The monogenic genes Ol-1, ol-2, and Ol-4 confer resistance to tomato powdery mildew Oidium neolycopersici via different mechanisms. The biochemical mechanisms involved in these monogenic resistances were studied by monitoring through time the association of H2O2 and callose accumulation with hypersensitive response (HR) and papilla formation. Our results showed that H2O2 and callose accumulation are coupled with both Ol-1- and Ol-4-mediated HR-associated resistance as well as with the ol-2-mediated papillae-associated resistance. Further, the transcriptomal changes related to these monogenic resistances were studied by using cDNA-amplification fragment length polymorphism. The expression profiling clarified that 81% of DE-TDF (differentially expressed transcript-derived fragments) were up-regulated upon inoculation with O. neolycopersici in both the compatible and Ol-1-mediated incompatible interactions, though with a difference in expression timing. Of these DE-TDF, more than 70% were not detected in the Ol-4-mediated resistance, while 58% were expressed in the ol-2-mediated resistance, generally at later timepoints. Sequence information suggested that most of these DE-TDF are related to genes involved in either basal defense or establishment of compatibility. In addition, DE-TDF (19%) specifically expressed in different incompatible interactions were identified. Expression patterns of some DE-TDF and marker gene GluB suggested that papillae-associated resistance exploits a different defense pathway from that of HR-associated resistance.
SUMMARYThe taxonomy of the genus Rosa is complex, not least because of hybridisations between species. We aimed to develop a method to connect the diploid Rosa taxa to the allopolyploid taxa to which they contributed, based on the sharing of haplotypes. For this we used an SNPSTR marker, which combines a short tandem repeat (STR; microsatellite) marker with single nucleotide polymorphisms (SNPs) in the flanking sequences. In total, 53 different sequences (haplotypes) were obtained for the SNPSTR marker, Rc06, from 20 diploid and 35 polyploid accessions from various species of Rosa. Most accessions of the diploid species had only one allele, while accessions of the polyploid species each contained two-to-five different alleles. Twelve SNPs were detected in the flanking sequences, which alone formed a total of 18 different haplotypes. A maximum likelihood dendrogram revealed five groups of haplotypes. Diploid species in the same Section of the genus Rosa contained SNP haplotypes from only one haplotype group. In contrast, polyploid species contained haplotypes from different haplotype groups. Identical SNP haplotypes were shared between polyploid species and diploid species from more than one Section of the genus Rosa. There were three different polymorphic repeat regions in the STR region. The STR repeat contained eight additional SNPs, but these contributed little to the resolution of the haplotype groups. Our results support hypotheses on diploid Rosa species that contributed to polyploid taxa. Finding different sets of haplotypes in different groups of species within the Sections Synstylae and Pimpinellifoliae supports the hypothesis that these may be paraphyletic.
On the short arm of tomato chromosome 6, a cluster of disease resistance (R) genes have evolved harboring the Mi-1 and Cf genes. The Mi-1 gene confers resistance to root-knot nematodes, aphids, and whiteflies. Previously, we mapped two genes, Ol-4 and Ol-6, for resistance to tomato powdery mildew in this cluster. The aim of this study was to investigate whether Ol-4 and Ol-6 are homologues of the R genes located in this cluster. We show that near-isogenic lines (NIL) harboring Ol-4 (NIL-Ol-4) and Ol-6 (NIL-Ol-6) are also resistant to nematodes and aphids. Genetically, the resistance to nematodes cosegregates with Ol-4 and Ol-6, which are further fine-mapped to the Mi-1 cluster. We provide evidence that the composition of Mi-1 homologues in NIL-Ol-4 and NIL-Ol-6 is different from other nematode-resistant tomato lines, Motelle and VFNT, harboring the Mi-1 gene. Furthermore, we demonstrate that the resistance to both nematodes and tomato powdery mildew in these two NIL is governed by linked (if not the same) Mi-1 homologues in the Mi-1 gene cluster. Finally, we discuss how Solanum crops exploit Mi-1 homologues to defend themselves against distinct pathogens.
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