that many more sequences of symbiotic regulatory genes remain to be identified. Transcriptomics approaches and genome-wide sequencing could help address this challenge.

Innovative transcriptome-based genotyping highlights environmentally responsive genes for phenology, growth and yield in a non-model grain legume

The narrow-leafed lupin, Lupinus angustifolius L., is a grain legume crop, cultivated both as a green manure and as a source of protein for animal feed and human food production. During its domestication process, numerous agronomic traits were improved, however, only two trait-related genes were identified hitherto, both by linkage mapping. Genome-wide association studies (GWAS), exploiting genomic sequencing, did not select any novel candidate gene. In the present study, an innovative method of 3'-end reduced representation transcriptomic profiling, a massive analysis of cDNA ends, has been used for genotyping of 126 L. angustifolius lines surveyed by field phenotyping. Significant genotype × environment interactions were identified for all phenology and yield traits analysed. Principal component analysis of population structure evidenced European domestication bottlenecks, visualized by clustering of breeding materials and cultivars. GWAS provided contribution towards deciphering vernalization pathway in legumes, and, apart from highlighting known domestication loci (Ku/Julius and mol), designated novel candidate genes for L. angustifolius traits. Early phenology was associated with genes from vernalization, cold-responsiveness and phosphatidylinositol signalling pathways whereas high yield with genes controlling photosynthesis performance and abiotic stress (drought or heat) tolerance. PCR-based toolbox was developed and validated to enable tracking desired alleles in marker-assisted selection. Narrow-leafed lupin was genotyped with an innovative method of transcriptome profiling and phenotyped for phenology, growth and yield traits in field. Early phenology was found associated with genes from cold-response, vernalization and phosphatidylinositol signalling pathways, whereas high yield with genes running photosystem II and drought or heat stress response. Key loci were supplied with PCR-based toolbox for marker-assisted selection.

Mutational analysis indicates that abnormalities in rhizobial infection and subsequent plant cell and bacteroid differentiation in pea (Pisum sativum) nodules coincide with abnormal cytokinin responses and localization

BACKGROUND AND AIMS

Recent findings indicate that Nod factor signalling is tightly interconnected with phytohormonal regulation that affects the development of nodules. Since the mechanisms of this interaction are still far from understood, here the distribution of cytokinin and auxin in pea (Pisum sativum) nodules was investigated. In addition, the effect of certain mutations blocking rhizobial infection and subsequent plant cell and bacteroid differentiation on cytokinin distribution in nodules was analysed.

METHODS

Patterns of cytokinin and auxin in pea nodules were profiled using both responsive genetic constructs and antibodies.

KEY RESULTS

In wild-type nodules, cytokinins were found in the meristem, infection zone and apical part of the nitrogen fixation zone, whereas auxin localization was restricted to the meristem and peripheral tissues. We found significantly altered cytokinin distribution in sym33 and sym40 pea mutants defective in IPD3/CYCLOPS and EFD transcription factors, respectively. In the sym33 mutants impaired in bacterial accommodation and subsequent nodule differentiation, cytokinin localization was mostly limited to the meristem. In addition, we found significantly decreased expression of LOG1 and A-type RR11 as well as KNOX3 and NIN genes in the sym33 mutants, which correlated with low cellular cytokinin levels. In the sym40 mutant, cytokinins were detected in the nodule infection zone but, in contrast to the wild type, they were absent in infection droplets.

CONCLUSIONS

In conclusion, our findings suggest that enhanced cytokinin accumulation during the late stages of symbiosis development may be associated with bacterial penetration into the plant cells and subsequent plant cell and bacteroid differentiation.

Candidate Domestication-Related Genes Revealed by Expression Quantitative Trait Loci Mapping of Narrow-Leafed Lupin (Lupinus angustifolius L.)

The last century has witnessed rapid domestication of the narrow-leafed lupin (Lupinus angustifolius L.) as a grain legume crop, exploiting discovered alleles conferring low-alkaloid content (iucundus), vernalization independence (Ku and Julius), and reduced pod shattering (lentus and tardus). In this study, a L. angustifolius mapping population was subjected to massive analysis of cDNA ends (MACE). The MACE yielded 4185 single nucleotide polymorphism (SNP) markers for linkage map improvement and 30,595 transcriptomic profiles for expression quantitative trait loci (eQTL) mapping. The eQTL highlighted a high number of cis- and trans-regulated alkaloid biosynthesis genes with gene expression orchestrated by a regulatory agent localized at iucundus locus, supporting the concept that ETHYLENE RESPONSIVE TRANSCRIPTION FACTOR RAP2-7 may control low-alkaloid phenotype. The analysis of Ku shed light on the vernalization response via FLOWERING LOCUS T and FD regulon in L. angustifolius, providing transcriptomic evidence for the contribution of several genes acting in C-repeat binding factor (CBF) cold responsiveness and in UDP-glycosyltransferases pathways. Research on lentus selected a DUF1218 domain protein as a candidate gene controlling the orientation of the sclerified endocarp and a homolog of DETOXIFICATION14 for purplish hue of young pods. An ABCG transporter was identified as a hypothetical contributor to sclerenchyma fortification underlying tardus phenotype.