Numbers of genes in the NBS and RLK families vary by more than four-fold within a plant species and are regulated by multiple factors

Plant Virus Impacts on Yield and Plant-Pollinator Interactions Are Phylogenetically Modulated Independently of Domestication in Cucurbita spp

Plant defenses are conserved among closely related species, but domestication can alter host genotypes through artificial selection with potential losses in host defenses. Therefore, both domestication and host phylogenetic structure may influence plant virus infection outcomes. Here, we examined the association of phylogeny and domestication with the fitness of infected plants. We inoculated three pairs of domesticated and wild/noncultivated squash (Cucurbita spp.) with a combination of two viruses commonly found to coinfect cucurbits, zucchini yellow mosaic virus and squash mosaic virus, and recorded fitness traits related to flowers, pollination, fruit, and seed viability in the field over 2 separate years. In an additional field experiment, we recorded the relative abundance of both viruses via RT-qPCR. We found a gradient of susceptibility across the six tested lineages, and phylogenetic structure, but not domestication, contributed to differences in infection outcomes and impacts on several fitness traits, including fruit number, fruit weight, and germination. Plant virus infection also impacted the quantity and quality of floral rewards and visitation rates of specialist bee pollinators. There were no detectable differences in viral load between the six host taxa for either virus individually or the ratio of zucchini yellow mosaic virus to squash mosaic virus. Our results highlight the importance of phylogenetic structure in predicting host susceptibility to disease across wild and domesticated plants and the ability of several hosts to maintain fitness in the field despite infection. Broader consequences of plant pathogens for beneficial insects, such as pollinators, should also be considered in future research.

Identification of the AP2/ERF transcription factor family of Eleutherococcus senticosus and its expression correlation with drought stress

In this study, through analysis of the genome of Eleutherococcus senticosus (ES). 228 AP2/ERF genes were identified and classified into 5 groups AP2 (47 genes), ERF (108 genes), RAV (6 genes), DREB (64 genes), and soloist (3 genes). According to the AP2/ERF classification of Arabidopsis thaliana, the ES AP2/ERF proteins were subdivided into 15 groups. The gene structure and motifs of each group of AP2/ERF in ES were highly similar, which confirmed the conservation of AP2/ERF genes. The ES AP2/ERF genes were unevenly distributed on chromosomes, and a total of four pairs of tandem repeats, and 84 co-linear gene pairs were found, so the AP2/ERF genes expanded in a fragment replication manner, and dominated by pure selection during evolution. By analyzing the transcriptome data of ES under different drought stress conditions, 87 AP2/ERF genes with differential expression were obtained, of which 10 genes with highly significant differences were further analyzed and screened for qRT-PCR validation. To the best of our knowledge, this is the first report on the AP2/ERF gene of Eleutherococcus senticosus, and the bioinformatics analysis and experimental validation provided valuable information about them, which is of great significance for further research on the molecular mechanisms of ES in response to drought stress.

AT-Hook Transcription Factors Show Functions in Liriodendron chinense under Drought Stress and Somatic Embryogenesis

AT-hook motif nuclear localized (AHL) is a transcription factor that can directly induce plant somatic embryogenesis without adding exogenous hormones. One of its functional domains, the AT-hook motif, has a chromatin-modifying function and participates in various cellular processes, including DNA replication and repair and gene transcription leading to cell growth. Liriodendron chinense (Hemsl.) Sargent is an important ornamental and timber tree in China. However, its low drought-resistant ability further leads to a low natural growth rate of its population. Based on bioinformatics analysis, this study identified a total of 21 LcAHLs in L. chinense. To explore the expression pattern of the AHL gene family under drought and somatic embryogenesis, we performed a systematic analysis including basic characteristics, gene structure, chromosome localization, replication event, cis-acting elements and phylogenetic analyses. According to the phylogenetic tree, the 21 LcAHL genes are divided into three separate clades (Clade I, II, and III). Cis-acting element analysis indicated the involvement of the LcAHL genes in drought, cold, light, and auxin regulation. In the generated drought stress transcriptome, a total of eight LcAHL genes showed increased expression levels, with their expression peaking at 3 h and leveling off after 1 d. Nearly all LcAHL genes were highly expressed in the process of somatic embryogenesis. In this study, we performed a genome-wide analysis of the LcAHL gene family and found that LcAHLs take part in resistance to drought stress and the development of somatic embryos. These findings will provide an important theoretical basis for understanding of the LcAHL gene function.

Genome-wide identification, characterization, and evolutionary analysis of NBS genes and their association with disease resistance in Musa spp

Banana is an important food crop that is susceptible to a wide range of pests and diseases that can reduce yield and quality. The primary objective of banana breeding programs is to increase disease resistance, which requires the identification of resistance (R) genes. Despite the fact that resistant sources have been identified in bananas, the genes, particularly the nucleotide-binding site (NBS) family, which play an important role in protecting plants against pathogens, have received little attention. As a result, this study included a thorough examination of the NBS disease resistance gene family's classification, phylogenetic analysis, genome organization, evolution, cis-elements, differential expression, regulation by microRNAs, and protein-protein interaction. A total of 116 and 43 putative NBS genes from M. acuminata and M. balbisiana, respectively, were identified and characterized, and were classified into seven sub-families. Structural analysis of NBS genes revealed the presence of signal peptides, their sub-cellular localization, molecular weight and pI. Eight commonly conserved motifs were found, and NBS genes were unevenly distributed across multiple chromosomes, with the majority of NBS genes being located in chr3 and chr1 of the A and B genomes, respectively. Tandem duplication occurrences have helped bananas' NBS genes spread throughout evolution. Transcriptome analysis of NBS genes revealed significant differences in expression between resistant and susceptible cultivars of fusarium wilt, eumusae leaf spot, root lesion nematode, and drought, implying that they can be used as candidate resistant genes. Ninety miRNAs were discovered to have targets in 104 NBS genes from the A genome, providing important insights into NBS gene expression regulation. Overall, this study offers a valuable genomic resource and understanding of the function and evolution of NBS genes in relation to rapidly evolving pathogens, as well as providing breeders with selection targets for fast-tracking breeding of banana varieties with more durable resistance to pathogens.

Genome-wide analysis of NBS-LRR genes in Rosaceae species reveals distinct evolutionary patterns

The nucleotide-binding site and leucine-rich repeat (NBS-LRR) genes, one of the largest gene families in plants, are evolving rapidly and playing a critical role in plant resistance to pathogens. In this study, a genome-wide search in 12 Rosaceae genomes screened out 2188 NBS-LRR genes, with the gene number varied distinctively across different species. The reconciled phylogeny revealed 102 ancestral genes (7 RNLs, 26 TNLs, and 69 CNLs), which underwent independent gene duplication and loss events during the divergence of the Rosaceae. The NBS-LRR genes exhibited dynamic and distinct evolutionary patterns in the 12 Rosaceae species due to independent gene duplication/loss events, which resulted the discrepancy of NBS-LRR gene number among Rosaceae species. Specifically, Rubus occidentalis, Potentilla micrantha, Fragaria iinumae and Gillenia trifoliata, displayed a “first expansion and then contraction” evolutionary pattern; Rosa chinensis exhibited a “continuous expansion” pattern; F. vesca had a “expansion followed by contraction, then a further expansion” pattern, three Prunus species and three Maleae species shared a “early sharp expanding to abrupt shrinking” pattern. Overall, this study elucidated the dynamic and complex evolutionary patterns of NBS-LRR genes in the 12 Rosaceae species, and could assist further investigation of mechanisms driving these evolutionary patterns.

