Comparative analysis of peanut NBS-LRR gene clusters suggests evolutionary innovation among duplicated domains and erosion of gene microsynteny

A chromosome-scale assembly of Brassica carinata (BBCC) accession HC20 containing resistance to multiple pathogens and an early generation assessment of introgressions into B. juncea (AABB)

Brassica carinata (BBCC) commonly referred to as Ethiopian mustard is a natural allotetraploid containing the genomes of Brassica nigra (BB) and Brassica oleracea (CC). It is an oilseed crop endemic to the northeastern regions of Africa. Although it is under limited cultivation, B. carinata is valuable as it is resistant/highly tolerant to most of the pathogens affecting widely cultivated Brassica species of the U's triangle. We report a chromosome-scale genome assembly of B. carinata accession HC20 using long-read Oxford Nanopore sequencing and Bionano optical maps. The assembly has a scaffold N50 of ~39.8 Mb and covers ~1.11 Gb of the genome. We compared the long-read genome assemblies of the U's triangle species and found extensive gene collinearity between the diploids and allopolyploids with no evidence of major gene losses. Therefore, B. juncea (AABB), B. napus (AACC), and B. carinata can be regarded as strict allopolyploids. We cataloged the nucleotide-binding and leucine-rich repeat immune receptor (NLR) repertoire of B. carinata and, identified 465 NLRs, and compared these with the NLRs in the other Brassica species. We investigated the extent and nature of early-generation genomic interactions between the constituent genomes of B. carinata and B. juncea in interspecific crosses between the two species. Besides the expected recombination between the constituent B genomes, extensive homoeologous exchanges were observed between the A and C genomes. Interspecific crosses, therefore, can be used for transferring disease resistance from B. carinata to B. juncea and broadening the genetic base of the two allotetraploid species.

The potential of genome editing to create novel alleles of resistance genes in rice

Rice, a staple food for a significant portion of the global population, faces persistent threats from various pathogens and pests, necessitating the development of resilient crop varieties. Deployment of resistance genes in rice is the best practice to manage diseases and reduce environmental damage by reducing the application of agro-chemicals. Genome editing technologies, such as CRISPR-Cas, have revolutionized the field of molecular biology, offering precise and efficient tools for targeted modifications within the rice genome. This study delves into the application of these tools to engineer novel alleles of resistance genes in rice, aiming to enhance the plant's innate ability to combat evolving threats. By harnessing the power of genome editing, researchers can introduce tailored genetic modifications that bolster the plant's defense mechanisms without compromising its essential characteristics. In this study, we synthesize recent advancements in genome editing methodologies applicable to rice and discuss the ethical considerations and regulatory frameworks surrounding the creation of genetically modified crops. Additionally, it explores potential challenges and future prospects for deploying edited rice varieties in agricultural landscapes. In summary, this study highlights the promise of genome editing in reshaping the genetic landscape of rice to confront emerging challenges, contributing to global food security and sustainable agriculture practices.

Evolution of NLR genes in genus Arachis reveals asymmetric expansion of NLRome in wild and domesticated tetraploid species

Arachis hypogaea is an allotetraploid crop widely grown in the world. Wild relatives of genus Arachis are the rich source of genetic diversity and high levels of resistance to combat pathogens and climate change. The accurate identification and characterization of plant resistance gene, nucleotide binding site leucine rich repeat receptor (NLRs) substantially contribute to the repertoire of resistances and improve production. In the current study, we have studied the evolution of NLR genes in genus Arachis and performed their comparative genomics among four diploids (A. duranensis, A. ipaensis, A. cardenasii, A. stenosperma) and two tetraploid (wild: A. monticola and domesticated: A. hypogaea) species. In total 521, 354, 284, 794, 654, 290 NLR genes were identified from A. cardenasii, A. stenosperma and A. duranensis, A. hypogaea, A. monticola and A. ipaensis respectively. Phylogenetic analysis and classification of NLRs revealed that they belong to 7 subgroups and specific subgroups have expanded in each genome leading towards divergent evolution. Gene gain and loss, duplication assay reveals that wild and domesticated tetraploids species have shown asymmetric expansion of NLRome in both sub-genome (AA and BB). A-subgenome of A. monticola exhibited significant contraction of NLRome while B-subgenome shows expansion and vice versa in case of A. hypogaea probably due to distinct natural and artificial selection pressure. In addition, diploid species A. cardenasii revealed the largest repertoire of NLR genes due to higher frequency of gene duplication and selection pressure. A. cardenasii and A. monticola can be regarded as putative resistance resources for peanut breeding program for introgression of novel resistance genes. Findings of this study also emphasize the application neo-diploids and polyploids due to higher quantitative expression of NLR genes. To the best of our knowledge, this is the first study that studied the effect of domestication and polyploidy on the evolution of NLR genes in genus Arachis to identify genomic resources for improving resistance of polyploid crop with global importance on economy and food security.

Advances in omics research on peanut response to biotic stresses

Peanut growth, development, and eventual production are constrained by biotic and abiotic stresses resulting in serious economic losses. To understand the response and tolerance mechanism of peanut to biotic and abiotic stresses, high-throughput Omics approaches have been applied in peanut research. Integrated Omics approaches are essential for elucidating the temporal and spatial changes that occur in peanut facing different stresses. The integration of functional genomics with other Omics highlights the relationships between peanut genomes and phenotypes under specific stress conditions. In this review, we focus on research on peanut biotic stresses. Here we review the primary types of biotic stresses that threaten sustainable peanut production, the multi-Omics technologies for peanut research and breeding, and the recent advances in various peanut Omics under biotic stresses, including genomics, transcriptomics, proteomics, metabolomics, miRNAomics, epigenomics and phenomics, for identification of biotic stress-related genes, proteins, metabolites and their networks as well as the development of potential traits. We also discuss the challenges, opportunities, and future directions for peanut Omics under biotic stresses, aiming sustainable food production. The Omics knowledge is instrumental for improving peanut tolerance to cope with various biotic stresses and for meeting the food demands of the exponentially growing global population.

