A QTL that confers resistance to Colorado potato beetle (Leptinotarsa decemlineata [Say]) in tetraploid potato populations segregating for leptine

Tetraose steroidal glycoalkaloids from potato provide resistance against Alternaria solani and Colorado potato beetle

Plants with innate disease and pest resistance can contribute to more sustainable agriculture. Natural defence compounds produced by plants have the potential to provide a general protective effect against pathogens and pests, but they are not a primary target in resistance breeding. Here, we identified a wild relative of potato, Solanum commersonii, that provides us with unique insight in the role of glycoalkaloids in plant immunity. We cloned two atypical resistance genes that provide resistance to Alternaria solani and Colorado potato beetle through the production of tetraose steroidal glycoalkaloids (SGA). Moreover, we provide in vitro evidence to show that these compounds have potential against a range of different (potato pathogenic) fungi. This research links structural variation in SGAs to resistance against potato diseases and pests. Further research on the biosynthesis of plant defence compounds in different tissues, their toxicity, and the mechanisms for detoxification, can aid the effective use of such compounds to improve sustainability of our food production.

Mapping Solanum chacoense mediated Colorado potato beetle (Leptinotarsa decemlineata) resistance in a self-compatible F2 diploid population

A major QTL on chromosome 2 associated with leptine biosynthesis and Colorado potato beetle resistance was identified in a diploid S. chacoense F₂ population using linkage mapping and bulk-segregant analysis. We examined the genetic features underlying leptine glycoalkaloid mediated Colorado potato beetle (Leptinotarsa decemlineata) host plant resistance in a diploid F₂ mapping population of 233 individuals derived from Solanum chacoense lines USDA8380-1 and M6. The presence of foliar leptine glycoalkaloids in this population segregated as a single dominant gene and displayed continuous distribution of accumulated quantity in those individuals producing the compound. Using biparental linkage mapping, a major overlapping QTL region with partial dominance effects was identified on chromosome 2 explaining 49.3% and 34.1% of the variance in Colorado potato beetle field resistance and leptine accumulation, respectively. Association of this putative resistance region on chromosome 2 was further studied in an expanded F₂ population in a subsequent field season. Loci significantly associated with leptine synthesis colocalized to chromosome 2. Significant correlation between increased leptine content and decreased Colorado potato beetle defoliation suggests a single QTL on chromosome 2. Additionally, a minor QTL with overdominance effects explaining 6.2% associated with Colorado potato beetle resistance donated by susceptible parent M6 was identified on chromosome 7. Bulk segregant whole genome sequencing of the same F₂ population detected QTL associated with Colorado potato beetle resistance on chromosomes 2, 4, 6, 7, and 12. Weighted gene co-expression network analysis of parental lines and resistant and susceptible F₂ individuals identified a tetratricopeptide repeat containing protein with a putative regulatory function and a previously uncharacterized acetyltransferase within the QTL region on chromosome 2, possibly under the control of a regulatory Tap46 subunit within the minor QTL on chromosome 12.

Host plant quantitative trait loci affect specific behavioral sequences in oviposition by a stem-mining insect

Genetic diversity in quantitative loci associated with plant traits used by insects as cues for host selection can influence oviposition behavior and maternal choice. Host plant selection for oviposition is an important determinant of progeny performance and survival for phytophagous insects. Specific cues from the plant influence insect oviposition behavior; but, to date, no set of host plant quantitative trait loci (QTLs) have been shown to have an effect on behavioral sequences leading to oviposition. Three QTLs in wheat (Triticum aestivum L.) have been identified as influencing resistance to the wheat stem sawfly (WSS) (Cephus cinctus Norton). Wheat near-isogenic lines (NILs) for each of the three QTLs were used to test whether foraging WSS were able to discriminate variation in plant cues resulting from allelic changes. A QTL on chromosome 3B (Qss-msub-3BL) previously associated with stem solidness and larval antibiosis was shown to affect WSS oviposition behavior, host preference, and field infestation. Decreased preference for oviposition was also related to a QTL allele on chromosome 2D (Qwss.msub-2D). A QTL on chromosome 4A (Qwss.msub-4A.1) affected host plant attractiveness to foraging females, but did not change oviposition preference after females landed on the stem. These findings show that oviposition decisions regarding potential plant hosts require WSS females to discriminate signals from the plant associated with allelic variation at host plant quantitative loci. Allele types in a host plant QTL associated with differential survival of immature progeny can affect maternal choices for oviposition. The multidisciplinary approach used here may lead to the identification of plant genes with important community consequences, and may complement the use of antibiosis due to solid stems to control the wheat stem sawfly in agroecosystems.

