Genetic linkage map construction and QTL mapping of cadmium accumulation in radish (Raphanus sativus L.)

Construction of a high-density genetic map based on specific-locus amplified fragment sequencing and identification of loci controlling anthocyanin pigmentation in Yunnan red radish

Radish (Raphanus sativus L.) belongs to the family Brassicaceae. The Yunnan red radish variety contains fairly relatively large amounts of anthocyanins, making them important raw materials for producing edible red pigment. However, the genetic mechanism underlying this pigmentation has not been fully characterized. Herein, the radish inbred line YAAS-WR1 (white root-skin and white root-flesh) was crossed with the inbred line YAAS-RR1 (red root-skin and red root-flesh) to produce F1, F2, BC1P1, and BC1P2 populations. Genetic analyses revealed that the pigmented/non-pigmented (PiN) and purple/red (PR) traits were controlled by two genetic loci. The F2 population and the specific-locus amplified fragment sequencing (SLAF-seq) technique were used to construct a high-density genetic map (1230.16 cM), which contained 4032 markers distributed in nine linkage groups, with a mean distance between markers of 0.31 cM. Additionally, two QTL (QAC1 and QAC2) considerably affecting radish pigmentation were detected. A bioinformatics analysis of the QAC1 region identified 58 predicted protein-coding genes. Of these genes, RsF3'H, which is related to anthocyanin biosynthesis, was revealed as a likely candidate gene responsible for the PR trait. The results were further verified by analyzing gene structure and expression. Regarding QAC2, RsMYB1.3 was determined to be a likely candidate gene important for the PiN trait, with a 4-bp insertion in the first exon that introduced a premature termination codon in the YAAS-WR1 sequence. Assays demonstrated that RsMYB1.3 interacted with RsTT8 and activates RsTT8 and RsUFGT expression. These findings may help clarify the complex regulatory mechanism underlying radish anthocyanin synthesis. Furthermore, this study's results may be relevant for the molecular breeding of radish to improve the anthocyanin content and appearance of the taproots.

Melatonin confers cadmium tolerance by modulating critical heavy metal chelators and transporters in radish plants

Cadmium (Cd) is an environmental pollutant that causes health hazard to living organisms. Melatonin (MT) has emerged as a ubiquitous pleiotropic molecule capable of coordinating heavy metal (HM) stresses in plants. However, it remains unclear how melatonin mediates Cd homeostasis and detoxification at transcriptional and/or post-transcriptional levels in radish. Herein, the activities of five key antioxidant enzymes were increased, while root and shoot Cd contents were dramatically decreased by melatonin. A combined small RNA and transcriptome sequencing analysis showed that 14 differentially expressed microRNAs (DEMs) and 966 differentially expressed genes (DEGs) were shared between the Cd and Cd + MT conditions. In all, 23 and ten correlated miRNA-DEG pairs were identified in Con vs. Cd and Con vs. Cd + MT comparisons, respectively. Several DEGs encoding yellow stripe 1-like (YSL), heavy metal ATPases (HMA), and ATP-binding cassette (ABC) transporters were involved in Cd transportation and sequestration in radish. Root exposure to Cd²⁺ induced several specific signaling molecules, which consequently trigger some HM chelators, transporters, and antioxidants to achieve reactive oxygen species (ROS) scavenging and detoxification and eliminate Cd toxicity in radish plants. Notably, transgenic analysis revealed that overexpression of the RsMT1 (Metallothionein 1) gene could enhance Cd tolerance of tobacco plants, indicating that the exogenous melatonin confers Cd tolerance, which might be attributable to melatonin-mediated upregulation of RsMT1 gene in radish plants. These results could contribute to dissecting the molecular basis governing melatonin-mediated Cd stress response in plants and pave the way for high-efficient genetically engineering low-Cd-content cultivars in radish breeding programs.

