QTL Analysis of Tuber Dormancy and Sprouting in Potato

Tuber dormancy and sprouting are important traits in commercial potato varieties. Since wild Solanum species can contribute new alleles in breeding, we combined Solanum chacoense and S. berthaultii species in a mapping population to develop a linkage map with genome wide markers and identify quantitative trait loci (QTL) for tuber dormancy and sprouting. There was genetic variation for the evaluated traits and high correlation (0.80) between dormancy and apical dominance release. QTL for tuber dormancy and sprouting traits were mapped to the same or similar positions on chromosomes 2, 3, 5, and 7, explaining from 9.5% to 16.3% of the phenotypic variance. We identified candidate genes associated with all major phytohormone regulation in close linkage with the QTL. The QTL on chromosomes 2, 3 and 7 are good candidates for marker assisted breeding, since they were associated with dormancy and apical dominance release and should be useful to delay or accelerate tuber sprouting.     

A genetic model for tuberization in potato haploid-wild species hybrids grown under long-day conditions

Wild diploidSolanum species contain valuable genes for potato improvement, but do not tuberize under the long-day conditions of temperate growing regions. Crosses to haploids (2n=2x=24) of the cultivated potato (Solanum tuberosum) produce hybrids that often tuberize under long days. The objectives of this work are (1) to document high levels of tuberization in haploid-wild species hybrid populations, (2) to evaluate the genetic basis of tuberization in haploid-wild species hybrids grown under long-day conditions, and (3) to propose a genetic model for tuberization in haploid-wild species hybrids. Tuberization under long-day conditions was evaluated in 154 haploid-wild species hybrid families. An average of 68% of plants in these families tuberized. Two major genes exhibiting duplicate dominant epistasis appear to regulate tuberization under long-day conditions. Based on this model, the haploid parent genotypes are A-B-, aaB-, or A-bb, while the wild species are aabb. Future studies are planned to identify the genetic components of tuberization.     

Effects of photoperiod on kinetin-induced tuberization of isolated potato stolons culturedin vitro

Are short-day (SD) conditions inductive or permissive for potato tuberization? This question was addressed using anin vitro culture system. Kinetin induction ofin vitro tuberization in potato stolons cultured in the dark can be partially inhibited by light treatments. Photoperiod required for a major inhibition varied with cultivars: 8 hours for cv. Red Pontiac and 16 hours for cv. Kennebec. Short photoperiods decreased kinetininduced tuberization for all cultivars. Stolons cultured on kinetin-free medium generally did not tuberize, regardless of light treatment. Effects of kinetin and photoperiod on stolon and tuber development are observed. Relationships among kinetin, photoperiod and tuber-inducing stimulus produced under short days are discussed.     

Physiological regulation of potato tuber dormancy

At harvest, potato (Solanum tuberosum L.) tubers are dormant and will not sprout. As the period of postharvest storage is extended, tuber dormancy is broken and sprout growth commences. The loss of tuber dormancy and onset of sprout growth is accompanied by numerous biochemical changes, many of which are detrimental to the nutritional and processing qualities of potatoes. Endogenous hormones have been proposed to play a significant role in tuber dormancy regulation. The involvement of all major classes of endogenous hormones in tuber dormancy is reviewed. Based on available evidence, it is concluded that both ABA and ethylene are required for dormancy induction, but only ABA is needed to maintain bud dormancy. An increase in cytokinin sensitivity and content appear to be the principal factors leading to the loss of dormancy. Changes in endogenous IAA and GA content appear to be more closely related to the regulation of subsequent sprout growth.     

Silencing of G1-1 and A2-1 genes. Effects on general plant phenotype and on tuber dormancy inSolanum tuberosum L.

