A high-resolution map of the <Emphasis Type="Italic">Grp1</Emphasis> locus on chromosome V of potato harbouring broad-spectrum resistance to the cyst nematode species <Emphasis Type="Italic">Globodera pallida</Emphasis> and <Emphasis Type="Italic">Globodera rostochiensis</Emphasis>

The Grp1 locus confers broad-spectrum resistance to the potato cyst nematode species Globodera pallida and Globodera rostochiensis and is located in the GP21-GP179 interval on the short arm of chromosome V of potato. A high-resolution map has been developed using the diploid mapping population RHAM026, comprising 1,536 genotypes. The flanking markers GP21 and GP179 have been used to screen the 1,536 genotypes for recombination events. Interval mapping of the resistances to G. pallida Pa2 and G. rostochiensis Ro5 resulted in two nearly identical LOD graphs with the highest LOD score just north of marker TG432. Detailed analysis of the 44 recombinant genotypes showed that G. pallida and G. rostochiensis resistance could not be separated and map to the same location between marker SPUD838 and TG432. It is suggested that the quantitative resistance to both nematode species at the Grp1 locus is mediated by one or more tightly linked R genes that might belong to the NBS-LRR class. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. A. Finkers-Tomczak and S. Danan contributed equally to this research.     

Overexpression of <Emphasis Type="Italic">AP1-</Emphasis>like genes from <Emphasis Type="Italic">Asteraceae</Emphasis> induces early-flowering in transgenic <Emphasis Type="Italic">Chrysanthemum</Emphasis> plants

Chrysanthemum is one of the most important commercial cut flowers in the world. Early-flowering cultivars are required to produce quality chrysanthemum flowers with a lower cost of production. To shorten the vegetative growth phase of chrysanthemum, three AP1-like genes from Asteraceae were constitutively overexpressed in 80 independent transgenic chrysanthemum lines. All lines were characterized by PCR and RT-PCR and demonstrated that overexpression of compositae AP1-homologs in transgenic chrysanthemum under long-day conditions had no effect on plant development compared to non-transgenic controls. Conversely, under short-day conditions, transgenic plants commenced bud initiation 2 wk earlier than non-transgenic chrysanthemum plants. Subsequently, transgenic chrysanthemum flowers showed color earlier and resulted in full opening of inflorescences 3 wk prior to non-transgenic control plants. These results open new possibilities for genetic improvement and breeding of chrysanthemum cultivars.     

High resolution DNA melting markers for identification of <Emphasis Type="Italic">H1</Emphasis>-linked resistance to potato cyst nematode

Although Sequence-Characterized Amplified Region (SCAR) markers linked to the potato H1 locus, which confers resistance to pathotypes Ro1 and Ro4 of the potato cyst nematode (PCN) Globodera rostochiensis, have been reported, robust markers that enable estimation of allele dosage would improve the quality of information obtained from genotyping parental accessions (cultivars/breeding lines) and progeny populations within breeding programmes. With this in mind, we have developed single nucleotide polymorphism (SNP)-based molecular markers flanking the H1 resistance gene, using genomic re-sequence data from five elite tetraploid accessions. The published TG689 and 57R primer sequences were used in a Basic Local Alignment Search Tool (BLAST) examination of the reference potato genome, and SNPs within the vicinity of these primer regions were identified and targeted for designing probe-based High Resolution Melting (HRM) SNP assays. Evaluation of the subsequently developed HRM markers, TG689_1P and 57R_1P, against the publicly available SCAR markers, TG689 and 57R, indicated that the HRM markers enabled more reliable marker-trait association than the SCARs. Additionally, allelic dosage estimates for the H1 locus were also derived using the TG689_1P marker, providing a tool to optimise parental and progeny selections in PCN resistance breeding.     

