Translational Genomics in Legumes Allowed Placing In Silico 5460 Unigenes on the Pea Functional Map and Identified Candidate Genes in Pisum sativum L

Reference genome sequence and population genomic analysis of peas provide insights into the genetic basis of Mendelian and other agronomic traits

Peas are essential for human nutrition and played a crucial role in the discovery of Mendelian laws of inheritance. In this study, we assembled the genome of the elite vegetable pea cultivar 'Zhewan No. 1' at the chromosome level and analyzed resequencing data from 314 accessions, creating a comprehensive map of genetic variation in peas. We identified 235 candidate loci associated with 57 important agronomic traits through genome-wide association studies. Notably, we pinpointed the causal gene haplotypes responsible for four Mendelian traits: stem length (Le/le), flower color (A/a), cotyledon color (I/i) and seed shape (R/r). Additionally, we discovered the genes controlling pod form (Mendelian P/p) and hilum color. Our study also involved constructing a gene expression atlas across 22 tissues, highlighting key gene modules related to pod and seed development. These findings provide valuable pea genomic information and will facilitate the future genome-informed improvement of pea crops.

afila, the origin and nature of a major innovation in the history of pea breeding

The afila (af) mutation causes the replacement of leaflets by a branched mass of tendrils in the compound leaves of pea - Pisum sativum L. This mutation was first described in 1953, and several reports of spontaneous af mutations and induced mutants with a similar phenotype exist. Despite widespread introgression into breeding material, the nature of af and the origin of the alleles used remain unknown. Here, we combine comparative genomics with reverse genetic approaches to elucidate the genetic determinants of af. We also investigate haplotype diversity using a set of AfAf and afaf cultivars and breeding lines and molecular markers linked to seven consecutive genes. Our results show that deletion of two tandemly arranged genes encoding Q-type Cys(2)His(2) zinc finger transcription factors, PsPALM1a and PsPALM1b, is responsible for the af phenotype in pea. Eight haplotypes were identified in the af-harbouring genomic region on chromosome 2. These haplotypes differ in the size of the deletion, covering more or less genes. Diversity at the af locus is valuable for crop improvement and sheds light on the history of pea breeding for improved standing ability. The results will be used to understand the function of PsPALM1a/b and to transfer the knowledge for innovation in related crops.

Fine mapping and identification of a Fusarium wilt resistance gene FwS1 in pea

KEY MESSAGE

A Fusarium wilt resistance gene FwS1 on pea chromosome 6 was identified and mapped to a 91.4 kb region by a comprehensive genomic-based approach, and the gene Psat6g003960 harboring NB-ARC domain was identified as the putative candidate gene. Pea Fusarium wilt, incited by Fusarium oxysporum f. sp. pisi (Fop), has always been a devastating disease that causes severe yield losses and economic damage in pea-growing regions worldwide. The utilization of pea cultivars carrying resistance gene is the most efficient approach for managing this disease. In order to finely map resistance gene, F2 populations were established through the cross between Shijiadacaiwan 1 (resistant) and Y4 (susceptible). The resistance genetic analysis indicated that the Fop resistance in Shijiadacaiwan 1 was governed by a single dominant gene, named FwS1. Based on the bulked segregant analysis sequencing analyses, the gene FwS1 was initially detected on chromosome 6 (i.e., linking group II, chr6LG2), and subsequent linkage mapping with 589 F2 individuals fine-mapped the gene FwS1 into a 91.4 kb region. The further functional annotation and haplotype analysis confirmed that the gene Psat6g003960, characterized by a NB-ARC (nucleotide-binding adaptor shared by APAF-1, R proteins, and CED-4) domain, was considered as the most promising candidate gene. The encoding amino acids were altered by a "T/C" single-nucleotide polymorphism (SNP) in the first exon of the Psat6g003960, and based on this SNP locus, the molecular marker A016180 was determined to be a diagnostic marker for FwS1 by validating its specificity in both pea accessions and genetic populations with different genetic backgrounds. The FwS1 with diagnostic KASP marker A016180 could facilitate marker-assisted selection in resistance pea breeding in pea. In addition, a comparison of the candidate gene Psat6g003960 in 74SN3B and SJ1 revealed the same sequences. This finding indicated that 74SN3B carried the candidate gene for FwS1, suggesting that FwS1 and Fwf may be closely linked or an identical resistant gene against Fusarium wilt.

Physical seed dormancy in pea is genetically separable from seed coat thickness and roughness

Introduction

The seeds of wild pea (Pisum) exhibit marked physical dormancy due to impermeability of the seed coat to water, and the loss of this dormancy is thought to have been critical for domestication. Wild pea seed coats are also notably thick and rough, traits that have also reduced during domestication and are anecdotally linked to increased permeability. However, how these traits specifically interact with permeability is unclear.

Methods

To investigate this, we examined the genetic control of differences in seed coat characteristics between wild P. sativum ssp. humile and a non-dormant domesticated P. s. sativum accession in a recombinant inbred population. QTL effects were confirmed and their locations refined in segregating F4/5 populations.

Results

In this population we found a moderate correlation between testa thickness and permeability, and identified loci that affect them independently, suggesting no close functional association. However, the major loci affecting both testa thickness and permeability collocated closely with Mendel's pigmentation locus A, suggesting flavonoid compounds under its control might contribute significantly to both traits. We also show that seed coat roughness is oligogenic in this population, with the major locus independent of both testa thickness and permeability, suggesting selection for smooth seed was unlikely to be due to effects on either of these traits.

Discussion

Results indicate loss of seed coat dormancy during domestication was not primarily driven by reduced testa thickness or smooth seededness. The close association between major permeability and thickness QTL and Mendel's 'A' warrant further study, particularly regarding the role of flavonoids.

Five Regions of the Pea Genome Co-Control Partial Resistance to D. pinodes, Tolerance to Frost, and Some Architectural or Phenological Traits

Evidence for reciprocal links between plant responses to biotic or abiotic stresses and architectural and developmental traits has been raised using approaches based on epidemiology, physiology, or genetics. Winter pea has been selected for years for many agronomic traits contributing to yield, taking into account architectural or phenological traits such as height or flowering date. It remains nevertheless particularly susceptible to biotic and abiotic stresses, among which Didymella pinodes and frost are leading examples. The purpose of this study was to identify and resize QTL localizations that control partial resistance to D. pinodes, tolerance to frost, and architectural or phenological traits on pea dense genetic maps, considering how QTL colocalizations may impact future winter pea breeding. QTL analysis revealed five metaQTLs distributed over three linkage groups contributing to both D. pinodes disease severity and frost tolerance. At these loci, the haplotypes of alleles increasing both partial resistance to D. pinodes and frost tolerance also delayed the flowering date, increased the number of branches, and/or decreased the stipule length. These results question both the underlying mechanisms of the joint control of biotic stress resistance, abiotic stress tolerance, and plant architecture and phenology and the methods of marker-assisted selection optimizing stress control and productivity in winter pea breeding.

Mendel: From genes to genome

Two hundred years after the birth of Gregor Mendel, it is an appropriate time to reflect on recent developments in the discipline of genetics, particularly advances relating to the prescient friar's model species, the garden pea (Pisum sativum L.). Mendel's study of seven characteristics established the laws of segregation and independent assortment. The genes underlying four of Mendel's loci (A, LE, I, and R) have been characterized at the molecular level for over a decade. However, the three remaining genes, influencing pod color (GP), pod form (V/P), and the position of flowers (FA/FAS), have remained elusive for a variety of reasons, including a lack of detail regarding the loci with which Mendel worked. Here, we discuss potential candidate genes for these characteristics, in light of recent advances in the genetic resources for pea. These advances, including the pea genome sequence and reverse-genetics techniques, have revitalized pea as an excellent model species for physiological-genetic studies. We also discuss the issues that have been raised with Mendel's results, such as the recent controversy regarding the discrete nature of the characters that Mendel chose and the perceived overly-good fit of his segregations to his hypotheses. We also consider the relevance of these controversies to his lasting contribution. Finally, we discuss the use of Mendel's classical results to teach and enthuse future generations of geneticists, not only regarding the core principles of the discipline, but also its history and the role of hypothesis testing.

Molecular Characterizations of the er1 Alleles Conferring Resistance to Erysiphe pisi in Three Chinese Pea (Pisum sativum L.) Landraces

Powdery mildew caused by Erysiphe pisi DC. is a major disease affecting pea worldwide. This study aimed to confirm the resistance genes contained in three powdery mildew-resistant Chinese pea landraces (Suoshadabaiwan, Dabaiwandou, and Guiwan 1) and to develop the functional markers of the novel resistance genes. The resistance genes were identified by genetic mapping and PsMLO1 gene sequence identification. To confirm the inheritance of powdery mildew resistance in the three Landraces, the susceptible cultivars Bawan 6, Longwan 1, and Chengwan 8 were crossed with Suoshadabaiwan, Dabaiwandou, and Guiwan 1 to produce F1, F2, and F2:3 populations, respectively. All F1 plants were susceptible to E. pisi, and phenotypic segregation patterns in all the F2 and F2:3 populations fit the 3:1 (susceptible: resistant) and 1:2:1 (susceptible homozygotes: heterozygotes: resistant homozygotes) ratios, respectively, indicating powdery mildew resistance in the three Landraces were controlled by a single recessive gene, respectively. The analysis of er1-linked markers and genetic mapping in the F2 populations suggested that the recessive resistance genes in three landraces could be er1 alleles. The cDNA sequences of 10 homologous PsMLO1 cDNA clones from the contrasting parents were obtained. A known er1 allele, er1-4, was identified in Suoshadabaiwan. Two novel er1 alleles were identified in Dabaiwandou and Guiwan 1, which were designated as er1-13 and er1-14, respectively. Both novel alleles were characterized with a 1-bp deletion (T) in positions 32 (exon 1) and 277 (exon 3), respectively, which caused a frame-shift mutation to result in premature termination of translation of PsMLO1 protein. The co-dominant functional markers specific for er1-13 and er1-14, KASPar-er1-13, and KASPar-er1-14 were developed and effectively validated in populations and pea germplasms. Here, two novel er1 alleles were characterized and their functional markers were validated. These results provide powerful tools for marker-assisted selection in pea breeding.