Copy Number Variation among Resistance Genes Analogues in Brassica napus

Copy number variations (CNVs) are defined as deletions, duplications and insertions among individuals of a species. There is growing evidence that CNV is a major factor underlining various autoimmune disorders and diseases in humans; however, in plants, especially oilseed crops, the role of CNVs in disease resistance is not well studied. Here, we investigate the genome-wide diversity and genetic properties of CNVs in resistance gene analogues (RGAs) across eight Brassica napus lines. A total of 1137 CNV events (704 deletions and 433 duplications) were detected across 563 RGAs. The results show CNVs are more likely to occur across clustered RGAs compared to singletons. In addition, 112 RGAs were linked to a blackleg resistance QTL, of which 25 were affected by CNV. Overall, we show that the presence and abundance of CNVs differ between lines, suggesting that in B. napus, the distribution of CNVs depends on genetic background. Our findings advance the understanding of CNV as an important type of genomic structural variation in B. napus and provide a resource to support breeding of advanced canola lines.

Construction of a single nucleotide polymorphism linkage map and identification of quantitative trait loci controlling heat tolerance in cowpea, Vigna unguiculata (L.) Walp

Plant tolerance to heat or high temperature is crucial to crop production, especially in the situation of elevated temperature resulting from global climate change. Cowpea, Vigna unguiculata (L.) Walp., is an internationally important legume food crop and an excellent pool of genes for numerous traits resilient to environmental extremes, particularly heat and drought. Here, we report a single nucleotide polymorphism (SNP) genetic map for cowpea and identification of the loci controlling the heat tolerance in the species. The SNP map consists of 531 bins containing 4,154 SNPs grouped into 11 linkage groups, and collectively spans 1,084.7 cM, thus having a density of one SNP in 0.26 cM or 149 kb. The 11 linkage groups of the map were aligned to the 11 cowpea chromosomes. Quantitative trait locus (QTL) mapping identified nine QTLs responsible for the cowpea heat tolerance on seven of the 11 chromosomes, with each QTL explaining 6.5-21.8% of heat tolerance phenotypic variation. Moreover, we aligned these nine QTLs to the cowpea genome. Each of the QTLs was positioned in a genomic region ranging from 209,000 bp to 12,590,450 bp, and the QTL with the largest effect (21.8%) on heat tolerance, qHT4-1, was located within an interval of only 234,195 bp. These results provide SNP markers useful for marker-assisted selection for heat tolerance and lay a foundation for cloning, characterization, and applications of the genes controlling the cowpea heat tolerance for heat tolerance genetic improvement in cowpea and related crops.

Identification, characterization and functional differentiation of the NAC gene family and its roles in response to cold stress in ginseng, Panax ginseng C.A. Meyer

The NAC gene family is one of the important plant-specific transcription factor families involved in variety of physiological processes. It has been found in several plant species; however, little is known about the gene family in ginseng, Panax ginseng C.A. Meyer. Here we report identification and systematic analysis of this gene family in ginseng. A total of 89 NAC genes, designated PgNAC01 to PgNAC89, are identified. These genes are alternatively spliced into 251 transcripts at fruiting stage of a four-year-old ginseng plant. The genes of this gene family have five conserved motifs and are clustered into 11 subfamilies, all of which are shared with the genes of the NAC gene families identified in the dicot and monocot model plant species, Arabidopsis and rice. This result indicates that the PgNAC gene family is an ancient and evolutionarily inactive gene family. Gene ontology (GO) analysis shows that the functions of the PgNAC gene family have been substantially differentiated; nevertheless, over 86% the PgNAC transcripts remain functionally correlated. Finally, five of the PgNAC genes, PgNAC05-2, PgNAC41-2, PgNAC48, PgNAC56-1, and PgNAC59, are identified to be involved in plant response to cold stress, suggesting that this gene family plays roles in response to cold stress in ginseng. These results, therefore, provide new insights into functional differentiation and evolution of a gene family in plants and gene resources necessary to comprehensively determine the functions of the PgNAC gene family in response to cold and other abiotic stresses in ginseng.

The bHLH gene family and its response to saline stress in Jilin ginseng, Panax ginseng C.A. Meyer

Basic helix-loop-helix (bHLH) gene family is a gene family of transcription factors that plays essential roles in plant growth and development, secondary metabolism and response to biotic and abiotic stresses. Therefore, a comprehensive knowledge of the bHLH gene family is paramount to understand the molecular mechanisms underlying these processes and develop advanced technologies to manipulate the processes efficiently. Ginseng, Panax ginseng C.A. Meyer, is a well-known medicinal herb; however, little is known  about the bHLH genes (PgbHLH) in the species. Here, we identified 137 PgbHLH genes from Jilin ginseng cultivar, Damaya, widely cultivated in Jilin, China, of which 50 are newly identified by pan-genome analysis. These 137 PgbHLH genes were phylogenetically classified into 26 subfamilies, suggesting their sequence diversification. They are alternatively spliced into 366 transcripts in a 4-year-old plant and involved in 11 functional subcategories of the gene ontology, indicating their functional differentiation in ginseng. The expressions of the PgbHLH genes dramatically vary spatio-temporally and across 42 genotypes, but they are still somehow functionally correlated. Moreover, the PgbHLH gene family, at least some of its genes, is shown to have roles in plant response to the abiotic stress of saline. These results provide a new insight into the evolution and functional differentiation of the bHLH gene family in plants, new bHLH genes to the PgbHLH gene family, and saline stress-responsive genes for genetic improvement in ginseng and other plant species.

Structural variation, functional differentiation and expression characteristics of the AP2/ERF gene family and its response to cold stress and methyl jasmonate in Panax ginseng C.A. Meyer

The APETALA2/Ethylene Responsive Factor (AP2/ERF) gene family has been shown to play a crucial role in plant growth and development, stress responses and secondary metabolite biosynthesis. Nevertheless, little is known about the gene family in ginseng (Panax ginseng C.A. Meyer), an important medicinal herb in Asia and North America. Here, we report the systematic analysis of the gene family in ginseng using several transcriptomic databases. A total of 189 putative AP2/ERF genes, defined as PgERF001 through PgERF189, were identified and these PgERF genes were spliced into 397 transcripts. The 93 PgERF genes that have complete AP2 domains in open reading frame were classified into five subfamilies, DREB, ERF, AP2, RAV and Soloist. The DREB subfamily and ERF subfamily were further clustered into four and six groups, respectively, compared to the 12 groups of these subfamilies found in Arabidopsis thaliana. Gene ontology categorized these 397 transcripts of the 189 PgERF genes into eight functional subcategories, suggesting their functional differentiation, and they have been especially enriched for the subcategory of nucleic acid binding transcription factor activity. The expression activity and networks of the 397 PgERF transcripts have substantially diversified across tissues, developmental stages and genotypes. The expressions of the PgERF genes also significantly varied, when ginseng was subjected to cold stress, as tested using six PgERF genes, PgERF073, PgERF079, PgERF110, PgERF115, PgERF120 and PgERF128, randomly selected from the DREB subfamily. This result suggests that the DREB subfamily genes play an important role in plant response to cold stress. Finally, we studied the responses of the PgERF genes to methyl jasmonate (MeJA). We found that 288 (72.5%) of the 397 PgERF gene transcripts responded to the MeJA treatment, with 136 up-regulated and 152 down-regulated, indicating that most members of the PgERF gene family are responsive to MeJA. These results, therefore, provide new resources and knowledge necessary for family-wide functional analysis of the PgERF genes in ginseng and related species.