Phenotype changes inherited by crossing pyrethroid susceptible and resistant genotypes from the cattle tick <Emphasis Type="Italic">Riphicephalus</Emphasis> (<Emphasis Type="Italic">Boophilus</Emphasis>) <Emphasis Type="Italic">microplus</Emphasis>

Dialelic crosses and backcrosses of pyrethroid resistant (RR) and susceptible (SS) Rhipicephalus (Boophilus) microplus tick strains were carried out and the substitution (Phe-Ile) within the sodium channel gene was monitored in order to analyze the effects of the genotype on the pyrethroid resistance phenotype as measured by the larval packet test (LPT). Parental strains: susceptible (SS) and resistant (RR); dialelic crosses: RS (♂RR × ♀SS), and SR (♂SS × ♀RR); and backcrosses: RS × SS, RS × RR, SR × SS and SR × RR were infested on 280 kg calves. Resistance type (monogenic or polygenic) and effective dominance were determined based on the discriminant concentration (DC) for cipermethrine (0.5%), deltamethrine (0.09%) and flumethrine (0.01%). Allele specific PCR (AS-PCR) was used for genotyping, looking at a sodium channel mutation (Phe-Ile substitution). The mortality rates and allele frequency of susceptible and pyrethroid resistant reference strains were 0% mortality and 90% RR alleles for resistant strain, and 100% mortality and 0% RR alleles as measured by the larval packet test (LPT) and allele specific PCR (AS-PCR) respectively. Backcrossed strain SR × RR showed an effective dominance (D_ML) of 0.605 for cypermethrin, 0.639 for deltamethrin and 0.498 for flumethrin, while survival of backcrosses RS × SS, RS × RR and SR × SS showed a significant tendency to recesivity. Backcrossed strain SR × RR (69.4%) also showed a higher RR genotype frequency with regards to RS × SS (25.5%), RS × RR (36.7%) and SR × SS (32.0%), however, susceptible allele was inherited in general as an incomplete dominant trait. Monogenic inheritance hypothesis was tested and the results showed monogenic inheritance for cypermethrin and flumethrin (P < 0.05) but not for deltamethrin (P > 0.05). However, significant correlation was found between RR genotype and the survival rate for all three pyrethroids used (P < 0.05), suggesting that a single substitution on the sodium channel gene can be responsible for resistance to pyrethroids as a class, due to the high frequency for RR genotypes. Combination with different mutations or metabolic resistance mechanisms cannot be excluded.     

Molecular mapping of leaf rust resistance gene <Emphasis Type="Italic">Rph14</Emphasis> in <Emphasis Type="Italic">Hordeum vulgare</Emphasis>

An incompletely dominant gene conferring resistance to Puccinia hordei, Rph14, identified previously in an accession of Hordeum vulgare, confers resistance to all known pathotypes of P. hordei in Australia. Knowledge of the chromosomal location of Rph14 and the identification of DNA markers closely linked to it will facilitate combining it with other important leaf rust resistance genes to achieve long lasting resistance. The inheritance of Rph14 was confirmed using 146 and 106 F₃ lines derived from the crosses ‘Baudin’/‘PI 584760’ (Rph14) and ‘Ricardo’/‘PI 584760’ (Rph14), respectively. Bulk segregant analysis on DNA from the parental genotypes and resistant and susceptible DNA bulks using DArT markers located Rph14 to the short arm of chromosome 2H. DArT marker bPb-1664 was identified as having the closest genetic association with Rph14. PCR based marker analysis identified a single SSR marker, Bmag692, linked closely to Rph14 at a map distance of 2.1 and 3.8 cm in the ‘Baudin’/‘PI 584760’and ‘Ricardo’/‘PI 584760’ populations, respectively.     

Linkage of a RAPD marker with powdery mildew resistance <Emphasis Type="Italic">er-1</Emphasis> gene in <Emphasis Type="Italic">Pisum sativum</Emphasis> L.

