Field resistance to late blight (Phytophthora infestans (Mont.) De Bary) in potato (Solanum tuberosum L.) somaclones associated with instability and pleiotropic effects

Summary Adventitious regenerants (somaclones) of Bintje and their vegetative progeny were screened for field resistance to Phytophthora infestans as follows: the area under the disease progress curve was computed and correlated with resistance rating in Bintje and reference varieties. The resistance rating of the somaclones was determined from this relationship.Clones with stable improved field resistance in successive years' trials were detected, however, most of such clones were also maturation mutants. Variation in resistance rating in clone replicates and between years was detected in most clones.The possible basis of the field resistance and reasons for its instability are discussed.Abbreviations AUDPC area under the disease progress curve     

Molecular breeding for resistance to Phytophthora infestans (Mont.) de Bary in potato (Solanum tuberosum L.): a perspective of cisgenesis

Late blight caused by Phytophthora infestans is one of the most devastating diseases in potato cultivation and is mostly controlled by the application of chemicals. However, introduction of combinations of resistance ( R ) genes conferring broad-spectrum resistance from wild Solanum species into cultivated potatoes is considered the most practical and promising approach to achieve durable resistance. This can be realized via classical breeding or genetic modification (GM). Because classical breeding is very time-consuming and is often hampered by linkage drag, a GM approach seems logic in this heterozygous and vegetatively propagated crop. During the last decades, many R genes have been identified in several wild Solanum species. Some have been cloned and more will follow. When these genes are derived from species crossable with cultivated potato (so-called cisgenes), application in resistance breeding using a GM approach is similar to an introgression breeding approach, in that the exploited genes are indigenous to the crop. Pending deregulation or derogation of cisgenesis, the use of cisgenic R genes would be an ideal strategy to accomplish durable resistance in potato.     

Resistance to potato late blight (Phytophthora infestans (Mont.) de Bary) in Solanum nigrum,S. villosum and their sexual hybrids with S. tuberosum and S. demissum

Summary Solanum nigrum and S. villosum, and their sexual hybrids with S. tuberosum and S. demissum respectively, were inoculated with a complex race of Phytophthora infestans. No visible reaction was seen on S. villosum and one genotype of S. nigrum. Another genotype of S. nigrum and the hybrids showed a hypersensitive response on most inoculated leaves. In one experiment, some sporulation was observed on detached leaves of a hybrid derived from S. nigrum. Microscopical examination of infections in S. nigrum and in a hybrid from S. nigrum, showed that penetration of epidermal cells and subsequent intercellular growth of the pathogen into the spongy mesophyll occurred, but without the formation of haustoria, and that invaded and neighbouring cells became necrotic. Callose appositions were found in epidermis and mesophyll cells of all inoculated genotypes, and also in epidermal cells of the unrelated nonhost species Brassica campestris.     

Inheritance of resistance to late blight (Phytophthora infestans) in potato

Under epiphytotic conditions for late blight in spring seasons, data were recorded on its intensity four times at 4 days intervals from the start of the disease in the field, in 114 (19 females × 6 males) progenies and their parents planted in randomized complete block design in the years 2005 and 2006. Regression and stepwise regression analysis showed that observations during the rapidly increasing phase of disease between initial and last phase of disease are more important than the observations at initial and last phase of the disease. Combining ability analysis on a sub-set of 68 progenies showed that the additive component of genetic variance was more important than the non-additive component of variance in inheritance of quantitative resistance to late blight. The per se performance of the parents does give an idea about their general combining ability (GCA). However, selection of parents based on their GCA will be very useful for breeding for quantitative resistance to late blight. Parents JX 90, JF 4841, CP 3356, CP 1358, CP 3290, JN 1197 and CP 3125 were found to have good GCA for quantitative resistance for late blight and the best six crosses for late bilght resistance based on mean performance involved parents with good combining ability only.     

Field resistance to late blight and potato root eelworm in group tuberosum dihaploids

Summary An examination of the feasibility of obtaining dihaploids with high levels of field resistance to important potato pathogens, showed that dihaploids may be produced which are more resistant to late blight or to potato root eelworm pathotype E than are their tetraploid parents. The significance of this is discussed in relation to the possible types of gene action involved in conferring field resistance.     