Genome-wide identification and analysis of the NLR gene family in Medicago ruthenica

Medicago ruthenica, important forage in the legume family, possesses high nutritional value and carries abundant tolerance genes. This study used whole-genome data of M. ruthenica to perform a genome-wide analysis of the nucleotide-binding site-leucine-rich repeat receptor (NLR) gene family, which is the largest family of plant disease resistance genes (R genes). A total of 338 NLR genes were identified in the M. ruthenica genome, including 160 typical genes that contained 80 coiled-coil (CC)-NBS-LRR (CNL) genes, 76 toll/interleukin-1 receptor (TIR)-NBS-LRR (TNL) genes, four resistance to powdery mildew 8 (RPW8)-NBS-LRR (RNL) subclass genes, and 178 atypical NLR genes encoding proteins without at least one important domain. Among its eight chromosomes, M. ruthenica chromosomes 3 and 8 contained most of the NLR genes. More than 40% of all NLR genes were located on these two chromosomes, mainly in multigene clusters. The NLR proteins of M. ruthenica had six highly conserved motifs: P-loop, GLPL, RNBS-D, kinase-2, RNBS-C, and MHDV. Phylogenetic analysis revealed that the NLR genes of M. ruthenica formed three deeply separated clades according to the N-terminal domain of the proteins encoded by these genes. Gene duplication and syntenic analysis suggested four gene duplication types in the NLR genes of M. ruthenica, namely, tandem, proximal, dispersed, and segmental duplicates, which involved 189, 49, 59, and 41 genes, respectively. A total of 41 segmental duplication genes formed 23 NLR gene pairs located on syntenic chromosomal blocks mainly between chromosomes 6 and 7. In addition, syntenic analysis between M. truncatula and M. ruthenica revealed 193 gene pairs located on syntenic chromosomal blocks of the two species. The expression analysis of M. ruthenica NLR genes showed that 303 (89.6%) of the NLR genes were expressed in different varieties. Overall, this study described the full NLR profile of the M. ruthenica genome to provide an important resource for mining disease-resistant genes and disease-resistant breeding.

Genome-wide identification and characterization profile of phosphatidy ethanolamine-binding protein family genes in carrot

Members of the family of Phosphatidy Ethanolamine-Binding Protein (PEBP) have been shown to be key regulators of the transition of plants from vegetative to reproductive phases. Here, a total of 12 PEBP proteins were identified in the carrot (Daucus carota L.) genome and classified into FT-like (4), TFL1-like (6), and MFT-like 2) subfamilies, that had different lengths (110-267 aa) and were distributed unevenly across seven chromosomes. Moreover, 13 and 31 PEBP proteins were identified in other two Apiaceae species, celery (Apium graveolens L.) and coriander (Coriandrum sativum L.). The phylogenetic and evolutionary results of these PEBP family proteins were obtained based on the protein sequences. In the three Apiaceae species, purifying selection was the main evolutionary force, and WGD, segmental duplication, and dispersed duplication have played key roles in the PEBP family expansion. The expression analysis showed that carrot PEBP genes exhibited relatively broad expression patterns across various tissues. In the period of bolting to flowering, the carrot FT-like subfamily genes were upregulated as positive regulators, and TFL1-like subfamily genes remained at lower expression levels as inhibitors. More interestingly, the members of carrot FT-like genes had different temporal-spatial expression characteristics, suggesting that they have different regulatory functions in the carrot reproductive phase. In summary, this study contributes to our understanding of the PEBP family proteins and provides a foundation for exploring the mechanism of carrot bolting and flowering for the breeding of cultivars with bolting resistance.

Integrative genomics analysis of the ever-shrinking pectin methylesterase (PME) gene family in foxtail millet (Setaria italica)

Pectin methylesterase (PME) plays a vital role in the growth and development of plants. Their genes can be classified into two types, with Type-1 having an extra domain, PMEI. PME genes in foxtail millet (Setaria italica L.) have not been identified, and their sequence features and evolution have not been explored. Here, we identified 41 foxtail millet PME genes. Decoding the pro-region, containing the PMEI domain, revealed its more active nature than the DNA encoding PME domain, easier to be lost to produce Type-2 PME genes. We inferred that the active nature of the pro-region could be related to its harbouring more repetitive DNA sequences. Further, we revealed that though whole-genome duplication and tandem duplication contributed to producing new copies of PME genes, phylogenetic analysis provided clear evidence of ever-shrinking gene family size in foxtail millet and the other grasses in the past 100 million years. Phylogenetic analysis also supports the existence of two gene groups, Group I and Group II, with genes in Group II being more conservative. Our research contributes to understanding how DNA sequence structure affects the functional innovation and evolution of PME genes.

The Genome of the Mimosoid Legume Prosopis cineraria, a Desert Tree

The mimosoid legumes are a clade of ~40 genera in the Caesalpinioideae subfamily of the Fabaceae that grow in tropical and subtropical regions. Unlike the better studied Papilionoideae, there are few genomic resources within this legume group. The tree Prosopis cineraria is native to the Near East and Indian subcontinent, where it thrives in very hot desert environments. To develop a tool to better understand desert plant adaptation mechanisms, we sequenced the P. cineraria genome to near-chromosomal assembly, with a total sequence length of ~691 Mb. We predicted 77,579 gene models (76,554 CDS, 361 rRNAs and 664 tRNAs) from the assembled genome, among them 55,325 (~72%) protein-coding genes that were functionally annotated. This genome was found to consist of over 58% repeat sequences, primarily long terminal repeats (LTR-)-retrotransposons. We find an expansion of terpenoid metabolism genes in P. cineraria and its relative Prosopis alba, but not in other legumes. We also observed an amplification of NBS-LRR disease-resistance genes correlated with LTR-associated retrotransposition, and identified 410 retrogenes with an active burst of chimeric retrogene creation that approximately occurred at the same time of divergence of P. cineraria from a common lineage with P. alba~23 Mya. These retrogenes include many biotic defense responses and abiotic stress stimulus responses, as well as the early Nodulin 93 gene. Nodulin 93 gene amplification is consistent with an adaptive response of the species to the low nitrogen in arid desert soil. Consistent with these results, our differentially expressed genes show a tissue specific expression of isoprenoid pathways in shoots, but not in roots, as well as important genes involved in abiotic salt stress in both tissues. Overall, the genome sequence of P. cineraria enriches our understanding of the genomic mechanisms of its disease resistance and abiotic stress tolerance. Thus, it is a very important step in crop and legume improvement.

Insights into the plateau adaptation of Salvia castanea by comparative genomic and WGCNA analyses

INTRODUCTION

Salvia castanea, a wild plant species is adapted to extreme Qinghai-Tibetan plateau (QTP) environments. It is also used for medicinal purposes due to high ingredient of tanshinone IIA (T-IIA). Despite its importance to Chinese medicinal industry, the mechanisms associated with secondary metabolites accumulation (i.e. T-IIA and rosmarinic acid (RA)) in this species have not been characterized. Also, the role of special underground tissues in QTP adaptation of S. castanea is still unknown.

OBJECTIVES

We explored the phenomenon of periderm-like structure in underground stem center of S. castanea with an aim to unravel the molecular evolutionary mechanisms of QTP adaptation in this species.

METHODS

Morphologic observation and full-length transcriptome of S. castanea plants were conducted. Comparative genomic analyses of S. castanea with other 14 representative species were used to reveal its phylogenetic position and molecular evolutionary mechanisms. RNA-seq and WGCNA analyses were applied to understand the mechanisms of high accumulations of T-IIA and RA in S. castanea tissues.