The aim of this study was to investigate the inheritance of powdery mildew disease and to tag it with a DNA marker to utilize for the marker-assisted selection (MAS) breeding program. The powdery mildew resistant genotype Fallon ^er and susceptible genotype 11760-3 ^ER were selected from 177 genotypes by heavy infestation of germplasm with Erysiphe pisi through artificial inoculation The F₁ plants of the cross Fallon/11760-3 indicated the dominance of the susceptible allele, while F₂ plants segregated in 3: 1 ratio (susceptible: resistant) that fit for goodness of fitness by χ² (P > 0.07), indicating monogenic recessive inheritance for powdery mildew resistance in Pisum sativum. A novel RAPD marker OPB18 (5′-CCACAGCAGT-3′) was linked to the er-1 gene with 83% probability with a LOD score of 4.13, and was located at a distance of 11.2 cM from the er-1 gene.     

Pea powdery mildew <Emphasis Type="Italic">er1</Emphasis> resistance is associated to loss-of-function mutations at a <Emphasis Type="Italic">MLO</Emphasis> homologous locus

The powdery mildew disease affects several crop species and is also one of the major threats for pea (Pisum sativum L.) cultivation all over the world. The recessive gene er1, first described over 60 years ago, is well known in pea breeding, as it still maintains its efficiency as a powdery mildew resistance source. Genetic and phytopathological features of er1 resistance are similar to those of barley, Arabidopsis, and tomato mlo powdery mildew resistance, which is caused by the loss of function of specific members of the MLO gene family. Here, we describe the obtainment of a novel er1 resistant line by experimental mutagenesis with the alkylating agent diethyl sulfate. This line was found to carry a single nucleotide polymorphism in the PsMLO1 gene sequence, predicted to result in premature termination of translation and a non-functional protein. A cleaved amplified polymorphic sequence (CAPS) marker was developed on the mutation site and shown to be fully co-segregating with resistance in F₂ individuals. Sequencing of PsMLO1 from three powdery mildew resistant cultivars also revealed the presence of loss-of-function mutations. Taken together, results reported in this study strongly indicate the identity between er1 and mlo resistances and are expected to be of great breeding importance for the development of resistant cultivars via marker-assisted selection.     

Gaps and perspectives of pathotype and race determination in <Emphasis Type="Italic">Golovinomyces cichoracearum</Emphasis> and <Emphasis Type="Italic">Podosphaera xanthii</Emphasis>

Golovinomyces cichoracearum and Podosphaera xanthii (family Erysiphaceae) are the most important species causing cucurbit powdery mildew (CPM), a serious disease of field and greenhouse cucurbits. Both species are highly variable in their pathogenicity and virulence, as indicated by the existence of large number of different pathotypes and races. Various independent systems of CPM pathotype and race determinations and denominations are used worldwide. CPM pathotype identification is based on intergeneric and interspecific differences in host-CPM interactions. The most commonly used set of CPM pathotype differentials includes one genotype from four species representing three agriculturally important cucurbit genera plus two genotypes from a fifth species, melon Cucumis melo L. CPM races are characterized by specialization on different cultivars or lines of one host species and have, to date, been differentiated only on melon (C. melo L.). The most frequently used set of melon differentials includes 11 genotypes that can differentiate CPM races originating from melon and other cucurbits, e.g., cucumber, Cucurbita spp., and watermelon. In this paper, we critically review the current state, gaps, and perspectives in our understanding of pathogenicity variation in these two CPM pathogens at the pathotype and race levels.     

Multiple alleles for resistance and susceptibility modulate the defense response in the interaction of tetraploid potato (<Emphasis Type="Italic">Solanum tuberosum</Emphasis>) with <Emphasis Type="Italic">Synchytrium endobioticum</Emphasis> pathotypes 1, 2, 6 and 18