Identification of somatic hybrids of dihaploid Solanum tuberosum lines and S. brevidens by species specific RAPD patterns and assessment of disease resistance of the hybrids

Summary Symmetric somatic hybrids were produced by electrofusion of protoplasts of two dihaploid tuber-bearing potato (Solanum tuberosum L.) lines and Solanum brevidens Phil., a diploid non-tuber-bearing wild potato species. A total of 985 plants was obtained. Verification of nuclear hybridity of putative hybrids was based on additive RAPD patterns, general morphological characteristics and chromosome counts. 53 (90%) calli regenerated into plants which were identified as somatic hybrids. Most of the hybrids were aneuploids at the tetraploid (4×) or hexaploid (6×) level. The 20 hybrids tested expressed a high level of resistance to potato virus Y (PVY ^N ) characteristic of the S. brevidens parent. Resistance to late blight (Phytophthora infestans (Mont.) de Bary) varied between hybrids, but was on average better than that of the fusion parents. Resistance of hybrids to bacterial stem rot (Erwinia carotovora subsp. atroseptica (van Hall) Dye) was not superior to that of commercial potato cultivars.     

Fertile somatic hybrids of Solanum etuberosum (+) dihaploid Solanum tuberosum and their backcrossing progenies: relationships of genome dosage with tuber development and resistance to potato virus Y

The wild non-tuberous species Solanumetuberosum is resistant to biotic andabiotic stresses, but is very difficult tocross with cultivated potato. Therefore,interspecific somatic hybrids between adihaploid clone of potato S.tuberosum (2n=2x=24, AA genome) and thediploid species S. etuberosum(2n=2x=24, EE genome) were produced byprotoplast fusion. Among the 7 fertilefusion hybrids analysed by genomic insitu hybridisation (GISH), three groups ofplants were found with the genomicconstitution of AAEE, AAEEEE and AAAAEE.Four fusion hybrids had exactly theexpected chromosome composition, while eachof the three aneuploid hybrids had lost twochromosomes of S. etuberosum. Twobackcross progenies were developed, andGISH analysis was applied to analysetransmission of the parental chromosomesinto the sexual generations. BC₁hybrids derived from the crosses of thehexaploid somatic hybrids with tetraploidpotato were pentaploid with thetheoretically expected genomic compositionor with slight deviation from thisexpectation. In the three BC₂ hybridsanalysed by GISH seven to 12 chromosomes ofS. etuberosum were detected in thepredominant S. tuberosum background.No recombinant chromosomes in the hybridswere detected. Genome dosage affects tuberformation in hybrids and their progenies,but has less effect on resistance to potatovirus Y (PVY) in fusion hybrids. Severalgenotypes of the fusion hybrids andBC₁ progeny did not show viralinfection even in the graftingexperiments.     

Correlation between late blight resistance and foliage maturity type in potato

The genetics of race-non-specific foliage resistance against Phytophthora infestans, of foliage maturity type, and of their association in potato (Solanum tuberosum) were studied. Six progenies were derived from a half-diallel set of crosses between diploid potato clones that represented a broad pool within the genus Solanum and were free of any of the 11 known R genes for late blight resistance. The progenies were evaluated for resistance to late blight and for foliage maturity type, and five of them showed a significant correlation between the two traits. The correlation did not account for all variation that was present for both traits, as reflected in the analysis in which the relative AUDPC values were adjusted for foliage maturity type. The present study adds to previous results: resistance against P. infestans always coincides with late foliage maturity. However, the results also indicate that some selection for late blight resistance without affecting the foliage maturity type should be possible.     

Introgression of resistance to late blight (Phytophthora infestans) from Solanum pinnatisectum into S. tuberosum using embryo rescue and double pollination