Transcriptome-Wide Identification, Evolutionary Analysis, and GA Stress Response of the GRAS Gene Family in Panax ginseng C. A. Meyer

GRAS transcription factors are a kind of plant-specific transcription factor that have been found in a variety of plants. According to previous studies, GRAS proteins are widely involved in the physiological processes of plant signal transduction, stress, growth and development. The Jilin ginseng (Panax ginseng C.A. Meyer) is a heterogeneous tetraploid perennial herb of the Araliaceae family, ginseng genus. Important information regarding the GRAS transcription factors has not been reported in ginseng. In this study, 59 Panax ginseng GRAS (PgGRAS) genes were obtained from the Jilin ginseng transcriptome data and divided into 13 sub-families according to the classification of Arabidopsis thaliana. Through systematic evolution, structural variation, function and gene expression analysis, we further reveal GRAS’s potential function in plant growth processes and its stress response. The expression of PgGRAS genes responding to gibberellin acids (GAs) suggests that these genes could be activated after application concentration of GA. The qPCR analysis result shows that four PgGRAS genes belonging to the DELLA sub-family potentially have important roles in the GA stress response of ginseng hairy roots. This study provides not only a preliminary exploration of the potential functions of the GRAS genes in ginseng, but also valuable data for further exploration of the candidate PgGRAS genes of GA signaling in Jilin ginseng, especially their roles in ginseng hairy root development and GA stress response.

Analysis of the Rdr1 gene family in different Rosaceae genomes reveals an origin of an R-gene cluster after the split of Rubeae within the Rosoideae subfamily

The Rdr1 gene confers resistance to black spot in roses and belongs to a large TNL gene family, which is organized in two major clusters at the distal end of chromosome 1. We used the recently available chromosome scale assemblies for the R. chinensis 'Old Blush' genome, re-sequencing data for nine rose species and genome data for Fragaria, Rubus, Malus and Prunus to identify Rdr1 homologs from different taxa within Rosaceae. Members of the Rdr1 gene family are organized into two major clusters in R. chinensis and at a syntenic location in the Fragaria genome. Phylogenetic analysis indicates that the two clusters existed prior to the split of Rosa and Fragaria and that one cluster has a more recent origin than the other. Genes belonging to cluster 2, such as the functional Rdr1 gene muRdr1A, were subject to a faster evolution than genes from cluster 1. As no Rdr1 homologs were found in syntenic positions for Prunus persica, Malus x domestica and Rubus occidentalis, a translocation of the Rdr1 clusters to the current positions probably happened after the Rubeae split from other groups within the Rosoideae approximately 70-80 million years ago during the Cretaceous period.

Genome- Wide Analysis of the Nucleotide Binding Site Leucine-Rich Repeat Genes of Four Orchids Revealed Extremely Low Numbers of Disease Resistance Genes

Orchids are one of the most diverse flowering plant families, yet possibly maintain the smallest number of the nucleotide-binding site-leucine-rich repeat (NBS-LRR) type plant resistance (R) genes among the angiosperms. In this study, a genome-wide search in four orchid taxa identified 186 NBS-LRR genes. Furthermore, 214 NBS-LRR genes were identified from seven orchid transcriptomes. A phylogenetic analysis recovered 30 ancestral lineages (29 CNL and one RNL), far fewer than other angiosperm families. From the genetics aspect, the relatively low number of ancestral R genes is unlikely to explain the low number of R genes in orchids alone, as historical gene loss and scarce gene duplication has continuously occurred, which also contributes to the low number of R genes. Due to recent sharp expansions, Phalaenopsis equestris and Dendrobium catenatum having 52 and 115 genes, respectively, and exhibited an “early shrinking to recent expanding” evolutionary pattern, while Gastrodia elata and Apostasia shenzhenica both exhibit a “consistently shrinking” evolutionary pattern and have retained only five and 14 NBS-LRR genes, respectively. RNL genes remain in extremely low numbers with only one or two copies per genome. Notably, all of the orchid RNL genes belong to the ADR1 lineage. A separate lineage, NRG1, was entirely absent and was likely lost in the common ancestor of all monocots. All of the TNL genes were absent as well, coincident with the RNL NRG1 lineage, which supports the previously proposed notion that a potential functional association between the TNL and RNL NRG1 genes.

Molecular evolution of SUN-domain containing proteins in diverse plant species and their expression profiling in response to developmental and perturbation stimuli

SUN (Sad1/UNC-84) domain-containing proteins are highly conserved throughout evolution. They are localized to the inner membrane of the nuclear envelope and are involved in nuclear migration and nucleoskeleton formation. In the present study, a genome-wide investigation was performed in three dicotyledonous (Arabidopsis thaliana, Glycine max and Medicago truncatula) and three monocotyledonous (Oryza sativa, Zea mays and Sorghum bicolor) plants. A total of 56 SUN proteins encoded by 30 genes were identified. Based on their length, transmembrane topology, conserved domains and phylogenetic relationships, they could be divided into two previously defined groups- Cter-SUN and mid-SUN proteins. Expression of these genes was analyzed in different developmental stages, tissues and various unfavorable conditions such as salinity, drought, and hormonal treatment. Analyses indicated that the expression of SUN1/2 transcripts are ubiquitous; that of SUN3/4 are development/tissue regulated, and SUN5 are inflorescence stage-specific. This study provides an initial framework for the characterization and functional validation of the plant SUN family.

Structural Variation, Functional Differentiation, and Activity Correlation of the Cytochrome P450 Gene Superfamily Revealed in Ginseng

Ginseng ( C.A. Mey.) is one of the most important medicinal herbs for human health and medicine, for which ginsenosides are the major bioactive components. The cytochrome P450 genes, , form a large gene superfamily; however, only three genes have been identified from ginseng and shown to be involved in ginsenoside biosynthesis, indicating the importance of the gene superfamily in the process. Here we report genome-wide identification and systems analysis of the genes in ginseng, defined as genes. We identified 414 genes, including the three published genes. These genes formed a superfamily consisting of 41 gene families, with a substantial diversity in phylogeny and dramatic variation in spatiotemporal expression. Gene ontology (GO) analysis categorized the gene superfamily into 12 functional subcategories distributing among all three primary functional categories, suggesting its functional differentiation. Nevertheless, the majority of its gene members expressed correlatively and tended to form a coexpression network and some of them were commonly regulated in expression across tissues and developmental stages. These results have led to genome-wide identification of genes useful for genome-wide identification of the genes involved in ginsenoside biosynthesis in ginseng and provided the first insight into how a gene superfamily functionally differentiates and acts correlatively in plants.

