BackgroundThe increasingly narrow genetic background characteristic of modern crop germplasm presents a challenge for the breeding of cultivars that require adaptation to the anticipated change in climate. Thus, high priority research aims at the identification of relevant allelic variation present both in the crop itself as well as in its progenitors. This study is based on the characterization of genetic variation in barley, with a view to enhancing its response to terminal drought stress.ResultsThe expression patterns of drought regulated genes were monitored during plant ontogeny, mapped and the location of these genes was incorporated into a comprehensive barley SNP linkage map. Haplotypes within a set of 17 starch biosynthesis/degradation genes were defined, and a particularly high level of haplotype variation was uncovered in the genes encoding sucrose synthase (types I and II) and starch synthase. The ability of a panel of 50 barley accessions to maintain grain starch content under terminal drought conditions was explored.ConclusionThe linkage/expression map is an informative resource in the context of characterizing the response of barley to drought stress. The high level of haplotype variation among starch biosynthesis/degradation genes in the progenitors of cultivated barley shows that domestication and breeding have greatly eroded their allelic diversity in current elite cultivars. Prospective association analysis based on core drought-regulated genes may simplify the process of identifying favourable alleles, and help to understand the genetic basis of the response to terminal drought.

Sugarcane mosaic virus (SCMV) is an important virus disease of maize (Zea mays) in Europe. In this study, we mapped and characterized quantitative trait loci (QTL) affecting resistance to SCMV in a maize population consisting of 219 F(3) or immortalized F(2) families from the cross of two European maize inbreds, D32 (resistant) x D145 (susceptible). Resistance was evaluated in replicated field trials across two environments under artificial inoculation. The method of composite interval mapping was employed for QTL detection with a linkage map based on 87 restriction fragment length polymorphism and 7 mapped microsatellite markers. Genotypic and genotype x environment interaction variances for SCMV resistance were highly significant in the population. Heritabilities ranged from 0.77 to 0.94 for disease scores recorded on seven consecutive dates. Five QTL for SCMV resistance were identified on chromosomes 1, 3, 5, 6, and 10 in the joint analyses. Two major QTL on chromosomes 3 and 6 were detected consistently in both environments. Significant epistatic effects were found among some of these QTL. A simultaneous fit with all QTL in the joint analyses explained between 70 and 77% of the phenotypic variance observed at various stages of plant development. Resistance to SCMV was correlated with plant height and days to anthesis.

Sugarcane mosaic virus (SCMV) is one of the most important virus diseases of maize in Europe. In this study, the gene action at two major quantitative trait loci (QTL) affecting resistance to SCMV in maize was mapped and characterized. A total of 121 F3 lines from cross F7 (susceptible) × FAP1360A (resistant) were evaluated for SCMV resistance in replicated field trials across two environments under artificial inoculation at seven scoring dates. The genotypic variance was always highly significant and heritability increased up to 0.92 for later scoring dates. The method of composite interval mapping was employed for QTL mapping using four simple sequence repeat (SSR) markers flanking two regions identified in a previous study with cross D145 × D32. The presence of two QTL for SCMV resistance, one on chromosome 6 (Scml region) and one on chromosome 2 (Scm2 region), was confirmed. These two QTL together explained between 15% (first score) and 62% (final score) of the phenotypic variance at various stages of plant development. Gene action was additive for the Scm1 region but completely dominant for the Scm2 region. Comparison of results of this study with those obtained for cross D145 × D32 suggested that the resistance alleles in the two populations are identical for the Scm1 region but different for the Scm2 region.

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The level and type of resistance against leaf rust (Puccinia hordei) was determined in modern spring barley germplasm. In field trials all over Europe most accessions were in some locations and years significantly less infected than the moderately resistant reference 'Grit'. Differentiating P. hordei isolates indicated that most accessions carried hypersensitivity (Rph) genes. A virulence survey indicated that among the known resistance genes, only Rph7 is still fully effective in Europe. Some accessions carried undetermined hypersensitivity resistance gene(s) that were effective to all isolates tested. The level of non-hypersensitivity or partial resistance was assessed from the latency period of the fungus and the percentage of early aborted infection units not associated with plant cell necrosis. These parameters indicated that several accessions had a level of partial resistance higher than that of the highly partially resistant 'Vada'. We concluded that barley breeders have achieved very high levels of partial resistance against P. hordei in spring barley germplasm. barley / leaf rust / partial resistance / virulence / hypersensitivityRésumé -Résistance à la rouille brune (Puccinia hordei) du germplasme d'orge de printemps d'Europe de l'ouest. Le niveau et le type de résistance contre la rouille brune (Puccinia hordei) ont été déterminé chez le germplasme d'orge de printemps. Dans des champs expérimentaux présents dans l'Europe entière, la plupart des accessions ont été pour certains sites et années, moins infectées comparé à la référence 'Grit' qui est modérément résistante. Des isolats de P. hordei différenciés ont indiqué que la plupart des accessions contenaient les gènes d'hypersensitivité Rph. Un test de virulence a indiqué que des gènes Rph connus, seul Rph7 est encore complètement efficace en Europe. Certaines Agronomie 20 (2000) 769-782 769 © INRA, EDP Sciences 2000 Communicated by Hanne Østergård (Roskilde, Denmark) * Correspondence and reprints rients.niks@pv.dpw.wau.nl ** Affiliations are in Appendix