Summary We have evaluated the effect that the, silencing of two genes specifically expressed in conditions of dormancy (A2-1) and sprouting (G1-1) had on tuber dormancy. For this purpose potato plants (Solanum tuberosum L. cv. Désirée) were transformed with the antisense of the genes G1-1 and A2-1 under the control of constitutive 35S CaMV promoter. A first generation of transgenic plants was propagated from axenic stem cuttings and a second generation by tuber planting. The plants obtined were analyzed for the length of dormancy, plant morphology and agronomic characteristics. Statistical analysis of dormancy in lines obtained from the original transformants for the antisense of G1-1 gene showed a significant increase in length as compared with different types of control plants, with few effects on plant vegetative habit and tuber production. In contrast, results obtained on A2-1 antisense transformed plants did not reveal any significant change on the length of dormancy. Here we report small-scale field trials performed with the aim to select and regenerate commercially exploitable potato plants with a stable transgene-dependent phenotype, affecting the length of dormancy.     

Molecular marker-assisted selection for potato breeding

The use of molecular markers in potato breeding offers new opportunities for the selection of genotypes. To date, many markers linked to useful traits have been found. Indeed, the potato molecular map is saturated with more than 350 markers uniformly distributed on 12 chromosomes. More than 25 single dominant genes have been localized on the potato map, most of them being pest-resistance genes, together with some quantitative trait loci (QTL) controlling yield and tuber quality traits. Molecular markers linked to these genes are now available for rapid, efficient assisted selection (positive assisted selection). Moreover, in order to transfer useful genes, interspecific crosses between wild and cultivated genotypes are often performed, and in this case the use of species-specific molecular markers would allow the wild genomic content to be reduced in few backcross generations (negative assisted selection). The following review discusses the progress achieved in potato breeding through molecular marker-assisted selection.     

Tolerance to potato leafhopper inSolanum tuberosum L. gp. Tuberosum clones

We identified clones inSolanum tuberosum L. gp. Tuberosum breeding populations that, although susceptible to infestation by potato leafhopper,Empoasca fabae (Harris), exhibited a tolerance which was manifested by the ability to produce higher than expected tuber yields when heavily infested with leafhoppers. Such tolerance was noted in a number of clones in varying degrees, but one, B6930-1, was exceptional. It sustained only a 19% loss in tuber yield compared to ‘Katahdin,’ a susceptible cultivar, that averaged 33% lower tuber yields under similar infestation levels. We studied the seasonal growth patterns of this clone and compared them to those of Katahdin, and B6705-10, a breeding clone with known resistance to leafhopper infestation. We found that much of B6930-l’s tolerance results from its early, vigorous vine growth and its early and rapid rate of tuber bulking, enabling this clone to escape some of the effects of leafhopper feeding, whereas Katahdin was usually just beginning to tuberize at the height of leafhopper infestation.     

Effects of Foliar and Tuber Sprout Suppressants on Storage of Ware Potatoes under Tropical Conditions

Potato (Solanum tuberosum L.) is an important source of dietary carbohydrate and cash income generation for farmers in the tropical highlands of Kenya. The feasibility for cold storage at the farm level is limited due to the high costs of maintaining such a facility and there is limited data on the long-term post-harvest storage and quality of tubers of tropical-adapted cultivars. Application of sprout suppressants to control premature sprouting of ware potato is an attractive proposition. The objectives of this study were to evaluate the efficacy of pre-harvest foliar applications of paclobutrazol (PBZ) and ethephon for sprout suppression on ware potato tubers in storage. Post-harvest spray applications of Isopropyl N-(3-chlorophenyl carbamate) chloropropham (CIPC) and 1,4-dimethylnaphthalene (DMN) on tubers as fog was also evaluated. Potato cultivars had varying levels of tuber dormancy. The tubers were stored at ambient temperature (23 C) and evaluated weekly for 24 weeks for percent of tubers sprouting, length of longest sprouts, tuber weight loss and assessed for dormancy for 24 weeks. Paclobutrazol prolonged tuber dormancy by 21–31 days and reduced tuber weight loss. Ethephon treatment had no effect on dormancy and tuber weight loss. Potato tubers treated with CIPC had greater sprout control than the other treatments in storage. Tuber response to DMN treatment varied among the three potato cultivars evaluated. The findings from this study imply that PBZ is effective in prolonging potato tuber dormancy for short-term basis at 23 C, while CIPC applied on tubers was effective for long term storage. Optimization of post-harvest potato storage can improve food security in the highland tropics.     

