Plant genes for pathogen resistance can be used to engineer disease resistant crops. Oligonucleotides were designed from sequence motifs conserved between resistance genes of tobacco and Arabidopsis thaliana and used as PCR primers in potato DNA. Amplification products were obtained that were homologous to known resistance genes and linked without recombination with the nematode resistance locus Gro1 and the Phytophthora infestans resistance locus R7 of potato. Map positions of PCR–derived potato gene fragments were also correlated with resistance loci of the related tomato and tobacco genomes. Our results indicate that plant resistance genes that are effective against nematodes, fungi.viruses and bacteria may be isolated based on common sequence motifs and PCR methodology.

A PCR-based approach for isolating pathogen resistance genes from potato with potential for wide application in plants

In order to construct an RFLP map of barley, two populations were analyzed using 251 genomic and cDNA markers: one population comprised 71 F1 antherderived double haploid (DH) individuals of an intraspecific cross (IGRI x FRANKA), and the other 135 individuals of an interspecific F2/F3 progeny (VADA x H. spontaneum). The distribution of nonrepetitive clones over the seven barley chromosomes revealed a maximum for chromosome 2H and a minimum for 6H. The polymorphism of the interspecific progeny (76%) clearly exceeded that of the intraspecific progeny (26%) although, based on their pedigrees, IGRI and FRANKA are only distantly related. The contribution of individual chromosomes of the DH parents to the overall polymorphism varied between 8% and 50%. A significant portion (44% versus 10% of the interspecific progeny) of the markers mapped on the DH offspring showed distorted segregation, caused mainly by the prevalence of variants originating from the parent that better responded to in vitro culture (IGRI). In contrast to the interspecific map, probes displaying skewed segregation were clustered on the DH map on discrete segments. The colinear arrangement of both maps covers a distance of 1,453 cM and identifies regions of varying map distances.

Construction of an RFLP map of barley.

Nineteen single dominant genes (R genes) for resistance to viruses, nematodes, and fungi have been positioned on the molecular map of potato using DNA markers. Fourteen of those genes are located in five "hotspots" for resistance in the potato genome. Quantitative trait loci (QTL) for resistance to late blight caused by the oomycete Phytophthora infestans, to tuber rot caused by the bacterium Erwinia carotovora ssp. atroseptica, and to root cyst nematodes have been identified on all 12 potato chromosomes. Some QTL for resistance to different pathogens are linked to each other and/or to resistance hotspots. Based on the genetic clustering with R genes, we propose that some QTL for resistance have a molecular basis similar to single R genes. Mapping potato genes with sequence similarity to cloned R genes of other plants and other defense-related genes reveals linkage between candidate genes, R genes, and resistance QTL. To explain the molecular basis of polygenic resistance in potato we propose (a) genes having structural similarity with cloned R genes and (b) genes involved in the defense response. The "candidate gene approach" enables the identification of markers highly useful for marker-assisted selection in potato breeding.

Organization of genes controlling disease resistance in the potato genome

An RFLP linkage map of the potato is presented which comprises 304 loci derived from 230 DNA probes and one morphological marker (tuber skin color) The self-incompatibility locus of potato was mapped to chromosome I, which is homoeologous to tomato chromosome I By mapping chromosome-specific tomato RFLP markers in potato and, vice versa, potato markers in tomato, the different potato and tomato RFLP maps were aligned to each other and the similarity of the potato and tomato genome was confirmed The numbers given to the 12 potato chromosomes are now in accordance with the established tomato nomenclature Comparisons between potato RFLP maps derived from different genetic backgrounds revealed conservation of marker order but differences in chromosome and total map length In particular, significant reduction of map length was observed in interspecific compared to intraspecific crosses The distribution of regions with distorted segregation ratios in the genome was analyzed for four potato parents The most prominent distortion of recombination was found to be caused by the self-incompatibility locus

RFLP maps of potato and their alignment with the homoeologous tomato genome

Summary

We have established culture conditions for successful infection and development of several economically important cyst-forming and root-knot nematodes on Arabidopsis thaliana under monoxenic conditions. Complete life cycles were obtained with the sedentary cyst nematodes Heterodera schachtii, H. trifolii, H. cajani and the root-knot nematodes Meloidogyne incognita and M. arenariaas well as with the migratory nematode Pratylenchus penetrans. In contrast, H. goettingiana and Globodera rostochiensis were unable to develop on Arabidopsis roots. Tissue-culture quality agar and medium conditions optimized for hydroponic root culture were essential for successful infections. Detailed in-vivo observations were made inside Arabidopsis roots during the early infection stages of M. incognita and during complete development of H. schachtii. Seventy-four different ecotypes of Arabidopsis were screened for their susceptibility towards H. schachtii resulting in a range of infection rates. None of the ecotypes tested showed complete resistance in vitro. The use of Arabidopsis as a host for plant-parasitic nematodes will provide a new model system for the molecular genetic analysis of this interaction.

Arabidopsis thaliana as a new model host for plant‐parasitic nematodes

A major dominant locus conferring resistance against several pathotypes of the root cyst nematode Globodera rostochiensis was mapped on the linkage map of potato using restriction fragment length polymorphism (RFLP) markers. The assessment of resistance versus susceptibility of the plants in the experimental population considered was based on an in vivo (pot) and an in vitro (petri dish) test. By linkage to nine RFLP markers the resistance locus Gro1 was assigned to the potato linkage group IX which is homologous to the tomato linkage group 7. Deviations from the additivity of recombination frequencies between Gro1 and its neighbouring markers in the pot test led to the detection of a few phenotypic misclassifications of small plants with poor root systems that limited the observation of cysts on susceptible roots. Pooled data from both tests provided better estimates of recombination frequencies in the linkage interval defined by the markers flanking the resistance locus.

Localization by restriction fragment length polymorphism mapping in potato of a major dominant gene conferring resistance to the potato cyst nematode Globodera rostochiensis

Amalia Barone

Papers

Localization by restriction fragment length polymorphism mapping in potato of a major dominant gene conferring resistance to the potato cyst nematode Globodera rostochiensis