Potato Steroidal Glycoalkaloids: Biosynthesis and Genetic Manipulation

The potato steroidal glycoalkaloids (SGAs) are important components of plant resistance against pests and pathogens but can be toxic to humans at high levels. SGAs derive their toxicity from anticholinesterase activity affecting the central nervous system and the disruptive effects on cell membrane integrity affecting the digestive system and other organs. Accordingly, current safety regulations limit their content in the edible tuber to 20 mg per 100 g fresh weight. SGA composition and level are genetically determined, with unfavourable growth conditions and inappropriate postharvest management inducing the accumulation of SGAs at levels in the tubers of “safe” cultivars beyond the maximum level set by the industry. Hence, genetic alteration of potato to prevent toxic levels of SGAs in tubers is highly desirable. At the same time, maintaining high SGA levels in other plant organs will contribute to plant resistance against pathogen and pest attacks. To this end, SGA biosynthesis and degradation should be manipulated precisely to exploit tissue-specific expression rather than whole-plant suppression of SGA production, to produce potato cultivars with SGA content enriched in the foliage but diminished in the edible tubers. Only a few details are known about the SGA biosynthetic pathway, its genes and intermediates. Research on factors that regulate SGA biosynthesis and catabolism as well as searches for genetic markers linked to total and specific SGA levels have only recently been pursued. The present review summarizes current data on these issues to encourage further discussion on SGA manipulation for safer food products.     

Potato viruses in China

Potato (Solanum tuberosum L.) is globally the fourth largest staple crop after rice, wheat and maize. China is the largest potato producer worldwide, accounting for 26.3% and 22.2% of the global total area and yield, respectively. Viral diseases have been a limiting factor for sustainable production of potato in China. This article provides update and comprehensive information on the most important viruses, their effect on yield and their control in China. Meristem culture is currently the most widely used technique for production of virus-free potato plants and these plants are used as nuclear stock for the production of certified seed tubers. Although a significant increase in tuber yield has been achieved using certified seed potatoes, lack of application of a national seed potato production scheme has hindered progress. Conventional breeding using virus-resistant cultivars introduced from other countries as parents has already led to the production of Chinese cultivars. Use of virus-resistant wild Solanum species and genetic transformation techniques will provide new virus-resistant varieties in the future to further enhance control of potato viruses in China.     

An Online Potato Pedigree Database Resource

At the Laboratory of Plant Breeding of Wageningen University, a large amount of pedigree data on current and historic potato cultivars and progenitors has been collected over many years. The sources and reasons for collection of these data are discussed. To allow others to use this data set for their own purposes, we have created a Web-accessible interface that allows querying of these data, e.g., for the ancestry of a cultivar of interest. This Web interface was recently extended and improved to allow users to create reports and dynamically created pedigree-tree images. Availability of this resource and the options provided by the new Web interface are presented.     

Application of Soluble Chitosan in vitro and in the Greenhouse to Increase Yield and Seed Quality of Potato Minitubers

The aim of this work was to investigate whether the application of soluble chitosan in potato micropropagation can improve microplant quality in vitro, help acclimatisation ex vitro, and increase yield and seed quality of minitubers. Potato cv. Désirée microplants were treated in vitro with soluble chitosan added to the semisolid tissue culture medium in different concentrations. Microplants were subsequently transferred to the greenhouse and sprayed with chitosan solutions or remained unsprayed. Untreated microplants were also established ex vitro and sprayed with chitosan, or left unsprayed as a control. Morphological and physiological parameters of plant growth were assessed in vitro and ex vitro. Plantlet quality was evaluated using a ranking system. Minitubers derived from greenhouse plantlets were planted in the field in subtropical conditions, and growth and yield parameters evaluated. The chitosan concentration most beneficial to the in vitro growth of microplants varied between years. In treatments with the best in vitro growth, minituber number and yield in the greenhouse was also increased. Foliar chitosan application at the acclimatisation phase stabilised the effect of in vitro treatment on yield parameters. The seed quality of minitubers derived from chitosan treatments in vitro alone and in combination with foliar treatment at acclimatisation was improved, giving rise to field plants with increased tuber numbers and yields. The combination of chitosan in vitro and foliar application during the acclimatisation phase ex vitro had a greater effect on minituber seed quality than the concentration applied in vitro.     

