An elicitor-and pathogen-induced cdna from potato encodes a stress-responsive cyclophilin

Differential mRNA display was used to identify pathogen-responsive, stress-related genes in potato cell suspensions treated with salicylic acid and a cell wall-derived elicitor from Phytophthora infestans. Among the positive clones identified, one was found to be expressed at a significantly higher level in elicited cells than in control cells. DNA sequencing of this amplicon revealed high homology and identified it as a potato cyclophilin cDNA. The maximum amount of the cyclophilin mRNA was found 9 to 12 h after elicitation. Cyclophilin (CyP) mRNA synthesis was also up-regulated from 12 to 24 h in potato leaves locally infected with zoospores from Phytophthora infestans. However, untreated leaves responding systemically to the pathogen showed only a weak, delayed response at 24 h post infection. The observed accumulation of potato CyP mRNA in response to salicylic acid, P. infestans elicitor and P. infestans infection, suggest that CyPs play an important role in plant stress responses.     

Two related genotypes of Solanum microdontum carry different segregating alleles for field resistance to Phytophthora infestans

Six F₁ populations produced from crosses between two Solanum microdontum genotypes and three S. tuberosum genotypes have been assessed for resistance to Phytophthora infestans (late blight; LB) in the field and proved to segregate for P. infestans resistance. The six populations were subjected to genetic mapping using AFLP markers in combination with a core map consisting of RFLPs and a few PCR-based markers. The two S. microdontum parents MCD167 and MCD178 harbour different alleles contributing to resistance. In the MCD167 set of populations a major QTL contributing to resistance was located on chromosome 4. This locus was not detected in the MCD178 set of populations. In the latter set of populations, a major QTL for resistance was detected on chromosome 10. This locus for resistance was not detected in the MCD167 set of populations. A third QTL contributing strongly to resistance was detected on chromosome 5. However, this locus was not consistently detected in all populations. For the loci on chromosome 5 and 10, segregation for resistance alleles from the susceptible parents was also observed. The results provide a starting point for future combinations of genes, to predict the value of each combination and finally to obtain potato germplasm carrying high levels of durable resistance to LB.     

Genetic mapping revealed two loci for soybean aphid resistance in PI 567301B

The soybean aphid (Aphis glycines Matsumura) is the most damaging insect pest of soybean [Glycine max (L.) Merr.] in North America. New soybean aphid biotypes have been evolving quickly and at least three confirmed biotypes have been reported in USA. These biotypes are capable of defeating most known aphid resistant soybean genes indicating the need for identification of new genes. Plant Introduction (PI) 567301B was earlier identified to have antixenosis resistance against biotype 1 and 2 of the soybean aphid. Two hundred and three F_7:9 recombinant inbred lines (RILs) developed from a cross of soybean aphid susceptible cultivar Wyandot and resistant PI 567301B were used for mapping aphid resistance genes using the quantitative trait loci (QTL) mapping approach. A subset of 94 RILs and 516 polymorphic SNP makers were used to construct a genome-wide molecular linkage map. Two candidate QTL regions for aphid resistance were identified on this linkage map. Fine mapping of the QTL regions was conducted with SSR markers using all 203 RILs. A major gene on chromosome 13 was mapped near the previously identified Rag2 gene. However, an earlier study revealed that the detached leaves of PI 567301B had no resistance against the soybean aphids while the detached leaves of PI 243540 (source of Rag2) maintained aphid resistance. These results and the earlier finding that PI 243540 showed antibiosis resistance and PI 567301B showed antixenosis type resistance, indicating that the aphid resistances in the two PIs are not controlled by the same gene. Thus, we have mapped a new gene near the Rag2 locus for soybean aphid resistance that should be useful in breeding for new aphid-resistant soybean cultivars. Molecular markers closely linked to this gene are available for marker-assisted breeding. Also, the minor locus found on chromosome 8 represents the first reported soybean aphid-resistant locus on this chromosome.     