Marker-assisted genetic analysis of non-acclimated freezing tolerance and cold acclimation capacity in a backcrossSolarium population

Random amplified polymorphic DNA (RAPD) and simple sequence repeat (SSR) markers were used to construct a partial genetic linkage map in a potato backcross population. The population, derived from two diploid wildSolatium species (frost tolerant, able to cold acclimateS. commersonii; frost sensitive, unable to cold acclimateS. cardiophyllum), was used to map quantitative trait loci (QTL) of non-acclimated relative freezing tolerance (NARFT) and cold acclimation capacity (CAC). Precise assessment of these traits allowed distinction of small but significant differences among 35 backcross genotypes. NARFT and CAC were not correlated in the segregating population, suggesting independent genetic control for these two major components of freezing tolerance. The linkage map spanned 479.4 cM and included 77 RAPD markers and two SSR markers, with 38 RAPD and 10 SSR unassigned markers. Two QTLs for NARFT were detected in two different linkage groups, accounting for 44.0% of the phenotypic variation for this trait. Two QTLs for CAC were detected, accounting for 24.9% of the phenotypic variation for this trait. QTLs for NARFT and CAC were detected at separate genomic regions, in support of the independent genetic control of these two traits. QTLs for NARFT and CAC were detected in a linkage group identified as part of chromosome V, suggesting that such chromosome constitutes a prime candidate for fine-mapping. Due to the relatively small progeny size evaluated in this study, additional QTLs for NARFT and CAC could have been involved but not identified. Therefore, the conclusions derived from this study should be considered preliminary.     

Gelling agent and light effects onin vitro tuberization of potato cultivars

The effects of Gelrite and agar onin vitro tuberization of six potato cultivars, Atlantic, Chaleur, Cherokee, Desiree, Kennebec and Red Pontiac under low light intensity and in the dark were examined. The medium consisted of MS salts, vitamins, 6% sucrose, 2.5 mg/l kinetin and either 2 g/l Gelrite or 6 g/l agar. The cultures were kept either under diffused light with a 16 h photoperiod or in darkness at 22/19 C day/night temperatures for eight weeks. Tuberization was earlier and more uniform (a higher proportion of tubers ≥ 5 mm) on Gelrite than on agar solidified medium and earlier in the darkness than in the light. Light effects varied from cultivar to cultivar. The slowest to tuberize in light were Atlantic and Red Pontiac, the fastest, Chaleur and Cherokee. Chaleur, Cherokee and Kennebec produced significantly more large tubers and higher total tuber mass under low light than in darkness. Atlantic and Red Pontiac reacted in the opposite way. Desiree produced significantly more large tubers in the dark, but the differences in tuber mass between the dark and light conditions were not significant. Light also stimulated shoot and root growth in most of the cultures. No relation was observed between the relative maturities of the tested cultivars and their tuberization responsesin vitro.     

Potato (Solanum spp.) in the hot tropics. VI. Plant population effects on soil temperature,plant development and tuber yield

Nine experiments were run at three hot tropical sites (5–12°S, 180–800 m) within Peru to quantify the influence of plant population on soil temperature and growth and yield of the potato.Radiation interception was greatest at the highest plant populations and soil cooling was directly proportional to the amount of crop cover over the soil, but no appreciable effect on the timing of tuber initiation was apparent. More stems per unit land area leading to a higher leaf area index (lai) were primarily responsible for greater interception of radiation at the higher plant populations, although some compensation in stem number per unit land area and in lai at lower populations was evident later in the season.In general, tuber yield increased linearly with increases in planted population over the range studied (2.7–12.5 plants m⁻², and in one experiment to 31.7 plants m⁻²), and was proportional to increases in the amount of intercepted radiation. Tuber yields ranged from 8 to 60 t ha⁻¹ over sites and populations. Vigorous clones with Solanum tuberosum spp. andigena in their genetic background constituted the exceptions to this linear trend, and for these clones yields declined at the highest populations, particularly when the rectangularity of planting vastly deviated from square patterns. Tuber yield of Solanum tuberosum spp. tuberosum and Neotuberosum (S. tuberosum spp. andigena selected for tuberization under long-day conditions) clones did not respond to variations in rectangularity of planting and, probably due to their small stature and early maturity, did not demonstrate signs of intense between-plant competition for tuber yield as measured with the Kira competition density index. In contrast, for clones with Solanum tuberosum ssp. andigena in their genetic background, maximum tuber yield at populations greater than 5.5 plants m⁻² was dependent on the rectangularity of planting, and declined as the latter deviated from squareness.Since the proportion of marketable tubers was scarcely affected by the planting densities, plant population of S. tuberosum ssp. tuberosum clones planted in hot climates should be as close as possible without limiting the amount of soil available for hilling-up.     