Cisgenesis Does Not Solve the Late Blight Problem of Organic Potato Production: Alternative Breeding Strategies

Average potato yields in Dutch organic farming systems vary from 15 to 29 Mg/ha and are limited by low input of nitrogen and severe late blight attacks caused by the oomycete Phytophthora infestans. Under Dutch late blight regulations it is mandatory to kill the haulm at 7% infestation. The late blight attacks have been so early in the organic potato production of the past few years that its acreage is now gradually decreasing whereas consumer demand is increasing. Agronomic control strategies have limited success. First priority lies in breeding for highly resistant varieties to safeguard organic potato production in the Netherlands. Cisgenesis, however, is not an option for the organic sector. Although the product of cisgenesis does not contain genes from non-crossable species it is a result of a genetic engineering process which is excluded from use in organic agriculture. As the principles and standards of organic agriculture are process-based, cisgenesis does not comply with the norms and standards of organic agriculture. The arguments of the organic sector go well beyond the alleged risks of the gene technology and relate to the technology itself. Breeding at DNA-level, instead of at whole-plant level, violates the integrity of life as described in the concept of naturalness. The Dutch organic sector is now aiming at increasing the traditional breeding activities including the participation of farmer-breeders in close cooperation with the formal breeding companies. Additional selection methods need to be developed to include required traits other than late blight resistance, such as nutrient efficiency. Recently two varieties have been released with high resistance against late blight based on introgressing genes from Solanum bulbocastanum. Organic agriculture can benefit from marker assisted breeding to achieve adequate pyramiding of different, new sources of resistance.     

Sequential Path Analysis of Yield Components in Potato

In order to determine proper selection criteria for tuber yield in potato clones obtained from true potato seed (TPS) and to evaluate relationships between tuber yield and its components, an investigation was conducted using the five check cultivars Agria, Marfona, Draga, Agata, and Arinda, and 120 potato clones obtained from TPS. The study was conducted in an augmented randomized complete block design with three replications. Yield and its components were recorded. Correlation coefficient analysis showed that tuber yield had significant positive associations with number of tubers per plant, average weight per tuber (tuber size), plant height, diameter of main stem, and number of main and secondary stems per plant, whereas its correlation with tuber dry-matter concentration was significantly negative. Number of tubers per plant, tuber size, and plant height had significant standardized partial regression coefficients with very small tolerance and large variance inflation factor (VIF) values. These were considered as the first-order variables in sequential path analysis.     

Potato germplasm collecting expedition to the Guaitecas and chonos Archipelagos,Chile, 1990

Summary A joint Chilean, Italian, and United States potato (Solanum tuberosum) germplasm collecting expedition was conducted in the Guaitecas and Chonos Archipelagos, south of Chiloé Island, between March 4 – March 21, 1990. The expedition resulted in 30 collections ofSolanum tuberosum and 1 collection ofFragaria chiloensis L. Twenty-three of the potato collections were along the salt-water-swept shores of the islands. These beach populations, unlike the indigenous landraces in southern Chile, are self-perpetuating populations and provide useful new germplasm for researchers interested in the origin of cultivated potatoes and in the origin ofSolanum tuberosum in Chile.     

Marker-assisted Breeding for Disease Resistance in Potato

Sub-project 5 of BIOEXPLOIT aims to design durable disease resistance through marker-assisted breeding by converting existing markers for high-throughput application, developing and validating high-throughput marker technologies and pyramiding major R genes and/or quantitative trait loci into elite material. Activities include (1) the fine mapping of the quantitative trait locus PiXspg which accounts for a large proportion of the variation in late blight resistance, (2) converting SNP-based markers and an AFLP marker to easy-to-use-markers, (3) testing of progenies with combined sources of late blight resistance for presence of R genes and agronomic features, (4) backcrossing new sources of resistance to S. tuberosum and molecular screening of breeding materials with marker GP94 linked with gene Rpi-phu1 conferring late blight resistance, (5) evaluating potato clones with enhanced resistance against Phytophthora infestans under field conditions of Toluca (México), and (6) developing populations and marker-assisted breeding for disease resistance.     

Methods to Minimize the Effect of Ethylene Sprout Inhibitor on Potato Fry Colour

Ethylene is an effective potato (Solanum tuberosum L.) sprout inhibitor, but it often darkens fry colour. Trials were conducted over nine consecutive storage seasons to identify ethylene application methods which would mitigate darkening while retaining adequate sprout inhibition, using cv. Russet Burbank plus cvs Shepody, Asterix and Santana in some years. Tubers were stored for up to 35 weeks in closed chambers with ethylene gas delivered via the ventilation airstream. Exposure to continuous 4 μl l⁻¹ ethylene after suberization and cooling were completed was designated the ethylene check. Alternative ethylene treatments included commencing exposure either before or after suberization was completed; gradually introducing the ethylene either by a concentration gradient of eight steps over 4 or 8 weeks or by increasing the duration of exposure from 6 to 24 h per day in four weekly steps; repeatedly interrupting exposure for several hours per day or for durations of 1 or more days; and warm storage. Selected ethylene treatment combinations were applied in each year, plus untreated controls, chlorpropham-treated (CIPC) checks and ethylene checks. Sprout growth, fry colour, loss of mass and disease incidence were evaluated at regular intervals. In all cultivars and all years, the ethylene check darkened fry colour more than the other treatments. Commencing before suberization ended, gradually introducing the ethylene by either concentration or time gradient and interrupting the exposure all reduced the negative effect of ethylene sprout inhibitor on fry colour. Continuous ethylene treatments inhibited sprout growth as effectively as CIPC, except at 13 °C storage. Interruptions of 18 h and 2 or more days reduced sprout inhibition. Regardless of cultivar variations, an early start using either a concentration or time-increment gradient had the least effect on fry colour while maintaining good sprout inhibition.     