The obligate biotrophic, soil-borne fungus Synchytrium endobioticum causes wart disease of potato (Solanum tuberosum), which is a serious problem for crop production in countries with moderate climates. S. endobioticum induces hypertrophic cell divisions in plant host tissues leading to the formation of tumor-like structures. Potato wart is a quarantine disease and chemical control is not possible. From 38 S. endobioticum pathotypes occurring in Europe, pathotypes 1, 2, 6 and 18 are the most relevant. Genetic resistance to wart is available but only few current potato varieties are resistant to all four pathotypes. The phenotypic evaluation of wart resistance is laborious, time-consuming and sometimes ambiguous, which makes breeding for resistance difficult. Molecular markers diagnostic for genes for resistance to S. endobioticum pathotypes 1, 2, 6 and 18 would greatly facilitate the selection of new, resistant cultivars. Two tetraploid half-sib families (266 individuals) segregating for resistance to S. endobioticum pathotypes 1, 2, 6 and 18 were produced by crossing a resistant genotype with two different susceptible ones. The families were scored for five different wart resistance phenotypes. The distribution of mean resistance scores was quantitative in both families. Resistance to pathotypes 2, 6 and 18 was correlated and independent from resistance to pathotype 1. DNA pools were constructed from the most resistant and most susceptible individuals and screened with genome wide simple sequence repeat (SSR), inverted simple sequence region (ISSR) and randomly amplified polymorphic DNA (RAPD) markers. Bulked segregant analysis identified three SSR markers that were linked to wart resistance loci (Sen). Sen1-XI on chromosome XI conferred partial resistance to pathotype 1, Sen18-IX on chromosome IX to pathotype 18 and Sen2/6/18-I on chromosome I to pathotypes 2,6 and 18. Additional genotyping with 191 single nucleotide polymorphism (SNP) markers confirmed the localization of the Sen loci. Thirty-three SNP markers linked to the Sen loci permitted the dissection of Sen alleles that increased or decreased resistance to wart. The alleles were inherited from both the resistant and susceptible parents.     

Development of marker-free transgenic lettuce resistant to <Emphasis Type="Italic">Mirafiori lettuce big</Emphasis>-<Emphasis Type="Italic">vein virus</Emphasis>

Lettuce big-vein disease caused by Mirafiori lettuce big-vein virus (MLBVV) is found in major lettuce production areas worldwide, but highly resistant cultivars have not yet been developed. To produce MLBVV-resistant marker-free transgenic lettuce that would have a transgene with a promoter and terminator of lettuce origin, we constructed a two T-DNA binary vector, in which the first T-DNA contained the selectable marker gene neomycin phosphotransferase II, and the second T-DNA contained the lettuce ubiquitin gene promoter and terminator and inverted repeats of the coat protein (CP) gene of MLBVV. This vector was introduced into lettuce cultivars ‘Watson’ and ‘Fuyuhikari’ by Agrobacterium tumefaciens-mediated transformation. Regenerated plants (T₀ generation) that were CP gene-positive by PCR analysis were self-pollinated, and 312 T₁ lines were analyzed for resistance to MLBVV. Virus-negative plants were checked for the CP gene and the marker gene, and nine lines were obtained which were marker-free and resistant to MLBVV. Southern blot analysis showed that three of the nine lines had two copies of the CP gene, whereas six lines had a single copy and were used for further analysis. Small interfering RNAs, which are indicative of RNA silencing, were detected in all six lines. MLBVV infection was inhibited in all six lines in resistance tests performed in a growth chamber and a greenhouse, resulting in a high degree of resistance to lettuce big-vein disease. Transgenic lettuce lines produced in this study could be used as resistant cultivars or parental lines for breeding.     