Introgression and stabilization oferwinia tuber soft rot resistance into potato after somatic hybridization ofsolanum tuberosum ands. brevidens

Resistance to potato tuber soft rot caused byErwinia carotovora was transferred fromSolanum brevidens to the cultivated potato over the course of four backcross generations originating from a somatic hybrid. Soft rot reactions were determined via a tuber plug inoculation method developed during the course of these experiments. Soft rot resistance was highest in the somatic hybrid (only ca. 20% of tubers and plugs showed evidence of severe rotting) and lowest among progeny of control potato x potato crosses (ca. 80% of tuber plugs showed severe rot). Backcross generations involving somatic hybrids were intermediate in their reaction, and resistance stabilized to about 60% of tuber plugs showing severe rot in the BC₂ through the BC₄. Reciprocal crosses showed no difference in the inheritance of soft rot resistance, indicating that neitherS. brevidens norS. tuberosum donor cytoplasm had a significant effect on the expression of resistance. Crosses between BC₃ siblings where noS. brevidens genetic markers were detected but resistance was segregating demonstrated a dosage effect for soft rot resistance. We conclude that introgression of soft rot resistance has occurred and that at least one locus responsible for resistance inS. brevidens now resides in theS. tuberosum genome.     

Molecular Diagnostics for Complex Pest and Disease Resistance and Tuber Quality Traits: Concept,Achievements and Perspectives

We summarize the concept of molecular diagnostic of complex traits related to pest and disease resistance and to tuber quality of potato, and describe recent achievements and perspectives. Many potato characteristics are controlled by multiple genetic and environmental factors. Knowing the genes and their allelic variants that underlay these characteristics allows developing molecular diagnostic tools to select for improved potato cultivars. Diagnostic DNA-based markers can be used to identify superior genotypes (precision breeding). Diagnostic markers can be identified by combining quantitative trait locus mapping, candidate gene mapping and association mapping using functional and positional candidate genes as markers. This approach was successfully used to identify loci, which contribute to the natural variation of important agronomic traits, including resistance against root cyst nematodes, late blight and wart disease and tuber quality (resistance to bruising and chip colour). In the future, whole genome association mapping based on single-nucleotide polymorphism genotyping methods in combination with the annotated potato genome sequence will allow identifying additional genes and gene variants controlling agronomic performance in potato. Prerequisites are accurate phenotyping under field conditions of advanced breeding materials, cost-effective and reliable genome-wide genotyping methods, and user-friendly software tools allowing to extract knowledge from massive quantities of data. This will further facilitate molecular diagnosis, selection and combination of superior alleles in potato-breeding programmes.     

Some sprouting characteristics of microtubers

Summary Microtubers can be stored longer, transported and handled easier than plantlets, but they have some disadvantages related to long dormancy. Dormancy, number of sprouts per tuber and loss of microtubers under different storage temperatures were studied. Microtubers of four genotypes originated from different tuberization treatments (photoperiod combinations) were observed. We found that dormancy depended on cultivar and — in some cases — on the photoperiod treatment applied during tuberization. Generally, the dormacy was long and was greatly elongated by low storage temperature. One of the photoperiod treatments shortened while the other treatment prolonged the rest-period compared to the control, maybe due to a change in temperature accompanying the change in daily light (photoperiod combination). Besides, these treatments affected the number of sprouts per tuber. There was no significant difference in duration of dormancy between different tuber-size groups, but the loss increased significanctly with a decrease in tuber size. Based on this information we can use microtubers more effectively in the seed potato production programme.     