Merging genotyping-by-sequencing data from two ex situ collections provides insights on the pea evolutionary history

Pea (Pisum sativum L. subsp. sativum) is one of the oldest domesticated species and a widely cultivated legume. In this study, we combined next generation sequencing (NGS) data referring to two genotyping-by-sequencing (GBS) libraries, each one prepared from a different Pisum germplasm collection. The selection of single nucleotide polymorphism (SNP) loci called in both germplasm collections caused some loss of information; however, this did not prevent the obtainment of one of the largest datasets ever used to explore pea biodiversity, consisting of 652 accessions and 22 127 markers. The analysis of population structure reflected genetic variation based on geographic patterns and allowed the definition of a model for the expansion of pea cultivation from the domestication centre to other regions of the world. In genetically distinct populations, the average decay of linkage disequilibrium (LD) ranged from a few bases to hundreds of kilobases, thus indicating different evolutionary histories leading to their diversification. Genome-wide scans resulted in the identification of putative selective sweeps associated with domestication and breeding, including genes known to regulate shoot branching, cotyledon colour and resistance to lodging, and the correct mapping of two Mendelian genes. In addition to providing information of major interest for fundamental and applied research on pea, our work describes the first successful example of integration of different GBS datasets generated from ex situ collections - a process of potential interest for a variety of purposes, including conservation genetics, genome-wide association studies, and breeding.

De novo super-early progeny in interspecific crosses Pisum sativum L. × P. fulvum Sibth. et Sm

Earliness in crop plants has a crucial role in avoiding the stress of drought and heat, which are the most important challenging stressors in crop production and are predicted to increase in the near future due to global warming. Furthermore, it provides a guarantee of vegetable production in the short growing season of agricultural lands in the northern hemisphere and at high altitudes. The growing human population needs super early plant cultivars for these agricultural lands to meet future global demands. This study examined de novo super-early progeny, referred to as much earlier than that of the earlier parent, which flowered in 13-17 days and pod setting in 18-29 days after germination, discovered in F2 and studied up to F5 derived from interspecific crosses between garden pea (P. sativum L.) and the most distant relative of pea (P. fulvum Sibth. et Sm.). De novo super-early progeny were found to be earlier by about one month than P. sativum and two months than P. fulvum under short day conditions in the F5 population. In respect of days to flowering and pod setting, de novo super-early progeny had a relatively high level of narrow sense heritability (h2 = 82% and 80%, respectively), indicating that the selections for earliness in segregating populations was effective for improvement of extreme early maturing varieties. De novo super-early progeny could be grown under heat stress conditions due to the escape ability. Vegetable types were not only high yielding but also free of any known undesirable traits from the wild species, such as pod dehiscence and non-uniform maturity. It could be considered complementary to "speed breeding", possibly obtaining more than six generations per year in a suitable climate chamber. Not only de novo super-early progeny but also transgressive segregation for agro-morphological traits can be created via interspecific crosses between P. sativum and P. fulvum, a precious unopened treasure in the second gene pool. Useful progeny obtained from crossing wild species with cultivated species reveal the importance of wild species.

Integration of the SMXL/D53 strigolactone signalling repressors in the model of shoot branching regulation in Pisum sativum

DWARF53 (D53) in rice (Oryza sativa) and its homologs in Arabidopsis (Arabidopsis thaliana), SUPPRESSOR OF MAX2-LIKE 6 (SMXL6), SMXL7 and SMXL8, are well established negative regulators of strigolactone (SL) signalling in shoot branching regulation. Little is known of pea (Pisum sativum) homologs and whether D53 and related SMXLs are specific to SL signalling pathways. Here, we identify two allelic pea mutants, dormant3 (dor3), and demonstrate through gene mapping and sequencing that DOR3 corresponds to a homolog of D53 and SMXL6/SMXL7, designated PsSMXL7. Phenotype analysis, gene expression, protein and hormone quantification assays were performed to determine the role of PsSMXL7 in regulation of bud outgrowth and the role of PsSMXL7 and D53 in integrating SL and cytokinin (CK) responses. Like D53 and related SMXLs, we show that PsSMXL7 can be degraded by SL and induces feedback upregulation of PsSMXL7 transcript. Here we reveal a system conserved in pea and rice, whereby CK also upregulates PsSMXL7/D53 transcripts, providing a clear mechanism for SL and CK cross-talk in the regulation of branching. To further deepen our understanding of the branching network in pea, we provide evidence that SL acts via PsSMXL7 to modulate auxin content via PsAFB5, which itself regulates expression of SL biosynthesis genes. We therefore show that PsSMXL7 is key to a triple hormone network involving an auxin-SL feedback mechanism and SL-CK cross-talk.

Annual and perennial Medicago show signatures of parallel adaptation to climate and soil in highly conserved genes

Human induced environmental change may require rapid adaptation of plant populations and crops, but the genomic basis of environmental adaptation remain poorly understood. We analysed polymorphic loci from the perennial crop Medicago sativa (alfalfa or lucerne) and the annual legume model species M. truncatula to search for a common set of candidate genes that might contribute to adaptation to abiotic stress in both annual and perennial Medicago species. We identified a set of candidate genes of adaptation associated with environmental gradients along the distribution of the two Medicago species. Candidate genes for each species were detected in homologous genomic linkage blocks using genome-environment (GEA) and genome-phenotype association analyses. Hundreds of GEA candidate genes were species-specific, of these, 13.4% (M. sativa) and 24% (M. truncatula) were also significantly associated with phenotypic traits. A set of 168 GEA candidates were shared by both species, which was 25.4% more than expected by chance. When combined, they explained a high proportion of variance for certain phenotypic traits associated with adaptation. Genes with highly conserved functions dominated among the shared candidates and were enriched in gene ontology terms that have shown to play a central role in drought avoidance and tolerance mechanisms by means of cellular shape modifications and other functions associated with cell homeostasis. Our results point to the existence of a molecular basis of adaptation to abiotic stress in Medicago determined by highly conserved genes and gene functions. We discuss these results in light of the recently proposed omnigenic model of complex traits.

Omics resources and omics-enabled approaches for achieving high productivity and improved quality in pea (Pisum sativum L.)

Pea (Pisum sativum L.), a cool-season legume crop grown in more than 85 countries, is the second most important grain legume and one of the major green vegetables in the world. While pea was historically studied as the genetic model leading to the discovery of the laws of genetics, pea research has lagged behind that of other major legumes in the genomics era, due to its large and complex genome. The evolving climate change and growing population have posed grand challenges to the objective of feeding the world, making it essential to invest research efforts to develop multi-omics resources and advanced breeding tools to support fast and continuous development of improved pea varieties. Recently, the pea researchers have achieved key milestones in omics and molecular breeding. The present review provides an overview of the recent important progress including the development of genetic resource databases, high-throughput genotyping assays, reference genome, genes/QTLs responsible for important traits, transcriptomic, proteomic, and phenomic atlases of various tissues under different conditions. These multi-faceted resources have enabled the successful implementation of various markers for monitoring early-generation populations as in marker-assisted backcrossing breeding programs. The emerging new breeding approaches such as CRISPR, speed breeding, and genomic selection are starting to change the paradigm of pea breeding. Collectively, the rich omics resources and omics-enable breeding approaches will enhance genetic gain in pea breeding and accelerate the release of novel pea varieties to meet the elevating demands on productivity and quality.

Genome-wide association study identified candidate genes for seed size and seed composition improvement in M. truncatula

Grain legumes are highly valuable plant species, as they produce seeds with high protein content. Increasing seed protein production and improving seed nutritional quality represent an agronomical challenge in order to promote plant protein consumption of a growing population. In this study, we used the genetic diversity, naturally present in Medicago truncatula, a model plant for legumes, to identify genes/loci regulating seed traits. Indeed, using sequencing data of 162 accessions from the Medicago HAPMAP collection, we performed genome-wide association study for 32 seed traits related to seed size and seed composition such as seed protein content/concentration, sulfur content/concentration. Using different GWAS and postGWAS methods, we identified 79 quantitative trait nucleotides (QTNs) as regulating seed size, 41 QTNs for seed composition related to nitrogen (i.e. storage protein) and sulfur (i.e. sulfur-containing amino acid) concentrations/contents. Furthermore, a strong positive correlation between seed size and protein content was revealed within the selected Medicago HAPMAP collection. In addition, several QTNs showed highly significant associations in different seed phenotypes for further functional validation studies, including one near an RNA-Binding Domain protein, which represents a valuable candidate as central regulator determining both seed size and composition. Finally, our findings in M. truncatula represent valuable resources to be exploitable in many legume crop species such as pea, common bean, and soybean due to its high synteny, which enable rapid transfer of these results into breeding programs and eventually help the improvement of legume grain production.