Using SNP markers to dissect linkage disequilibrium at a major quantitative trait locus for resistance to the potato cyst nematode Globodera pallida on potato chromosome V

The damage caused by the parasitic root cyst nematode Globodera pallida is a major yield-limiting factor in potato cultivation . Breeding for resistance is facilitated by the PCR-based marker ‘HC’, which is diagnostic for an allele conferring high resistance against G. pallida pathotype Pa2/3 that has been introgressed from the wild potato species Solanum vernei into the Solanum tuberosum tetraploid breeding pool. The major quantitative trait locus (QTL) controlling this nematode resistance maps on potato chromosome V in a hot spot for resistance to various pathogens including nematodes and the oomycete Phytophthora infestans. An unstructured sample of 79 tetraploid, highly heterozygous varieties and breeding clones was selected based on presence (41 genotypes) or absence (38 genotypes) of the HC marker. Testing the clones for resistance to G. pallida confirmed the diagnostic power of the HC marker. The 79 individuals were genotyped for 100 single nucleotide polymorphisms (SNPs) at 10 loci distributed over 38 cM on chromosome V. Forty-five SNPs at six loci spanning 2 cM in the interval between markers GP21-GP179 were associated with resistance to G. pallida. Based on linkage disequilibrium (LD) between SNP markers, six LD groups comprising between 2 and 18 SNPs were identified. The LD groups indicated the existence of multiple alleles at a single resistance locus or at several, physically linked resistance loci. LD group C comprising 18 SNPs corresponded to the ‘HC’ marker. LD group E included 16 SNPs and showed an association peak, which positioned one nematode resistance locus physically close to the R1 gene family. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The R1 gene conferring race-specific resistance to Phytophthora infestans in potato is located on potato chromosome V.

Summary Late blight in potato is caused by the fungusPhytophthora infestans and can inflict severe damage on the potato crop. Resistance toP. infestans is either based on major dominantR genes conferring vertical, race-specific resistance or on minor genes inducing horizontal, unspecific resistance. A dihaploid potato line was identified which carried theR1 gene, conferring vertical resistance to allP. infestans races, with the exception of those homozygous for the recessive virulence allele of the locusV1. The F₁ progeny of a cross between this resistant parent P(R1) and P(r), a line susceptible to all races, was analysed for segregation ofR1 and of restriction fragment length polymorphism (RFLP) markers distributed on the potato RFLP map comprising more than 300 loci. TheR1 locus was mapped to chromosome V in the interval between RFLP markers GP21 and GP179. The map position ofR1 was found to be very similar to the one ofRx2, a dominant locus inducing extreme resistance to potato virus X.     

Late blight resistance linkages in a novel cross of the wild potato species Solanum paucissectum (series Piurana)

The cultivated potato, Solanum tuberosum, is affected by a variety of diseases with late blight, caused by Phytophthora infestans, being the most severe. Wild potato species have proven to be a continuing source of resistance, sometimes of an extreme type, to this disease. The present study constructs the first late blight linkage map of a member of series Piurana, S. paucissectum, a tuber-bearing relative of potato, using probes for conserved sequences from potato and tomato. Eight probes mapped to unexpected linkage groups, but syntenic differences with prior maps of potato were not supported by any blocks of rearranged chromosome segments. All 12 linkage groups were resolved and significant associations with late blight resistance were found on chromosomes 10, 11 and 12. A major quantitative trait locus (QTL) on chromosome 11 accounts for more than 25% of the phenotypic variance measured in a field trial. Crossing of S. paucissectum with cultivated potato resulted in very few seeds indicating partial reproductive barriers. Differential reactions of accessions of this potential donor species with simple and complex isolates of P. infestans suggest that it carries major resistance genes that are not those previously described from the Mexican species, S. demissum. However, the additivity of the QTL effects argues for the quantitative nature of resistance in this cross.     

Genetic localisation of transformation competence in diploid potato

In the course of improving diploid potato genotypes for transformation ability, selection for specific components affecting regeneration and transformation was carried out. From a segregating population between two good regenerating clones a selection was made to yield an optimal well-transforming and fertile genotype J92-6400-A16. This plant yielded predominantly diploid transformants and was heterozygous for the gene R1, conferring resistance to Phytophthora infestans. The speed of, and competence for, regeneration and transformation on both sides of the stem explant were improved. A competence factor for tranformation was found to be linked with the R1 locus and a molecular marker on chromosome 5. The male fertility of transformants was frequently decreased to a great extent, whereas female fertility was not so markedly affected.     

Inoculation of susceptible and resistant potato plants with the late blight pathogen Phytophthora infestans: effects on an aphid and its parasitoid