Tuberization in potato at high temperatures: Responses to exogenous gibberellin,cytokinin and ethylene

Summary In potato (Solanum tuberosum L., cv. Sebago), benzyladenine (BA) promoted tuberization at high day/night temperatures (32°/18°C), while gibberrellic acid (GA) and chlorethylphosphonic acid (CEPA) reduced tuberization at low day/night temperatures (22°/18°C). These results are consistent with the hypothesis that temperature exerts its influence on tuber formation by altering the balance between endogenous gibberellins, cytokinins and inhibitors, but not ethylene.     

Screening of Iranian Potato Germplasm for Resistance to the Potato Tuberworm Phthorimaea operculella (Lepidoptera: Gelechiidae)

The potato tuberworm, Phthorimaea operculella (Zeller), is an important insect pest of potato, Solanum tuberosum L., both in storage and in the field. In this research, tubers of eight commercial potato cultivars and four Iranian selections with equal weight and dormancy were exposed to 10 pairs of adult P. operculella in a climate chamber set at 25?±?1 °C, 65?±?5 % RH and total darkness. In a free-choice situation, oviposition was lower on 397082–2, Khavaran and Morene compared to the other germplasm that were exposed to adults of P. operculella. Number of mines per tuber, length of mines per tuber, time of development of larvae, number of pre-pupae produced per tuber, weight of pre-pupae and number of eggs developed in ovaries per female were counted and/or measured on each commercial cultivar and selection. There were fewer and shorter mines on tubers of 397082–2, Khavaran and Morene compared to the other potato germplasm. The number of pre-pupae produced per tuber and the weight of pre-pupae were lower when P. operculella was reared on tubers of 397082–2, Khavaran and Morene. Also the development, survival and fecundity were lower when P. operculella was reared on those same germplasm. Flesh firmness was negatively correlated with larval survival (r ²?=?0.87); in addition, the percentage of starch and macronutrient composition was low on these three germplasm. Thus, tuber flesh firmness of these germplasm could delay larval penetration and lower establishment 397082–2, Khavaran and Morene showed promising traits that can be integral component of potato breeding for resistance to P. operculella and pest management programs.     

Stability Analysis of Agronomic Traits in Potato Cultivars of Different Origin

Potato (Solanum tuberosum ssp. tuberosum) cultivars are expected to express a stable level for traits important for growers and consumers. To investigate how this expectation was met by a set of 21 cultivars bred in Hungary, Poland and Spain, 2-year field experiments were carried out in these countries for the evaluation of tuber yield, starch content and yield, and occurrence of secondary growth of tubers. Stability in an agronomic sense was evaluated by the analysis of genotype by environment interaction (GE) using the Scheffé-Caliński mixed model. Unstable trait expression was indicated by the statistically significant share of GE in the variability contributed by a specific cultivar. This instability could lead to either complete or partial unpredictability. Stable trait expression was observed for 6–11 cultivars, depending on the trait. A significant genetic factor, which indicates broad adaptation, was rarely found. Stable expression of tuber yield occurred together with stable or predictable expression of both starch content and yield. Unstable expressions of tuber and starch yield were also associated. The stability or instability of secondary growth was not associated with stability or instability of the other measured traits. Analysis of GE interaction was useful for identifying stable or unstable responses and revealed the presence of incomplete stability or partial unpredictability as intermediate types of reaction.     

Estimation of genetic parameters for characters related to potato propagation by true seed (TPS) in an andigena population

Genetic variances were estimated for a population ofSolarium tuberosum L. subsp.andigena, selected for adaptation to north temperate climatic conditions. Estimates for 15 traits, including some unique to true seed (TPS) propagation, were obtained from trials conducted under short day conditions in Peru. Only non-additive variance was found for yield, but heritability estimates were relatively high for tuber number and tuber size. The results indicated that yield could be increased through selection utilizing additive variance for the component traits, especially tuber size. Estimates of genetic variability for tuber uniformity were low. TPS yield was positively correlated with most other traits except tuber number.