Linkage disequilibrium mapping of a<Emphasis Type="Italic"> Verticillium dahliae</Emphasis> resistance quantitative trait locus in tetraploid potato (<Emphasis Type="Italic">Solanum tuberosum</Emphasis>) through a candidate gene approach

We have used the linkage disequilibrium mapping method to test for an association between a candidate gene marker and resistance to Verticillium dahliae in tetraploid potato. A probe derived from the tomato Verticillium resistance gene (Ve1) identified homologous sequences (StVe1) in potato, which in a diploid population map to chromosome 9, in a position analogous to that of the tomato resistance gene. When a molecular marker closely linked (1.5 cM) to the homologues was used as a candidate gene marker on 137 tetraploid potato genotypes (mostly North American cultivars), the association between the marker and resistance was confirmed (P<0.001). The amount of phenotypic variation in resistance explained by the allele of the STM1051 marker was greater than 10% and 25% in two subpopulations that were inferred from coancestry data matrix. Cloning of homologues from the highly resistant potato cv. Reddale indicates that the resistance quantitative trait locus (QTL) comprises at least an eleven-member family, encoding plant-specific leucine-rich repeat proteins highly similar to the tomato Ve genes. The sequence analysis shows that all homologues are uninterrupted open reading frames and thus represent putative functional resistance genes. This is the first time that the linkage disequilibrium method has been used to find an association between a resistance gene and a candidate gene marker in tetraploid potato. We have shown that it is possible to map QTL directly on already available potato cultivars, without developing a new mapping population.Communicated by F. SalaminiAn erratum to this article can be found at     

Cold-hardening of winter triticale (<Emphasis Type="Italic">x Triticosecale</Emphasis> Wittm.) results in increased resistance to pink snow mould <Emphasis Type="Italic">Microdochium nivale</Emphasis> (Fr., Samuels &; Hallett) and genotype-dependent chlorophyll fluorescence modulations

The resistance of triticale (x Triticosecale Wittm.) to infection of snow mould Microdochium nivale (Fr., Samuels & Hallett) was examined under different temperature pre-treatment regimes. The results of laboratory “cold chamber” resistance tests correlated with the breeders’ report from field experiments. Studied genotypes differed substantially in their resistance to infection. Two cultivars: ‘Magnat’ (susceptible) and ‘Hewo’ (relatively resistant) were further studied as a plant model to test the role of pre-hardening and cold-hardening induction of resistance expression. Both model cultivars were susceptible to M. nivale infection without cold pre-treatment and gained genotype-depended level of resistance after 4 weeks treatment at 4°C, moreover the resistance grew gradually. Simultaneously to the resistance tests, the measurements of chlorophyll fluorescence parameters were taken. The results showed that higher vitality index Rfd of cold-hardened triticale seedlings correlated with increased pink snow mould resistance while differences in other parameters of fluorescence were not distinctly significant. Establishment of Rfd in 4 weeks hardened triticale seedlings could be used for a large scale screening of breeding material in order to select potentially resistant genotypes. Such analyses have not been reported for triticale before.     

Development of wheat lines carrying stem rust resistance gene <Emphasis Type="Italic">Sr39</Emphasis> with reduced <Emphasis Type="Italic">Aegilops speltoides</Emphasis> chromatin and simple PCR markers for marker-assisted selection

The use of major resistance genes is a cost-effective strategy for preventing stem rust epidemics in wheat crops. The stem rust resistance gene Sr39 provides resistance to all currently known pathotypes of Puccinia graminis f. sp. tritici (Pgt) including Ug99 (TTKSK) and was introgressed together with leaf rust resistance gene Lr35 conferring adult plant resistance to P. triticina (Pt), into wheat from Aegilops speltoides. It has not been used extensively in wheat breeding because of the presumed but as yet undocumented negative agronomic effects associated with Ae. speltoides chromatin. This investigation reports the production of a set of recombinants with shortened Ae. speltoides segments through induction of homoeologous recombination between the wheat and the Ae. speltoides chromosome. Simple PCR-based DNA markers were developed for resistant and susceptible genotypes (Sr39#22r and Sr39#50s) and validated across a set of recombinant lines and wheat cultivars. These markers will facilitate the pyramiding of ameliorated sources of Sr39 with other stem rust resistance genes that are effective against the Pgt pathotype TTKSK and its variants.     