Effect of temperature on dry rot development of potato tubers inoculated with differentFusarium spp.

Summary Dry rot of stored potatoes in the main dryland production regions of South Africa is caused by eightFusarium spp. An investigation of the effects of temperature showed that maximum decay occurred at 25°C and that the levels of decay were higher than those reported in other countries. Local isolates of pathogenic fusaria are possibly adapted to the local high prevailing temperatures. Part of an M.Sc. (Agric.) thesis submitted by the first author to the University of Stellenbosch, Stellenbosch 7600, South Africa.     

Assessing progenies of potato for resistance to skin spot (Polyscytalum pustulans) in the glasshouse

Summary Crosses were made between six parents differing in susceptibility to skin spot and glasshouse-grown tubers from true seedlings of the nine resulting progenies assessed for resistance to the disease. The assessment was evaluated by a two-year comparison with the behaviour of field-grown tubers of the progenies and their parents. The correlations between progenies in the glasshouse and field were r=0.911 and 0.753 and those between the mid-parental values and the progeny means were high (r>0.864), indicating that parental differences were heritable. It is concluded that correlations were sufficient for glasshouse-grown tubers to be used for assessing large numbers of progenies in genetical studies and for eliminating highly susceptible progenies at an early stage in a breeding programme. R.L. Wastie died 16.01.96.     

The influence of 2,4-D and kinetin on leaf callus formation in different potato species

Summary Leaf segments ofSolanum tuberosum andS. acaule can proliferate callus. However, callus formation and growth for a specific species depends on different concentrations of 2,4-D and kinetin (6-furfurylamino purine) used in a medium. Large amounts of creamy white friable callus could be proliferated fromS. tuberosum when explants were cultured in media containing 3 mg of 2,4-D and 0.3 mg of kinetin per liter. WhenS. acaule explants were cultured in the similar medium, the segment tended to become yellow; however, when it was subcultured onto a similar fresh medium, friable and creamy white callus was then formed. In a medium containing 0.3 mg of 2,4-D and 1 mg of kinetin per liter, the compact callus could be directly proliferated fromS. acaule leaf segments. Segments incubated under light formed less callus than those incubated in darkness. Scientific Journal Series No 10559 of the Minnesota Agricultural Experiment Station. This research was supported in part by a research contract from the International Potato Center, Lima. Peru.     

Potato tuber sugar content during development and storage (10°C): possible predictors of storage potential and the role of sucrose in storage hexose accumulation

Summary Reducing sugar and sucrose contents of potato tubers (Solanum tuberosum L.) from six cultivars were monitored both during the growing season and after a four months storage period at 10°C. Significant correlations were found between tuber sugar contents (sucrose, reducing and total) measured at harvest and the reducing sugar content after storage. Similarly, in five of the six cultivars, the sucrose loss and the corresponding reducing sugar gain during storage were significantly correlated.     

Potato minituber production from nodal cuttings compared to wholein vitro plantlets using low volume media in a greenhouse

Summary A greenhouse minituber production system involving low inputs of in vitro potato plantlets and propagation media is described, in vitro plantlets of six potato cultivars were sectioned into nodal cuttings and separately planted into moist peat based growing medium in shallow plastic trays. Cultivar differences were evident with respect to node viability, shoot regrowth and minituber yields. Nodal viability for shoot regrowth varied between 80–100%. Maximum shoot heights were recorded with whole in vitro plantlets (WIP) and the terminal Node-5 cluster. All cuttings produced minitubers. The terminal Node-5 cutting and WIP produced significantly larger minitubers >3.0 g as compared to single node cuttings. Greater numbers of minitubers were produced by the cvs Norchip, Red Pontiac and Conestoga as compared to cvs Eramosa. G8610-4PY and Shepody. Total numbers of minitubers were 3 to 5 times higher from each in vitro plantlet that was sectioned into nodal cuttings as compared with intact WIP: the yield ratios depended on cultivar.     