RESULTS

Based on anatomical observations, we proposed a "trunk-branches" developmental model to explain periderm-like structure in the center of underground stem of S. castanea. Our study suggested that S. castanea branched off from cultivated Danshen around 16 million years ago. During the evolutionary process, significantly expanded orthologous gene groups, 24 species-specific and 64 positively selected genes contributed to morphogenesis and QTP adaptation in S. castanea. RNA-seq and WGCNA analyses unraveled underlying mechanisms of high accumulations of T-IIA and RA in S. castanea and identified NAC29 and TGA22 as key transcription factors.

CONCLUSION

We proposed a "trunk-branches" developmental model for the underground stem in S. castanea. Adaptations to extreme QTP environment in S. castanea are associated with accumulations of high secondary metabolites in this species.

Conversion between 100-million-year-old duplicated genes contributes to rice subspecies divergence

BACKGROUND

Duplicated gene pairs produced by ancient polyploidy maintain high sequence similarity over a long period of time and may result from illegitimate recombination between homeologous chromosomes. The genomes of Asian cultivated rice Oryza sativa ssp. indica (XI) and Oryza sativa ssp. japonica (GJ) have recently been updated, providing new opportunities for investigating ongoing gene conversion events and their impact on genome evolution.

RESULTS

Using comparative genomics and phylogenetic analyses, we evaluated gene conversion rates between duplicated genes produced by polyploidization 100 million years ago (mya) in GJ and XI. At least 5.19-5.77% of genes duplicated across the three rice genomes were affected by whole-gene conversion after the divergence of GJ and XI at ~ 0.4 mya, with more (7.77-9.53%) showing conversion of only portions of genes. Independently converted duplicates surviving in the genomes of different subspecies often use the same donor genes. The ongoing gene conversion frequency was higher near chromosome termini, with a single pair of homoeologous chromosomes, 11 and 12, in each rice genome being most affected. Notably, ongoing gene conversion has maintained similarity between very ancient duplicates, provided opportunities for further gene conversion, and accelerated rice divergence. Chromosome rearrangements after polyploidization are associated with ongoing gene conversion events, and they directly restrict recombination and inhibit duplicated gene conversion between homeologous regions. Furthermore, we found that the converted genes tended to have more similar expression patterns than nonconverted duplicates. Gene conversion affects biological functions associated with multiple genes, such as catalytic activity, implying opportunities for interaction among members of large gene families, such as NBS-LRR disease-resistance genes, contributing to the occurrence of the gene conversion.

CONCLUSION

Duplicated genes in rice subspecies generated by grass polyploidization ~ 100 mya remain affected by gene conversion at high frequency, with important implications for the divergence of rice subspecies.

Novel disease resistance gene paralogs created by CRISPR/Cas9 in soy

KEY MESSAGE

Novel disease resistance gene paralogues are generated by targeted chromosome cleavage of tandem duplicated NBS-LRR gene complexes and subsequent DNA repair in soybean. This study demonstrates accelerated diversification of innate immunity of plants using CRISPR. Nucleotide-binding-site-leucine-rich-repeat (NBS-LRR) gene families are key components of effector-triggered immunity. They are often arranged in tandem duplicated arrays in the genome, a configuration that is conducive to recombinations that will lead to new, chimeric genes. These rearrangements have been recognized as major sources of novel disease resistance phenotypes. Targeted chromosome cleavage by CRISPR/Cas9 can conceivably induce rearrangements and thus emergence of new resistance gene paralogues. Two NBS-LRR families of soy have been selected to demonstrate this concept: a four-copy family in the Rpp1 region (Rpp1L) and a large, complex locus, Rps1 with 22 copies. Copy-number variations suggesting large-scale, CRISPR/Cas9-mediated chromosome rearrangements in the Rpp1L and Rps1 complexes were detected in up to 58.8% of progenies of primary transformants using droplet-digital PCR. Sequencing confirmed development of novel, chimeric paralogs with intact open reading frames. These novel paralogs may confer new disease resistance specificities. This method to diversify innate immunity of plants by genome editing is readily applicable to other disease resistance genes or other repetitive loci.

Comparison and evolutionary analysis of Brassica nucleotide binding site leucine rich repeat (NLR) genes and importance for disease resistance breeding

The Brassica genus contains many agriculturally significant oilseed and vegetable crops, however the crop yield is threatened by a range of fungal and bacterial pathogens. Nucleotide Binding Site Leucine Rich Repeat (NLR) genes play important roles in plant innate immunity. The evolution of NLR genes is influenced by genomic processes and pathogen selection. At the whole genome level, whole genome duplications (WGDs) generate abundant gene copies, most of which are lost during genome fractionation. At sub-genomic levels, some retained copies undergo duplication forming clusters which facilitate rapid evolution through recombination. The number, distribution and genetic variations of the NLR genes vary among Brassica species and within populations suggesting differential selection pressure exerted by pathogen populations throughout the evolutionary history. A study of the evolution of disease resistance genes in agriculturally important plants such as Brassicas helps gain insights into their function and inform the identification of resistance genes for breeding of resistant lines.

Whole Genome Re-sequencing of Soybean Accession EC241780 Providing Genomic Landscape of Candidate Genes Involved in Rust Resistance

BACKGROUND

In this study, whole genome re-sequencing of rust resistant soybean genotype EC241780 was performed to understand the genomic landscape involved in the resistance mechanism.

METHODS

A total of 374 million raw reads were obtained with paired-end sequencing performed with Illumina HiSeq 2500 instrument, out of which 287.3 million high quality reads were mapped to Williams 82 reference genome. Comparative sequence analysis of EC241780 with rust susceptible cultivars Williams 82 and JS 335 was performed to identify sequence variation and to prioritise the candidate genes.

RESULTS

Comparative analysis indicates that genotype EC241780 has high sequence similarity with rust resistant genotype PI 200492 and the resistance in EC241780 is conferred by the Rpp1 locus. Based on the sequence variations and functional annotations, three genes Glyma18G51715, Glyma18G51741 and Glyma18G51765 encoding for NBS-LRR family protein were identified as the most prominent candidate for Rpp1 locus.

CONCLUSION

The study provides insights of genome-wide sequence variation more particularly at Rpp1 loci which will help to develop rust resistant soybean cultivars through efficient exploration of the genomic resource.

In silico guided structural and functional analysis of genes with potential involvement in resistance to coffee leaf rust: A functional marker based approach

Physiology-based differentiation of S_H genes and Hemileia vastatrix races is the principal method employed for the characterization of coffee leaf rust resistance. Based on the gene-for-gene theory, nine major rust resistance genes (S_H1-9) have been proposed. However, these genes have not been characterized at the molecular level. Consequently, the lack of molecular data regarding rust resistance genes or candidates is a major bottleneck in coffee breeding. To address this issue, we screened a BAC library with resistance gene analogs (RGAs), identified RGAs, characterized and explored for any S_H related candidate genes. Herein, we report the identification and characterization of a gene (gene 11), which shares conserved sequences with other S_H genes and displays a characteristic polymorphic allele conferring different resistance phenotypes. Furthermore, comparative analysis of the two RGAs belonging to CC-NBS-LRR revealed more intense diversifying selection in tomato and grape genomes than in coffee. For the first time, the present study has unveiled novel insights into the molecular nature of the S_H genes, thereby opening new avenues for coffee rust resistance molecular breeding. The characterized candidate RGA is of particular importance for further biological function analysis in coffee.

