Ivan Šimko

Genetic mapping from field tests of qualitative and quantitative resistance to Phytophthora infestans in a population derived from Solanum tuberosum and Solanum berthaultii.

Under controlled field conditions, a Solanum backcross population segregated for resistance to Phytophthora infestans. The population (`BCT) had been derived previously by crossing the Solanum tuberosum dihaploid USW2230 × Solanum berthaultii PI473331 to obtain the hybrid M200-30, and then backcrossing the hybrid to the S.xa0tuberosum dihaploid HH1-9. Resistance was assessed from analyses of epidemics in small plots of each individual genotype, and data were recorded as area under the disease progress curve (AUDPC). The parents of the original cross (USW2230 and a selection from PI473331) were not included in the test, but the hybrid was incompatible and HH1-9 was compatible with the tester strain of P.xa0infestans (US-8 lineage). Somewhat more than half of the progeny also were incompatible with the tester strain, indicating the presence of an R gene. This gene segregated from the S.xa0berthaultii parent and mapped 4.8xa0cm from the RFLP marker TG63 on chromosome 10. We deduce that the Rxa0gene is not R-1, R-2, R-3, R-6, or R-7 and is probably not R-4, R-5, or R-10. Among the remaining, compatible progeny, there was a wide range of quantitative resistance. All were more resistant than the susceptible cultivar Superior, and most individuals were much more resistant than the moderately resistant cultivar Kennebec. AUDPC values among the sub-population of compatible genotypes ranged from about 400 to 1500 units the first year and from 400 to 1760 units the second year. At least five quantitative trait loci (QTLs) were detected in this sub-population in both 1997 and 1998, including one detected through segregation of alleles from both the hybrid parent and the recurrent S.xa0tuberosum parent. A model of main and epistatic effects explained 56% and 66% of the variation observed for quantitative resistance to late blight in 1997 and 1998, respectively. Several of the QTLs for late blight resistance were located in regions of the genome to which QTLs for late maturity have previously been mapped.

Characterization and mapping of R Pi-ber , a novel potato late blight resistance gene from Solanum berthaultii

Phytophthora infestans, the causal agent of late blight, threatens potato production worldwide. An important tool in the management of the disease is the use of resistant varieties. Eleven major resistance genes have been identified and introgressed from Solanum demissum. However, new sources of resistance are continually sought. Here, we report the characterization and refined genetic localization of a resistance gene previously identified as Rber in a backcross progeny of Solanum tuberosum and Solanum berthaultii. In order to further characterize Rber, we developed a set of P. infestans isolates capable of identifying each of the 11 R-genes known to confer resistance to late blight in potato. Our results indicate that Rber is a new resistance gene, different from those recognized in S. demissum, and therefore, it has been named RPi-ber according to the current system of nomenclature. In order to add new molecular markers around RPi-ber, we used a PCR-based mapping technique, named MASP-map, which located RPi-ber in a 3.9xa0cM interval between markers CT240 and TG63 on potato chromosome X. The location of RPi-ber coincides with an area involved in resistance to different pathogens of potato and tomato.

Evidence from Polygene Mapping for a Causal Relationship between Potato Tuber Dormancy and Abscisic Acid Content

In previous studies polygene mapping of a backcross population derived from haploid potato (Solanum tuberosum) and a diploid wild species (Solanum berthaultii) showed at least eight quantitative trait loci (QTLs) associated with tuber dormancy. The same population was mapped for abscisic acid (ABA) content in tubers so that any QTLs identified could be compared with those detected previously. At least three distinct loci on three chromosomes (2, 4, and 7) were associated with variation in ABA content. One of the QTLs was detected only as a main (single locus) effect, and two QTLs were found through two-locus interaction analysis (epistasis). Interaction between QTLs at markers TG234 (chromosome 2) and TG155 (chromosome 4) explained 20% of total phenotypic variance for this trait. The interaction closely resembled one previously detected for dormancy, suggesting an association between high ABA content and long tuber dormancy. Although relationships between ABA level and dormancy could be demonstrated through polygene mapping, there was no indication of a relationship between these traits when they were subjected to a conventional correlation test. This illustrates the usefulness of polygene mapping as a tool to identify possible associations between hormone levels and plant development.

Morphology and [14C]Gibberellin A12 Metabolism in WildType and Dwarf Solanum tuberosum ssp. Andigena Grown under Long and Short Photoperiods

Summary Potato tuberization is promoted by short days and inhibited by applications of gibberellin (GA). Wildtype Solanum tuberosum ssp. andigena required short days for tuberization, whereas their dwarf siblings tuberized under long days. Nevertheless, shortening the photoperiod also enhanced tuberization of dwarf plants and caused an increase in leaf angle and a reduction in internode length of both genotypes. The potential role of GA metabolism in tuberization as influenced by dwarfing and photoperiod was investigated by studying the metabolism of [ 14 C]GA 12 in shoots of wild-type and dwarf potato plants. Measurement of metabolite levels was by radiocounting after HPLC. The data indicated a reduced conversion of GA 12 to GA 53 in dwarf plants, a difference which may be responsible for the dwarf morphology, although other explanations cannot be ruled out. Dwarf plants contained much less GA 1 , as measured by GC-MS. We did not detect differences in [ 14 C]GA 12 metabolism between plants grown under 10-h versus 16-h photoperiods, in either wild-type or dwarf plants, even though other wild-type plants contained less GA 1 , under short photoperiods than under long photoperiods.