Analysis and Identification of QTL for Resistance to Sclerotinia sclerotiorum in Pea (Pisum sativum L.)

White mold caused by Sclerotinia sclerotiorum is an important constraint to field pea (Pisum sativum L.) production worldwide. To transfer white mold resistance into an adapted background, and study the genetics of the disease, two recombinant inbred line (RIL) populations (PRIL17 and PRIL19) were developed by crossing two partially resistant plant introductions with two susceptible pea cultivars. PRIL17 (Lifter × PI240515), and PRIL19 (PI169603 × Medora) were evaluated for resistance to white mold by measuring lesion expansion inhibition (LEI) and nodal transmission inhibition (NTI) at 3, 7, and 14 days post inoculation (dpi) under controlled environmental conditions. Lesion expansion inhibition percentage (LEIP), survival rate (SR), and area under disease progress curves (AUDPC) were also calculated accordingly. Because of a positive correlation between LEI and NTI with height, short and long internode individuals of each population were analyzed separately to avoid any confounding effect of height to pathogen response. A total of 22 short genotypes demonstrated partial resistance based on at least two Porter's resistance criteria. Only two pea genotypes with partial resistance to white mold (PRIL19-18 and PRIL19-124) had both semi-leafless (afila) and short internode traits. Both the RIL populations were genotyped using genotyping by sequencing (GBS). For PRIL17 and PRIL19, genetic maps were constructed from a total of 1,967 and 1,196 single nucleotide polymorphism (SNP) and spanned over 1,494 cM and 1,415 cM representing seven and nine linkage groups, respectively. A consensus map constructed using data from both populations, had 1,486 unique SNPs over 2,461 cM belonging to seven linkage groups. Inclusive composite interval mapping (ICIM) identified thirteen quantitative trait loci (QTL) associated with white mold resistance traits in both populations. Three of them were co-located with height genes (a morphological trait that reduces infection risk and acts as disease avoidance) and the other ten QTL were associated with two forms of physiological resistance (seven for LEI and three for NTI) with LOD and r2 ranging from 3.0 to 28.5 and 5.1 to 64.3, respectively. The development of resistance lines, genetic dissection and identification of markers associated will help accelerate breeding efforts for white mold resistance using molecular breeding approaches.

Identification of potential candidate genes controlling pea aphid tolerance in a Pisum fulvum high-density integrated DArTseq SNP-based genetic map

BACKGROUND

Pea (Pisum sativum) is one of the most important temperate grain legumes in the world, and its production is severely constrained by the pea aphid (Acyrthosiphon pisum). Wild relatives, such as P. fulvum, are valuable sources of allelic diversity to improve the genetic resistance of cultivated pea species against A. pisum attack. To unravel the genetic control underlying resistance to the pea aphid attack, a quantitative trait loci (QTL) analysis was performed using the previously developed high density integrated genetic linkage map originated from an intraspecific recombinant inbred line (RIL) population (P. fulvum: IFPI3260 × IFPI3251).

RESULTS

We accurately evaluated specific resistance responses to pea aphid that allowed the identification, for the first time, of genomic regions that control plant damage and aphid reproduction. Eight QTLs associated with tolerance to pea aphid were identified in LGs I, II, III, IV and V, which individually explained from 17.0% to 51.2% of the phenotypic variation depending on the trait scored, and as a whole from 17.0% to 88.6%. The high density integrated genetic linkage map also allowed the identification of potential candidate genes co-located with the QTLs identified.

CONCLUSIONS

Our work shows how the survival of P. fulvum after the pea aphid attack depends on the triggering of a multi-component protection strategy that implies a quantitative tolerance. The genomic regions associated with the tolerance responses of P. fulvum during A. pisum infestation have provided six potential candidate genes that could be useful in marker-assisted selection (MAS) and genomic assisted breeding (GAB) after functional validation in the future. © 2019 Society of Chemical Industry.

Identification of quantitative trait loci (QTL) controlling resistance to pea weevil (Bruchus pisorum) in a high-density integrated DArTseq SNP-based genetic map of pea

Pea weevil (Bruchus pisorum) is a damaging insect pest affecting pea (Pisum sativum) production worldwide. No resistant cultivars are available, although some levels of incomplete resistance have been identified in Pisum germplasm. To decipher the genetic control underlying the resistance previously identify in P. sativum ssp. syriacum, a recombinant inbred line (RIL F8:9) population was developed. The RIL was genotyped through Diversity Arrays Technology PL's DArTseq platform and screened under field conditions for weevil seed infestation and larval development along 5 environments. A newly integrated genetic linkage map was generated with a subset of 6,540 markers, assembled into seven linkage groups, equivalent to the number of haploid pea chromosomes. An accumulated distance of 2,503 cM was covered with an average density of 2.61 markers cM-1. The linkage map allowed the identification of three QTLs associated to reduced seed infestation along LGs I, II and IV. In addition, a QTL for reduced larval development was also identified in LGIV. Expression of these QTLs varied with the environment, being particularly interesting QTL BpSI.III that was detected in most of the environments studied. This high-saturated pea genetic map has also allowed the identification of seven potential candidate genes co-located with QTLs for marker-assisted selection, providing an opportunity for breeders to generate effective and sustainable strategies for weevil control.

Two Novel er1 Alleles Conferring Powdery Mildew (Erysiphe pisi) Resistance Identified in a Worldwide Collection of Pea (Pisum sativum L.) Germplasms

Powdery mildew caused by Erysiphe pisi DC. severely affects pea crops worldwide. The use of resistant cultivars containing the er1 gene is the most effective way to control this disease. The objectives of this study were to reveal er1 alleles contained in 55 E. pisi-resistant pea germplasms and to develop the functional markers of novel alleles. Sequences of 10 homologous PsMLO1 cDNA clones from each germplasm accession were used to determine their er1 alleles. The frame shift mutations and various alternative splicing patterns were observed during transcription of the er1 gene. Two novel er1 alleles, er1-8 and er1-9, were discovered in the germplasm accessions G0004839 and G0004400, respectively, and four known er1 alleles were identified in 53 other accessions. One mutation in G0004839 was characterized by a 3-bp (GTG) deletion of the wild-type PsMLO1 cDNA, resulting in a missing valine at position 447 of the PsMLO1 protein sequence. Another mutation in G0004400 was caused by a 1-bp (T) deletion of the wild-type PsMLO1 cDNA sequence, resulting in a serine to leucine change of the PsMLO1 protein sequence. The er1-8 and er1-9 alleles were verified using resistance inheritance analysis and genetic mapping with respectively derived F2 and F2:3 populations. Finally, co-dominant functional markers specific to er1-8 and er1-9 were developed and validated in populations and pea germplasms. These results improve our understanding of E. pisi resistance in pea germplasms worldwide and provide powerful tools for marker-assisted selection in pea breeding.

Development of gene-based molecular markers tagging low alkaloid pauper locus in white lupin (Lupinus albus L.)

White lupin (Lupinus albus L.) is a legume grain crop cultivated since ancient Greece and Egypt. Modern white lupin cultivars are appreciated as a source of protein with positive nutraceutical impact. However, white lupins produce anti-nutritional compounds, quinolizidine alkaloids, which provide bitter taste and have a negative influence on human health. During domestication of this species, several recessive alleles at unlinked loci controlling low alkaloid content were selected. One of these loci, pauper, was exploited worldwide providing numerous low-alkaloid cultivars. However, molecular tracking of pauper has been hampered due to the lack of diagnostic markers. In the present study, the synteny-based approach was harnessed to target pauper locus. Single-nucleotide polymorphisms flanking pauper locus on white lupin linkage map as well as candidate gene sequences elucidated from the narrow-leafed lupin (L. angustifolius L.) chromosome segment syntenic to the pauper linkage group region were transformed to PCR-based molecular markers. These markers were analyzed both in the mapping population and world germplasm collection. From fourteen markers screened, eleven were localized at a distance below 1.5 cM from this locus, including five co-segregating with pauper. The linkage of these markers was confirmed by high LOD values (up to 58.4). Validation performed in the set of 127 bitter and 23 sweet accessions evidenced high applicability of one marker, LAGI01_35805_F1_R1, for pauper locus selection, highlighted by the low ratio of false-positive scores (2.5%). LAGI01_35805 represents a homolog of L. angustifolius acyltransferase-like (LaAT) gene which might hypothetically participate in the alkaloid biosynthesis process in lupins.

Towards Exploitation of Adaptive Traits for Climate-Resilient Smart Pulses

Pulses are the main source of protein and minerals in the vegetarian diet. These are primarily cultivated on marginal lands with few inputs in several resource-poor countries of the world, including several in South Asia. Their cultivation in resource-scarce conditions exposes them to various abiotic and biotic stresses, leading to significant yield losses. Furthermore, climate change due to global warming has increased their vulnerability to emerging new insect pests and abiotic stresses that can become even more serious in the coming years. The changing climate scenario has made it more challenging to breed and develop climate-resilient smart pulses. Although pulses are climate smart, as they simultaneously adapt to and mitigate the effects of climate change, their narrow genetic diversity has always been a major constraint to their improvement for adaptability. However, existing genetic diversity still provides opportunities to exploit novel attributes for developing climate-resilient cultivars. The mining and exploitation of adaptive traits imparting tolerance/resistance to climate-smart pulses can be accelerated further by using cutting-edge approaches of biotechnology such as transgenics, genome editing, and epigenetics. This review discusses various classical and molecular approaches and strategies to exploit adaptive traits for breeding climate-smart pulses.