Draft genome sequence of Solanum aethiopicum provides insights into disease resistance, drought tolerance, and the evolution of the genome

BACKGROUND

The African eggplant (Solanum aethiopicum) is a nutritious traditional vegetable used in many African countries, including Uganda and Nigeria. It is thought to have been domesticated in Africa from its wild relative, Solanum anguivi. S. aethiopicum has been routinely used as a source of disease resistance genes for several Solanaceae crops, including Solanum melongena. A lack of genomic resources has meant that breeding of S. aethiopicum has lagged behind other vegetable crops.

RESULTS

We assembled a 1.02-Gb draft genome of S. aethiopicum, which contained predominantly repetitive sequences (78.9%). We annotated 37,681 gene models, including 34,906 protein-coding genes. Expansion of disease resistance genes was observed via 2 rounds of amplification of long terminal repeat retrotransposons, which may have occurred ∼1.25 and 3.5 million years ago, respectively. By resequencing 65 S. aethiopicum and S. anguivi genotypes, 18,614,838 single-nucleotide polymorphisms were identified, of which 34,171 were located within disease resistance genes. Analysis of domestication and demographic history revealed active selection for genes involved in drought tolerance in both "Gilo" and "Shum" groups. A pan-genome of S. aethiopicum was assembled, containing 51,351 protein-coding genes; 7,069 of these genes were missing from the reference genome.

CONCLUSIONS

The genome sequence of S. aethiopicum enhances our understanding of its biotic and abiotic resistance. The single-nucleotide polymorphisms identified are immediately available for use by breeders. The information provided here will accelerate selection and breeding of the African eggplant, as well as other crops within the Solanaceae family.

The genome of cultivated peanut provides insight into legume karyotypes, polyploid evolution and crop domestication

High oil and protein content make tetraploid peanut a leading oil and food legume. Here we report a high-quality peanut genome sequence, comprising 2.54 Gb with 20 pseudomolecules and 83,709 protein-coding gene models. We characterize gene functional groups implicated in seed size evolution, seed oil content, disease resistance and symbiotic nitrogen fixation. The peanut B subgenome has more genes and general expression dominance, temporally associated with long-terminal-repeat expansion in the A subgenome that also raises questions about the A-genome progenitor. The polyploid genome provided insights into the evolution of Arachis hypogaea and other legume chromosomes. Resequencing of 52 accessions suggests that independent domestications formed peanut ecotypes. Whereas 0.42-0.47 million years ago (Ma) polyploidy constrained genetic variation, the peanut genome sequence aids mapping and candidate-gene discovery for traits such as seed size and color, foliar disease resistance and others, also providing a cornerstone for functional genomics and peanut improvement.

Resistance to Thrips in Peanut and Implications for Management of Thrips and Thrips-Transmitted Orthotospoviruses in Peanut

Thrips are major pests of peanut (Arachis hypogaea L.) worldwide, and they serve as vectors of devastating orthotospoviruses such as Tomato spotted wilt virus (TSWV) and Groundnut bud necrosis virus (GBNV). A tremendous effort has been devoted to developing peanut cultivars with resistance to orthotospoviruses. Consequently, cultivars with moderate field resistance to viruses exist, but not much is known about host resistance to thrips. Integrating host plant resistance to thrips in peanut could suppress thrips feeding damage and reduce virus transmission, will decrease insecticide usage, and enhance sustainability in the production system. This review focuses on details of thrips resistance in peanut and identifies future directions for incorporating thrips resistance in peanut cultivars. Research on thrips-host interactions in peanut is predominantly limited to field evaluations of feeding damage, though, laboratory studies have revealed that peanut cultivars could differentially affect thrips feeding and thrips biology. Many runner type cultivars, field resistant to TSWV, representing diverse pedigrees evaluated against thrips in the greenhouse revealed that thrips preferred some cultivars over others, suggesting that antixenosis "non-preference" could contribute to thrips resistance in peanut. In other crops, morphological traits such as leaf architecture and waxiness and spectral reflectance have been associated with thrips non-preference. It is not clear if foliar morphological traits in peanut are associated with reduced preference or non-preference of thrips and need to be evaluated. Besides thrips non-preference, thrips larval survival to adulthood and median developmental time were negatively affected in some peanut cultivars and in a diploid peanut species Arachis diogoi (Hoehne) and its hybrids with a Virginia type cultivar, indicating that antibiosis (negative effects on biology) could also be a factor influencing thrips resistance in peanut. Available field resistance to orthotospoviruses in peanut is not complete, and cultivars can suffer substantial yield loss under high thrips and virus pressure. Integrating thrips resistance with available virus resistance would be ideal to limit losses. A discussion of modern technologies such as transgenic resistance, marker assisted selection and RNA interference, and future directions that could be undertaken to integrate resistance to thrips and to orthotospoviruses in peanut cultivars is included in this article.

Comparative root transcriptome of wild Arachis reveals NBS-LRR genes related to nematode resistance

BACKGROUND

The Root-Knot Nematode (RKN), Meloidogyne arenaria, significantly reduces peanut grain quality and yield worldwide. Whilst the cultivated species has low levels of resistance to RKN and other pests and diseases, peanut wild relatives (Arachis spp.) show rich genetic diversity and harbor high levels of resistance to many pathogens and environmental constraints. Comparative transcriptome analysis can be applied to identify candidate resistance genes.

RESULTS

Transcriptome analysis during the early stages of RKN infection of two peanut wild relatives, the highly RKN resistant Arachis stenosperma and the moderately susceptible A. duranensis, revealed genes related to plant immunity with contrasting expression profiles. These included genes involved in hormone signaling and secondary metabolites production and also members of the NBS-LRR class of plant disease resistance (R) genes. From 345 NBS-LRRs identified in A.duranensis reference genome, 52 were differentially expressed between inoculated and control samples, with the majority occurring in physical clusters unevenly distributed on eight chromosomes with preferential tandem duplication. The majority of these NBS-LRR genes showed contrasting expression behaviour between A. duranensis and A. stenosperma, particularly at 6 days after nematode inoculation, coinciding with the onset of the Hypersensitive Response in the resistant species. The physical clustering of some of these NBS-LRR genes correlated with their expression patterns in the contrasting genotypes. Four NBS-LRR genes exclusively expressed in A. stenosperma are located within clusters on chromosome Aradu. A09, which harbors a QTL for RKN resistance, suggesting a functional role for their physical arrangement and their potential involvement in this defense response.