Effect of haulm treatments on the formation of microsclerotia ofVerticillium dahliae Kleb. on potato

Summary In four pot experiments, potato plants of cv. Element were artificially infected withV. dahliae. At an early and a late harvest haulms were killed chemically, by burning or by various other treatments, including cutting them into pieces of different lengths and keeping the debris on the soil surface or covering with soil. After 4 weeks the plant material was air-dried and the number of microsclerotia per mg was determined. At the early harvest, in two experiments, the chemical treatment yielded more microsclerotia than the cutting treatments. Covering colonised haulm tissue with non-sterilised soil was effective in inhibiting microsclerotia formation. Shorter haulm pieces led to fewer microsclerotia at the later harvest if the material was kept on the soil surface. The variation in microsclerotial yield and in treatment effects among the different experiments was large.     

Resistance to <Emphasis Type="Italic">Potato virus Y</Emphasis> in a Multitrait Potato Breeding Scheme without Direct Selection in Each Generation

In an experimental breeding scheme to improve late blight (Phytophthora infestans) and white potato cyst nematode (Globodera pallida) resistance of tetraploid potato over three generations of crossing and selection, 15 clones survived the final selection, and these were derived from 15 great-grandparents. There was no direct selection for resistance to Potato virus Y (PVY), but 14 out of the 15 great-grandparents were resistant to PVY and three had extreme resistance. Thirteen of the 15 descendants had PVY resistance and one extreme resistance. This was within the range expected for a random (unselected) sample from the genotypes of the great-grandparents. Hence, we found no evidence for any positive or negative association between PVY resistance and the attributes selected. The conclusion is that laborious selection is not required in every generation when many parents have PVY resistance, including some with more than one copy of a PVY resistance gene or resistance at more than one locus. However, in the future, determining the major virus resistance genes present in potential parents in each generation using diagnostic molecular markers would prevent susceptible × susceptible crosses being made and maximise the number of resistant × resistant ones.     

New isolates of the necrotic strain of potato virus Y (PVYN) found recently in Poland

Summary In comparison to the previously known isolates of potato virus Y^N (PVY^N), some isolates found in Poland since 1984 are more infectious to potato plants, reach faster a higher concentration and induce milder disease symptoms. Potato cultivars resistant to the standard type of PVY_N may be susceptible to the new isolates whereas those that are extremely resistant to PVY remain extremely resistant to the new isolates. The potato cultivar Elipsa is suitable for the differentiation of PVY^N isolates.     

Current status of the TPS technology in the world

Summary The availability of improved True Potato Seed (TPS) varieties for short and long photoperiod conditions has recently increased the interest in the TPS technology. A summarized overview is presented of the TPS technology for potato tuber production, TPS breeding and production. TPS competes successfully with clonal cultivars in Egypt and India. In other developing countries higher yields and better adaptation are needed to make the TPS technology economically attractive; the case of TPS in Indonesia is used to illustrate this. In industrialised countries, well-performing seed tuber programmes, productivity and legislative restrictions are difficult to beat by TPS. However, the use of TPS can be economically attractive where disease pressure is high. It is concluded that TPS has a place within potato systems where agroecological conditions for seed tuber systems and steady supply of good quality tubers from a formal seed programme are most constrained. Further genetic improvement will increase the areas where TPS provides better and cheaper planting material.     

Reaction toPhytophthora infestans of potato genotypes with race-specific resistance

Summary Phytophthora infestans isolates collected in Poland in 1987–1995 were evaluated on detached leaflets of a differential series consisting of potato genotypes possessing resistance genes R1–R11. On the same differentials was evaluated over three years a highly virulentP. infestans isolate MP 245. Isolate MP 245 and those from natural populations ofP. infestans were seldom or inconsistently pathogenic to R5, R8 and R9. When leaflets of the differentials were infected with MP 245 the lesions were often small and sporulation weak. A statistically significant interaction was found between years and pathogenicity of MP 245 to individual differentials. Attempts to train the isolate MP 245 for pathogenicity to R9 were unsuccessful. The variation in specific resistance of potato genotypes may complicate the evaluation of their resistance toP. infestans, as well as the evaluation of the virulence spectrum of natural fungus populations. This variation may be due to changes in specific pathogenicity of the fungus or in specific resistance of the potato. Some differentials have a possibly useful resistance toP. infestans.     

Exploring the Natural Biodiversity of Potato for Late Blight Resistance

This paper gives an overview of the activities in Sub-project 4 of the BIOEXPLOIT programme. The European genebank collections of potato need to be narrowed down into (customized) core collections by focusing on loci associated with disease resistance. Therefore, new and more efficient molecular methods are being developed to identify these loci, extensive genotyping of resistance to Phytophthora infestans and signalling loci is being performed, and genotyped selections of accessions are phenotypically analysed for their resistance specificities. The results of these activities are being assembled in an integrated database.