Quantitative Trait Loci (QTLs) Associated with Microspore Culture in Raphanus sativus L. (Radish)

The radish is a highly self-incompatible plant, and consequently it is difficult to produce homozygous lines. Bud pollination in cross-fertilization plants should be done by opening immature pollen and attaching pollen to mature flowers. It accordingly takes a lot of time and effort to develop lines with fixed alleles. In the current study, a haploid breeding method has been applied to obtain homozygous plants in a short period of time by doubling chromosomes through the induction of a plant body in the haploid cells, in order to shorten the time to breed inbred lines. We constructed genetic maps with an F1 population derived by crossing parents that show a superior and inferior ability to regenerate microspores, respectively. Genetic maps were constructed from the maternal and parental maps, separately, using the two-way pseudo-testcross model. The phenotype of the regeneration rate was examined by microspore cultures and a quantitative trait loci (QTL) analysis was performed based on the regeneration rate. From the results of the culture of microspores in the F1 population, more than half of the group did not regenerate, and only a few showed a high regeneration rate. A total of five significant QTLs were detected in the F1 population, and five candidate genes were found based on the results. These candidate genes are divided into two classes, and appear to be related to either PRC2 subunits or auxin synthesis.

An ultra-high-density genetic map provides insights into genome synteny, recombination landscape and taproot skin colour in radish (Raphanus sativus L.)

High-density genetic map is a valuable tool for exploring novel genomic information, quantitative trait locus (QTL) mapping and gene discovery of economically agronomic traits in plant species. However, high-resolution genetic map applied to tag QTLs associated with important traits and to investigate genomic features underlying recombination landscape in radish (Raphanus sativus) remains largely unexplored. In this study, an ultra-high-density genetic map with 378 738 SNPs covering 1306.8 cM in nine radish linkage groups (LGs) was developed by a whole-genome sequencing-based approach. A total of 18 QTLs for 11 horticulture traits were detected. The map-based cloning data indicated that the R2R3-MYB transcription factor RsMYB90 was a crucial candidate gene determining the taproot skin colour. Comparative genomics analysis among radish, Brassica rapa and B. oleracea genome revealed several genomic rearrangements existed in the radish genome. The highly uneven distribution of recombination was observed across the nine radish chromosomes. Totally, 504 recombination hot regions (RHRs) were enriched near gene promoters and terminators. The recombination rate in RHRs was positively correlated with the density of SNPs and gene, and GC content, respectively. Functional annotation indicated that genes within RHRs were mainly involved in metabolic process and binding. Three QTLs for three traits were found in the RHRs. The results provide novel insights into the radish genome evolution and recombination landscape, and facilitate the development of effective strategies for molecular breeding by targeting and dissecting important traits in radish.

Construction of a genetic linkage map in Pyropia yezoensis (Bangiales, Rhodophyta) and QTL analysis of several economic traits of blades

Pyropia yezoensis is an economically important seaweed but its molecular genetics is poorly understood. In the present study, we used a doubled haploid (DH) population that was established in our previous work to construct a genetic linkage map of P. yezoensis and analyze the quantitative trait loci (QTLs) of blades. The DH population was genotyped with fluorescent sequence-related amplified polymorphism (SRAP) markers. A chi-square test identified 301 loci with normal segregation (P ≥ 0.01) and 96 loci (24.18%) with low-level skewed segregation (0.001 ≤ P < 0.01). The genetic map was constructed after a total of 92 loci were assembled into three linkage groups (LGs). The map spanned 557.36 cM covering 93.71% of the estimated genome, with a mean interlocus space of 6.23 cM. Kolmogorov-Smirnov test (α = 5%) showed a uniform distribution of the markers along each LG. On the genetic map, 10 QTLs associated with five economic traits of blades were detected. One QTL was for length, one for width, two for fresh weight, two for specific growth rate of length and four for specific growth rate of fresh weight. These QTLs could explain 2.29–7.87% of the trait variations, indicating that their effects were all minor. The results may serve as a framework for future marker-assisted breeding in P. yezoensis.

Genome-Wide Association Study of Cadmium Accumulation at the Seedling Stage in Rapeseed (Brassica napus L.)