CONCLUSION

The identification of functional novel R genes in wild Arachis species responsible for triggering effective defense cascades can contribute to the crop genetic improvement and enhance peanut resilience to RKN.

Evolutionary Divergence of TNL Disease-Resistant Proteins in Soybean (Glycine max) and Common Bean (Phaseolus vulgaris)

Disease-resistant genes (R genes) encode proteins that are involved in protecting plants from their pathogens and pests. Availability of complete genome sequences from soybean and common bean allowed us to perform a genome-wide identification and analysis of the Toll interleukin-1 receptor-like nucleotide-binding site leucine-rich repeat (TNL) proteins. Hidden Markov model (HMM) profiling of all protein sequences resulted in the identification of 117 and 77 regular TNL genes in soybean and common bean, respectively. We also identified TNL gene homologs with unique domains, and signal peptides as well as nuclear localization signals. The TNL genes in soybean formed 28 clusters located on 10 of the 20 chromosomes, with the majority found on chromosome 3, 6 and 16. Similarly, the TNL genes in common bean formed 14 clusters located on five of the 11 chromosomes, with the majority found on chromosome 10. Phylogenetic analyses of the TNL genes from Arabidopsis, soybean and common bean revealed less divergence within legumes relative to the divergence between legumes and Arabidopsis. Syntenic blocks were found between chromosomes Pv10 and Gm03, Pv07 and Gm10, as well as Pv01 and Gm14. The gene expression data revealed basal level expression and tissue specificity, while analysis of available microRNA data showed 37 predicted microRNA families involved in targeting the identified TNL genes in soybean and common bean.

Evolution and characterisation of the AhRAF4 NB-ARC gene family induced by Aspergillus flavus inoculation and abiotic stresses in peanut

Aflatoxin contamination in peanut is a serious food safety issue to human health around the world. Finding disease resistance genes is a key strategy for genetic improvement in breeding to deal with this issue. We identified an Aspergillus flavus-induced NBS-LRR gene, AhRAF4, using a microarray-based approach. By comparison of 23 sequences from three species using phytogenetics, protein secondary structure and three-dimensional structural analyses, AhRAF4 was revealed to be derived from Arachis duranensis by recombination, and has newly evolved into a family of several members, characterised by duplications and point mutations. However, the members of the family descended from A. ipaensis were lost following tetraploidisation. AhRAF4 was slightly up-regulated by low temperature, drought, salicylic acid and ethylene, but down-regulated by methyl jasmonate. The distinct responses upon As. flavus inoculation and the differential reactions between resistant and susceptible varieties indicate that AhRAF4 might play a role in defence responses. Temporal and spatial expression and the phenotype of transformed protoplasts suggest that AhRAF4 may also be associated with pericarp development. Because tetraploid cultivated peanuts are vulnerable to many pathogens, an exploration of R-genes may provide an effective method for genetic improvement of peanut cultivars.

Common Bean Subtelomeres Are Hot Spots of Recombination and Favor Resistance Gene Evolution

Subtelomeres of most eukaryotes contain fast-evolving genes usually involved in adaptive processes. In common bean (Phaseolus vulgaris), the Co-2 anthracnose resistance (R) locus corresponds to a cluster of nucleotide-binding-site leucine-rich-repeat (NL) encoding sequences, the prevalent class of plant R genes. To study the recent evolution of this R gene cluster, we used a combination of sequence, genetic and cytogenetic comparative analyses between common bean genotypes from two distinct gene pools (Andean and Mesoamerican) that diverged 0.165 million years ago. Co-2 is a large subtelomeric cluster on chromosome 11 comprising from 32 (Mesoamerican) to 52 (Andean) NL sequences embedded within khipu satellite repeats. Since the recent split between Andean and Mesoamerican gene pools, the Co-2 cluster has experienced numerous gene-pool specific NL losses, leading to distinct NL repertoires. The high proportion of solo-LTR retrotransposons indicates that the Co-2 cluster is located in a hot spot of unequal intra-strand homologous recombination. Furthermore, we observe large segmental duplications involving both Non-Homologous End Joining and Homologous Recombination double-strand break repair pathways. Finally, the identification of a Mesoamerican-specific subtelomeric sequence reveals frequent interchromosomal recombinations between common bean subtelomeres. Altogether, our results highlight that common bean subtelomeres are hot spots of recombination and favor the rapid evolution of R genes. We propose that chromosome ends could act as R gene incubators in many plant genomes.

New reference genome sequences of hot pepper reveal the massive evolution of plant disease-resistance genes by retroduplication

Background

Transposable elements are major evolutionary forces which can cause new genome structure and species diversification. The role of transposable elements in the expansion of nucleotide-binding and leucine-rich-repeat proteins (NLRs), the major disease-resistance gene families, has been unexplored in plants.

Results

We report two high-quality de novo genomes (Capsicum baccatum and C. chinense) and an improved reference genome (C. annuum) for peppers. Dynamic genome rearrangements involving translocations among chromosomes 3, 5, and 9 were detected in comparison between C. baccatum and the two other peppers. The amplification of athila LTR-retrotransposons, members of the gypsy superfamily, led to genome expansion in C. baccatum. In-depth genome-wide comparison of genes and repeats unveiled that the copy numbers of NLRs were greatly increased by LTR-retrotransposon-mediated retroduplication. Moreover, retroduplicated NLRs are abundant across the angiosperms and, in most cases, are lineage-specific.

Conclusions

Our study reveals that retroduplication has played key roles for the massive emergence of NLR genes including functional disease-resistance genes in pepper plants.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-017-1341-9) contains supplementary material, which is available to authorized users.

New reference genome sequences of hot pepper reveal the massive evolution of plant disease-resistance genes by retroduplication

BACKGROUND

Transposable elements are major evolutionary forces which can cause new genome structure and species diversification. The role of transposable elements in the expansion of nucleotide-binding and leucine-rich-repeat proteins (NLRs), the major disease-resistance gene families, has been unexplored in plants.

RESULTS

We report two high-quality de novo genomes (Capsicum baccatum and C. chinense) and an improved reference genome (C. annuum) for peppers. Dynamic genome rearrangements involving translocations among chromosomes 3, 5, and 9 were detected in comparison between C. baccatum and the two other peppers. The amplification of athila LTR-retrotransposons, members of the gypsy superfamily, led to genome expansion in C. baccatum. In-depth genome-wide comparison of genes and repeats unveiled that the copy numbers of NLRs were greatly increased by LTR-retrotransposon-mediated retroduplication. Moreover, retroduplicated NLRs are abundant across the angiosperms and, in most cases, are lineage-specific.

CONCLUSIONS

Our study reveals that retroduplication has played key roles for the massive emergence of NLR genes including functional disease-resistance genes in pepper plants.

