Abstract
BackgroundThe nematode Pratylenchus neglectus has a wide host range and is able to feed on the root systems of cereals, oilseeds, grain and pasture legumes. Under the Mediterranean low rainfall environments of Australia, annual Medicago pasture legumes are used in rotation with cereals to fix atmospheric nitrogen and improve soil parameters. Considerable efforts are being made in breeding programs to improve resistance and tolerance to Pratylenchus neglectus in the major crops wheat and barley, which makes it vital to develop appropriate selection tools in medics.ResultsA strong source of tolerance to root damage by the root lesion nematode (RLN) Pratylenchus neglectus had previously been identified in line RH-1 (strand medic, M. littoralis). Using RH-1, we have developed a single seed descent (SSD) population of 138 lines by crossing it to the intolerant cultivar Herald. After inoculation, RLN-associated root damage clearly segregated in the population. Genetic analysis was performed by constructing a genetic map using simple sequence repeat (SSR) and gene-based SNP markers. A highly significant quantitative trait locus (QTL), QPnTolMl.1, was identified explaining 49% of the phenotypic variation in the SSD population. All SSRs and gene-based markers in the QTL region were derived from chromosome 1 of the sequenced genome of the closely related species M. truncatula. Gene-based markers were validated in advanced breeding lines derived from the RH-1 parent and also a second RLN tolerance source, RH-2 (M. truncatula ssp. tricycla). Comparative analysis to sequenced legume genomes showed that the physical QTL interval exists as a synteny block in Lotus japonicus, common bean, soybean and chickpea. Furthermore, using the sequenced genome information of M. truncatula, the QTL interval contains 55 genes out of which five are discussed as potential candidate genes responsible for the mapped tolerance.ConclusionThe closely linked set of SNP-based PCR markers is directly applicable to select for two different sources of RLN tolerance in breeding programs. Moreover, genome sequence information has allowed proposing candidate genes for further functional analysis and nominates QPnTolMl.1 as a target locus for RLN tolerance in economically important grain legumes, e.g. chickpea.
Highlights
The nematode Pratylenchus neglectus has a wide host range and is able to feed on the root systems of cereals, oilseeds, grain and pasture legumes
Phenotypic analysis of tolerance to Pratylenchus neglectus Plants that were not inoculated with nematodes had negligible root damage (Score < 0.06) indicating that root damage symptoms observed in inoculated treatments was caused by the nematode
In the present study, a genetic mapping approach led to the identification of a major quantitative trait locus (QTL) for tolerance to the root lesion nematode Pratylenchus neglectus in Medicago littoralis
Summary
The nematode Pratylenchus neglectus has a wide host range and is able to feed on the root systems of cereals, oilseeds, grain and pasture legumes. Neutral to alkaline soils are widespread in the southern cropping regions of Australia and annual medics (Medicago spp.) are widely grown on these soils with Medicago littoralis and M. truncatula (barrel medic) as the predominant annual medics [2,3] These conditions appear favourable to the root lesion nematode (RLN) Pratylenchus neglectus that is widely distributed in South Australian cropping soils [4] and affects all major legumes and cereal crops [5]. Experience based on field assays suggests that each nematode per gram soil translates into one percent of loss in yield in annual medics and severe damage to wheat, especially under low rainfall conditions where nematode-affected root systems are unable to use the sparsely available water [6,9]
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