Cadmium is a potentially toxic heavy metal to human health. Rapeseed (Brassica napus L.), a vegetable and oilseed crop, might also be a Cd hyperaccumulator, but there is little information on this trait in rapeseed. We evaluated Cd accumulation in different oilseed accessions and employed a genome-wide association study to identify quantitative trait loci (QTLs) related to Cd accumulation. A total of 419 B. napus accessions and inbred lines were genotyped with a 60K Illumina Infinium SNP array of Brassica. Wide genotypic variations in Cd concentration and translocation were found. Twenty-five QTLs integrated with 98 single-nucleotide polymorphisms (SNPs) located at 15 chromosomes were associated with Cd accumulation traits. These QTLs explained 3.49-7.57% of the phenotypic variation observed. Thirty-two candidate genes were identified in these genomic regions, and they were 0.33-497.97 kb away from the SNPs. We found orthologs of Arabidopsis thaliana located near the significant SNPs on the B. napus genome, including NRAMP6 (natural resistance-associated macrophage protein 6), IRT1 (iron-regulated transporter 1), CAD1 (cadmium-sensitive 1), and PCS2 (phytochelatin synthase 2). Of them, four candidate genes were verified by qRT-PCR, the expression levels of which were significantly higher after exposure to Cd than in the controls. Our results might facilitate the study of the genetic basis of Cd accumulation and the cloning of candidate Cd accumulation genes, which could be used to help reduce Cd levels in edible plant parts and/or create more efficient hyperaccumulators.

Comparative transcriptomics uncovers alternative splicing and molecular marker development in radish (Raphanus sativus L.)

Background

Alternative splicing (AS) plays important roles in gene expression and proteome diversity. Single nucleotide polymorphism (SNP) and insertion/deletion (InDel) are abundant polymorphisms and co-dominant inheritance markers, which have been widely used in germplasm identification, genetic mapping and marker-assisted selection in plants. So far, however, little information is available on utilization of AS events and development of SNP and InDel markers from transcriptome in radish.

Results

In this study, three radish transcriptome datasets were collected and aligned to the reference radish genome. A total of 56,530 AS events were identified from three radish genotypes with intron retention (IR) being the most frequent AS type, which accounted for 59.4% of the total expressed genes in radish. In all, 22,412 SNPs and 9436 InDels were identified with an average frequency of 1 SNP/17.9 kb and 1 InDel/42.5 kb, respectively. A total of 43,680 potential SSRs were identified in 31,604 assembled unigenes with a density of 1 SSR/2.5 kb. The ratio of SNPs with nonsynonymous/synonymous mutations was 1.05:1. Moreover, 35 SNPs and 200 InDels were randomly selected and validated by Sanger sequencing, 83.9% of the SNPs and 70% of the InDels exhibited polymorphism among these three genotypes. In addition, the 15 SNPs and 125 InDels were found to be unevenly distributed on 9 linkage groups. Furthermore, 40 informative InDel markers were successfully used for the genetic diversity analysis on 32 radish accessions.

Conclusions

These results would not only provide new insights into transcriptome complexity and AS regulation, but also furnish large amount of molecular marker resources for germplasm identification, genetic mapping and further genetic improvement of radish in breeding programs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3874-4) contains supplementary material, which is available to authorized users.

Dehydration-responsive miRNAs in foxtail millet: genome-wide identification, characterization and expression profiling

A set of novel and known dehydration-responsive miRNAs have been identified in foxtail millet. These findings provide new insights into understanding the functional role of miRNAs and their respective targets in regulating plant response to dehydration stress. MicroRNAs perform significant regulatory roles in growth, development and stress response of plants. Though the miRNA-mediated gene regulatory networks under dehydration stress remain largely unexplored in plant including foxtail millet (Setaria italica), which is a natural abiotic stress tolerant crop. To find out the dehydration-responsive miRNAs at the global level, four small RNA libraries were constructed from control and dehydration stress treated seedlings of two foxtail millet cultivars showing contrasting tolerance behavior towards dehydration stress. Using Illumina sequencing technology, 55 known and 136 novel miRNAs were identified, representing 22 and 48 miRNA families, respectively. Eighteen known and 33 novel miRNAs were differentially expressed during dehydration stress. After the stress treatment, 32 dehydration-responsive miRNAs were up-regulated in tolerant cultivar and 22 miRNAs were down-regulated in sensitive cultivar, suggesting that miRNA-mediated molecular regulation might play important roles in providing contrasting characteristics to these cultivars. Predicted targets of identified miRNAs were found to encode various transcription factors and functional enzymes, indicating their involvement in broad spectrum regulatory functions and biological processes. Further, differential expression patterns of seven known miRNAs were validated by northern blot and expression of ten novel dehydration-responsive miRNAs were confirmed by SL-qRT PCR. Differential expression behavior of five miRNA-target genes was verified under dehydration stress treatment and two of them also validated by RLM RACE. Overall, the present study highlights the importance of dehydration stress-associated post-transcriptional regulation governed by miRNAs and their targets in a naturally stress-tolerant model crop.