Comparative analysis of NBS-LRR genes and their response to Aspergillus flavus in Arachis

Studies have demonstrated that nucleotide-binding site-leucine-rich repeat (NBS-LRR) genes respond to pathogen attack in plants. Characterization of NBS-LRR genes in peanut is not well documented. The newly released whole genome sequences of Arachis duranensis and Arachis ipaënsis have allowed a global analysis of this important gene family in peanut to be conducted. In this study, we identified 393 (AdNBS) and 437 (AiNBS) NBS-LRR genes from A. duranensis and A. ipaënsis, respectively, using bioinformatics approaches. Full-length sequences of 278 AdNBS and 303 AiNBS were identified. Fifty-one orthologous, four AdNBS paralogous, and six AiNBS paralogous gene pairs were predicted. All paralogous gene pairs were located in the same chromosomes, indicating that tandem duplication was the most likely mechanism forming these paralogs. The paralogs mainly underwent purifying selection, but most LRR 8 domains underwent positive selection. More gene clusters were found in A. ipaënsis than in A. duranensis, possibly owing to tandem duplication events occurring more frequently in A. ipaënsis. The expression profile of NBS-LRR genes was different between A. duranensis and A. hypogaea after Aspergillus flavus infection. The up-regulated expression of NBS-LRR in A. duranensis was continuous, while these genes responded to the pathogen temporally in A. hypogaea.

Comparative genomic de-convolution of the cotton genome revealed a decaploid ancestor and widespread chromosomal fractionation

The 'apparently' simple genomes of many angiosperms mask complex evolutionary histories. The reference genome sequence for cotton (Gossypium spp.) revealed a ploidy change of a complexity unprecedented to date, indeed that could not be distinguished as to its exact dosage. Herein, by developing several comparative, computational and statistical approaches, we revealed a 5× multiplication in the cotton lineage of an ancestral genome common to cotton and cacao, and proposed evolutionary models to show how such a decaploid ancestor formed. The c. 70% gene loss necessary to bring the ancestral decaploid to its current gene count appears to fit an approximate geometrical model; that is, although many genes may be lost by single-gene deletion events, some may be lost in groups of consecutive genes. Gene loss following cotton decaploidy has largely just reduced gene copy numbers of some homologous groups. We designed a novel approach to deconvolute layers of chromosome homology, providing definitive information on gene orthology and paralogy across broad evolutionary distances, both of fundamental value and serving as an important platform to support further studies in and beyond cotton and genomics communities.

QTL mapping for bacterial wilt resistance in peanut (Arachis hypogaea L.)

Bacterial wilt (BW) caused by Ralstonia solanacearum is a serious, global, disease of peanut (Arachis hypogaea L.), but it is especially destructive in China. Identification of DNA markers linked to the resistance to this disease will help peanut breeders efficiently develop resistant cultivars through molecular breeding. A F₂ population, from a cross between disease-resistant and disease-susceptible cultivars, was used to detect quantitative trait loci (QTL) associated with the resistance to this disease in the cultivated peanut. Genome-wide SNPs were identified from restriction-site-associated DNA sequencing tags using next-generation DNA sequencing technology. SNPs linked to disease resistance were determined in two bulks of 30 resistant and 30 susceptible plants along with two parental plants using bulk segregant analysis. Polymorphic SSR and SNP markers were utilized for construction of a linkage map and for performing the QTL analysis, and a moderately dense linkage map was constructed in the F₂ population. Two QTL (qBW-1 and qBW-2) detected for resistance to BW disease were located in the linkage groups LG1 and LG10 and account for 21 and 12 % of the bacterial wilt phenotypic variance. To confirm these QTL, the F₈ RIL population with 223 plants was utilized for genotyping and phenotyping plants by year and location as compared to the F₂ population. The QTL qBW-1 was consistent in the location of LG1 in the F₈ population though the QTL qBW-2 could not be clarified due to fewer markers used and mapped in LG10. The QTL qBW-1, including four linked SNP markers and one SSR marker within 14.4-cM interval in the F₈, was closely related to a disease resistance gene homolog and was considered as a candidate gene for resistance to BW. QTL identified in this study would be useful to conduct marker-assisted selection and may permit cloning of resistance genes. Our study shows that bulk segregant analysis of genome-wide SNPs is a useful approach to expedite the identification of genetic markers linked to disease resistance traits in the allotetraploidy species peanut.

A haplotype map of allohexaploid wheat reveals distinct patterns of selection on homoeologous genomes

Background

Bread wheat is an allopolyploid species with a large, highly repetitive genome. To investigate the impact of selection on variants distributed among homoeologous wheat genomes and to build a foundation for understanding genotype-phenotype relationships, we performed population-scale re-sequencing of a diverse panel of wheat lines.

Results

A sample of 62 diverse lines was re-sequenced using the whole exome capture and genotyping-by-sequencing approaches. We describe the allele frequency, functional significance, and chromosomal distribution of 1.57 million single nucleotide polymorphisms and 161,719 small indels. Our results suggest that duplicated homoeologous genes are under purifying selection. We find contrasting patterns of variation and inter-variant associations among wheat genomes; this, in addition to demographic factors, could be explained by differences in the effect of directional selection on duplicated homoeologs. Only a small fraction of the homoeologous regions harboring selected variants overlapped among the wheat genomes in any given wheat line. These selected regions are enriched for loci associated with agronomic traits detected in genome-wide association studies.

Conclusions

Evidence suggests that directional selection in allopolyploids rarely acted on multiple parallel advantageous mutations across homoeologous regions, likely indicating that a fitness benefit could be obtained by a mutation at any one of the homoeologs. Additional advantageous variants in other homoelogs probably either contributed little benefit, or were unavailable in populations subjected to directional selection. We hypothesize that allopolyploidy may have increased the likelihood of beneficial allele recovery by broadening the set of possible selection targets.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-015-0606-4) contains supplementary material, which is available to authorized users.

A novel approach for multi-domain and multi-gene family identification provides insights into evolutionary dynamics of disease resistance genes in core eudicot plants

Background

Recent advances in DNA sequencing techniques resulted in more than forty sequenced plant genomes representing a diverse set of taxa of agricultural, energy, medicinal and ecological importance. However, gene family curation is often only inferred from DNA sequence homology and lacks insights into evolutionary processes contributing to gene family dynamics. In a comparative genomics framework, we integrated multiple lines of evidence provided by gene synteny, sequence homology and protein-based Hidden Markov Modelling to extract homologous super-clusters composed of multi-domain resistance (R)-proteins of the NB-LRR type (for NUCLEOTIDE BINDING/LEUCINE-RICH REPEATS), that are involved in plant innate immunity.

