Development of SCAR markers linked to three disease resistances based on AFLP within Nicotiana tabacum L

Genetic Mapping of the HLA1 Locus Causing Hybrid Lethality in Nicotiana Interspecific Hybrids

Hybrid lethality, a postzygotic mechanism of reproductive isolation, is a phenomenon that causes the death of F₁ hybrid seedlings. Hybrid lethality is generally caused by the epistatic interaction of two or more loci. In the genus Nicotiana, N. debneyi has the dominant allele Hla1-1 at the HLA1 locus that causes hybrid lethality in F₁ hybrid seedlings by interaction with N. tabacum allele(s). Here, we mapped the HLA1 locus using the F₂ population segregating for the Hla1-1 allele derived from the interspecific cross between N. debneyi and N. fragrans. To map HLA1, several DNA markers including random amplified polymorphic DNA, amplified fragment length polymorphism, and simple sequence repeat markers, were used. Additionally, DNA markers were developed based on disease resistance gene homologs identified from the genome sequence of N. benthamiana. Linkage analysis revealed that HLA1 was located between two cleaved amplified polymorphic sequence markers Nb14-CAPS and NbRGH1-CAPS at a distance of 10.8 and 10.9 cM, respectively. The distance between these markers was equivalent to a 682 kb interval in the genome sequence of N. benthamiana.

Development, Identification, and Application of a Germplasm Specific SCAR Marker for Dendrobium officinale Kimura et Migo

The stems of Dendrobium officinale have been used as a rare and valuable Chinese tonic medicine, known as "Tiepi Fengdou", since the Qing dynasty. Because of the increased market demand and continued exploitation of this plant, the reserves of wild D. officinale resources have been depleted, and D. officinale products on the market are being increasingly adulterated. Such changes have strongly affected the sustainable utilization of this valuable medicinal plant resource and the development of related industries. In this study, a species-specific DNA marker was developed for the rapid and accurate authentication of D. officinale. In total, 36 start codon-targeted (SCoT) polymorphism primers were screened in 36 definite Dendrobium species, and a distinct species-specific DNA amplicon (SCoT13-215) for D. officinale was obtained. After the sequence was cloned and sequenced, a sequence-characterized amplified region marker was developed (named SHF/SHR) and validated through PCR amplification of all 38 Dendrobium samples. The marker's specificity for D. officinale was confirmed through the consistent amplification of a clear 197-bp band. This SCAR marker can be used to rapidly, effectively, and reliably identify D. officinale among various Dendrobium species and may play an important role in ensuring the quality of medicinal preparations and protecting the germplasm of this important medicinal species.

Omics: The way forward to enhance abiotic stress tolerance in Brassica napus L

Plant abiotic stresses negative affects growth and development, causing a massive reduction in global agricultural production. Rapeseed (Brassica napus L.) is a major oilseed crop because of its economic value and oilseed production. However, its productivity has been reduced by many environmental adversities. Therefore, it is a prime need to grow rapeseed cultivars, which can withstand numerous abiotic stresses. To understand the various molecular and cellular mechanisms underlying the abiotic stress tolerance and improvement in rapeseed, omics approaches have been extensively employed in recent years. This review summarized the recent advancement in genomics, transcriptomics, proteomics, metabolomics, and their imploration in abiotic stress regulation in rapeseed. Some persisting bottlenecks have been highlighted, demanding proper attention to fully explore the omics tools. Further, the potential prospects of the CRISPR/Cas9 system for genome editing to assist molecular breeding in developing abiotic stress-tolerant rapeseed genotypes have also been explained. In short, the combination of integrated omics, genome editing, and speed breeding can alter rapeseed production worldwide.