Transcriptome-based gene expression profiling identifies differentially expressed genes critical for salt stress response in radish (Raphanus sativus L.)

Transcriptome-based gene expression analysis identifies many critical salt-responsive genes in radish and facilitates further dissecting the molecular mechanism underlying salt stress response. Salt stress severely impacts plant growth and development. Radish, a moderately salt-sensitive vegetable crop, has been studied for decades towards the physiological and biochemical performances under salt stress. However, no systematic study on isolation and identification of genes involved in salt stress response has been performed in radish, and the molecular mechanism governing this process is still indistinct. Here, the RNA-Seq technique was applied to analyze the transcriptomic changes on radish roots treated with salt (200 mM NaCl) for 48 h in comparison with those cultured in normal condition. Totally 8709 differentially expressed genes (DEGs) including 3931 up- and 4778 down-regulated genes were identified. Functional annotation analysis indicated that many genes could be involved in several aspects of salt stress response including stress sensing and signal transduction, osmoregulation, ion homeostasis and ROS scavenging. The association analysis of salt-responsive genes and miRNAs exhibited that 36 miRNA-mRNA pairs had negative correlationship in expression trends. Reverse-transcription quantitative PCR (RT-qPCR) analysis revealed that the expression profiles of DEGs were in line with results from the RNA-Seq analysis. Furthermore, the putative model of DEGs and miRNA-mediated gene regulation was proposed to elucidate how radish sensed and responded to salt stress. This study represents the first comprehensive transcriptome-based gene expression profiling under salt stress in radish. The outcomes of this study could facilitate further dissecting the molecular mechanism underlying salt stress response and provide a valuable platform for further genetic improvement of salt tolerance in radish breeding programs.

Functional and Integrative Analysis of the Proteomic Profile of Radish Root under Pb Exposure

Lead (Pb) is one of the most abundant heavy metal (HM) pollutants, which can penetrate the plant through the root and then enter the food chain causing potential health risks for human beings. Radish is an important root vegetable crop worldwide. To investigate the mechanism underlying plant response to Pb stress in radish, the protein profile changes of radish roots respectively upon Pb(NO₃)₂ at 500 mg L^-1(Pb500) and 1000 mg L^-1(Pb1000), were comprehensively analyzed using iTRAQ (Isobaric Tag for Relative and Absolute Quantification). A total of 3898 protein species were successfully detected and 2141 were quantified. Among them, a subset of 721 protein species were differentially accumulated upon at least one Pb treatment, and 135 ones showed significantly abundance changes under both two Pb-stressed conditions. Many critical protein species related to protein translation, processing, and degradation, reactive oxygen species (ROS) scavenging, photosynthesis, and respiration and carbon metabolism were successfully identified. Gene Ontology (GO) and pathway enrichment analysis of the 135 differential abundance protein species (DAPS) revealed that the overrepresented GO terms included "cell wall," "apoplast," "response to metal ion," "vacuole," and "peroxidase activity," and the critical enriched pathways were involved in "citric acid (TCA) cycle and respiratory electron transport," "pyruvate metabolism," "phenylalanine metabolism," "phenylpropanoid biosynthesis," and "carbon metabolism." Furthermore, the integrative analysis of transcriptomic, miRNA, degradome, metabolomics and proteomic data provided a strengthened understanding of radish response to Pb stress at multiple levels. Under Pb stress, many key enzymes (i.e., ATP citrate lyase, Isocitrate dehydrogenase, fumarate hydratase and malate dehydrogenase) involved in the glycolysis and TCA cycle were severely affected, which ultimately cause alteration of some metabolites including glucose, citrate and malate. Meanwhile, a series of other defense responses including ascorbate (ASA)-glutathione (GSH) cycle for ROS scavenging and Pb-defense protein species (glutaredoxin, aldose 1-epimerase malate dehydrogenase and thioredoxin), were triggered to cope with Pb-induced injuries. These results would be helpful for further dissecting molecular mechanism underlying plant response to HM stresses, and facilitate effective management of HM contamination in vegetable crops by genetic manipulation.