Paleogenomics: reconstruction of plant evolutionary trajectories from modern and ancient DNA

How contemporary plant genomes originated and evolved is a fascinating question. One approach uses reference genomes from extant species to reconstruct the sequence and structure of their common ancestors over deep timescales. A second approach focuses on the direct identification of genomic changes at a shorter timescale by sequencing ancient DNA preserved in subfossil remains. Merged within the nascent field of paleogenomics, these complementary approaches provide insights into the evolutionary forces that shaped the organization and regulation of modern genomes and open novel perspectives in fostering genetic gain in breeding programs and establishing tools to predict future population changes in response to anthropogenic pressure and global warming.

Genomic dissection of pod shattering in common bean: mutations at non-orthologous loci at the basis of convergent phenotypic evolution under domestication of leguminous species

The complete or partial loss of shattering ability occurred independently during the domestication of several crops. Therefore, the study of this trait can provide an understanding of the link between phenotypic and molecular convergent evolution. The genetic dissection of 'pod shattering' in Phaseolus vulgaris is achieved here using a population of introgression lines and next-generation sequencing techniques. The 'occurrence' of the indehiscent phenotype (indehiscent versus dehiscent) depends on a major locus on chromosome 5. Furthermore, at least two additional genes are associated with the 'level' of shattering (number of shattering pods per plant: low versus high) and the 'mode' of shattering (non-twisting versus twisting pods), with all of these loci contributing to the phenotype by epistatic interactions. Comparative mapping indicates that the major gene identified on common bean chromosome 5 corresponds to one of the four quantitative trait loci for pod shattering in Vigna unguiculata. None of the loci identified comprised genes that are homologs of the known shattering genes in Glycine max. Therefore, although convergent domestication can be determined by mutations at orthologous loci, this was only partially true for P. vulgaris and V. unguiculata, which are two phylogenetically closely related crop species, and this was not the case for the more distant P. vulgaris and G. max. Conversely, comparative mapping suggests that the convergent evolution of the indehiscent phenotype arose through mutations in different genes from the same underlying gene networks that are involved in secondary cell-wall biosynthesis and lignin deposition patterning at the pod level.

Construction of high-density linkage maps for mapping quantitative trait loci for multiple traits in field pea (Pisum sativum L.)

BACKGROUND

The objective of this research was to map quantitative trait loci (QTLs) of multiple traits of breeding importance in pea (Pisum sativum L.). Three recombinant inbred line (RIL) populations, PR-02 (Orb x CDC Striker), PR-07 (Carerra x CDC Striker) and PR-15 (1-2347-144 x CDC Meadow) were phenotyped for agronomic and seed quality traits under field conditions over multiple environments in Saskatchewan, Canada. The mapping populations were genotyped using genotyping-by-sequencing (GBS) method for simultaneous single nucleotide polymorphism (SNP) discovery and construction of high-density linkage maps.

RESULTS

After filtering for read depth, segregation distortion, and missing values, 2234, 3389 and 3541 single nucleotide polymorphism (SNP) markers identified by GBS in PR-02, PR-07 and PR-15, respectively, were used for construction of genetic linkage maps. Genetic linkage groups were assigned by anchoring to SNP markers previously positioned on these linkage maps. PR-02, PR-07 and PR-15 genetic maps represented 527, 675 and 609 non-redundant loci, and cover map distances of 951.9, 1008.8 and 914.2 cM, respectively. Based on phenotyping of the three mapping populations in multiple environments, 375 QTLs were identified for important traits including days to flowering, days to maturity, lodging resistance, Mycosphaerella blight resistance, seed weight, grain yield, acid and neutral detergent fiber concentration, seed starch concentration, seed shape, seed dimpling, and concentration of seed iron, selenium and zinc. Of all the QTLs identified, the most significant in terms of explained percentage of maximum phenotypic variance (PVmax) and occurrence in multiple environments were the QTLs for days to flowering (PVmax = 47.9%), plant height (PVmax = 65.1%), lodging resistance (PVmax = 35.3%), grain yield (PVmax = 54.2%), seed iron concentration (PVmax = 27.4%), and seed zinc concentration (PVmax = 43.2%).

CONCLUSION

We have identified highly significant and reproducible QTLs for several agronomic and seed quality traits of breeding importance in pea. The QTLs identified will be the basis for fine mapping candidate genes, while some of the markers linked to the highly significant QTLs are useful for immediate breeding applications.

A High-Density Integrated DArTseq SNP-Based Genetic Map of Pisum fulvum and Identification of QTLs Controlling Rust Resistance

Pisum fulvum, a wild relative of pea is an important source of allelic diversity to improve the genetic resistance of cultivated species against fungal diseases of economic importance like the pea rust caused by Uromyces pisi. To unravel the genetic control underlying resistance to this fungal disease, a recombinant inbred line (RIL) population was generated from a cross between two P. fulvum accessions, IFPI3260 and IFPI3251, and genotyped using Diversity Arrays Technology. A total of 9,569 high-quality DArT-Seq and 8,514 SNPs markers were generated. Finally, a total of 12,058 markers were assembled into seven linkage groups, equivalent to the number of haploid chromosomes of P. fulvum and P. sativum. The newly constructed integrated genetic linkage map of P. fulvum covered an accumulated distance of 1,877.45 cM, an average density of 1.19 markers cM^-1 and an average distance between adjacent markers of 1.85 cM. The composite interval mapping revealed three QTLs distributed over two linkage groups that were associated with the percentage of rust disease severity (DS%). QTLs UpDSII and UpDSIV were located in the LGs II and IV respectively and were consistently identified both in adult plants over 3 years at the field (Córdoba, Spain) and in seedling plants under controlled conditions. Whenever they were detected, their contribution to the total phenotypic variance varied between 19.8 and 29.2. A third QTL (UpDSIV.2) was also located in the LGIVand was environmentally specific as was only detected for DS % in seedlings under controlled conditions. It accounted more than 14% of the phenotypic variation studied. Taking together the data obtained in the study, it could be concluded that the expression of resistance to fungal diseases in P. fulvum originates from the resistant parent IFPI3260.

High-Throughput RNA-Seq Data Analysis of the Single Nucleotide Polymorphisms (SNPs) and Zygomorphic Flower Development in Pea (Pisum sativum L.)

Pea (Pisum sativum L.) is a model plant that has been used in classical genetics and organ development studies. However, its large and complex genome has hindered research investigations in pea. Here, we generated transcriptomes from different tissues or organs of three pea accessions using next-generation sequencing to assess single nucleotide polymorphisms (SNPs), and further investigated petal differentially expressed genes to elucidate the mechanisms regulating floral zygomorphy. Eighteen samples were sequenced, which yielded a total of 617 million clean reads, and de novo assembly resulted in 87,137 unigenes. A total of 9044 high-quality SNPs were obtained among the three accessions, and a consensus map was constructed. We further discovered several dorsoventral asymmetrically expressed genes that were confirmed by qRT-PCR among different petals, including previously reported three CYC-like proliferating cell factor (TCP) genes. One MADS-box gene was highly expressed in dorsal petals, and several MYB factors were predominantly expressed among dorsal, lateral, and/or ventral petals, together with a ventrally expressed TCP gene. In sum, our comprehensive database complements the existing resources for comparative genetic mapping and facilitates future investigations in legume zygomorphic flower development.

The pea branching RMS2 gene encodes the PsAFB4/5 auxin receptor and is involved in an auxin-strigolactone regulation loop

Strigolactones (SLs) are well known for their role in repressing shoot branching. In pea, increased transcript levels of SL biosynthesis genes are observed in stems of highly branched SL deficient (ramosus1 (rms1) and rms5) and SL response (rms3 and rms4) mutants indicative of negative feedback control. In contrast, the highly branched rms2 mutant has reduced transcript levels of SL biosynthesis genes. Grafting studies and hormone quantification led to a model where RMS2 mediates a shoot-to-root feedback signal that regulates both SL biosynthesis gene transcript levels and xylem sap levels of cytokinin exported from roots. Here we cloned RMS2 using synteny with Medicago truncatula and demonstrated that it encodes a putative auxin receptor of the AFB4/5 clade. Phenotypes similar to rms2 were found in Arabidopsis afb4/5 mutants, including increased shoot branching, low expression of SL biosynthesis genes and high auxin levels in stems. Moreover, afb4/5 and rms2 display a specific resistance to the herbicide picloram. Yeast-two-hybrid experiments supported the hypothesis that the RMS2 protein functions as an auxin receptor. SL root feeding using hydroponics repressed auxin levels in stems and down-regulated transcript levels of auxin biosynthesis genes within one hour. This auxin down-regulation was also observed in plants treated with the polar auxin transport inhibitor NPA. Together these data suggest a homeostatic feedback loop in which auxin up-regulates SL synthesis in an RMS2-dependent manner and SL down-regulates auxin synthesis in an RMS3 and RMS4-dependent manner.