NtRLK5, a novel RLK-like protein kinase from Nitotiana tobacum, positively regulates drought tolerance in transgenic Arabidopsis

Receptor-like protein kinase (RLKs) plays pivotal roles in plant growth and development as well as stress responses. However, little is known about the function of RLKs in Nitotiana tobacum. In the present study, we present data on NtRLK5, a novel RLK-like gene isolated from Hongda (Nitotiana tobacum L.). Expression profile analysis revealed that NtRLK5 was strongly induced by drought and salt stresses. Transient expression of NtRLK5-GFP fusion protein in protoplast showed that NtRLK5 was localized to plasma membrane. Overexpression of NtRLK5 conferred enhanced drought tolerance in transgenic Arabidopsis plants, which was attributed to not only the lower malondialdehyde (MDA) and H2O2 contents, but also the higher antioxidant enzymes activities. Moreover, the expression of several antioxidation- and stress-related genes was also significantly up-regulated in NtRLK5 transgenic plants under drought condition. Taken together, the results suggest that NtRLK5 functions as a positive regulator in drought tolerance.

Evolution, functional differentiation, and co-expression of the RLK gene family revealed in Jilin ginseng, Panax ginseng C.A. Meyer

Most genes in a genome exist in the form of a gene family; therefore, it is necessary to have knowledge of how a gene family functions to comprehensively understand organismal biology. The receptor-like kinase (RLK)-encoding gene family is one of the most important gene families in plants. It plays important roles in biotic and abiotic stress tolerances, and growth and development. However, little is known about the functional differentiation and relationships among the gene members within a gene family in plants. This study has isolated 563 RLK genes (designated as PgRLK genes) expressed in Jilin ginseng (Panax ginseng C.A. Meyer), investigated their evolution, and deciphered their functional diversification and relationships. The PgRLK gene family is highly diverged and formed into eight types. The LRR type is the earliest and most prevalent, while only the Lec type originated after P. ginseng evolved. Furthermore, although the members of the PgRLK gene family all encode receptor-like protein kinases and share conservative domains, they are functionally very diverse, participating in numerous biological processes. The expressions of different members of the PgRLK gene family are extremely variable within a tissue, at a developmental stage and in the same cultivar, but most of the genes tend to express correlatively, forming a co-expression network. These results not only provide a deeper and comprehensive understanding of the evolution, functional differentiation and correlation of a gene family in plants, but also an RLK genic resource useful for enhanced ginseng genetic improvement.

Functional differentiation and spatial-temporal co-expression networks of the NBS-encoding gene family in Jilin ginseng, Panax ginseng C.A. Meyer

Ginseng, Panax ginseng C.A. Meyer, is one of the most important medicinal plants for human health and medicine. It has been documented that over 80% of genes conferring resistance to bacteria, viruses, fungi and nematodes are contributed by the nucleotide binding site (NBS)-encoding gene family. Therefore, identification and characterization of NBS genes expressed in ginseng are paramount to its genetic improvement and breeding. However, little is known about the NBS-encoding genes in ginseng. Here we report genome-wide identification and systems analysis of the NBS genes actively expressed in ginseng (PgNBS genes). Four hundred twelve PgNBS gene transcripts, derived from 284 gene models, were identified from the transcriptomes of 14 ginseng tissues. These genes were classified into eight types, including TNL, TN, CNL, CN, NL, N, RPW8-NL and RPW8-N. Seven conserved motifs were identified in both the Toll/interleukine-1 receptor (TIR) and coiled-coil (CC) typed genes whereas six were identified in the RPW8 typed genes. Phylogenetic analysis showed that the PgNBS gene family is an ancient family, with a vast majority of its genes originated before ginseng originated. In spite of their belonging to a family, the PgNBS genes have functionally dramatically differentiated and been categorized into numerous functional categories. The expressions of the across tissues, different aged roots and the roots of different genotypes. However, they are coordinating in expression, forming a single co-expression network. These results provide a deeper understanding of the origin, evolution and functional differentiation and expression dynamics of the NBS-encoding gene family in plants in general and in ginseng particularly, and a NBS gene toolkit useful for isolation and characterization of disease resistance genes and for enhanced disease resistance breeding in ginseng and related species.

Transcript Profiling Reveals the Presence of Abiotic Stress and Developmental Stage Specific Ascorbate Oxidase Genes in Plants

Abiotic stress and climate change is the major concern for plant growth and crop yield. Abiotic stresses lead to enhanced accumulation of reactive oxygen species (ROS) consequently resulting in cellular damage and major losses in crop yield. One of the major scavengers of ROS is ascorbate (AA) which acts as first line of defense against external oxidants. An enzyme named ascorbate oxidase (AAO) is known to oxidize AA and deleteriously affect the plant system in response to stress. Genome-wide analysis of AAO gene family has led to the identification of five, three, seven, four, and six AAO genes in Oryza sativa, Arabidopsis, Glycine max, Zea mays, and Sorghum bicolor genomes, respectively. Expression profiling of these genes was carried out in response to various abiotic stresses and during various stages of vegetative and reproductive development using publicly available microarray database. Expression analysis in Oryza sativa revealed tissue specific expression of AAO genes wherein few members were exclusively expressed in either root or shoot. These genes were found to be regulated by both developmental cues as well as diverse stress conditions. The qRT-PCR analysis in response to salinity and drought stress in rice shoots revealed OsAAO2 to be the most stress responsive gene. On the other hand, OsAAO3 and OsAAO4 genes showed enhanced expression in roots under salinity/drought stresses. This study provides lead about important stress responsive AAO genes in various crop plants, which could be used to engineer climate resilient crop plants.

Morphological features of different polyploids for adaptation and molecular characterization of CC-NBS-LRR and LEA gene families in Agave L

Polyploidy has been widely described in many Agave L. species, but its influence on environmental response to stress is still unknown. With the objective of knowing the morphological adaptations and regulation responses of genes related to biotic (LEA) and abiotic (NBS-LRR) stress in species of Agave with different levels of ploidy, and how these factors contribute to major response of Agave against environmental stresses, we analyzed 16 morphological trials on five accessions of three species (Agave tequilana Weber, Agave angustifolia Haw. and Agave fourcroydes Lem.) with different ploidy levels (2n=2x=60 2n=3x=90, 2n=5x=150, 2n=6x=180) and evaluated the expression of NBS-LRR and LEA genes regulated by biotic and abiotic stress. It was possible to associate some morphological traits (spines, nuclei, and stomata) to ploidy level. The genetic characterization of stress-related genes NBS-LRR induced by pathogenic infection and LEA by heat or saline stresses indicated that amino acid sequence analysis in these genes showed more substitutions in higher ploidy level accessions of A. fourcroydes Lem. 'Sac Ki' (2n=5x=150) and A. angustifolia Haw. 'Chelem Ki' (2n=6x=180), and a higher LEA and NBS-LRR representativeness when compared to their diploid and triploid counterparts. In all studied Agave accessions expression of LEA and NBS-LRR genes was induced by saline or heat stresses or by infection with Erwinia carotovora, respectively. The transcriptional activation was also higher in A. angustifolia Haw. 'Chelem Ki' (2n=6x=180) and A. fourcroydes 'Sac Ki' (2n=5x=150) than in their diploid and triploid counterparts, which suggests higher adaptation to stress. Finally, the diploid accession A. tequilana Weber 'Azul' showed a differentiated genetic profile relative to other Agave accessions. The differences include similar or higher genetic representativeness and transcript accumulation of LEA and NBS-LRR genes than in polyploid (2n=5x=150 and 2n=6x=180) Agave accessions, thus suggesting a differentiated selection pressure for overcoming the lower ploidy level of the diploid A. tequilana Weber 'Azul'.