Quantitative Trait Loci for Morphological Traits and their Association with Functional Genes in Raphanus sativus

Identification of quantitative trait loci (QTLs) governing morphologically important traits enables to comprehend their potential genetic mechanisms in the genetic breeding program. In this study, we used 210 F2 populations derived from a cross between two radish inbred lines (Raphanus sativus) "835" and "B2," including 258 SSR markers were used to detect QTLs for 11 morphological traits that related to whole plant, leaf, and root yield in 3 years of replicated field test. Total 55 QTLs were detected which were distributed on each linkage group of the Raphanus genome. Individual QTLs accounted for 2.69-12.6 of the LOD value, and 0.82-16.25% of phenotypic variation. Several genomic regions have multiple traits that clustered together, suggested the existence of pleiotropy linkage. Synteny analysis of the QTL regions with A. thaliana genome selected orthologous genes in radish. InDels and SNPs in the parental lines were detected in those regions by Illumina genome sequence. Five identified candidate gene-based markers were validated by co-mapping with underlying QTLs affecting different traits. Semi-quantitative reverse transcriptase PCR analysis showed the different expression levels of these five genes in parental lines. In addition, comparative QTL analysis with B. rapa revealed six common QTL regions and four key major evolutionarily conserved crucifer blocks (J, U, R, and W) harboring QTL for morphological traits. The QTL positions identified in this study will provide a valuable resource for identifying more functional genes when whole radish genome sequence is released. Candidate genes identified in this study that co-localized in QTL regions are expected to facilitate in radish breeding programs.

Metabolomic analysis with GC-MS to reveal potential metabolites and biological pathways involved in Pb &Cd stress response of radish roots

The radish (Raphanus sativus L.) is an important root vegetable crop. In this study, the metabolite profiling analysis of radish roots exposed to lead (Pb) and cadmium (Cd) stresses has been performed using gas chromatography-mass spectrometry (GC-MS). The score plots of principal component analysis (PCA) and partial least squares-discriminate analysis (PLS-DA) showed clear discrimination between control and Pb- or Cd-treated samples. The metabolic profiling indicated Pb or Cd stress could cause large metabolite alteration mainly on sugars, amino acids and organic acids. Furthermore, an integrated analysis of the effects of Pb or Cd stress was performed on the levels of metabolites and gene transcripts from our previous transcriptome work in radish roots. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of integration data demonstrated that exposure of radish to Pb stress resulted in profound biochemical changes including carbohydrate metabolism, energy metabolism and glutathione metabolism, while the treatment of Cd stress caused significant variations in energy production, amino acid metabolism and oxidative phosphorylation-related pathways. These results would facilitate further dissection of the mechanisms of heavy metal (HM) accumulation/tolerance in plants and the effective management of HM contamination in vegetable crops by genetic manipulation.

De novo sequencing of root transcriptome reveals complex cadmium-responsive regulatory networks in radish (Raphanus sativus L.)

Cadmium (Cd) is a nonessential metallic trace element that poses potential chronic toxicity to living organisms. To date, little is known about the Cd-responsive regulatory network in root vegetable crops including radish. In this study, 31,015 unigenes representing 66,552 assembled unique transcripts were isolated from radish root under Cd stress based on de novo transcriptome assembly. In all, 1496 differentially expressed genes (DEGs) consisted of 3579 transcripts were identified from Cd-free (CK) and Cd-treated (Cd200) libraries. Gene Ontology and pathway enrichment analysis indicated that the up- and down-regulated DEGs were predominately involved in glucosinolate biosynthesis as well as cysteine and methionine-related pathways, respectively. RT-qPCR showed that the expression profiles of DEGs were in consistent with results from RNA-Seq analysis. Several candidate genes encoding phytochelatin synthase (PCS), metallothioneins (MTs), glutathione (GSH), zinc iron permease (ZIPs) and ABC transporter were responsible for Cd uptake, accumulation, translocation and detoxification in radish. The schematic model of DEGs and microRNAs-involved in Cd-responsive regulatory network was proposed. This study represents a first comprehensive transcriptome-based characterization of Cd-responsive DEGs in radish. These results could provide fundamental insight into complex Cd-responsive regulatory networks and facilitate further genetic manipulation of Cd accumulation in root vegetable crops.