Pea Marker Database (PMD) - A new online database combining known pea (Pisum sativum L.) gene-based markers

Pea (Pisum sativum L.) is the oldest model object of plant genetics and one of the most agriculturally important legumes in the world. Since the pea genome has not been sequenced yet, identification of genes responsible for mutant phenotypes or desirable agricultural traits is usually performed via genetic mapping followed by candidate gene search. Such mapping is best carried out using gene-based molecular markers, as it opens the possibility for exploiting genome synteny between pea and its close relative Medicago truncatula Gaertn., possessing sequenced and annotated genome. In the last 5 years, a large number of pea gene-based molecular markers have been designed and mapped owing to the rapid evolution of "next-generation sequencing" technologies. However, the access to the complete set of markers designed worldwide is limited because the data are not uniformed and therefore hard to use. The Pea Marker Database was designed to combine the information about pea markers in a form of user-friendly and practical online tool. Version 1 (PMD1) comprises information about 2484 genic markers, including their locations in linkage groups, the sequences of corresponding pea transcripts and the names of related genes in M. truncatula. Version 2 (PMD2) is an updated version comprising 15944 pea markers in the same format with several advanced features. To test the performance of the PMD, fine mapping of pea symbiotic genes Sym13 and Sym27 in linkage groups VII and V, respectively, was carried out. The results of mapping allowed us to propose the Sen1 gene (a homologue of SEN1 gene of Lotus japonicus (Regel) K. Larsen) as the best candidate gene for Sym13, and to narrow the list of possible candidate genes for Sym27 to ten, thus proving PMD to be useful for pea gene mapping and cloning. All information contained in PMD1 and PMD2 is available at www.peamarker.arriam.ru.

Genetic Variation Controlling Wrinkled Seed Phenotypes in Pisum: How Lucky Was Mendel?

One of the traits studied by Mendel in pea (Pisum sativum L.) was the wrinkled-seeded phenotype, and the molecular basis for a mutation underlying this phenotype was discovered in the 1990s. Although the starch-branching enzyme gene mutation identified at the genetic locus r is most likely to be that in seeds available to Mendel in the mid-1800s, it has remained an open question as to whether or not additional natural mutations in this gene exist within Pisum germplasm collections. Here, we explore this question and show that all but two wrinkled-seeded variants in one such collection correspond to either the mutant allele described previously for the r locus or a mutation at a second genetic locus, rb, affecting the gene encoding the large subunit of Adenosine diphosphoglucose (ADP-glucose) pyrophosphorylase; the molecular basis for the rb mutation is described here. The genetic basis for the phenotype of one (JI 2110) of the two lines which are neither r nor rb has been studied in crosses with a round-seeded variant (JI 281); for which extensive genetic marker data were expected. In marked contrast to the trait studied by Mendel and the rb phenotype; the data suggest that the wrinkled-seeded phenotype in JI 2110 is maternally determined, controlled by two genetic loci, and the extent to which it is manifested is very sensitive to the environment. Metabolite analysis of the cotyledons of JI 2110 revealed a profile for sucrose and sucrose-derived compounds that was more similar to that of wild-type round-seeded, than that of wrinkled-seeded r, pea lines. However, the metabolite profile of the seed coat (testa) of JI 2110 was distinct from that of other round-seeded genotypes tested which, together with analysis of recombinant inbred progeny lines, suggests an explanation for the seed phenotype.

Genome-wide association studies with proteomics data reveal genes important for synthesis, transport and packaging of globulins in legume seeds

Improving nutritional seed quality is an important challenge in grain legume breeding. However, the genes controlling the differential accumulation of globulins, which are major contributors to seed nutritional value in legumes, remain largely unknown. We combined a search for protein quantity loci with genome-wide association studies on the abundance of 7S and 11S globulins in seeds of the model legume species Medicago truncatula. Identified genomic regions and genes carrying polymorphisms linked to globulin variations were then cross-compared with pea (Pisum sativum), leading to the identification of candidate genes for the regulation of globulin abundance in this crop. Key candidates identified include genes involved in transcription, chromatin remodeling, post-translational modifications, transport and targeting of proteins to storage vacuoles. Inference of a gene coexpression network of 12 candidate transcription factors and globulin genes revealed the transcription factor ABA-insensitive 5 (ABI5) as a highly connected hub. Characterization of loss-of-function abi5 mutants in pea uncovered a role for ABI5 in controlling the relative abundance of vicilin, a sulfur-poor 7S globulin, in pea seeds. This demonstrates the feasibility of using genome-wide association studies in M. truncatula to reveal genes that can be modulated to improve seed nutritional value.

EARLY FLOWERING3 Redundancy Fine-Tunes Photoperiod Sensitivity

Three pea (Pisum sativum) loci controlling photoperiod sensitivity, HIGH RESPONSE (HR), DIE NEUTRALIS (DNE), and STERILE NODES (SN), have recently been shown to correspond to orthologs of Arabidopsis (Arabidopsis thaliana) circadian clock genes EARLY FLOWERING3 (ELF3), ELF4, and LUX ARRHYTHMO, respectively. A fourth pea locus, PHOTOPERIOD (PPD), also contributes to the photoperiod response in a similar manner to SN and DNE, and recessive ppd mutants on a spring-flowering hr mutant background show early, photoperiod-insensitive flowering. However, the molecular identity of PPD has so far remained elusive. Here, we show that the PPD locus also has a role in maintenance of diurnal and circadian gene expression rhythms and identify PPD as an ELF3 co-ortholog, termed ELF3b Genetic interactions between pea ELF3 genes suggest that loss of PPD function does not affect flowering time in the presence of functional HR, whereas PPD can compensate only partially for the lack of HR These results provide an illustration of how gene duplication and divergence can generate potential for the emergence of more subtle variations in phenotype that may be adaptively significant.

Genome-wide SNP identification, linkage map construction and QTL mapping for seed mineral concentrations and contents in pea (Pisum sativum L.)

BACKGROUND

Marker-assisted breeding is now routinely used in major crops to facilitate more efficient cultivar improvement. This has been significantly enabled by the use of next-generation sequencing technology to identify loci and markers associated with traits of interest. While rich in a range of nutritional components, such as protein, mineral nutrients, carbohydrates and several vitamins, pea (Pisum sativum L.), one of the oldest domesticated crops in the world, remains behind many other crops in the availability of genomic and genetic resources. To further improve mineral nutrient levels in pea seeds requires the development of genome-wide tools. The objectives of this research were to develop these tools by: identifying genome-wide single nucleotide polymorphisms (SNPs) using genotyping by sequencing (GBS); constructing a high-density linkage map and comparative maps with other legumes, and identifying quantitative trait loci (QTL) for levels of boron, calcium, iron, potassium, magnesium, manganese, molybdenum, phosphorous, sulfur, and zinc in the seed, as well as for seed weight.

RESULTS

In this study, 1609 high quality SNPs were found to be polymorphic between 'Kiflica' and 'Aragorn', two parents of an F6-derived recombinant inbred line (RIL) population. Mapping 1683 markers including 75 previously published markers and 1608 SNPs developed from the present study generated a linkage map of size 1310.1 cM. Comparative mapping with other legumes demonstrated that the highest level of synteny was observed between pea and the genome of Medicago truncatula. QTL analysis of the RIL population across two locations revealed at least one QTL for each of the mineral nutrient traits. In total, 46 seed mineral concentration QTLs, 37 seed mineral content QTLs, and 6 seed weight QTLs were discovered. The QTLs explained from 2.4% to 43.3% of the phenotypic variance.

CONCLUSION

The genome-wide SNPs and the genetic linkage map developed in this study permitted QTL identification for pea seed mineral nutrients that will serve as important resources to enable marker-assisted selection (MAS) for nutritional quality traits in pea breeding programs.

A Combined Comparative Transcriptomic, Metabolomic, and Anatomical Analyses of Two Key Domestication Traits: Pod Dehiscence and Seed Dormancy in Pea (Pisum sp.)

The origin of the agriculture was one of the turning points in human history, and a central part of this was the evolution of new plant forms, domesticated crops. Seed dispersal and germination are two key traits which have been selected to facilitate cultivation and harvesting of crops. The objective of this study was to analyze anatomical structure of seed coat and pod, identify metabolic compounds associated with water-impermeable seed coat and differentially expressed genes involved in pea seed dormancy and pod dehiscence. Comparative anatomical, metabolomics, and transcriptomic analyses were carried out on wild dormant, dehiscent Pisum elatius (JI64, VIR320) and cultivated, indehiscent Pisum sativum non-dormant (JI92, Cameor) and recombinant inbred lines (RILs). Considerable differences were found in texture of testa surface, length of macrosclereids, and seed coat thickness. Histochemical and biochemical analyses indicated genotype related variation in composition and heterogeneity of seed coat cell walls within macrosclereids. Liquid chromatography-electrospray ionization/mass spectrometry and Laser desorption/ionization-mass spectrometry of separated seed coats revealed significantly higher contents of proanthocyanidins (dimer and trimer of gallocatechin), quercetin, and myricetin rhamnosides and hydroxylated fatty acids in dormant compared to non-dormant genotypes. Bulk Segregant Analysis coupled to high throughput RNA sequencing resulted in identification of 770 and 148 differentially expressed genes between dormant and non-dormant seeds or dehiscent and indehiscent pods, respectively. The expression of 14 selected dormancy-related genes was studied by qRT-PCR. Of these, expression pattern of four genes: porin (MACE-S082), peroxisomal membrane PEX14-like protein (MACE-S108), 4-coumarate CoA ligase (MACE-S131), and UDP-glucosyl transferase (MACE-S139) was in agreement in all four genotypes with Massive analysis of cDNA Ends (MACE) data. In case of pod dehiscence, the analysis of two candidate genes (SHATTERING and SHATTERPROOF) and three out of 20 MACE identified genes (MACE-P004, MACE-P013, MACE-P015) showed down-expression in dorsal and ventral pod suture of indehiscent genotypes. Moreover, MACE-P015, the homolog of peptidoglycan-binding domain or proline-rich extensin-like protein mapped correctly to predicted Dpo1 locus on PsLGIII. This integrated analysis of the seed coat in wild and cultivated pea provides new insight as well as raises new questions associated with domestication and seed dormancy and pod dehiscence.