Exploring structural variants in environmentally sensitive gene families

Environmentally sensitive plant gene families like NBS-LRRs, receptor kinases, defensins and others, are known to be highly variable. However, most existing strategies for discovering and describing structural variation in complex gene families provide incomplete and imperfect results. The move to de novo genome assemblies for multiple accessions or individuals within a species is enabling more comprehensive and accurate insights about gene family variation. Earlier array-based genome hybridization and sequence-based read mapping methods were limited by their reliance on a reference genome and by misplacement of paralogous sequences. Variant discovery based on de novo genome assemblies overcome the problems arising from a reference genome and reduce sequence misplacement. As de novo genome sequencing moves to the use of longer reads, artifacts will be minimized, intact tandem gene clusters will be constructed accurately, and insights into rapid evolution will become feasible.

Large-Scale Analyses of Angiosperm Nucleotide-Binding Site-Leucine-Rich Repeat Genes Reveal Three Anciently Diverged Classes with Distinct Evolutionary Patterns

Nucleotide-binding site-leucine-rich repeat (NBS-LRR) genes make up the largest plant disease resistance gene family (R genes), with hundreds of copies occurring in individual angiosperm genomes. However, the expansion history of NBS-LRR genes during angiosperm evolution is largely unknown. By identifying more than 6,000 NBS-LRR genes in 22 representative angiosperms and reconstructing their phylogenies, we present a potential framework of NBS-LRR gene evolution in the angiosperm. Three anciently diverged NBS-LRR classes (TNLs, CNLs, and RNLs) were distinguished with unique exon-intron structures and DNA motif sequences. A total of seven ancient TNL, 14 CNL, and two RNL lineages were discovered in the ancestral angiosperm, from which all current NBS-LRR gene repertoires were evolved. A pattern of gradual expansion during the first 100 million years of evolution of the angiosperm clade was observed for CNLs. TNL numbers remained stable during this period but were eventually deleted in three divergent angiosperm lineages. We inferred that an intense expansion of both TNL and CNL genes started from the Cretaceous-Paleogene boundary. Because dramatic environmental changes and an explosion in fungal diversity occurred during this period, the observed expansions of R genes probably reflect convergent adaptive responses of various angiosperm families. An ancient whole-genome duplication event that occurred in an angiosperm ancestor resulted in two RNL lineages, which were conservatively evolved and acted as scaffold proteins for defense signal transduction. Overall, the reconstructed framework of angiosperm NBS-LRR gene evolution in this study may serve as a fundamental reference for better understanding angiosperm NBS-LRR genes.

Comparative analysis of resistance gene analogues encoding NBS-LRR domains in cotton

BACKGROUND

Plant production is severely affected by biotic and abiotic stresses R-genes exhibit resistance against a range of diseases and pathogens in plants. The nucleotide binding site and leucine rich repeat (NBS-LRR) class of R-genes is the most comprehensively studied in terms of sequence evolution and genome distribution. The differential response for resistance against biotic and abiotic stress has been observed in cultivated and wild relatives of the genus Gossypium.

RESULTS

Efforts have been made to address the recent evolution of NBS-LRR sequences within Gossypium hirsutum and resistance gene analogue (RGA) sequences derived from G. arboreum and G. raimondii. The % identity and phylogenetic analysis of NBS-LRR-encoded RGAs from tetraploid New World cotton and its diploid ancestors G. raimondii and G. arboreum suggest that the evolution of NBS-LRR-encoding sequences in G. hirsutum occurred by gradual accumulation of mutants that led to positive selection and a slow rate of divergence within distinct R-gene families.

CONCLUSION

The allotetraploid genome of cotton, after separating from its diploid parents, experienced polyploidisation, natural and artificial selection, hybrid necrosis, duplication and recombination which became the reason to shed off and evolve new genes for its survival. These driving forces influenced the development of genomic architecture that make it susceptible to diseases and pathogens as compared to donor parents.

Phylogenetic analysis of Gossypium L. using restriction fragment length polymorphism of repeated sequences

Cotton is the world's leading textile fiber crop and is also grown as a bioenergy and food crop. Knowledge of the phylogeny of closely related species and the genome origin and evolution of polyploid species is significant for advanced genomics research and breeding. We have reconstructed the phylogeny of the cotton genus, Gossypium L., and deciphered the genome origin and evolution of its five polyploid species by restriction fragment analysis of repeated sequences. Nuclear DNA of 84 accessions representing 35 species and all eight genomes of the genus were analyzed. The phylogenetic tree of the genus was reconstructed using the parsimony method on 1033 polymorphic repeated sequence restriction fragments. The genome origin of its polyploids was determined by calculating the diploid-polyploid restriction fragment correspondence (RFC). The tree is consistent with the morphological classification, genome designation and geographic distribution of the species at subgenus, section and subsection levels. Gossypium lobatum (D7) was unambiguously shown to have the highest RFC with the D-subgenomes of all five polyploids of the genus, while the common ancestor of Gossypium herbaceum (A1) and Gossypium arboreum (A2) likely contributed to the A-subgenomes of the polyploids. These results provide a comprehensive phylogenetic tree of the cotton genus and new insights into the genome origin and evolution of its polyploid species. The results also further demonstrate a simple, rapid and inexpensive method suitable for phylogenetic analysis of closely related species, especially congeneric species, and the inference of genome origin of polyploids that constitute over 70 % of flowering plants.

DNA is structured as a linear "jigsaw puzzle" in the genomes of Arabidopsis, rice, and budding yeast

Knowledge of how a genome is structured and organized from its constituent elements is crucial to understanding its biology and evolution. Here, we report the genome structuring and organization pattern as revealed by systems analysis of the sequences of three model species, Arabidopsis, rice and yeast, at the whole-genome and chromosome levels. We found that all fundamental function elements (FFE) constituting the genomes, including genes (GEN), DNA transposable elements (DTE), retrotransposable elements (RTE), simple sequence repeats (SSR), and (or) low complexity repeats (LCR), are structured in a nonrandom and correlative manner, thus leading to a hypothesis that the DNA of the species is structured as a linear "jigsaw puzzle". Furthermore, we showed that different FFE differ in their importance in the formation and evolution of the DNA jigsaw puzzle structure between species. DTE and RTE play more important roles than GEN, LCR, and SSR in Arabidopsis, whereas GEN and RTE play more important roles than LCR, SSR, and DTE in rice. The genes having multiple recognized functions play more important roles than those having single functions. These results provide useful knowledge necessary for better understanding genome biology and evolution of the species and for effective molecular breeding of rice.