Plants are exposed to microbial pathogens as well as herbivorous insects and their natural enemies. Here, we examined the effects of inoculation of potato plants, Solanum tuberosum L. (Solanaceae), with the late blight pathogen Phytophthora infestans (Mont.) de Bary (Peronosporales: Pythiaceae) on an aphid species commonly infesting potato crops and one of the aphid's major parasitoids. We observed the peach‐potato aphid, Myzus persicae Sulzer (Hemiptera: Aphididae), and its natural enemy, the biocontrol agent Aphidius colemani Viereck (Hymenoptera: Braconidae), on potato either inoculated with water or P. infestans. Population growth of the aphid, parasitism rate of its natural enemy, and other insect life‐history traits were compared on several potato genotypes, the susceptible cultivar Désirée and genetically modified (GM) isogenic lines carrying genes conferring resistance to P. infestans. Effects of P. infestans inoculation on the intrinsic rate of aphid population increase and the performance of the parasitoid were only found on the susceptible cultivar. Insect traits were similar when comparing inoculated with non‐inoculated resistant GM genotypes. We also tested how GM‐plant characteristics such as location of gene insertion and number of R genes could influence non‐target insects by comparing insect performance among GM events. Different transformation events leading to different positions of R‐gene insertion in the genome influenced aphids either with or without P. infestans infection, whereas effects of position of R‐gene insertion on the parasitoid A. colemani were evident only in the presence of inoculation with P. infestans. We conclude that it is important to study different transformation events before continuing with further stages of risk assessment of this GM crop. This provides important information on the effects of plant resistance to a phytopathogen on non‐target insects at various trophic levels.     

Autotetraploids and genetic mapping using common AFLP markers: the R2 allele conferring resistance to Phytophthora infestans mapped on potato chromosome 4

 Due to the complexity of tetrasomic inheritance, mapping studies in potato (Solanum tuberosum L.) are generally conducted at the diploid level. In the present study we tested the feasibility of Bulked Segregant Analysis (BSA) using a tetraploid offspring for the identification of AFLP markers linked to the R2 allele, which confers race-specific resistance to Phytophthora infestans. Eleven bulk-specific AFLP markers, detected in fingerprints of 205 AFLP primer combinations, could be mapped in a linkage group encompassing the R2 locus. The efficiency of BSA at the tetraploid level, determined by the frequency of single-dose restriction fragments (SDRF), was much higher than expected on the basis of overall genetic dissimilarity between the parental clones. The fortuitous detection of AFLPs with linkage to the R2 allele is explained on the basis of specific genetic dissimilarity between cultivated potato and the chromosomal segment introgressed from S. demissum carrying the resistant R2 allele. AFLP markers common to those with linkage to R2 were visually recognized by their electrophoretic mobility in the AFLP fingerprint in a parental clone of a reference mapping population. Using these common AFLP markers we anchored the linkage group comprising the R2 allele to potato chromosome 4. Received: 30 October 1997 / Accepted: 6 November 1997     

Pollen markers for gene-centromere mapping in diploid potato

The utility of two pollen genetic markers for estimating the extent of meiotic recombination between the centromere and a marker gene was tested in 2n pollen of diploid potato clones. One of these markers was the distal locus amylose-free (amf) on chromosome 8 and the other was the isozyme locus alcohol dehydrogenase (Adh-1) on chromosome 4. In the case of the amf locus, the gene-centromere distance was estimated in a normal synaptic and a desynaptic genotype. In both cases the genetic analysis was confined to: (1) a direct estimation of the phenotypic (blue vs red) segregation ratios in FDR (first-division restitution) 2n pollen and (2) a classification of the 4 x progeny from 4x (nulliplex amf) x 2x (Amf/amf) crosses into duplex, simplex and nulliplex classes. The recombination frequency between the centromere and the amf locus in the normal synaptic genotype B92-7015-4 corresponded to a gene-centromere distance of 48.8 cM, whereas this distance amounted to 13.3 cM in the desynaptic genotype RS93-8025-1. Hence desynapsis reduced crossing-over by 73%. The observed genetic distance of 48.8 cM in the normal synaptic clone, B92-7015-4, is the highest gene-centromere distance reported so far in potato and this could be explained on the assumption of absolute chiasma interference. For the Adh-1 locus, it was found that heterozygous 2n pollen grains could be detected in pollen samples of the diploid clones, because of the occurrence of a heterodimeric band of the isozyme. Unlike the amf locus, the genecentromere distance for the Adh-1 locus was estimated only on the basis of the duplex, simplex and nulliplex classes in the progenies from 4x (nulliplex Adh-1 ² )x B92-7015-4 (Adh-1 ¹ /Adh-1 ² )crosses and was found to be 19.4 cM. Because the accurate positions of centromeres in relation to other loci are not available in the existing genetic maps of potato, which are saturated with molecular markers, halftetrad analysis is a promising additional approach to the basic genetics of this crop.     