Host plant quantitative trait loci affect specific behavioral sequences in oviposition by a stem-mining insect

KEY MESSAGE

Genetic diversity in quantitative loci associated with plant traits used by insects as cues for host selection can influence oviposition behavior and maternal choice. Host plant selection for oviposition is an important determinant of progeny performance and survival for phytophagous insects. Specific cues from the plant influence insect oviposition behavior; but, to date, no set of host plant quantitative trait loci (QTLs) have been shown to have an effect on behavioral sequences leading to oviposition. Three QTLs in wheat (Triticum aestivum L.) have been identified as influencing resistance to the wheat stem sawfly (WSS) (Cephus cinctus Norton). Wheat near-isogenic lines (NILs) for each of the three QTLs were used to test whether foraging WSS were able to discriminate variation in plant cues resulting from allelic changes. A QTL on chromosome 3B (Qss-msub-3BL) previously associated with stem solidness and larval antibiosis was shown to affect WSS oviposition behavior, host preference, and field infestation. Decreased preference for oviposition was also related to a QTL allele on chromosome 2D (Qwss.msub-2D). A QTL on chromosome 4A (Qwss.msub-4A.1) affected host plant attractiveness to foraging females, but did not change oviposition preference after females landed on the stem. These findings show that oviposition decisions regarding potential plant hosts require WSS females to discriminate signals from the plant associated with allelic variation at host plant quantitative loci. Allele types in a host plant QTL associated with differential survival of immature progeny can affect maternal choices for oviposition. The multidisciplinary approach used here may lead to the identification of plant genes with important community consequences, and may complement the use of antibiosis due to solid stems to control the wheat stem sawfly in agroecosystems.

Genetic dissection of fruit quality traits in the octoploid cultivated strawberry highlights the role of homoeo-QTL in their control

Fruit quality traits are major breeding targets in the Rosaceae. Several of the major Rosaceae species are current or ancient polyploids. To dissect the inheritance of fruit quality traits in polyploid fleshy fruit species, we used a cultivated strawberry segregating population comprising a 213 full-sibling F1 progeny from a cross between the variety 'Capitola' and the genotype 'CF1116'. We previously developed the most comprehensive strawberry linkage map, which displays seven homoeology groups (HG), including each four homoeology linkage groups (Genetics 179:2045-2060, 2008). The map was used to identify quantitative trait loci (QTL) for 19 fruit traits related to fruit development, texture, colour, anthocyanin, sugar and organic acid contents. Analyses were carried out over two or three successive years on field-grown plants. QTL were detected for all the analysed traits. Because strawberry is an octopolyploid species, QTL controlling a given trait and located at orthologous positions on different homoeologous linkage groups within one HG are considered as homoeo-QTL. We found that, for various traits, about one-fourth of QTL were putative homoeo-QTL and were localised on two linkage groups. Several homoeo-QTL could be detected the same year, suggesting that several copies of the gene underlying the QTL are functional. The detection of some other homoeo-QTL was year-dependent. Therefore, changes in allelic expression could take place in response to environmental changes. We believe that, in strawberry as in other polyploid fruit species, the mechanisms unravelled in the present study may play a crucial role in the variations of fruit quality.

Molecular bases of plant resistance to arthropods

Arthropod-resistant crops provide significant ecological and economic benefits to global agriculture. Incompatible interactions involving resistant plants and avirulent pest arthropods are mediated by constitutively produced and arthropod-induced plant proteins and defense allelochemicals synthesized by resistance gene products. Cloning and molecular mapping have identified the Mi-1.2 and Vat arthropod resistance genes as CC-NBS-LRR (coiled coil-nucleotide binding site-leucine rich repeat) subfamily NBS-LRR resistance proteins, as well as several resistance gene analogs. Genetic linkage mapping has identified more than 100 plant resistance gene loci and linked molecular markers used in cultivar development. Rice and sorghum arthropod-resistant cultivars and, to a lesser extent, raspberry and wheat cultivars are components of integrated pest management (IPM) programs in Asia, Australia, Europe, and North America. Nevertheless, arthropod resistance in most food and fiber crops has not been integrated due primarily to the application of synthetic insecticides. Plant and arthropod genomics provide many opportunities to more efficiently develop arthropod-resistant plants, but integration of resistant cultivars into IPM programs will succeed only through interdisciplinary collaboration.

Linkage analysis of a rare alkaloid present in a tetraploid potato with Solanum chacoense background

The potato genotype ND4382-19 has Solanum chacoense Bitt. in its genetic background. Foliar alkaloid analysis of it and its progeny ND5873 (ND4382-19 × Chipeta) by gas chromatography-mass spectrometry (GC-MS) showed that, in addition to the expected alkaloids (solanidine, leptinidine, and acetyl-leptinidine), there was an aglycone of another rare alkaloid. Its molecular mass and some of the m/z fragment ions were similar to leptinidine, but the major fragment ion was the m/z 150 peak of solanidine. This fragmentation pattern suggested that this alkaloid is a solanidine-based compound with mass equal to leptinidine. Leptinidine differs from solanidine by an extra -OH group, but the GC-MS fragmentation pattern of the rare compound indicated hydroxylation at a different position than the C-23 of leptinidine. The exact chemical structure is still unknown, and further analysis, such as NMR will be necessary to determine the structure. Segregation analysis of ND5873 (ND4382-19 × Chipeta) showed that presence of this rare compound segregated in a 1:1 ratio, indicating that a single gene controlled its synthesis and/or accumulation in foliar tissue. Analysis with AFLP and microsatellite markers indicated that the locus-controlling presence of this alkaloid resided on potato chromosome I, with the nearest flanking AFLP markers 0.6 and 9.4 cM apart. This rare alkaloid was present in the foliage and not detected in potato tubers. Its presence in leaves did not affect resistance/susceptibility to Colorado potato beetle.