Characterisation and analysis of the Aegilops sharonensis transcriptome, a wild relative of wheat in the Sitopsis section

Aegilops sharonensis Eig (Sharon goatgrass) is a wild diploid relative of wheat within the Sitopsis section of Aegilops. This species represents an untapped reservoir of genetic diversity for traits of agronomic importance, especially as a source of novel disease resistance. To gain a foothold in this genetic resource, we sequenced the cDNA from leaf tissue of two geographically distinct Ae. sharonensis accessions (1644 and 2232) using the 454 Life Sciences platform. We compared the results of two different assembly programs using different parameter sets to generate 13 distinct assemblies in an attempt to maximize representation of the gene space in de novo transcriptome assembly. The most sensitive assembly (71,029 contigs; N50 674 nts) retrieved 18,684 unique best reciprocal BLAST hits (BRBH) against six previously characterised grass proteomes while the most specific assembly (30,609 contigs; N50 815 nts) retrieved 15,687 BRBH. We combined these two assemblies into a set of 62,243 non-redundant sequences and identified 139 belonging to plant disease resistance genes of the nucleotide binding leucine-rich repeat class. Based on the non-redundant sequences, we predicted 37,743 single nucleotide polymorphisms (SNP), equivalent to one per 1,142 bp. We estimated the level of heterozygosity as 1.6% in accession 1644 and 30.1% in 2232. The Ae. sharonensis leaf transcriptome provides a rich source of sequence and SNPs for this wild wheat relative. These sequences can be used with existing monocot genome sequences and EST sequence collections (e.g. barley, Brachypodium, wheat, rice, maize and Sorghum) to assist with genetic and physical mapping and candidate gene identification in Ae. sharonensis. These resources provide an initial framework to further build on and characterise the genetic and genomic structure of Ae. sharonensis.

A receptor-like kinase gene (GbRLK) from Gossypium barbadense enhances salinity and drought-stress tolerance in Arabidopsis

BACKGROUND

Cotton (Gossypium spp.) is widely cultivated due to the important economic value of its fiber. However, extreme environmental degradation impedes cotton growth and production. Receptor-like kinase (RLK) proteins play important roles in signal transduction and participate in a diverse range of processes in response to plant hormones and environmental cues. Here, we introduced an RLK gene (GbRLK) from cotton into Arabidopsis and investigated its role in imparting abiotic stress tolerance.

RESULTS

GbRLK transcription was induced by exogenously supplied abscisic acid (ABA), salicylic acid, methyl jasmonate, mock drought conditions and high salinity. We cloned the promoter sequence of this gene via self-formed adaptor PCR. Sequence analysis revealed that the promoter region contains many cis-acting stress-responsive elements such as ABRE, W-Box, MYB-core, W-Box core, TCA-element and others. We constructed a vector containing a 1,890-bp sequence in the 5' region upstream of the initiation codon of this promoter and transformed it into Arabidopsis thaliana. GUS histochemical staining analysis showed that GbRLK was expressed mainly in leaf veins, petioles and roots of transgenic Arabidopsis, but not in the cotyledons or root hairs. GbRLK promoter activity was induced by ABA, PEG, NaCl and Verticillium dahliae. Transgenic Arabidopsis with constitutive overexpression of GbRLK exhibited a reduced rate of water loss in leaves in vitro, along with improved salinity and drought tolerance and increased sensitivity to ABA compared with non-transgenic Col-0 Arabidopsis. Expression analysis of stress-responsive genes in GbRLK Arabidopsis revealed that there was increased expression of genes involved in the ABA-dependent signaling pathway (AtRD20, AtRD22 and AtRD26) and antioxidant genes (AtCAT1, AtCCS, AtCSD2 and AtCSD1) but not ion transporter genes (AtNHX1, AtSOS1).

CONCLUSIONS

GbRLK is involved in the drought and high salinity stresses pathway by activating or participating in the ABA signaling pathway. Overexpression of GbRLK may improve stress tolerance by regulating stress-responsive genes to reduce water loss. GbRLK may be employed in the genetic engineering of novel cotton cultivars in the future. Further studying of GbRLK will help elucidate abiotic stress signaling pathways.

Frequent loss of lineages and deficient duplications accounted for low copy number of disease resistance genes in Cucurbitaceae

Background

The sequenced genomes of cucumber, melon and watermelon have relatively few R-genes, with 70, 75 and 55 copies only, respectively. The mechanism for low copy number of R-genes in Cucurbitaceae genomes remains unknown.

Results

Manual annotation of R-genes in the sequenced genomes of Cucurbitaceae species showed that approximately half of them are pseudogenes. Comparative analysis of R-genes showed frequent loss of R-gene loci in different Cucurbitaceae species. Phylogenetic analysis, data mining and PCR cloning using degenerate primers indicated that Cucurbitaceae has limited number of R-gene lineages (subfamilies). Comparison between R-genes from Cucurbitaceae and those from poplar and soybean suggested frequent loss of R-gene lineages in Cucurbitaceae. Furthermore, the average number of R-genes per lineage in Cucurbitaceae species is approximately 1/3 that in soybean or poplar. Therefore, both loss of lineages and deficient duplications in extant lineages accounted for the low copy number of R-genes in Cucurbitaceae. No extensive chimeras of R-genes were found in any of the sequenced Cucurbitaceae genomes. Nevertheless, one lineage of R-genes from Trichosanthes kirilowii, a wild Cucurbitaceae species, exhibits chimeric structures caused by gene conversions, and may contain a large number of distinct R-genes in natural populations.

Conclusions

Cucurbitaceae species have limited number of R-gene lineages and each genome harbors relatively few R-genes. The scarcity of R-genes in Cucurbitaceae species was due to frequent loss of R-gene lineages and infrequent duplications in extant lineages. The evolutionary mechanisms for large variation of copy number of R-genes in different plant species were discussed.

Construction of a plant-transformation-competent BIBAC library and genome sequence analysis of polyploid Upland cotton (Gossypium hirsutum L.)

Background

Cotton, one of the world’s leading crops, is important to the world’s textile and energy industries, and is a model species for studies of plant polyploidization, cellulose biosynthesis and cell wall biogenesis. Here, we report the construction of a plant-transformation-competent binary bacterial artificial chromosome (BIBAC) library and comparative genome sequence analysis of polyploid Upland cotton (Gossypium hirsutum L.) with one of its diploid putative progenitor species, G. raimondii Ulbr.

Results

We constructed the cotton BIBAC library in a vector competent for high-molecular-weight DNA transformation in different plant species through either Agrobacterium or particle bombardment. The library contains 76,800 clones with an average insert size of 135 kb, providing an approximate 99% probability of obtaining at least one positive clone from the library using a single-copy probe. The quality and utility of the library were verified by identifying BIBACs containing genes important for fiber development, fiber cellulose biosynthesis, seed fatty acid metabolism, cotton-nematode interaction, and bacterial blight resistance. In order to gain an insight into the Upland cotton genome and its relationship with G. raimondii, we sequenced nearly 10,000 BIBAC ends (BESs) randomly selected from the library, generating approximately one BES for every 250 kb along the Upland cotton genome. The retroelement Gypsy/DIRS1 family predominates in the Upland cotton genome, accounting for over 77% of all transposable elements. From the BESs, we identified 1,269 simple sequence repeats (SSRs), of which 1,006 were new, thus providing additional markers for cotton genome research. Surprisingly, comparative sequence analysis showed that Upland cotton is much more diverged from G. raimondii at the genomic sequence level than expected. There seems to be no significant difference between the relationships of the Upland cotton D- and A-subgenomes with the G. raimondii genome, even though G. raimondii contains a D genome (D₅).

Conclusions

The library represents the first BIBAC library in cotton and related species, thus providing tools useful for integrative physical mapping, large-scale genome sequencing and large-scale functional analysis of the Upland cotton genome. Comparative sequence analysis provides insights into the Upland cotton genome, and a possible mechanism underlying the divergence and evolution of polyploid Upland cotton from its diploid putative progenitor species, G. raimondii.