Identification of novel and salt-responsive miRNAs to explore miRNA-mediated regulatory network of salt stress response in radish (Raphanus sativus L.)

Background

Salt stress is one of the most representative abiotic stresses that severely affect plant growth and development. MicroRNAs (miRNAs) are well known for their significant involvement in plant responses to abiotic stresses. Although miRNAs implicated in salt stress response have been widely reported in numerous plant species, their regulatory roles in the adaptive response to salt stress in radish (Raphanus sativus L.), an important root vegetable crop worldwide, remain largely unknown.

Results

Solexa sequencing of two sRNA libraries from NaCl-free (CK) and NaCl-treated (Na200) radish roots were performed for systematical identification of salt-responsive miRNAs and their expression profiling in radish. Totally, 136 known miRNAs (representing 43 miRNA families) and 68 potential novel miRNAs (belonging to 51 miRNA families) were identified. Of these miRNAs, 49 known and 22 novel miRNAs were differentially expressed under salt stress. Target prediction and annotation indicated that these miRNAs exerted a role by regulating specific stress-responsive genes, such as squamosa promoter binding-like proteins (SPLs), auxin response factors (ARFs), nuclear transcription factor Y (NF-Y) and superoxide dismutase [Cu-Zn] (CSD1). Further functional analysis suggested that these target genes were mainly implicated in signal perception and transduction, regulation of ion homeostasis, basic metabolic processes, secondary stress responses, as well as modulation of attenuated plant growth and development under salt stress. Additionally, the expression patterns of ten miRNAs and five corresponding target genes were validated by reverse-transcription quantitative PCR (RT-qPCR).

Conclusions

With the sRNA sequencing, salt-responsive miRNAs and their target genes in radish were comprehensively identified. The results provide novel insight into complex miRNA-mediated regulatory network of salt stress response in radish, and facilitate further dissection of molecular mechanism underlying plant adaptive response to salt stress in root vegetable crops.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1416-5) contains supplementary material, which is available to authorized users.

Soil contamination in China: current status and mitigation strategies

China faces great challenges in protecting its soil from contamination caused by rapid industrialization and urbanization over the last three decades. Recent nationwide surveys show that 16% of the soil samples, 19% for the agricultural soils, are contaminated based on China’s soil environmental quality limits, mainly with heavy metals and metalloids. Comparisons with other regions of the world show that the current status of soil contamination, based on the total contaminant concentrations, is not worse in China. However, the concentrations of some heavy metals in Chinese soils appear to be increasing at much greater rates. Exceedance of the contaminant limits in food crops is widespread in some areas, especially southern China, due to elevated inputs of contaminants, acidic nature of the soil and crop species or cultivars prone to heavy metal accumulation. Minimizing the transfer of contaminants from soil to the food chain is a top priority. A number of options are proposed, including identification of the sources of contaminants to agricultural systems, minimization of contaminant inputs, reduction of heavy metal phytoavailability in soil with liming or other immobilizing materials, selection and breeding of low accumulating crop cultivars, adoption of appropriate water and fertilizer management, bioremediation, and change of land use to grow nonfood crops. Implementation of these strategies requires not only technological advances, but also social-economic evaluation and effective enforcement of environmental protection law.