Emerging Genomic Tools for Legume Breeding: Current Status and Future Prospects

Legumes play a vital role in ensuring global nutritional food security and improving soil quality through nitrogen fixation. Accelerated higher genetic gains is required to meet the demand of ever increasing global population. In recent years, speedy developments have been witnessed in legume genomics due to advancements in next-generation sequencing (NGS) and high-throughput genotyping technologies. Reference genome sequences for many legume crops have been reported in the last 5 years. The availability of the draft genome sequences and re-sequencing of elite genotypes for several important legume crops have made it possible to identify structural variations at large scale. Availability of large-scale genomic resources and low-cost and high-throughput genotyping technologies are enhancing the efficiency and resolution of genetic mapping and marker-trait association studies. Most importantly, deployment of molecular breeding approaches has resulted in development of improved lines in some legume crops such as chickpea and groundnut. In order to support genomics-driven crop improvement at a fast pace, the deployment of breeder-friendly genomics and decision support tools seems appear to be critical in breeding programs in developing countries. This review provides an overview of emerging genomics and informatics tools/approaches that will be the key driving force for accelerating genomics-assisted breeding and ultimately ensuring nutritional and food security in developing countries.

A novel er1 allele and the development and validation of its functional marker for breeding pea (Pisum sativum L.) resistance to powdery mildew

KEY MESSAGE

A novel er1 allele, er1 -7, conferring pea powdery mildew resistance was characterized by a 10-bp deletion in PsMLO1 cDNA, and its functional marker was developed and validated in pea germplasms. Pea powdery mildew caused by Erysiphe pisi DC is a major disease worldwide. Pea cultivar 'DDR-11' is an elite germplasm resistant to E. pisi. To identify the gene conferring resistance in DDR-11, the susceptible Bawan 6 and resistant DDR-11 cultivars were crossed to produce F1, F2, and F(2:3) populations. The phenotypic segregation patterns in the F2 and F(2:3) populations fit the 3:1 (susceptible:resistant) and 1:2:1 (susceptible homozygotes:heterozygotes:resistant homozygotes) ratios, respectively, indicating that resistance was controlled by a single recessive gene. Analysis of er1-linked markers in the F2 population suggested that the recessive resistance gene in DDR-11 was an er1 allele, which was mapped between markers ScOPE16-1600 and c5DNAmet. To further characterize er1 allele, the cDNA sequences of PsMLO1 from the parents were obtained and a novel er1 allele in DDR-11 was identified and designated as er1-7, which has a 10-bp deletion in position 111-120. The er1-7 allele caused a frame-shift mutation, resulting in a premature termination of translation of PsMLO1 protein. A co-dominant functional marker specific for er1-7 was developed, InDel111-120, which co-segregated with E. pisi resistance in the mapping population. The marker was able to distinguish between pea germplasms with and without the er1-7. Of 161 pea germplasms tested by InDel111-120, seven were detected containing resistance allele er1-7, which was verified by sequencing their PsMLO1 cDNA. Here, a novel er1 allele was characterized and its an ideal functional marker was validated, providing valuable genetic information and a powerful tool for breeding pea resistance to powdery mildew.

Ascochyta blight disease of pea (Pisum sativum L.): defence-related candidate genes associated with QTL regions and identification of epistatic QTL

KEY MESSAGE

Advances have been made in our understanding of Ascochyta blight resistance genetics through mapping candidate genes associated with QTL regions and demonstrating the importance of epistatic interactions in determining resistance. Ascochyta blight disease of pea (Pisum sativum L.) is economically significant with worldwide distribution. The causal pathogens are Didymella pinodes, Phoma medicaginis var pinodella and, in South Australia, P. koolunga. This study aimed to identify candidate genes that map to quantitative trait loci (QTL) for Ascochyta blight field disease resistance and to explore the role of epistatic interactions. Candidate genes associated with QTL were identified beginning with 101 defence-related genes from the published literature. Synteny between pea and Medicago truncatula was used to narrow down the candidates for mapping. Fourteen pea candidate sequences were mapped in two QTL mapping populations, A26 × Rovar and A88 × Rovar. QTL peaks, or the intervals containing QTL peaks, for the Asc2.1, Asc4.2, Asc4.3 and Asc7.1 QTL were defined by four of these candidate genes, while another three candidate genes occurred within 1.0 LOD confidence intervals. Epistasis involving QTL × background marker and background marker × background marker interactions contributed to the disease response phenotypes observed in the two mapping populations. For each population, five pairwise interactions exceeded the 5% false discovery rate threshold. Two candidate genes were involved in significant pairwise interactions. Markers in three genomic regions were involved in two or more epistatic interactions. Therefore, this study has identified pea defence-related sequences that are candidates for resistance determination, and that may be useful for marker-assisted selection. The demonstration of epistasis informs breeders that the architecture of this complex quantitative resistance includes epistatic interactions with non-additive effects.

Ancestors of modern plant crops

Recent accumulation of plant genomic resources offers the opportunity to compare modern genomes and model their evolutionary history from their reconstructed Most Recent Common Ancestors (MRCAs) that can be used as a guide to unveil the forces driving the evolutionary success of angiosperms and ultimately to perform applied translational research from models to crops. This article reviews the current state of art of recent structural comparative genomics studies through ancestral genome reconstruction, that is, the field of in silico paleogenomics.

SNP discovery and genetic mapping using genotyping by sequencing of whole genome genomic DNA from a pea RIL population

BACKGROUND

Progress in genetics and breeding in pea still suffers from the limited availability of molecular resources. SNP markers that can be identified through affordable sequencing processes, without the need for prior genome reduction or a reference genome to assemble sequencing data would allow the discovery and genetic mapping of thousands of molecular markers. Such an approach could significantly speed up genetic studies and marker assisted breeding for non-model species.

RESULTS

A total of 419,024 SNPs were discovered using HiSeq whole genome sequencing of four pea lines, followed by direct identification of SNP markers without assembly using the discoSnp tool. Subsequent filtering led to the identification of 131,850 highly designable SNPs, polymorphic between at least two of the four pea lines. A subset of 64,754 SNPs was called and genotyped by short read sequencing on a subpopulation of 48 RILs from the cross 'Baccara' x 'PI180693'. This data was used to construct a WGGBS-derived pea genetic map comprising 64,263 markers. This map is collinear with previous pea consensus maps and therefore with the Medicago truncatula genome. Sequencing of four additional pea lines showed that 33 % to 64 % of the mapped SNPs, depending on the pairs of lines considered, are polymorphic and can therefore be useful in other crosses. The subsequent genotyping of a subset of 1000 SNPs, chosen for their mapping positions using a KASP™ assay, showed that almost all generated SNPs are highly designable and that most (95 %) deliver highly qualitative genotyping results. Using rather low sequencing coverages in SNP discovery and in SNP inferring did not hinder the identification of hundreds of thousands of high quality SNPs.

CONCLUSIONS

The development and optimization of appropriate tools in SNP discovery and genetic mapping have allowed us to make available a massive new genomic resource in pea. It will be useful for both fine mapping within chosen QTL confidence intervals and marker assisted breeding for important traits in pea improvement.

Discovery of a Novel er1 Allele Conferring Powdery Mildew Resistance in Chinese Pea (Pisum sativum L.) Landraces

Pea powdery mildew, caused by Erysiphe pisi D.C., is an important disease worldwide. Deployment of resistant varieties is the main way to control this disease. This study aimed to screen Chinese pea (Pisum sativum L.) landraces resistant to E. pisi, and to characterize the resistance gene(s) at the er1 locus in the resistant landraces, and to develop functional marker(s) specific to the novel er1 allele. The 322 landraces showed different resistance levels. Among them, 12 (3.73%), 4 (1.24%) and 17 (5.28%) landraces showed immunity, high resistance and resistance to E. pisi, respectively. The other landraces appeared susceptible or highly susceptible to E. pisi. Most of the immune and highly resistant landraces were collected from Yunnan province. To characterize the resistance gene at the er1 locus, cDNA sequences of PsMLO1 gene were determined in 12 immune and four highly resistant accessions. The cDNAs of PsMLO1 from the immune landrace G0005576 produced three distinct transcripts, characterized by a 129-bp deletion, and 155-bp and 220-bp insertions, which were consistent with those of er1-2 allele. The PsMLO1 cDNAs in the other 15 resistant landraces produced identical transcripts, which had a new point mutation (T→C) at position 1121 of PsMLO1, indicating a novel er1 allele, designated as er1-6. This mutation caused a leucine to proline change in the amino acid sequence. Subsequently, the resistance allele er1-6 in landrace G0001778 was confirmed by resistance inheritance analysis and genetic mapping on the region of the er1 locus using populations derived from G0001778 × Bawan 6. Finally, a functional marker specific to er1-6, SNP1121, was developed using the high-resolution melting technique, which could be used in pea breeding via marker-assisted selection. The results described here provide valuable genetic information for Chinese pea landraces and a powerful tool for pea breeders.