Engineering plant disease resistance based on TAL effectors

Transcription activator-like (TAL) effectors are encoded by plant-pathogenic bacteria and induce expression of plant host genes. TAL effectors bind DNA on the basis of a unique code that specifies binding of amino acid residues in repeat units to particular DNA bases in a one-to-one correspondence. This code can be used to predict binding sites of natural TAL effectors and to design novel synthetic DNA-binding domains for targeted genome manipulation. Natural mechanisms of resistance in plants against TAL effector-containing pathogens have given insights into new strategies for disease control.

Overview of tomato (Solanum lycopersicum) candidate pathogen recognition genes reveals important Solanum R locus dynamics

To investigate the genome-wide spatial arrangement of R loci, a complete catalogue of tomato (Solanum lycopersicum) and potato (Solanum tuberosum) nucleotide-binding site (NBS) NBS, receptor-like protein (RLP) and receptor-like kinase (RLK) gene repertories was generated. Candidate pathogen recognition genes were characterized with respect to structural diversity, phylogenetic relationships and chromosomal distribution. NBS genes frequently occur in clusters of related gene copies that also include RLP or RLK genes. This scenario is compatible with the existence of selective pressures optimizing coordinated transcription. A number of duplication events associated with lineage-specific evolution were discovered. These findings suggest that different evolutionary mechanisms shaped pathogen recognition gene cluster architecture to expand and to modulate the defence repertoire. Analysis of pathogen recognition gene clusters associated with documented resistance function allowed the identification of adaptive divergence events and the reconstruction of the evolution history of these loci. Differences in candidate pathogen recognition gene number and organization were found between tomato and potato. Most candidate pathogen recognition gene orthologues were distributed at less than perfectly matching positions, suggesting an ongoing lineage-specific rearrangement. Indeed, a local expansion of Toll/Interleukin-1 receptor (TIR)-NBS-leucine-rich repeat (LRR) (TNL) genes in the potato genome was evident. Taken together, these findings have implications for improved understanding of the mechanisms of molecular adaptive selection at Solanum R loci.

Impacts of resistance gene genetics, function, and evolution on a durable future

Studies on resistance gene function and evolution lie at the confluence of structural and molecular biology, genetics, and plant breeding. However, knowledge from these disparate fields has yet to be extensively integrated. This review draws on ideas and information from these different fields to elucidate the influences driving the evolution of different types of resistance genes in plants and the concurrent evolution of virulence in pathogens. It provides an overview of the factors shaping the evolution of recognition, signaling, and response genes in the context of emerging functional information along with a consideration of the new opportunities for durable resistance enabled by high-throughput DNA sequencing technologies.

Transgenic resistance confers effective field level control of bacterial spot disease in tomato

We investigated whether lines of transgenic tomato (Solanum lycopersicum) expressing the Bs2 resistance gene from pepper, a close relative of tomato, demonstrate improved resistance to bacterial spot disease caused by Xanthomonas species in replicated multi-year field trials under commercial type growing conditions. We report that the presence of the Bs2 gene in the highly susceptible VF 36 background reduced disease to extremely low levels, and VF 36-Bs2 plants displayed the lowest disease severity amongst all tomato varieties tested, including commercial and breeding lines with host resistance. Yields of marketable fruit from transgenic lines were typically 2.5 times that of the non-transformed parent line, but varied between 1.5 and 11.5 fold depending on weather conditions and disease pressure. Trials were conducted without application of any copper-based bactericides, presently in wide use despite negative impacts on the environment. This is the first demonstration of effective field resistance in a transgenic genotype based on a plant R gene and provides an opportunity for control of a devastating pathogen while eliminating ineffective copper pesticides.

Structural variants in the soybean genome localize to clusters of biotic stress-response genes

Genome-wide structural and gene content variations are hypothesized to drive important phenotypic variation within a species. Structural and gene content variations were assessed among four soybean (Glycine max) genotypes using array hybridization and targeted resequencing. Many chromosomes exhibited relatively low rates of structural variation (SV) among genotypes. However, several regions exhibited both copy number and presence-absence variation, the most prominent found on chromosomes 3, 6, 7, 16, and 18. Interestingly, the regions most enriched for SV were specifically localized to gene-rich regions that harbor clustered multigene families. The most abundant classes of gene families associated with these regions were the nucleotide-binding and receptor-like protein classes, both of which are important for plant biotic defense. The colocalization of SV with plant defense response signal transduction pathways provides insight into the mechanisms of soybean resistance gene evolution and may inform the development of new approaches to resistance gene cloning.

Evolution of a complex disease resistance gene cluster in diploid Phaseolus and tetraploid Glycine

We used a comparative genomics approach to investigate the evolution of a complex nucleotide-binding (NB)-leucine-rich repeat (LRR) gene cluster found in soybean (Glycine max) and common bean (Phaseolus vulgaris) that is associated with several disease resistance (R) genes of known function, including Rpg1b (for Resistance to Pseudomonas glycinea1b), an R gene effective against specific races of bacterial blight. Analysis of domains revealed that the amino-terminal coiled-coil (CC) domain, central nucleotide-binding domain (NB-ARC [for APAF1, Resistance genes, and CED4]), and carboxyl-terminal LRR domain have undergone distinct evolutionary paths. Sequence exchanges within the NB-ARC domain were rare. In contrast, interparalogue exchanges involving the CC and LRR domains were common, consistent with both of these regions coevolving with pathogens. Residues under positive selection were overrepresented within the predicted solvent-exposed face of the LRR domain, although several also were detected within the CC and NB-ARC domains. Superimposition of these latter residues onto predicted tertiary structures revealed that the majority are located on the surface, suggestive of a role in interactions with other domains or proteins. Following polyploidy in the Glycine lineage, NB-LRR genes have been preferentially lost from one of the duplicated chromosomes (homeologues found in soybean), and there has been partitioning of NB-LRR clades between the two homeologues. The single orthologous region in common bean contains approximately the same number of paralogues as found in the two soybean homeologues combined. We conclude that while polyploidization in Glycine has not driven a stable increase in family size for NB-LRR genes, it has generated two recombinationally isolated clusters, one of which appears to be in the process of decay.

A genome-wide comparison of NB-LRR type of resistance gene analogs (RGA) in the plant kingdom

Plants express resistance (R) genes to recognize invaders and prevent the spread of pathogens. To analyze nucleotide binding site, leucine-rich repeat (NB-LRR) genes, we constructed a fast pipeline to predict and classify the R gene analogs (RGAs) by applying in-house matrices. With predicted ~37,000 RGAs, we can directly compare RGA contents across entire plant lineages, from green algae to flowering plants. We focused on the highly divergent NBLRRs in land plants following the emergence of mosses. We identified entire loss of Toll/Interleukin-1 receptor, NBLRR (TNL) in Poaceae family of monocots and interestingly from Mimulus guttatus (a dicot), which leads to the possibility of species-specific TNL loss in other sequenced flowering plants. Using RGA maps, we have elucidated a positive correlation between the cluster sizes of NB-LRRs and their numbers. The cluster members were observed to consist of the same class of NB-LRRs or their variants, which were probably generated from a single locus for an R gene. Our website ( http://sol.kribb.re.kr/PRGA/ ), called plant resistance gene analog (PRGA), provides useful information, such as RGA annotations, tools for predicting RGAs, and analyzing domain profiles. Therefore, PRGA provides new insights into R-gene evolution and is useful in applying RGA as markers in breeding and or systematic studies.