Results

To assess the diversity of R-proteins within and between species, we screened twelve eudicot plant genomes including six major crops and found a total of 2,363 NB-LRR genes. Our curated R-proteins set shows a 50% average for tandem duplicates and a 22% fraction of gene copies retained from ancient polyploidy events (ohnologs). We provide evidence for strong positive selection and show significant differences in molecular evolution rates (Ka/Ks-ratio) among tandem- (mean = 1.59), ohnolog (mean = 1.36) and singleton (mean = 1.22) R-gene duplicates. To foster the process of gene-edited plant breeding, we report species-specific presence/absence of all 140 NB-LRR genes present in the model plant Arabidopsis and describe four distinct clusters of NB-LRR “gatekeeper” loci sharing syntenic orthologs across all analyzed genomes.

Conclusion

By curating a near-complete set of multi-domain R-protein clusters in an eudicot-wide scale, our analysis offers significant insight into evolutionary dynamics underlying diversification of the plant innate immune system. Furthermore, our methods provide a blueprint for future efforts to identify and more rapidly clone functional NB-LRR genes from any plant species.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-966) contains supplementary material, which is available to authorized users.

Comparative and evolutionary analysis of major peanut allergen gene families

Peanut (Arachis hypogaea L.) causes one of the most serious food allergies. Peanut seed proteins, Arah1, Arah2, and Arah3, are considered to be among the most important peanut allergens. To gain insights into genome organization and evolution of allergen-encoding genes, approximately 617 kb from the genome of cultivated peanut and 215 kb from a wild relative were sequenced including three Arah1, one Arah2, eight Arah3, and two Arah6 gene family members. To assign polarity to differences between homoeologous regions in peanut, we used as outgroups the single orthologous regions in Medicago, Lotus, common bean, chickpea, and pigeonpea, which diverged from peanut about 50 Ma and have not undergone subsequent polyploidy. These regions were also compared with orthologs in many additional dicot plant species to help clarify the timing of evolutionary events. The lack of conservation of allergenic epitopes between species, and the fact that many different proteins can be allergenic, makes the identification of allergens across species by comparative studies difficult. The peanut allergen genes are interspersed with low-copy genes and transposable elements. Phylogenetic analyses revealed lineage-specific expansion and loss of low-copy genes between species and homoeologs. Arah1 syntenic regions are conserved in soybean, pigeonpea, tomato, grape, Lotus, and Arabidopsis, whereas Arah3 syntenic regions show genome rearrangements. We infer that tandem and segmental duplications led to the establishment of the Arah3 gene family. Our analysis indicates differences in conserved motifs in allergen proteins and in the promoter regions of the allergen-encoding genes. Phylogenetic analysis and genomic organization studies provide new insights into the evolution of the major peanut allergen-encoding genes.

Comparative proteomic analysis of differentially expressed proteins induced by hydrogen sulfide in Spinacia oleracea leaves

Hydrogen sulfide (H2S), as a potential gaseous messenger molecule, has been suggested to play important roles in a wide range of physiological processes in plants. The aim of present study was to investigate which set of proteins is involved in H2S-regulated metabolism or signaling pathways. Spinacia oleracea seedlings were treated with 100 µM NaHS, a donor of H2S. Changes in protein expression profiles were analyzed by 2-D gel electrophoresis coupled with MALDI-TOF MS. Over 1000 protein spots were reproducibly resolved, of which the abundance of 92 spots was changed by at least 2-fold (sixty-five were up-regulated, whereas 27 were down-regulated). These proteins were functionally divided into 9 groups, including energy production and photosynthesis, cell rescue, development and cell defense, substance metabolism, protein synthesis and folding, cellular signal transduction. Further, we found that these proteins were mainly localized in cell wall, plasma membrane, chloroplast, mitochondria, nucleus, peroxisome and cytosol. Our results demonstrate that H2S is involved in various cellular and physiological activities and has a distinct influence on photosynthesis, cell defense and cellular signal transduction in S. oleracea leaves. These findings provide new insights into proteomic responses in plants under physiological levels of H2S.

Identification of expressed resistance gene analogs from peanut (Arachis hypogaea L.) expressed sequence tags

Low genetic diversity makes peanut (Arachis hypogaea L.) very vulnerable to plant pathogens, causing severe yield loss and reduced seed quality. Several hundred partial genomic DNA sequences as nucleotide-binding-site leucine-rich repeat (NBS-LRR) resistance genes (R) have been identified, but a small portion with expressed transcripts has been found. We aimed to identify resistance gene analogs (RGAs) from peanut expressed sequence tags (ESTs) and to develop polymorphic markers. The protein sequences of 54 known R genes were used to identify homologs from peanut ESTs from public databases. A total of 1,053 ESTs corresponding to six different classes of known R genes were recovered, and assembled 156 contigs and 229 singletons as peanut-expressed RGAs. There were 69 that encoded for NBS-LRR proteins, 191 that encoded for protein kinases, 82 that encoded for LRR-PK/transmembrane proteins, 28 that encoded for Toxin reductases, 11 that encoded for LRR-domain containing proteins and four that encoded for TM-domain containing proteins. Twenty-eight simple sequence repeats (SSRs) were identified from 25 peanut expressed RGAs. One SSR polymorphic marker (RGA121) was identified. Two polymerase chain reaction-based markers (Ahsw-1 and Ahsw-2) developed from RGA013 were homologous to the Tomato Spotted Wilt Virus (TSWV) resistance gene. All three markers were mapped on the same linkage group AhIV. These expressed RGAs are the source for RGA-tagged marker development and identification of peanut resistance genes.

Quantitative Trait Locus (QTL) meta-analysis and comparative genomics for candidate gene prediction in perennial ryegrass (Lolium perenne L.)

BACKGROUND

In crop species, QTL analysis is commonly used for identification of factors contributing to variation of agronomically important traits. As an important pasture species, a large number of QTLs have been reported for perennial ryegrass based on analysis of biparental mapping populations. Further characterisation of those QTLs is, however, essential for utilisation in varietal improvement programs.

RESULTS

A bibliographic survey of perennial ryegrass trait-dissection studies identified a total of 560 QTLs from previously published papers, of which 189, 270 and 101 were classified as morphology-, physiology- and resistance/tolerance-related loci, respectively. The collected dataset permitted a subsequent meta-QTL study and implementation of a cross-species candidate gene identification approach. A meta-QTL analysis based on use of the BioMercator software was performed to identify two consensus regions for pathogen resistance traits. Genes that are candidates for causal polymorphism underpinning perennial ryegrass QTLs were identified through in silico comparative mapping using rice databases, and 7 genes were assigned to the p150/112 reference map. Markers linked to the LpDGL1, LpPh1 and LpPIPK1 genes were located close to plant size, leaf extension time and heading date-related QTLs, respectively, suggesting that these genes may be functionally associated with important agronomic traits in perennial ryegrass.