Generation of expressed sequence tags under cadmium stress for gene discovery and development of molecular markers in chickpea

Chickpea is the world's third most important legume crop and belongs to Fabaceae family but suffered from severe yield loss due to various biotic and abiotic stresses. Development of modern genomic tools such as molecular markers and identification of resistant genes associated with these stresses facilitate improvement in chickpea breeding towards abiotic stress tolerance. In this study, 1597 high-quality expressed sequence tags (ESTs) were generated from a cDNA library of variety Pusa 1105 root tissue after cadmium (Cd) treatment. Assembly of ESTs resulted in a total of 914 unigenes of which putative homology was obtained for 38.8 % of unigenes after BLASTX search. In terms of species distribution, majority of sequences found similarity with Medicago truncatula followed by Glycine max, Vitis vinifera and Populus trichocarpa and Pisum sativum sequences. Functional annotation was assigned using Blast2Go, and the Gene Ontology (GO) terms were categorized into biological process, molecular function and cellular component. Approximately 10.83 % of unigenes were assigned at least one GO term. Moreover, in the distribution of transcripts into various biological pathways, 20 of the annotated transcripts were assigned to ten pathways in KEGG database. A majority of the genes were found to be involved in sulphur and nitrogen metabolism. In the quantitative real-time PCR analysis, five of the transcription factors and three of the transporter genes were found to be highly expressed after Cd treatment. Besides, the utility of ESTs was demonstrated by exploiting them for the development of 83 genic molecular markers including EST-simple sequence repeats and intron targeted polymorphism that would assist in tagging of genes related to metal stress for future prospects.

Genome-wide identification and characterization of cadmium-responsive microRNAs and their target genes in radish (Raphanus sativus L.) roots

MicroRNAs (miRNAs) are endogenous non-coding small RNAs that play vital regulatory roles in plant growth, development, and environmental stress responses. Cadmium (Cd) is a non-essential heavy metal that is highly toxic to living organisms. To date, a number of conserved and non-conserved miRNAs have been identified to be involved in response to Cd stress in some plant species. However, the miRNA-mediated gene regulatory networks responsive to Cd stress in radish (Raphanus sativus L.) remain largely unexplored. To dissect Cd-responsive miRNAs and their targets systematically at the global level, two small RNA libraries were constructed from Cd-treated and Cd-free roots of radish seedlings. Using Solexa sequencing technology, 93 conserved and 16 non-conserved miRNAs (representing 26 miRNA families) and 28 novel miRNAs (representing 22 miRNA families) were identified. In all, 15 known and eight novel miRNA families were significantly differently regulated under Cd stress. The expression patterns of a set of Cd-responsive miRNAs were validated by quantitative real-time PCR. Based on the radish mRNA transcriptome, 18 and 71 targets for novel and known miRNA families, respectively, were identified by the degradome sequencing approach. Furthermore, a few target transcripts including phytochelatin synthase 1 (PCS1), iron transporter protein, and ABC transporter protein were involved in plant response to Cd stress. This study represents the first transcriptome-based analysis of miRNAs and their targets responsive to Cd stress in radish roots. These findings could provide valuable information for functional characterization of miRNAs and their targets in regulatory networks responsive to Cd stress in radish.

Comprehensive analysis of expressed sequence tags from cultivated and wild radish (Raphanus spp.)

BACKGROUND

Radish (Raphanus sativus L., 2n = 2× = 18) is an economically important vegetable crop worldwide. A large collection of radish expressed sequence tags (ESTs) has been generated but remains largely uncharacterized.

RESULTS

In this study, approximately 315,000 ESTs derived from 22 Raphanus cDNA libraries from 18 different genotypes were analyzed, for the purpose of gene and marker discovery and to evaluate large-scale genome duplication and phylogenetic relationships among Raphanus spp. The ESTs were assembled into 85,083 unigenes, of which 90%, 65%, 89% and 89% had homologous sequences in the GenBank nr, SwissProt, TrEMBL and Arabidopsis protein databases, respectively. A total of 66,194 (78%) could be assigned at least one gene ontology (GO) term. Comparative analysis identified 5,595 gene families unique to radish that were significantly enriched with genes related to small molecule metabolism, as well as 12,899 specific to the Brassicaceae that were enriched with genes related to seed oil body biogenesis and responses to phytohormones. The analysis further indicated that the divergence of radish and Brassica rapa occurred approximately 8.9-14.9 million years ago (MYA), following a whole-genome duplication event (12.8-21.4 MYA) in their common ancestor. An additional whole-genome duplication event in radish occurred at 5.1-8.4 MYA, after its divergence from B. rapa. A total of 13,570 simple sequence repeats (SSRs) and 28,758 high-quality single nucleotide polymorphisms (SNPs) were also identified. Using a subset of SNPs, the phylogenetic relationships of eight different accessions of Raphanus was inferred.