In order to investigate the agricultural potential of the genus Vicia, and identify traits associated with productivity and responsiveness to environment, 34 undomesticated Mediterranean accessions representing Section Narbonensis (V. johannis, V. narbonensis) and V. sativa were grown in five contrasting environments in northern Syria (growing season rainfall: 76–290 mm).Highly significant genotype × environment interactions were observed for all traits. For most of the components of yield, accession mean performance (productivity)was highly correlated with responsiveness across environments (r = 0.59–0.96), as defined by joint linear regressions. Thus high yielding genotypes tended to be relatively more productive than low yielding genotypes under conditions that favoured high yields. Regression analysis revealed that mean site yields were positively correlated to rainfall (r = 0.85) and its attendant effect on growing season length as measured by cumulative season temperature and phenology (r = 0.59–0.81).In order to examine yield related traits independently of taxonomy, genotypes were grouped into three categories using K-means clustering based on productivity and responsiveness of seed, hay and biological yield. Highly productive/responsive genotypes were tall with high harvest index, large seeds and low fecundity (seeds and pods per plant), whereas unproductive/unresponsive plants tended to be short, highly fecund, with small seeds and low harvest index. Principal components analysis showed that responsiveness, in terms of seed, hay and biological yields, was closely related to phenological plasticity. Thus highly productive/responsive genotypes were able to start flowering earlier than unproductive/unresponsive genotypes in early environments, but significantly later in late, higher rainfall environments. Plant growth habit was also related to yield responsiveness. In environments with little biomass production the proportion of erect plants was high in all three categories. In more favourable, high biomass environments, the proportion of erect plants in unproductive/unresponsive genotypes fell dramatically, but was unchanged among productive/responsive genotypes. We suggest that for unproductive/unresponsive genotypes competition for light is increased under optimal growth conditions. We argue that the optimal combination of fixed and responsive traits in high yielding genotypes results in a `compound interest-type' response to more favourable environments. Highly productive and responsive genotypes can capture resources more effectively than their low yielding counterparts, leading to a positive relationship between performance and responsiveness for most components of yield. Differences in productivity and responsiveness for seed, hay and biological yield reflected Vicia taxonomy, increasing in the following order from low to high: V. johannis, V. sativa, the small seeded V. narbonensis (salmonea, jordanica, affinis) V. n. var. narbonensis, and finally V. n. var.aegyptiaca. V. n. var. aegyptiaca showed the most agricultural potential, since the taxon contained all the properties of productive/responsive genotypes listed above, yielding >1 t/ha under extremely arid conditions (104 mm),and >2.5 t/ha on 290 mm rainfall, confirming its potential for dry environments. This revised version was published online in August 2006 with corrections to the Cover Date.     

Gene action in the resistance of peppers (Capsicum annuum) to Phytophthora stem blight (Phytophthora capsici L.)

Summary The variation among single and three-way crosses derived from seven lines of pepper (Capsicum annuum) selected for resistance to Phytophthora capsici was analyzed into statistical-genetic components due to general and specific combining abilities and additive-by-additive epistasis. Indirect evidence of higher order epistasis was also estimated. Epistasis was a principal source of variation. It was correlated with the level of aggressiveness of the pathogen. A breeding method such as recurrent selection seems to be the most appropriate to improve resistance to the fungus.     

Race specific resistance against Phytophthora infestans in potato is controlled by more genetic factors than only R-genes

Summary For RFLP mapping of R-genes, determining resistance to specific races of Phytophthora infestans in tetraploid potato, it is necessary to develop well segregating populations at the 2x level. During mapping studies, evidence was obtained that more genetic factor(s) are involved in the expression of R-genes than conventionally believed. Two experiments are described in which such an additional genetic factor was suppressing or enhancing the expression of unknown R _nand R _ifactors. R _nand R _iappeared to be present in the investigated plant material, containing R4 and R10, or in one of the susceptible crossing parents. In a third experiment, the expression and the segregation of the well known R1 gene was influenced by an additional genetic factor. In that case there were indications for a dominant suppressor. This was established by the selection of susceptible plants carrying a RFLP allele of probe GP21 closely linked to R1. In three of the four F₁ populations, resulting from crosses between such susceptible plants and susceptible tester plants, resistnat progenies were found. The resistance appeared to be R1-specific. This clearly indicates that in three of the four investigated susceptible plants, the R1 gene was still present but not expressed.     

Genetic linkage of QTLs for late blight resistance and foliage maturity type in six related potato progenies

A set of test crosses of diploid potatoes was used to identify QTLs for foliage resistance against Phytophthora infestans and QTLs for foliage maturity type, and to assess their genetic relationship. The most important locus for both traits was found on chromosome 5 near marker GP21: the allele of marker GP21 that is associated with resistance to late blight is also associated with late foliage maturity. An additional QTL with a small effect on foliage maturity type was identified on chromosome 3, and additional QTLs for late blight resistance were found on chromosomes 3 and 10. Another QTL was detected on chromosome 7 when resistance was adjusted for the effect of foliage maturity type. The additional QTLs for resistance against P. infestans on chromosomes 3 and 10 seem to be independent of foliage maturity type and are not affected by epistatic effects of the locus on chromosome 5. The effects of the additional QTLs for resistance are small, but early maturing genotypes that necessarily have the allele for susceptibility for late blight on chromosome 5 may benefit from the resistance that is provided by these QTLs on chromosomes 3 and 10.     