CONCLUSIONS

Functional markers are valuable for QTL meta-analysis and comparative genomics. Enrichment of such genetic markers may permit further detailed characterisation of QTLs. The outcomes of QTL meta-analysis and comparative genomics studies may be useful for accelerated development of novel perennial ryegrass cultivars with desirable traits.

Family-based linkage and association mapping reveals novel genes affecting Plum pox virus infection in Arabidopsis thaliana

Sharka is a devastating viral disease caused by the Plum pox virus (PPV) in stone fruit trees and few sources of resistance are known in its natural hosts. Since any knowledge gained from Arabidopsis on plant virus susceptibility factors is likely to be transferable to crop species, Arabidopsis's natural variation was searched for host factors essential for PPV infection. To locate regions of the genome associated with susceptibility to PPV, linkage analysis was performed on six biparental populations as well as on multiparental lines. To refine quantitative trait locus (QTL) mapping, a genome-wide association analysis was carried out using 147 Arabidopsis accessions. Evidence was found for linkage on chromosomes 1, 3 and 5 with restriction of PPV long-distance movement. The most relevant signals occurred within a region at the bottom of chromosome 3, which comprises seven RTM3-like TRAF domain-containing genes. Since the resistance mechanism analyzed here is recessive and the rtm3 knockout mutant is susceptible to PPV infection, it suggests that other gene(s) present in the small identified region encompassing RTM3 are necessary for PPV long-distance movement. In consequence, we report here the occurrence of host factor(s) that are indispensable for virus long-distance movement.

Genome-wide mapping of NBS-LRR genes and their association with disease resistance in soybean

BACKGROUND

R genes are a key component of genetic interactions between plants and biotrophic bacteria and are known to regulate resistance against bacterial invasion. The most common R proteins contain a nucleotide-binding site and a leucine-rich repeat (NBS-LRR) domain. Some NBS-LRR genes in the soybean genome have also been reported to function in disease resistance. In this study, the number of NBS-LRR genes was found to correlate with the number of disease resistance quantitative trait loci (QTL) that flank these genes in each chromosome. NBS-LRR genes co-localized with disease resistance QTL. The study also addressed the functional redundancy of disease resistance on recently duplicated regions that harbor NBS-LRR genes and NBS-LRR gene expression in the bacterial leaf pustule (BLP)-induced soybean transcriptome.

RESULTS

A total of 319 genes were determined to be putative NBS-LRR genes in the soybean genome. The number of NBS-LRR genes on each chromosome was highly correlated with the number of disease resistance QTL in the 2-Mb flanking regions of NBS-LRR genes. In addition, the recently duplicated regions contained duplicated NBS-LRR genes and duplicated disease resistance QTL, and possessed either an uneven or even number of NBS-LRR genes on each side. The significant difference in NBS-LRR gene expression between a resistant near-isogenic line (NIL) and a susceptible NIL after inoculation of Xanthomonas axonopodis pv. glycines supports the conjecture that NBS-LRR genes have disease resistance functions in the soybean genome.

CONCLUSIONS

The number of NBS-LRR genes and disease resistance QTL in the 2-Mb flanking regions of each chromosome was significantly correlated, and several recently duplicated regions that contain NBS-LRR genes harbored disease resistance QTL for both sides. In addition, NBS-LRR gene expression was significantly different between the BLP-resistant NIL and the BLP-susceptible NIL in response to bacterial infection. From these observations, NBS-LRR genes are suggested to contribute to disease resistance in soybean. Moreover, we propose models for how NBS-LRR genes were duplicated, and apply Ks values for each NBS-LRR gene cluster.

Molecular mapping of the Pl(16) downy mildew resistance gene from HA-R4 to facilitate marker-assisted selection in sunflower

The major genes controlling sunflower downy mildew resistance have been designated as Pl genes. Ten of the more than 20 Pl genes reported have been mapped. In this study, we report the molecular mapping of gene Pl(16) in a sunflower downy mildew differential line, HA-R4. It was mapped on the lower end of linkage group (LG) 1 of the sunflower reference map, with 12 markers covering a distance of 78.9 cM. One dominant simple sequence repeat (SSR) marker, ORS1008, co-segregated with Pl(16), and another co-dominant expressed sequence tag (EST)-SSR marker, HT636, was located 0.3 cM proximal to the Pl(16) gene. The HT636 marker was also closely linked to the Pl(13) gene in another sunflower differential line, HA-R5. Thus the Pl(16) and Pl(13) genes were mapped to a similar position on LG 1 that is different from the previously reported Pl(14) gene. When the co-segregating and tightly linked markers for the Pl(16) gene were applied to other germplasms or hybrids, a unique band pattern for the ORS1008 marker was detected in HA-R4 and HA-R5 and their F(1) hybrids. This is the first report to provide two tightly linked markers for both the Pl(16) and Pl(13) genes, which will facilitate marker-assisted selection in sunflower resistance breeding, and provide a basis for the cloning of these genes.

Genome Wide Analysis of Nucleotide-Binding Site Disease Resistance Genes in Brachypodium distachyon

Nucleotide-binding site (NBS) disease resistance genes play an important role in defending plants from a variety of pathogens and insect pests. Many R-genes have been identified in various plant species. However, little is known about the NBS-encoding genes in Brachypodium distachyon. In this study, using computational analysis of the B. distachyon genome, we identified 126 regular NBS-encoding genes and characterized them on the bases of structural diversity, conserved protein motifs, chromosomal locations, gene duplications, promoter region, and phylogenetic relationships. EST hits and full-length cDNA sequences (from Brachypodium database) of 126 R-like candidates supported their existence. Based on the occurrence of conserved protein motifs such as coiled-coil (CC), NBS, leucine-rich repeat (LRR), these regular NBS-LRR genes were classified into four subgroups: CC-NBS-LRR, NBS-LRR, CC-NBS, and X-NBS. Further expression analysis of the regular NBS-encoding genes in Brachypodium database revealed that these genes are expressed in a wide range of libraries, including those constructed from various developmental stages, tissue types, and drought challenged or nonchallenged tissue.

Genome-enabled insights into legume biology

Legumes are the third-largest family of angiosperms, the second-most-important crop family, and a key source of biological nitrogen in agriculture. Recently, the genome sequences of Glycine max (soybean), Medicago truncatula, and Lotus japonicus were substantially completed. Comparisons among legume genomes reveal a key role for duplication, especially a whole-genome duplication event approximately 58 Mya that is shared by most agriculturally important legumes. A second and more recent genome duplication occurred only in the lineage leading to soybean. Outcomes of genome duplication, including gene fractionation and sub- and neofunctionalization, have played key roles in shaping legume genomes and in the evolution of legume-specific traits. Analysis of legume genome sequences also enables the discovery of legume-specific gene families and provides a framework for genome-wide association mapping that will target phenotypes of special importance in legumes. Translating genomic resources from sequenced species to less studied but still important "orphan" legumes will enhance prospects for world food production.