Development of two major resources for pea genomics: the GenoPea 13.2K SNP Array and a high-density, high-resolution consensus genetic map

Single nucleotide polymorphism (SNP) arrays represent important genotyping tools for innovative strategies in both basic research and applied breeding. Pea is an important food, feed and sustainable crop with a large (about 4.45 Gbp) but not yet available genome sequence. In the present study, 12 pea recombinant inbred line populations were genotyped using the newly developed GenoPea 13.2K SNP Array. Individual and consensus genetic maps were built providing insights into the structure and organization of the pea genome. Largely collinear genetic maps of 3918-8503 SNPs were obtained from all mapping populations, and only two of these exhibited putative chromosomal rearrangement signatures. Similar distortion patterns in different populations were noted. A total of 12 802 transcript-derived SNP markers placed on a 15 079-marker high-density, high-resolution consensus map allowed the identification of ohnologue-rich regions within the pea genome and the localization of local duplicates. Dense syntenic networks with sequenced legume genomes were further established, paving the way for the identification of the molecular bases of important agronomic traits segregating in the mapping populations. The information gained on the structure and organization of the genome from this research will undoubtedly contribute to the understanding of the evolution of the pea genome and to its assembly. The GenoPea 13.2K SNP Array and individual and consensus genetic maps are valuable genomic tools for plant scientists to strengthen pea as a model for genetics and physiology and enhance breeding.

Genomic Tools in Pea Breeding Programs: Status and Perspectives

Pea (Pisum sativum L.) is an annual cool-season legume and one of the oldest domesticated crops. Dry pea seeds contain 22-25% protein, complex starch and fiber constituents, and a rich array of vitamins, minerals, and phytochemicals which make them a valuable source for human consumption and livestock feed. Dry pea ranks third to common bean and chickpea as the most widely grown pulse in the world with more than 11 million tons produced in 2013. Pea breeding has achieved great success since the time of Mendel's experiments in the mid-1800s. However, several traits still require significant improvement for better yield stability in a larger growing area. Key breeding objectives in pea include improving biotic and abiotic stress resistance and enhancing yield components and seed quality. Taking advantage of the diversity present in the pea genepool, many mapping populations have been constructed in the last decades and efforts have been deployed to identify loci involved in the control of target traits and further introgress them into elite breeding materials. Pea now benefits from next-generation sequencing and high-throughput genotyping technologies that are paving the way for genome-wide association studies and genomic selection approaches. This review covers the significant development and deployment of genomic tools for pea breeding in recent years. Future prospects are discussed especially in light of current progress toward deciphering the pea genome.

Full-length de novo assembly of RNA-seq data in pea (Pisum sativum L.) provides a gene expression atlas and gives insights into root nodulation in this species

Next-generation sequencing technologies allow an almost exhaustive survey of the transcriptome, even in species with no available genome sequence. To produce a Unigene set representing most of the expressed genes of pea, 20 cDNA libraries produced from various plant tissues harvested at various developmental stages from plants grown under contrasting nitrogen conditions were sequenced. Around one billion reads and 100 Gb of sequence were de novo assembled. Following several steps of redundancy reduction, 46 099 contigs with N50 length of 1667 nt were identified. These constitute the 'Caméor' Unigene set. The high depth of sequencing allowed identification of rare transcripts and detected expression for approximately 80% of contigs in each library. The Unigene set is now available online (http://bios.dijon.inra.fr/FATAL/cgi/pscam.cgi), allowing (i) searches for pea orthologs of candidate genes based on gene sequences from other species, or based on annotation, (ii) determination of transcript expression patterns using various metrics, (iii) identification of uncharacterized genes with interesting patterns of expression, and (iv) comparison of gene ontology pathways between tissues. This resource has allowed identification of the pea orthologs of major nodulation genes characterized in recent years in model species, as a major step towards deciphering unresolved pea nodulation phenotypes. In addition to a remarkable conservation of the early transcriptome nodulation apparatus between pea and Medicago truncatula, some specific features were highlighted. The resource provides a reference for the pea exome, and will facilitate transcriptome and proteome approaches as well as SNP discovery in pea.

Molecular characterization of a mutation affecting abscisic acid biosynthesis and consequently stomatal responses to humidity in an agriculturally important species

Mutants deficient in the phytohormone abscisic acid (ABA) have been instrumental in determining not only the biosynthetic pathway for this hormone, but also its physiological role in land plants. The wilty mutant of Pisum sativum is one of the classical, well-studied ABA-deficient mutants; however, this mutant remains uncharacterized at a molecular level. Using a candidate gene approach, we show that the wilty mutation affects the xanthoxin dehydrogenase step in ABA biosynthesis. To date, this step has only been represented by mutants in the ABA2 gene of Arabidopsis thaliana. Functional ABA biosynthesis appears to be critical for normal stomatal responses to changes in humidity in angiosperms, with wilty mutant plants having no increase in foliar ABA levels in response to a doubling in vapour pressure deficit, and no closure of stomata. Phylogenetic analysis of the ABA2 gene family from diverse land plants indicates that an ABA-biosynthesis-specific short-chain dehydrogenase (ABA2) evolved in the earliest angiosperms. The relatively recent origin of specificity in this step has important implications for both the evolution of ABA biosynthesis and action in land plants.

Genetic diversity and trait genomic prediction in a pea diversity panel

BACKGROUND

Pea (Pisum sativum L.), a major pulse crop grown for its protein-rich seeds, is an important component of agroecological cropping systems in diverse regions of the world. New breeding challenges imposed by global climate change and new regulations urge pea breeders to undertake more efficient methods of selection and better take advantage of the large genetic diversity present in the Pisum sativum genepool. Diversity studies conducted so far in pea used Simple Sequence Repeat (SSR) and Retrotransposon Based Insertion Polymorphism (RBIP) markers. Recently, SNP marker panels have been developed that will be useful for genetic diversity assessment and marker-assisted selection.

RESULTS

A collection of diverse pea accessions, including landraces and cultivars of garden, field or fodder peas as well as wild peas was characterised at the molecular level using newly developed SNP markers, as well as SSR markers and RBIP markers. The three types of markers were used to describe the structure of the collection and revealed different pictures of the genetic diversity among the collection. SSR showed the fastest rate of evolution and RBIP the slowest rate of evolution, pointing to their contrasted mode of evolution. SNP markers were then used to predict phenotypes -the date of flowering (BegFlo), the number of seeds per plant (Nseed) and thousand seed weight (TSW)- that were recorded for the collection. Different statistical methods were tested including the LASSO (Least Absolute Shrinkage ans Selection Operator), PLS (Partial Least Squares), SPLS (Sparse Partial Least Squares), Bayes A, Bayes B and GBLUP (Genomic Best Linear Unbiased Prediction) methods and the structure of the collection was taken into account in the prediction. Despite a limited number of 331 markers used for prediction, TSW was reliably predicted.

CONCLUSION

The development of marker assisted selection has not reached its full potential in pea until now. This paper shows that the high-throughput SNP arrays that are being developed will most probably allow for a more efficient selection in this species.

Intra- and interchromosomal rearrangements between cowpea [Vigna unguiculata (L.) Walp.] and common bean (Phaseolus vulgaris L.) revealed by BAC-FISH

Cowpea (Vigna unguiculata) is an annual legume grown in tropical and subtropical regions, which is economically relevant due to high protein content in dried beans, green pods, and leaves. In this work, a comparative cytogenetic study between V. unguiculata and Phaseolus vulgaris (common bean) was conducted using BAC-FISH. Sequences previously mapped in P. vulgaris chromosomes (Pv) were used as probes in V. unguiculata chromosomes (Vu), contributing to the analysis of macrosynteny between both legumes. Thirty-seven clones from P. vulgaris 'BAT93' BAC library, corresponding to its 11 linkage groups, were hybridized in situ. Several chromosomal rearrangements were identified, such as translocations (between BACs from Pv1 and Pv8; Pv2 and Pv3; as well as Pv2 and Pv11), duplications (BAC from Pv3), as well as paracentric and pericentric inversions (BACs from Pv3, and Pv4, respectively). Two BACs (from Pv2 and Pv7), which hybridized at terminal regions in almost all P. vulgaris chromosomes, showed single-copy signal in Vu. Additionally, 17 BACs showed no signal in V. unguiculata chromosomes. The present results demonstrate the feasibility of using BAC libraries in comparative chromosomal mapping and karyotype evolution studies between Phaseolus and Vigna species, and revealed several macrosynteny and collinearity breaks among both legumes.