Transfer of resistance to Verticillium dahliae Kleb. from Solanum torvum S.W. into potato (Solanum tuberosum L.) by protoplast electrofusion

Summary Interspecific somatic hybrid plants were regenerated after electrofusion of mesophyll protoplasts with the objective of transferring resistance to Verticillium dahliae from Solanum torvum into potato. Early selection of the putative hybrids was based on differences in cultural behaviour of the parental and hybrid calli (particularly the ability of the latter to regenerate early) in combination with morphological markers. Four putative hybrids were recovered from hundreds of calli, probably resulting from complementation of the two parental genomes. The regenerates were tetraploids (2n=4×=48 chromosomes) and exhibited intermediate traits including leaf form, plant morphology and the presence of anthocyanin. The hybrid nature of the four selected plants was confirmed by examining isoenzyme patterns for isocitrate dehydrogenase (Idh), malate dehydrogenase (Mdh), phosphoglucoisomerase (Pgi) and 6-phosphogluconate dehydrogenase (6-Pgd). While the hybrid plants rooted readily and grew vigorously under in vitro conditions, in the greenhouse their development and growth were retarded by difficulties in rooting. When grafted on potato or S. torvum rootstocks, the hybrid plants recovered normal development and growth. Again, they exhibited intermediate morphological traits. Tests for resistance realized in vitro with medium containing 50% Verticillium wilt filtrate showed that all the somatic hybrids were resistant to the fungus filtrate.     

Characterization of morphological variation and cold resistance in interspecific somatic hybrids between potato (Solanum tuberosum L.) and S. brevidens Phil.

Summary Somatic hybrids between Solanum tuberosum L. cv. Gracia (2n=4x=48) and Solanum brevidens Phil. (2n=2x=24) were produced via fusion of mesophyll protoplasts. Selection of the protoplast derived putative hybrid calli was based on their vigorous growth. Additive isozyme patterns and chromosome numbers as well as the expression of parental morphological characters have proved the hybrid origin of the selected regenerants. Extensive chromosome loss during the regeneration process resulted in aneuploid hybrids with high frequency. Genomic instability could not be detected in these plants during the period of vegetative propagation. Regenerants from hybrid tissues exhibited wide morphological variation especially in tuber formation. The detailed morphological analysis based on the use of multivariate method (principal component analysis, PCA) enabled to identify morphological groups among the hybrid clones. The positioning of hybrid clones in the PCA space could not be correlated with chromosome numbers. The genomic ratio represented by the tetraploid and diploid parents influenced the morphology of somatic hybrid population according to the applied analytical system. Two selected hybrid clones have exhibited an intermediate degree of frost tolerance compared to the parents, based on the recovery of plants from lower buds after freezing of potted plants.     

Integration of transgenes into sexual polyploidization schemes for potato (Solanum tuberosum L.)

Most potato transgenic research has focused on development of resistance to pathogens and modification of potato physiology. Many transgenes, particularly those conferring pathogen resistance, could substantially lower potato production costs in developing countries. However, transgenes have not been reported in sexually propagated 4x-2x potato hybrids commonly grown in developing countries. Two transgenes,the Bacillus thuringiensis cry3Aa endotoxin protein gene and the PVY°coat protein gene, were engineered intodiploid and tetraploid potato using Agrobacterium tumefaciens-mediated transformation. Cry3Aa was produced at high levels in several lines while the PVY° coat protein was not expressed. Diplandroid and tetraploid genotypes were crossed to produce transgenic 4x-2x hybrids. Genetic transformation had no discernable effect on fertility ofthe primary transformants, germination of4x-2x seed derived from the transformants and agronomic performance(tuber set, average tuber weight and total tuber yield) of the 4x-2xhybrids. The transgenic 4x-2xhybrids produced non-viable pollen and could only be crossed as female parents. Results suggest that transgenes, such ascry3Aa, could be expressed in 4x-2x hybrids to lower costs of production with no significant effect on plant phenotype. This revised version was published online in July 2006 with corrections to the Cover Date.     