Molecular breeding approaches for production of disease-resilient commercially important tobacco

Tobacco is one of the most widely cultivated nonfood cash crops, a source of income, model organism for plant molecular research, a natural pesticide and of pharmaceutical importance. First domesticated in South Americas, the modern-day tobacco (Nicotiana tabacum) is now cultivated in more than 125 countries to generate revenues worth billions of dollars each year. However, the production of this crop is highly threatened by the global presence of devastating infectious agents, which cause huge fiscal loss. These threats have been battled through breeding for acquiring disease resilience in tobacco plants, first, via conventional and now with the use of modern molecular breeding approaches. For efficacy and precision, the characterization of the genetic components underlying disease resistance is the key tool in tobacco for resistance breeding programs. The past few decades have witnessed significant progress in resilience breeding through advanced molecular techniques. The current review discusses history of tobacco breeding since its time of origin till date, highlighting the most widely used techniques and recent advances in molecular research and strategies for resistance breeding. In addition, we narrate the budding possibilities for the future. This review will provide a comprehensive and valuable information for the tobacco growers and researchers to deal with the destructive infectious diseases.

Mapping Quantitative Trait Loci Controlling High Iron and Zinc Content in Self and Open Pollinated Grains of Pearl Millet [Pennisetum glaucum (L.) R. Br.]

Pearl millet is a multipurpose grain/fodder crop of the semi-arid tropics, feeding many of the world's poorest and most undernourished people. Genetic variation among adapted pearl millet inbreds and hybrids suggests it will be possible to improve grain micronutrient concentrations by selective breeding. Using 305 loci, a linkage map was constructed to map QTLs for grain iron [Fe] and zinc [Zn] using replicated samples of 106 pearl millet RILs (F6) derived from ICMB 841-P3 × 863B-P2. The grains of the RIL population were evaluated for Fe and Zn content using atomic absorption spectrophotometer. Grain mineral concentrations ranged from 28.4 to 124.0 ppm for Fe and 28.7 to 119.8 ppm for Zn. Similarly, grain Fe and Zn in open pollinated seeds ranged between 22.4-77.4 and 21.9-73.7 ppm, respectively. Mapping with 305 (96 SSRs; 208 DArT) markers detected seven linkage groups covering 1749 cM (Haldane) with an average intermarker distance of 5.73 cM. On the basis of two environment phenotypic data, two co-localized QTLs for Fe and Zn content on linkage group (LG) 3 were identified by composite interval mapping (CIM). Fe QTL explained 19% phenotypic variation, whereas the Zn QTL explained 36% phenotypic variation. Likewise for open pollinated seeds, the QTL analysis led to the identification of two QTLs for grain Fe content on LG3 and 5, and two QTLs for grain Zn content on LG3 and 7. The total phenotypic variance for Fe and Zn QTLs in open pollinated seeds was 16 and 42%, respectively. Analysis of QTL × QTL and QTL × QTL × environment interactions indicated no major epistasis.

Large-scale development of SSR markers in tobacco and construction of a linkage map in flue-cured tobacco

Tobacco (Nicotiana tabacum L.), particularly flue-cured tobacco, is one of the most economically important nonfood crops and is also an important model system in plant biotechnology. Despite its importance, only limited molecular marker resources are available for genome analysis, genetic mapping, and breeding. Simple sequence repeats (SSR) are one of the most widely-used molecular markers, having significant advantages including that they are generally co-dominant, easy to use, abundant in eukaryotic organisms, and produce highly reproducible results. In this study, based on the genome sequence data of flue-cured tobacco (K326), we developed a total of 13,645 mostly novel SSR markers, which were working in a set of eighteen tobacco varieties of four different types. A mapping population of 213 backcross (BC1) individuals, which were derived from an intra-type cross between two flue-cured tobacco varieties, Y3 and K326, was selected for mapping. Based on the newly developed SSR markers as well as published SSR markers, we constructed a genetic map consisting of 626 SSR loci distributed across 24 linkage groups and covering a total length of 1120.45 cM with an average distance of 1.79 cM between adjacent markers, which is the highest density map of flue-cured tobacco till date.

The tobacco genome sequence and its comparison with those of tomato and potato

The allotetraploid plant Nicotiana tabacum (common tobacco) is a major crop species and a model organism, for which only very fragmented genomic sequences are currently available. Here we report high-quality draft genomes for three main tobacco varieties. These genomes show both the low divergence of tobacco from its ancestors and microsynteny with other Solanaceae species. We identify over 90,000 gene models and determine the ancestral origin of tobacco mosaic virus and potyvirus disease resistance in tobacco. We anticipate that the draft genomes will strengthen the use of N. tabacum as a versatile model organism for functional genomics and biotechnology applications.