A new RNASeq-based reference transcriptome for sugar beet and its application in transcriptome-scale analysis of vernalization and gibberellin responses

BACKGROUND

Sugar beet (Beta vulgaris sp. vulgaris) crops account for about 30% of world sugar. Sugar yield is compromised by reproductive growth hence crops must remain vegetative until harvest. Prolonged exposure to cold temperature (vernalization) in the range 6 °C to 12 °C induces reproductive growth, leading to bolting (rapid elongation of the main stem) and flowering. Spring cultivation of crops in cool temperate climates makes them vulnerable to vernalization and hence bolting, which is initiated in the apical shoot meristem in processes involving interaction between gibberellin (GA) hormones and vernalization. The underlying mechanisms are unknown and genome scale next generation sequencing approaches now offer comprehensive strategies to investigate them; enabling the identification of novel targets for bolting control in sugar beet crops. In this study, we demonstrate the application of an mRNA-Seq based strategy for this purpose.

RESULTS

There is no sugar beet reference genome, or public expression array platforms. We therefore used RNA-Seq to generate the first reference transcriptome. We next performed digital gene expression profiling using shoot apex mRNA from two sugar beet cultivars with and without applied GA, and also a vernalized cultivar with and without applied GA. Subsequent bioinformatics analyses identified transcriptional changes associated with genotypic difference and experimental treatments. Analysis of expression profiles in response to vernalization and GA treatment suggested previously unsuspected roles for a RAV1-like AP2/B3 domain protein in vernalization and efflux transporters in the GA response.

CONCLUSIONS

Next generation RNA-Seq enabled the generation of the first reference transcriptome for sugar beet and the study of global transcriptional responses in the shoot apex to vernalization and GA treatment, without the need for a reference genome or established array platforms. Comprehensive bioinformatic analysis identified transcriptional programmes associated with different sugar beet genotypes as well as biological treatments; thus providing important new opportunities for basic scientists and sugar beet breeders. Transcriptome-scale identification of agronomically important traits as used in this study should be widely applicable to all crop plants where genomic resources are limiting.

A new RNASeq-based reference transcriptome for sugar beet and its application in transcriptome-scale analysis of vernalization and gibberellin responses

Background

Sugar beet (Beta vulgaris sp. vulgaris) crops account for about 30% of world sugar. Sugar yield is compromised by reproductive growth hence crops must remain vegetative until harvest. Prolonged exposure to cold temperature (vernalization) in the range 6°C to 12°C induces reproductive growth, leading to bolting (rapid elongation of the main stem) and flowering. Spring cultivation of crops in cool temperate climates makes them vulnerable to vernalization and hence bolting, which is initiated in the apical shoot meristem in processes involving interaction between gibberellin (GA) hormones and vernalization. The underlying mechanisms are unknown and genome scale next generation sequencing approaches now offer comprehensive strategies to investigate them; enabling the identification of novel targets for bolting control in sugar beet crops. In this study, we demonstrate the application of an mRNA-Seq based strategy for this purpose.

Results

There is no sugar beet reference genome, or public expression array platforms. We therefore used RNA-Seq to generate the first reference transcriptome. We next performed digital gene expression profiling using shoot apex mRNA from two sugar beet cultivars with and without applied GA, and also a vernalized cultivar with and without applied GA. Subsequent bioinformatics analyses identified transcriptional changes associated with genotypic difference and experimental treatments. Analysis of expression profiles in response to vernalization and GA treatment suggested previously unsuspected roles for a RAV1-like AP2/B3 domain protein in vernalization and efflux transporters in the GA response.

Conclusions

Next generation RNA-Seq enabled the generation of the first reference transcriptome for sugar beet and the study of global transcriptional responses in the shoot apex to vernalization and GA treatment, without the need for a reference genome or established array platforms. Comprehensive bioinformatic analysis identified transcriptional programmes associated with different sugar beet genotypes as well as biological treatments; thus providing important new opportunities for basic scientists and sugar beet breeders. Transcriptome-scale identification of agronomically important traits as used in this study should be widely applicable to all crop plants where genomic resources are limiting.

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.

Contrasting patterns of evolution following whole genome versus tandem duplication events in Populus

Comparative analysis of multiple angiosperm genomes has implicated gene duplication in the expansion and diversification of many gene families. However, empirical data and theory suggest that whole-genome and small-scale duplication events differ with respect to the types of genes preserved as duplicate pairs. We compared gene duplicates resulting from a recent whole genome duplication to a set of tandemly duplicated genes in the model forest tree Populus trichocarpa. We used a combination of microarray expression analyses of a diverse set of tissues and functional annotation to assess factors related to the preservation of duplicate genes of both types. Whole genome duplicates are 700 bp longer and are expressed in 20% more tissues than tandem duplicates. Furthermore, certain functional categories are over-represented in each class of duplicates. In particular, disease resistance genes and receptor-like kinases commonly occur in tandem but are significantly under-retained following whole genome duplication, while whole genome duplicate pairs are enriched for members of signal transduction cascades and transcription factors. The shape of the distribution of expression divergence for duplicated pairs suggests that nearly half of the whole genome duplicates have diverged in expression by a random degeneration process. The remaining pairs have more conserved gene expression than expected by chance, consistent with a role for selection under the constraints of gene balance. We hypothesize that duplicate gene preservation in Populus is driven by a combination of subfunctionalization of duplicate pairs and purifying selection favoring retention of genes encoding proteins with large numbers of interactions.

Improving immunity in crops: new tactics in an old game

Crop disease remains a major cause of yield loss and emerging diseases pose new threats to global food security. Despite the dearth of commercial development to date, progress in using our rapidly expanding knowledge of plant-pathogen interactions to invent new ways of controlling diseases in crops has been good. Many major resistance genes have now been shown to retain function when transferred between species, and evidence indicates that resistance genes are more effective when deployed in a background containing quantitative resistance traits. The EFR pattern-recognition receptor, present in only the Brassicaceae, functions to provide bacterial disease control in the Solanaceae. Knowledge of how transcription activator-like effectors bind DNA is leading to new methods for triggering disease resistance and broader applications in genome engineering.

Co-variation among major classes of LRR-encoding genes in two pairs of plant species

NBS-LRR (nucleotide-binding site-leucine-rich repeat), LRR-RLK (LRR-receptor-like kinase), and LRR-only are the three major LRR-encoding genes. Owing to the crucial role played by them in plant resistance, development, and growth, extensive studies have been performed on the NBS-LRR and LRR-RLK genes. However, few studies have focused on these genes collectively; they may co-vary as all of them contain LRR motifs. To investigate their common evolutionary patterns, all major classes of LRR-encoding genes were identified in 12 plant species, and particularly compared in two pairs of close relatives, Arabidopsis thaliana-A. lyrata (At-Al) and Zea mays-Sorghum bicolor. Our results showed that these genes co-vary significantly in terms of their numbers between species and that the genes with certain evolutionary parameters are most likely to have similar functions. The development-related genes have clear orthologous relationships between closely related species, as well as lower nucleotide divergence, and Ka/Ks ratio. In contrast, resistance-related genes have exactly opposite characteristics and favor 11-15 LRRs per gene. This association could be very useful in predicting the function of LRR-encoding genes. The presence of co-variation suggests that LRRs, combined with other domains, can work better in some common functions. In order to cooperate efficiently, there should be balanced gene numbers among the different gene classes.