Genomic resources in mungbean for future breeding programs

Among the legume family, mungbean (Vigna radiata) has become one of the important crops in Asia, showing a steady increase in global production. It provides a good source of protein and contains most notably folate and iron. Beyond the nutritional value of mungbean, certain features make it a well-suited model organism among legume plants because of its small genome size, short life-cycle, self-pollinating, and close genetic relationship to other legumes. In the past, there have been several efforts to develop molecular markers and linkage maps associated with agronomic traits for the genetic improvement of mungbean and, ultimately, breeding for cultivar development to increase the average yields of mungbean. The recent release of a reference genome of the cultivated mungbean (V. radiata var. radiata VC1973A) and an additional de novo sequencing of a wild relative mungbean (V. radiata var. sublobata) has provided a framework for mungbean genetic and genome research, that can further be used for genome-wide association and functional studies to identify genes related to specific agronomic traits. Moreover, the diverse gene pool of wild mungbean comprises valuable genetic resources of beneficial genes that may be helpful in widening the genetic diversity of cultivated mungbean. This review paper covers the research progress on molecular and genomics approaches and the current status of breeding programs that have developed to move toward the ultimate goal of mungbean improvement.

Gene-based SNP discovery and genetic mapping in pea

KEY MESSAGE

Gene-based SNPs were identified and mapped in pea using five recombinant inbred line populations segregating for traits of agronomic importance. Pea (Pisum sativum L.) is one of the world's oldest domesticated crops and has been a model system in plant biology and genetics since the work of Gregor Mendel. Pea is the second most widely grown pulse crop in the world following common bean. The importance of pea as a food crop is growing due to its combination of moderate protein concentration, slowly digestible starch, high dietary fiber concentration, and its richness in micronutrients; however, pea has lagged behind other major crops in harnessing recent advances in molecular biology, genomics and bioinformatics, partly due to its large genome size with a large proportion of repetitive sequence, and to the relatively limited investment in research in this crop globally. The objective of this research was the development of a genome-wide transcriptome-based pea single-nucleotide polymorphism (SNP) marker platform using next-generation sequencing technology. A total of 1,536 polymorphic SNP loci selected from over 20,000 non-redundant SNPs identified using deep transcriptome sequencing of eight diverse Pisum accessions were used for genotyping in five RIL populations using an Illumina GoldenGate assay. The first high-density pea SNP map defining all seven linkage groups was generated by integrating with previously published anchor markers. Syntenic relationships of this map with the model legume Medicago truncatula and lentil (Lens culinaris Medik.) maps were established. The genic SNP map establishes a foundation for future molecular breeding efforts by enabling both the identification and tracking of introgression of genomic regions harbouring QTLs related to agronomic and seed quality traits.

Identification of quantitative trait loci and candidate genes for specific cellular resistance responses against Didymella pinodes in pea

KEY MESSAGE

Phenotyping of specific cellular resistance responses and improvement of previous genetic map allowed the identification of novel genomic regions controlling cellular mechanisms involved in pea resistance to ascochyta blight and provided candidate genes suitable for MAS. Didymella pinodes, causing ascochyta blight, is a major pathogen of the pea crop and is responsible for serious damage and yield losses. Resistance is inherited polygenically and several quantitative trait loci (QTLs) have been already identified. However, the position of these QTLs should be further refined to identify molecular markers more closely linked to the resistance genes. In previous works, resistance was scored visually estimating the final disease symptoms; in this study, we have conducted a more precise phenotyping of resistance evaluating specific cellular resistance responses at the histological level to perform a more accurate QTL analysis. In addition, P665 × Messire genetic map used to identify the QTLs was improved by adding 117 SNP markers located in genes. This combined approach has allowed the identification, for the first time, of genomic regions controlling cellular mechanisms directly involved in pea resistance to ascochyta blight. Furthermore, the inclusion of the gene-based SNP markers has allowed the identification of candidate genes co-located with QTLs and has provided robust markers for marker-assisted selection.

QTL analysis of frost damage in pea suggests different mechanisms involved in frost tolerance

KEY MESSAGE

Avoidance mechanisms and intrinsic resistance are complementary strategies to improve winter frost tolerance and yield potential in field pea. The development of the winter pea crop represents a major challenge to expand plant protein production in temperate areas. Breeding winter cultivars requires the combination of freezing tolerance as well as high seed productivity and quality. In this context, we investigated the genetic determinism of winter frost tolerance and assessed its genetic relationship with yield and developmental traits. Using a newly identified source of frost resistance, we developed a population of recombinant inbred lines and evaluated it in six environments in Dijon and Clermont-Ferrand between 2005 and 2010. We developed a genetic map comprising 679 markers distributed over seven linkage groups and covering 947.1 cM. One hundred sixty-one quantitative trait loci (QTL) explaining 9-71 % of the phenotypic variation were detected across the six environments for all traits measured. Two clusters of QTL mapped on the linkage groups III and one cluster on LGVI reveal the genetic links between phenology, morphology, yield-related traits and frost tolerance in winter pea. QTL clusters on LGIII highlighted major developmental gene loci (Hr and Le) and the QTL cluster on LGVI explained up to 71 % of the winter frost damage variation. This suggests that a specific architecture and flowering ideotype defines frost tolerance in winter pea. However, two consistent frost tolerance QTL on LGV were independent of phenology and morphology traits, showing that different protective mechanisms are involved in frost tolerance. Finally, these results suggest that frost tolerance can be bred independently to seed productivity and quality.

The Pea Photoperiod Response Gene STERILE NODES Is an Ortholog of LUX ARRHYTHMO

The STERILE NODES (SN) locus in pea (Pisum sativum) was one of the first photoperiod response genes to be described and provided early evidence for the genetic control of long-distance signaling in flowering-time regulation. Lines homozygous for recessive sn mutations are early flowering and photoperiod insensitive, with an increased ability to promote flowering across a graft union in short-day conditions. Here, we show that SN controls developmental regulation of genes in the FT family and rhythmic regulation of genes related to circadian clock function. Using a positional and functional candidate approach, we identify SN as the pea ortholog of LUX ARRHYTHMO, a GARP transcription factor from Arabidopsis (Arabidopsis thaliana) with an important role in circadian clock function. In addition to induced mutants, sequence analysis demonstrates the presence of at least three other independent, naturally occurring loss-of-function mutations among known sn cultivars. Examination of genetic and regulatory interactions between SN and two other circadian clock genes, HIGH RESPONSE TO PHOTOPERIOD (HR) and DIE NEUTRALIS (DNE), suggests a complex relationship in which HR regulates expression of SN and the role of DNE and HR in control of flowering is dependent on SN. These results extend previous work to show that pea orthologs of all three Arabidopsis evening complex genes regulate clock function and photoperiod-responsive flowering and suggest that the function of these genes may be widely conserved.

Genomics-assisted breeding in four major pulse crops of developing countries: present status and prospects

KEY MESSAGE

Given recent advances in pulse molecular biology, genomics-driven breeding has emerged as a promising approach to address the issues of limited genetic gain and low productivity in various pulse crops. The global population is continuously increasing and is expected to reach nine billion by 2050. This huge population pressure will lead to severe shortage of food, natural resources and arable land. Such an alarming situation is most likely to arise in developing countries due to increase in the proportion of people suffering from protein and micronutrient malnutrition. Pulses being a primary and affordable source of proteins and minerals play a key role in alleviating the protein calorie malnutrition, micronutrient deficiencies and other undernourishment-related issues. Additionally, pulses are a vital source of livelihood generation for millions of resource-poor farmers practising agriculture in the semi-arid and sub-tropical regions. Limited success achieved through conventional breeding so far in most of the pulse crops will not be enough to feed the ever increasing population. In this context, genomics-assisted breeding (GAB) holds promise in enhancing the genetic gains. Though pulses have long been considered as orphan crops, recent advances in the area of pulse genomics are noteworthy, e.g. discovery of genome-wide genetic markers, high-throughput genotyping and sequencing platforms, high-density genetic linkage/QTL maps and, more importantly, the availability of whole-genome sequence. With genome sequence in hand, there is a great scope to apply genome-wide methods for trait mapping using association studies and to choose desirable genotypes via genomic selection. It is anticipated that GAB will speed up the progress of genetic improvement of pulses, leading to the rapid development of cultivars with higher yield, enhanced stress tolerance and wider adaptability.

Cross-family translational genomics of abiotic stress-responsive genes between Arabidopsis and Medicago truncatula

Cross-species translation of genomic information may play a pivotal role in applying biological knowledge gained from relatively simple model system to other less studied, but related, genomes. The information of abiotic stress (ABS)-responsive genes in Arabidopsis was identified and translated into the legume model system, Medicago truncatula. Various data resources, such as TAIR/AtGI DB, expression profiles and literatures, were used to build a genome-wide list of ABS genes. tBlastX/BlastP similarity search tools and manual inspection of alignments were used to identify orthologous genes between the two genomes. A total of 1,377 genes were finally collected and classified into 18 functional criteria of gene ontology (GO). The data analysis according to the expression cues showed that there was substantial level of interaction among three major types (i.e., drought, salinity and cold stress) of abiotic stresses. In an attempt to translate the ABS genes between these two species, genomic locations for each gene were mapped using an in-house-developed comparative analysis platform. The comparative analysis revealed that fragmental colinearity, represented by only 37 synteny blocks, existed between Arabidopsis and M. truncatula. Based on the combination of E-value and alignment remarks, estimated translation rate was 60.2% for this cross-family translation. As a prelude of the functional comparative genomic approaches, in-silico gene network/interactome analyses were conducted to predict key components in the ABS responses, and one of the sub-networks was integrated with corresponding comparative map. The results demonstrated that core members of the sub-network were well aligned with previously reported ABS regulatory networks. Taken together, the results indicate that network-based integrative approaches of comparative and functional genomics are important to interpret and translate genomic information for complex traits such as abiotic stresses.