Resistance to late blight (Phytophthora infestans) in ten wild Solanum species

Nineteen accessions of the tuber-bearing species Solanum berthaultii, S. chacoense, S. leptophyes, S. microdontum, S. sparsipilum, S. sucrense, S. venturii, S. vernei and S. verrucosum were tested for their resistance to late blight in two years of field experiments. Plants were artifically inoculated with zoospores of race 1.2.3.4.5.7.10.11 and the development of the disease was followed. Resistance ratings, calculated as the areas under the disease progress curves (ADPC), demonstrated a high resistance in all accessions except in S. sparsipilum, S. leptophyes and their interspecific hybrid. Segregations suggest that major genes for resistance are present in S. sucrense and S. venturii, and may also play a role in S. verrucosum. It is not yet certain wether the resistance of the other accessions is comparable to the partial and durable resistance of S. tuberosum cultivars like Pimpernel, as inheritance and mechanism have yet to be established. However, segregations suggesting the presence of single major genes with complete dominance were not found in these other accessions. Tuber initiation in the field occurred in only one accession, S. tuberosum ssp. andigena, and maturity of the clones was not related to their resistance. In the other accessions maturity types could not be assessed, as the clones require short day conditions for tuber initiation.     

Induction of novel organelle DNA variation and transfer of resistance to frost and Verticillium wilt in Solanum tuberosum through somatic hybridization with 1EBN S. commersonii

Somatic hybridization can be used to induce genetic variability in plastidial and mitochondrial genomes, and transfer multiple uncloned genes across sexual barriers. Somatic hybrids were produced between a dihaploid clone of the common potato, S. tuberosum subsp. tuberosum, and the wild sexually incongruent diploid species S. commersonii. Fusion products were selected on the basis of callus growth and regeneration in vitro. Genome composition of putative somatic hybrids was determined by flow cytometric analysis of nuclear DNA content, RAPD analysis, and Southern analysis with probes specific to organellar DNA. All regenerated fusion products proved to be hybrids based on RAPD analysis. Seventy per cent of somatic hybrids were (near) tetraploids, 22% (near) hexaploids and 8%(near) octoploids. A high correlation was found between the nuclear DNA content determined by flow cytometry and the number of chloroplasts in stomata guard cell pairs. Somatic hybrids inherited the parental plastids in a random manner. On the contrary, they preferentially inherited the mitochondrial DNA fragments of S. tuberosum. The majority of them had a rearranged mitochondrial genome with fragments from both parents. Hybrids were highly vigorous and morphologically more similar to the cultivated than to the wild parent, produced tubers on long stolons under long photoperiod conditions, showed a high degree of flowering, but did not produce pollen. In addition, somatic hybrids were generally more resistant to frost and Verticillium wilt than the cultivated parent, indicating the introgression of relevant resistance genes from the wild species into the genetic background of S. tuberosum. This revised version was published online in July 2006 with corrections to the Cover Date.     

The breeding values of potato parents for field resistance to late blight measured by whole seedlings

Summary Parents used in a potato breeding programme were intercrossed in a multiple mating scheme. The resulting progenies were assessed for resistance to late blight, using as criteria the size and frequencies of lesions on leaves, on petioles and on stems, and an overall score. Analysis of variance showed that all the statistically significant genetic variation was attributable to general combining ability (GCA) differences. The seven variates were highly correlated.     

Regarding the novelty and breeding value of protoplast-derived variants of Russet Burbank (Solanum tuberosum L.)

Summary Striking similarities appear to exist between selected protoplast-derived variants (PD-clones) of the potato cultivar Russet Burbank, and commonly occurring somatic potato sports, called bolters. A limited survey of individuals involved in the certification of Russet Burbank, revealed that bolters do commonly occur in this variety, are highly variable, and sometimes occur at high frequency. It is therefore suggested that bolters and the more promising selected PD-clones of Russest Burbank may be one and the same, arising from the same type of macromutation.One selected PD-clone, three different Russet Burbank bolters, and two controls were analyzed for 15 isozymes in the hope of determining if bolters and promising PD-clones belong to the same class of variants. No isozyme variation between Russet Burbank and its subclones was observed. The breeding value of protoplast-derived variation is discussed in this light.Approved by the Director of the New York State Agricultural Experiment Station for publication as Journal paper No. 3554, Oct. 19, 1983.     

Epistasis in the resistance of pepper to phytophthora stem blight (Phytophthora capsici L.) and its significance in the prediction of double cross performances

Summary Epistatic effects are present in the resistance to Phytophthora capsici in peppers (Capsicum annuum). Genetic statistical analysis shows that additive by additive epistasis as well as higher order epistatic effects contribute to the total variation of resistance, significantly. Higher order epistatic effects caused bias in the prediction of the performance of double crosses, the difference between observed and predicted performances being significant.The relative importance of higher order epistasis with respect to additive by additive epistasis seems to be correlated with the aggressiveness of the isolate of Phytophthora capsici.