Quantitative resistance to late blight from Solanum berthaultii cosegregates with RPi-ber: insights in stability through isolates and environment.

Genetic resistance is a valuable tool in the fight against late blight of potatoes but little is known about the stability and specificity of quantitative resistance including the effect of defeated major resistance genes. In the present study we investigated the effect of different isolates of Phytophthora infestans on the mode of action of RPi-ber, an R-gene originating from Solanum berthaultii. The experiments were conducted on progenies derived from two reciprocal inter-specific backcrosses of Solanum tuberosum and S. berthaultii. The plant–pathogen interaction was tested in diverse environments including field, greenhouse and growth chamber conditions. The RPi-ber gene provided complete resistance against a US8 isolate of P. infestans in all trials. When isolates compatible with RPi-ber were used for inoculation, a smaller, but significant resistance effect was consistently detected in the same map position as the R-gene. This indicates that this R-gene provides a residual resistance effect, and/or that additional resistance loci are located in this genomic region of chromosome X. Additional quantitative resistance loci (QRL) were identified in the analyzed progenies. While some of the QRL (such as those near TG130 on chromosome III) were effective against several isolates of the pathogen, others were isolate specific. With a single exception, the S. berthaultii alleles were associated with a decrease in disease severity. Resistance loci reported in the present study co-locate with previously reported R-genes and QRL to P. infestans and other pathogens.

Similarity of QTLs detected for in vitro and greenhouse development of potato plants

A relationship between quantitative trait loci (QTLs) detected for inxa0vitro and greenhouse growing conditions was studied in a backcross population of 155 genotypes derived from a haploid potato (Solanum tuberosum) and a diploid wild species (S.xa0berthaultii). Both plant height and tuberization earliness were characterized under two growing conditions. Main-effect QTLs and QTLs identified only through interaction were detected for each of the traits. For traits associated with plant height as well as for traits associated with early tuberization, the most significant QTL detected for greenhouse cultivated plants was also found when the population was grown inxa0vitro. The most significant QTL for earliness of tuberization inxa0vitro, which was located on chromosome 8, coincides with that detected for sucrose concentration in leaf exudate. The absence of a S.xa0berthaultii allele was associated both with a higher amount of sucrose in the exudate and with earlier inxa0vitro tuber formation. Epistasis was found to have a significant effect on all traits investigated. The QTL model that included main-effect QTLs and all significant interactions explained 83–88% of the total genetic variance for each of the developmental traits. The possibility of using an inxa0vitro system combined with marker-assisted selection for preliminary selection of early tuberizing clones is discussed.

Genetic control of aggressiveness in Phytophthora infestans to tomato

The genetic control of aggressiveness in Phytophthora infestans to tomato was investigated by analyzing hybrid progeny from five different crosses (four F1 crosses and one F2 cross). The parental isolates were all highly aggressive to potatoes, but some had low and others had high aggressiveness to tomato. Of 225 single-oospore progenies tested, 198 were demonstrated to be recombinants on the basis of mating type, allozyme genotype, and (in some cases) nuclear-DNA fingerprint. Trisomy for some parental and progeny isolates was suggested from analysis of allozyme data. The recombinants were evaluated for aggressiveness using detached leaflets of both tomato and potato. From these assays, we suggest that there is a locus with strong influence on P. infestans aggressiveness to tomato, with low aggressiveness being dominant. Other loci of less influence may also be involved, but were not detectable. Some progenies were not aggressive to potatoes, which was consistent with the hypothesis that aggressiveness to potato is dominant. However, because sib crosses and backcrosses are very difficult to achieve, and because even F1 progenies are generally less fit than either parent, conclusions about the genetic control of aggressiveness to potato remain tentative.Key words: late blight, oomycete, genetics.

Tuberonic (12-OH-jasmonic) acid glucoside and its methyl ester in potato

Abstract Tuberonic acid glucoside and tuberonic acid glucoside Me ester were detected in the leaves of Solanum tuberosum. This is the first report of the isolation of the latter from potatoes. Higher concentrations of both substances were detected in leaves of plants grown under long than under short days. Methylation of tuberonic acid glucoside prior to GC-mass spectrometry increased the threshold for detection 12–50-fold, depending upon whether Ac or TMSi derivatives were made.