Molecular Linkage Mapping and Marker-Trait Associations with NlRPT, a Downy Mildew Resistance Gene in Nicotiana langsdorffii

Nicotiana langsdorffii is one of two species of Nicotiana known to express an incompatible interaction with the oomycete Peronospora tabacina, the causal agent of tobacco blue mold disease. We previously showed that incompatibility is due to the hypersensitive response (HR), and plants expressing the HR are resistant to P. tabacina at all stages of growth. Resistance is due to a single dominant gene in N. langsdorffii accession S-4-4 that we have named NlRPT. In further characterizing this unique host-pathogen interaction, NlRPT has been placed on a preliminary genetic map of the N. langsdorffii genome. Allelic scores for five classes of DNA markers were determined for 90 progeny of a "modified backcross" involving two N. langsdorffii inbred lines and the related species N. forgetiana. All markers had an expected segregation ratio of 1:1, and were scored in a common format. The map was constructed with JoinMap 3.0, and loci showing excessive transmission distortion were removed. The linkage map consists of 266 molecular marker loci defined by 217 amplified fragment length polymorphisms (AFLPs), 26 simple-sequence repeats (SSRs), 10 conserved orthologous sequence markers, nine inter-simple sequence repeat markers, and four target region amplification polymorphism markers arranged in 12 linkage groups with a combined length of 1062 cM. NlRPT is located on linkage group three, flanked by four AFLP markers and one SSR. Regions of skewed segregation were detected on LGs 1, 5, and 9. Markers developed for N. langsdorffii are potentially useful genetic tools for other species in Nicotiana section Alatae, as well as in N. benthamiana. We also investigated whether AFLPs could be used to infer genetic relationships within N. langsdorffii and related species from section Alatae. A phenetic analysis of the AFLP data showed that there are two main lineages within N. langsdorffii, and that both contain populations expressing dominant resistance to P. tabacina.

COSII genetic maps of two diploid Nicotiana species provide a detailed picture of synteny with tomato and insights into chromosome evolution in tetraploid N. tabacum

Using single-copy conserved ortholog set (COSII) and simple sequence repeat (SSR) markers, we have constructed two genetic maps for diploid Nicotiana species, N. tomentosiformis and N. acuminata, respectively. N. acuminata is phylogenetically closer to N. sylvestris than to N. tomentosiformis, the latter two of which are thought to contribute the S-genome and T-genome, respectively, to the allotetraploid tobacco (N. tabacum L., 2n = 48). A comparison of the two maps revealed a minimum of seven inversions and one translocation subsequent to the divergence of these two diploid species. Further, comparing the diploid maps with a dense tobacco map revealed that the tobacco genome experienced chromosomal rearrangements more frequently than its diploid relatives, supporting the notion of accelerated genome evolution in allotetraploids. Mapped COSII markers permitted the investigation of Nicotiana-tomato syntenic relationships. A minimum of 3 (and up to 10) inversions and 11 reciprocal translocations differentiate the tomato genome from that of the last common ancestor of N. tomentosiformis and N. acuminata. Nevertheless, the marker/gene order is well preserved in 25 conserved syntenic segments. Molecular dating based on COSII sequences suggested that tobacco was formed 1.0 MYA or later. In conclusion, these COSII and SSR markers link the cultivated tobacco map to those of wild diploid Nicotiana species and tomato, thus providing a platform for cross-reference of genetic and genomic information among them as well as other solanaceous species including potato, eggplant, pepper and the closely allied coffee (Rubiaceae). Therefore they will facilitate genetic research in the genus Nicotiana.

Tobacco blue mould disease caused by Peronospora hyoscyami f. sp. tabacina

Blue mould [Peronospora hyoscyami f. sp. tabacina (Adam) Skalicky 1964] is one of the most important foliar diseases of tobacco that causes significant losses in the Americas, south-eastern Europe and the Middle East. This review summarizes the current knowledge of the mechanisms employed by this oomycete pathogen to colonize its host, with emphasis on molecular aspects of pathogenicity. In addition, key biochemical and molecular mechanisms involved in tobacco resistance to blue mould are discussed.