CONCLUSION

Comprehensive analysis of radish ESTs provided new insights into radish genome evolution and the phylogenetic relationships of different radish accessions. Moreover, the radish EST sequences and the associated SSR and SNP markers described in this study represent a valuable resource for radish functional genomics studies and breeding.

Comparative mapping of Raphanus sativus genome using Brassica markers and quantitative trait loci analysis for the Fusarium wilt resistance trait

Fusarium wilt (FW), caused by the soil-borne fungal pathogen Fusarium oxysporum is a serious disease in cruciferous plants, including the radish (Raphanus sativus). To identify quantitative trait loci (QTL) or gene(s) conferring resistance to FW, we constructed a genetic map of R. sativus using an F2 mapping population derived by crossing the inbred lines '835' (susceptible) and 'B2' (resistant). A total of 220 markers distributed in 9 linkage groups (LGs) were mapped in the Raphanus genome, covering a distance of 1,041.5 cM with an average distance between adjacent markers of 4.7 cM. Comparative analysis of the R. sativus genome with that of Arabidopsis thaliana and Brassica rapa revealed 21 and 22 conserved syntenic regions, respectively. QTL mapping detected a total of 8 loci conferring FW resistance that were distributed on 4 LGs, namely, 2, 3, 6, and 7 of the Raphanus genome. Of the detected QTL, 3 QTLs (2 on LG 3 and 1 on LG 7) were constitutively detected throughout the 2-year experiment. QTL analysis of LG 3, flanked by ACMP0609 and cnu_mBRPGM0085, showed a comparatively higher logarithm of the odds (LOD) value and percentage of phenotypic variation. Synteny analysis using the linked markers to this QTL showed homology to A. thaliana chromosome 3, which contains disease-resistance gene clusters, suggesting conservation of resistance genes between them.

Construction of a chromosome-assigned, sequence-tagged linkage map for the radish, Raphanus sativus L. and QTL analysis of morphological traits

The radish displays great morphological variation but the genetic factors underlying this variability are mostly unknown. To identify quantitative trait loci (QTLs) controlling radish morphological traits, we cultivated 94 F4 and F5 recombinant inbred lines derived from a cross between the rat-tail radish and the Japanese radish cultivar 'Harufuku' inbred lines. Eight morphological traits (ovule and seed numbers per silique, plant shape, pubescence and root formation) were measured for investigation. We constructed a map composed of 322 markers with a total length of 673.6 cM. The linkage groups were assigned to the radish chromosomes using disomic rape-radish chromosome-addition lines. On the map, eight and 10 QTLs were identified in 2008 and 2009, respectively. The chromosome-linkage group correspondence, the sequence-specific markers and the QTLs detected here will provide useful information for further genetic studies and for selection during radish breeding programs.

RadishBase: a database for genomics and genetics of radish

Radish is an economically important vegetable crop. During the past several years, large-scale genomics and genetics resources have been accumulated for this species. To store, query, analyze and integrate these radish resources efficiently, we have developed RadishBase (http://bioinfo.bti.cornell.edu/radish), a genomics and genetics database of radish. Currently the database contains radish mitochondrial genome sequences, expressed sequence tag (EST) and unigene sequences and annotations, biochemical pathways, EST-derived single nucleotide polymorphism (SNP) and simple sequence repeat (SSR) markers, and genetic maps. RadishBase is designed to enable users easily to retrieve and visualize biologically important information through a set of efficient query interfaces and analysis tools, including the BLAST search and unigene annotation query interfaces, and tools to classify unigenes functionally, to identify enriched gene ontology (GO) terms and to visualize genetic maps. A database containing radish pathways predicted from unigene sequences is also included in RadishBase. The tools and interfaces in RadishBase allow efficient mining of recently released and continually expanding large-scale radish genomics and genetics data sets, including the radish genome sequences and RNA-seq data sets.