Genetic mapping of the novel <Emphasis Type="Italic">Turnip mosaic virus</Emphasis> resistance gene <Emphasis Type="Italic">TuRB03</Emphasis> in <Emphasis Type="Italic">Brassica napus</Emphasis>

A new source of resistance to the pathotype 4 isolate of Turnip mosaic virus (TuMV) CDN 1 has been identified in Brassica napus (oilseed rape). Analysis of segregation of resistance to TuMV isolate CDN 1 in a backcross generation following a cross between a resistant and a susceptible B. napus line showed that the resistance was dominant and monogenic. Molecular markers linked to this dominant resistance were identified using amplified fragment length polymorphism (AFLP) and microsatellite bulk segregant analysis. Bulks consisted of individuals from a BC₁ population with the resistant or the susceptible phenotype following challenge with CDN 1. One AFLP and six microsatellite markers were associated with the resistance locus, named TuRB03, and these mapped to the same region on chromosome N6 as a previously mapped TuMV resistance gene TuRB01. Further testing of TuRB03 with other TuMV isolates showed that it was not effective against all pathotype 4 isolates. It was effective against some, but not all pathotype 3 isolates tested. It provided further resolution of TuMV pathotypes by sub-dividing pathotypes 3 and 4. TuRB03 also provides a new source of resistance for combining with other resistances in our attempts to generate durable resistance to this virus.     

Fine genetic mapping localizes cucumber scab resistance gene <Emphasis Type="Italic">Ccu</Emphasis> into an <Emphasis Type="Italic">R</Emphasis> gene cluster

Scab, caused by Cladosporium cucumerinum, is an important disease of cucumber, Cucumis sativus. In this study, we conducted fine genetic mapping of the single dominant scab resistance gene, Ccu, with 148 F₉ recombinant inbred lines (RILs) and 1,944 F₂ plants derived from the resistant cucumber inbred line 9110Gt and the susceptible line 9930, whose draft genome sequence is now available. A framework linkage map was first constructed with simple sequence repeat markers placing Ccu into the terminal 670 kb region of cucumber Chromosome 2. The 9110Gt genome was sequenced at 5× genome coverage with the Solexa next-generation sequencing technology. Sequence analysis of the assembled 9110Gt contigs and the Ccu region of the 9930 genome identified three insertion/deletion (Indel) markers, Indel01, Indel02, and Indel03 that were closely linked with the Ccu locus. On the high-resolution map developed with the F₂ population, the two closest flanking markers, Indel01 and Indel02, were 0.14 and 0.15 cM away from the target gene Ccu, respectively, and the physical distance between the two markers was approximately 140 kb. Detailed annotation of the 180 kb region harboring the Ccu locus identified a cluster of six resistance gene analogs (RGAs) that belong to the nucleotide binding site (NBS) type R genes. Four RGAs were in the region delimited by markers Indel01 and Indel02, and thus were possible candidates of Ccu. Comparative DNA analysis of this cucumber Ccu gene region with a melon (C. melo) bacterial artificial chromosome (BAC) clone revealed a high degree of micro-synteny and conservation of the RGA tandem repeats in this region.     

An <Emphasis Type="Italic">in vitro</Emphasis> method to evaluate grapevine cultivars for <Emphasis Type="Italic">Erysiphe necator</Emphasis> susceptibility

Powdery mildew, caused by the obligate biotrophic ascomycete Erysiphe necator, is one of the most destructive grapevine diseases worldwide. Cultivars of Vitis vinifera L, for wine and table grape production, are all susceptible to E. necator, whose attacks result in severe epidemics under the warm and dry conditions of the Mediterranean basin. The aim of the present study was to compare the susceptibility of different grapevine cultivars to E. necator by an in vitro assay for assessing the potentiality of this method in breeding programs for resistance to the pathogen. Leaves of 12 grapevine cultivars were spot-inoculated in vitro with about 10 conidia from five different isolates of E. necator, using colony growth and conidiation 3 wk post-inoculation as indicators of susceptibility to the disease. A remarkable difference was observed between highly susceptible cultivars like ‘Baresana’, ‘Malvasia’, ‘Bianca’, and ‘Italia’, and the less susceptible ‘Alphonse Lavallée’ and ‘Ohanez’, in accordance with their behavior in the field. No statistically significant differences were found in the virulence of E. necator isolates.     