The novel, major locus Rpi-phu1 for late blight resistance maps to potato chromosome IX and is not correlated with long vegetation period

Despite the long history of breeding potatoes resistant to Phytophthora infestans, this oomycete is still economically the most important pathogen of potato worldwide. The correlation of high levels of resistance to late blight with a long vegetation period is one of the bottlenecks for progress in breeding resistant cultivars of various maturity types. Solanum phureja was identified as a source of effective late blight resistance, which was transferred to the cultivated gene pool by interspecific crosses with dihaploids of Solanum tuberosum. A novel major resistance locus, Rpi-phu1, derived most likely from S. phureja and conferring broad-spectrum resistance to late blight, was mapped to potato chromosome IX, 6.4 cM proximal to the marker GP94. Rpi-phu1 was highly effective in detached leaflet, tuber slice and whole tuber tests during 5 years of quantitative phenotypic assessment. The resistance did not show significant correlation with vegetation period length. Our findings provide a well-characterized new source of resistance for breeding early and resistant-to-P. infestans potatoes.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Influence of day-length and isolates of Phytophthora infestans on field resistance to late blight of potato

Main and interaction effects of day-length and pathogen isolate on the reaction and expression of field resistance to Phytophthora infestans were analyzed in a sample of standard clones for partial resistance to potato late blight, and in the BCT mapping population derived from a backcross of Solanum berthaultii to Solanum tuberosum. Detached leaves from plants grown in field plots exposed to short- and long day-length conditions were independently inoculated with two P. infestans isolates and incubated in chambers under short- and long photoperiods, respectively. Lesion growth rate (LGR) was used for resistance assessment. Analysis of variance revealed a significant contribution of genotype × isolate × day-length interaction to variation in LGR indicating that field resistance of genotypes to foliar late blight under a given day-length depended on the infecting isolate. An allele segregating from S. berthaultii with opposite effects on foliar resistance to late blight under long- and short day-lengths, respectively, was identified at a quantitative trait locus (QTL) that mapped on chromosome 1. This allele was associated with positive (decreased resistance) and negative (increased resistance) additive effects on LGR, under short- and long day-length conditions, respectively. Disease progress on whole plants inoculated with the same isolate under field conditions validated the direction of its effect in short day-length regimes. The present study suggests the occurrence of an isolate-specific QTL that displays interaction with isolate behavior under contrasting environments, such as those with different day-lengths. This study highlights the importance of exposing genotypes to a highly variable population of the pathogen under contrasting environments when stability to late blight resistance is to be assessed or marker-assisted selection is attempted for the manipulation of quantitative resistance to late blight.     

Yield,Yield‐related Traits and Response of Potato Clones to Late Blight Disease,in North‐Western Highlands of Ethiopia

Late blight disease of potato caused by Phytophthora infestans poses a significant threat to potato production in Ethiopia. The development of new high yielding genotypes with adequate late blight disease resistance will provide a strong component of an integrated management strategy for farmers. The objective of this study was to determine late blight resistance and yield of potato clones under field condition in north‐western Ethiopia. Twenty‐four clones (17 from the International Potato Centre B3C2 population and seven widely grown cultivars) were evaluated at three locations. The experiment was laid in a randomized complete block design with two replications. Late blight resistance and yield‐related traits were determined. Results showed that clones differ significantly for all traits across locations. The following five clones combine high to moderate resistance to late blight with high yields: 396029.250, 395017.229, 396004.263, 396034.103 and 395077.12. These clones are useful genetic resources for resistance breeding against late blight disease and for enhanced yields.     

Novel potato C2H2‐type zinc finger protein gene,StZFP1, which responds to biotic and abiotic stress,plays a role in salt tolerance

Many TFIIIA‐type zinc finger proteins (ZFPs) play important roles in stress responses in plants. In the present study, a novel zinc finger protein gene, StZFP1, was cloned from potato. StZFP1 is a typical TFIIIA‐type two‐finger zinc finger gene with one B‐box domain, one L‐box domain and a DLN‐box/EAR motif. The StZFP1 genes belong to a small gene family with an estimated copy number of four or five, located on chromosome I. StZFP1 is constitutively expressed in leaves, stems, roots, tubers and flowers of adult plants. Expression of StZFP1 can be induced by salt, dehydration and exogenously applied ABA. StZFP1 expression is also responsive to infection by the late blight pathogen Phytophthora infestans. Transient expression analysis of StZFP1:GFP fusion protein revealed that StZFP1 is preferentially localised in the nucleus. Ectopic expression of StZFP1, driven by the Arabidopsis rd29A promoter in transgenic tobacco, increased plant tolerance to salt stress. These results demonstrate that StZFP1 might be involved in potato responses to salt and dehydration stresses through an ABA‐dependent pathway.