TAXONOMY

Kingdom: Chromista (Straminipila); Phylum: Heterokontophyta; Class: Oomycete; Order: Peronosporales; Family: Peronosporaceae; Genus: Peronospora; Species: Peronospora hyoscyami f. sp. tabacina.

DISEASE SYMPTOMS

The pathogen typically causes localized lesions on tobacco leaves that appear as single, or groups of, yellow spots that often coalesce to form light-brown necrotic areas. Some of the leaves exhibit grey to bluish downy mould on their lower surfaces. Diseased leaves can become twisted, such that the lower surfaces turn upwards. In such cases, the bluish colour of the diseased plants becomes quite conspicuous, especially under moist conditions when sporulation is abundant. Hence the name of the disease: tobacco blue mould.

INFECTION PROCESS

The pathogen develops haustoria within plant cells that are thought to establish the transfer of nutrients from the host cell, and may also act in the delivery of effector proteins during infection.

RESISTANCE

Several defence responses have been reported to occur in the Nicotiana tabacum-P. hyoscyami f. sp. tabacina interaction. These include the induction of pathogenesis-related genes, and a correlated increase in the activities of typical pathogenesis-related proteins, such as peroxidases, chitinases, beta-1,3-glucanases and lipoxygenases. Systemic acquired resistance is one of the best characterized tobacco defence responses activated on pathogen infection.

Differential impact of retrotransposon populations on the genome of allotetraploid tobacco (Nicotiana tabacum)

LTR-retrotransposons contribute substantially to the structural diversity of plant genomes. Recent models of genome evolution suggest that retrotransposon amplification is offset by removal of retrotransposon sequences, leading to a turnover of retrotransposon populations. While bursts of amplification have been documented, it is not known whether removal of retrotransposon sequences occurs continuously, or is triggered by specific stimuli over short evolutionary periods. In this work, we have characterized the evolutionary dynamics of four populations of copia-type retrotransposons in allotetraploid tobacco (Nicotiana tabacum) and its two diploid progenitors Nicotiana sylvestris and Nicotiana tomentosiformis. We have used SSAP (Sequence-Specific Amplification Polymorphism) to evaluate the contribution retrotransposons have made to the diversity of tobacco and its diploid progenitor species, to quantify the contribution each diploid progenitor has made to tobacco's retrotransposon populations, and to estimate losses or amplifications of retrotransposon sequences subsequent to tobacco's formation. Our results show that the tobacco genome derives from a turnover of retrotransposon sequences with removals concomitant with new insertions. We have detected unique behaviour specific to each retrotransposon population, with differences likely reflecting distinct evolutionary histories and activities of particular elements. Our results indicate that the retrotransposon content of a given plant species is strongly influenced by the host evolutionary history, with periods of rapid turnover of retrotransposon sequences stimulated by allopolyploidy.

A microsatellite marker based linkage map of tobacco

We report the first linkage map of tobacco (Nicotiana tabacum L.) generated through microsatellite markers. The microsatellite markers were predominantly derived from genomic sequences of the Tobacco Genome Initiative (TGI) through bioinformatics screening for microsatellite motives. A total of 684 primer pairs were screened for functionality in a panel of 16 tobacco lines. Of those, 637 primer pairs were functional. Potential parents for mapping populations were evaluated for their polymorphism level through genetic similarity analysis. The similarity analysis revealed that the known groups of tobacco varieties (Burley, Flue-cured, Oriental and Dark) form distinct clusters. A mapping population, based on a cross between varieties Hicks Broad Leaf and Red Russian, and consisting of 186 F2 individuals, was selected for mapping. A total of 282 functional microsatellite markers were polymorphic in this population and 293 loci could be mapped together with the morphological trait flower color. Twenty-four tentative linkage groups spanning 1,920 cM could be identified. This map will provide the basis for the genetic mapping of traits in tobacco and for further analyses of the tobacco genome.