Molecular Variability and Evolution of the Pectate Lyase (<Emphasis Type="Italic">pel</Emphasis>-2) Parasitism Gene in Cyst Nematodes Parasitizing Different <Emphasis Type="Italic">Solanaceous</Emphasis> Plants

While pectate lyases are major parasitism factors in plant-parasitic nematodes, there is little information on the variability of these genes within species and their utility as pathotype or host range molecular markers. We have analysed polymorphisms of pectate lyase 2 (pel-2) gene, which degrades the unesterified polygalacturonate (pectate) of the host cell-wall, in the genus Globodera. Molecular variability of the pel-2 gene and the predicted protein was evaluated in populations of G. rostochiensis, G. pallida, G. “mexicana” and G. tabacum. Seventy eight pel-2 sequences were obtained and aligned. Point mutations were observed at 373 positions, 57% of these affect the coding part of the gene and produce 129 aa replacements. The observed polymorphism does not correlate either to the pathotypes proposed in potato cyst nematodes (PCN) or the subspecies described in tobacco cyst nematodes. The trees reveal a topology different from the admitted species topology as G. rostochiensis and G. pallida sequences are more similar to each other than to G. tabacum. Species-specific sites, potentially applicable for identification, and sites distinguishing PCN from tobacco cyst nematodes, were identified. As both G. rostochiensis and G. pallida display the same host range, but distinct from G. tabacum, which cannot parasitize potato plants, it is tempting to speculate that pel-2 genes polymorphism may be implicated in this adaptation, a view supported by the fact that no active pectate lyase 2 was found in G. “mexicana”, a close relative of G. pallida that is unable to develop on cultivated potato varieties.     

Molecular tagging and validation of microsatellite markers linked to the low germination stimulant gene (<Emphasis Type="Italic">lgs</Emphasis>) for <Emphasis Type="Italic">Striga</Emphasis> resistance in sorghum [<Emphasis Type="Italic">Sorghum bicolor</Emphasis> (L.) Moench]

Striga is a devastating parasitic weed in Africa and parts of Asia. Low Striga germination stimulant activity, a well-known resistance mechanism in sorghum, is controlled by a single recessive gene (lgs). Molecular markers linked to the lgs gene can accelerate development of Striga-resistant cultivars. Using a high density linkage map constructed with 367 markers (DArT and SSRs) and an in vitro assay for germination stimulant activity towards Striga asiatica in 354 recombinant inbred lines derived from SRN39 (low stimulant) × Shanqui Red (high stimulant), we precisely tagged and mapped the lgs gene on SBI-05 between two tightly linked microsatellite markers SB3344 and SB3352 at a distance of 0.5 and 1.5 cM, respectively. The fine-mapped lgs region was delimited to a 5.8 cM interval with the closest three markers SB3344, SB3346 and SB3343 positioned at 0.5, 0.7 and 0.9 cM, respectively. We validated tightly linked markers in a set of 23 diverse sorghum accessions, most of which were known to be Striga resistant, by genotyping and phenotyping for germination stimulant activity towards both S. asiatica and S. hermonthica. The markers co-segregated with Striga germination stimulant activity in 21 of the 23 tested lines. The lgs locus similarly affected germination stimulant activity for both Striga species. The identified markers would be useful in marker-assisted selection for introgressing this trait into susceptible sorghum cultivars. Examination of the sorghum genome sequence and comparative analysis with the rice genome suggests some candidate genes in the fine-mapped region (400 kb) that may affect strigolactone biosynthesis or exudation. This work should form a foundation for map-based cloning of the lgs gene and aid in elucidation of an exact mechanism for resistance based on low Striga germination stimulant activity.