INTRODUCTION

Most dicotyledonous plants are susceptible to tumour formation as a result of infection of wounded sites by the Gram-negative bacterium Agrobacterium tumefaciens (for a general review see Caplan et al. 1983). The bacterium is necessary for tumour induction but not for tumour maintenance and growth. The ability of the bacteria to induce tumour formation is strictly limited to those Agrobacteria species harbouring a large extrachromosomal DNA element, the so-called Ti-(tumour-inducing) plasmid, which has a size of 120-180 kb. The molecular basis underlying the neoplastic transformation is the transfer and stable integration of a well-defined part of the Ti-plasmid, the so-called T-DNA (T=Transfer or Tumour) into the plant nuclear DNA, leading to the formation of a gall. The Ti-plasmid therefore represents a natural gene vector to plant cells. As it is essential for the practical use of this system for gene transfer to obtain normal plants; mutants of the Ti-plasmid have been constructed with all the T-DNA genes leading to tumour formation deleted. Normal plants which are fertile and which sexually transmit the introduced genes as a single, dominant Mendelian locus (Zambryski et al. 1983) have been obtained.

The main limitation at the present time concerning the use of Ti-plasmid mediated gene transfer is the limited host range of Agrobacterium species.

INTRODUCTION

Screening for resistance to virus diseases based on both qualitative and quantitative measurements of virus in plants grown under controlled conditions may increase the efficiency of resistance breeding. Moreover, a measurement of changes in virus content in host plants could be of great help in studying the nature of resistance. It has been suggested that the enzyme-linked immunosorbent assay (ELISA) is a suitable technique for the assessment of virus content and a negative correlation between the extinction values in ELISA and the level of resistance to PVYN in some potato varieties has been found. However the assessment may be influenced by a number of factors. This paper describes the results of an investigation on the effect of time interval between inoculation and assessment on the relative content of virus in leaves of plants infected with PVYN.

MATERIALS AND METHODS

Three experiments were carried out in 1984 and 1985 at the Foundation for Agricultural Plant Breeding (SVP) in Wageningen. Plants grown in a glasshouse at 22 to 24°C and a daily photoperiod of 16h were inoculated mechanically at different plant ages and assessed for relative virus content by ELISA at different time intervals after inoculation. Leaves of inoculated plants were sampled at different positions according to an appropriate experimental design. ELISA was done according to the method of Clark and Adams (1977) with the modification of De Bokx and Maat (1979). The varieties used in the experiments differed in resistance to PVYN (Anon. 1985).

Improved potato varieties are obtained by bringing together desirable combinations of genes. Generally this is achieved by sexual hybridization followed by selection of plants with the desired characters. New techniques are now in existence that enable specific genes to be inserted into all the cells of a particular plant, leaving the residing somatic genome largely intact. In principle, such foreign genes can originate from any organism. They may be naturally existing genes, genetically engineered derivatives or may even consist of newly designed and chemically and/or biochemically synthesized DNA.

In this chapter three aspects of introducing specific genes into potato are discussed. Firstly the transformation of potato cells by direct infection of wounded shoot cultures of cultivars with Agrobacterium, the acquired growth properties of such transformed cells and the isolation of transformed plants. Secondly, the initial analysis of the expression of genes introduced into potato and finally implications for future genetic manipulations in potato, based on the known effects of genes already introduced.

Possible alternative ways of obtaining potato plants transformed with specific foreign DNA are omitted or only mentioned in passing. This is not to express a preference for direct Agrobacterium infections but simply because experimental progress in potato using other systems is less advanced.

The Caithness breeding programme was initiated in 1976 by an experienced potato breeder turned Scottish seed grower (the Writer) who first linked up with a London-based exporter and then with three of the larger Scottish grower/dealers. Thus the Caithness Group, which aspires to breed new varieties and sell seed of them worldwide, could be described as a vertically integrated seed potato business on the Dutch model, the only one in the United Kingdom.

The first breeding objective was to improve upon Desiree, the Dutch red-tubered variety which is a leading maincrop in the UK, as well as in important seed potato export markets. Desiree lacks resistance to cyst nematode (Globodera spp.) and common scab (Streptomyces spp.).

Maris Piper, a leading British variety from the Plant Breeding Institute, Cambridge, has the gene H1 for resistance to Globodera rostochiensis, so the first cross was Desiree X Maris Piper, or DXMP in the short notation which will be developed in a self-evident way. Pentland Crown, a leading variety bred at the Scottish Plant Breeding Station near Edinburgh was used as the source of resistance to common scab in the second generation cross D.MPXPC. Up to this point, therefore, the breeding had the advantage of relying solely on three leading varieties of English, Dutch and Scottish origin.

INTRODUCTION

The potato is the most important field crop for arable farming in the Netherlands. In the past 10 years the area under potato has varied from 160 000 to 173 000 ha. Well over 40% is used for the cultivation of ware potatoes, nearly 40% for starch potatoes and about 20% for seed potatoes. In each case a large part of the produce must be exported, either fresh or in processed form. The cost price therefore has to be low and the quality requirements high.

Breeding has done a great deal for farmers trying to meet these requirements. Furthermore, a comprehensive system of assessment of the value for cultivation and use has made it possible for new, improved, products of breeding to be identified rapidly, while on the basis of the same objective data the introduction of less promising entries has been prevented.

The importance of potato varieties in the Netherlands is demonstrated by the fact that 96 of the 262 recommended varieties of field crops which are registered in the Descriptive List of Varieties of Field Crops are potato varieties. A few of these 96 are grown on a very large area, and as many as 48 are grown on more than 500 ha and/or cover 100 ha for the production of seed potatoes (Anon. 1985; Daemen 1985).

INTRODUCTION

The use of true potato (TPS) seed for potato production has been considered a good alternative to seed tubers, since it minimizes the transmission of viruses and other pathogens. Using TPS also means that the total crop is available for consumption and consequently potatoes are grown at a relatively lower cost. In addition, storage and transfer of TPS is generally easy and inexpensive compared with seed tubers.

To further develop the use of TPS, research is needed to identify breeding methods which will generate high yielding and uniform families and to produce hybrid seed at low cost by using natural pollinators. These objectives can be obtained by using the breeding schemes proposed by Peloquin (1979, 1983).

The purpose of this study is to explore the possibility of using TPS in the south of Italy.

MATERIALS AND METHODS

Sixty-four TPS families obtained from different types of crosses and open pollinations were evaluated in a trial near Naples.

The 64 families consisted of 23 Dts (4x progenies from crosses between 4x cultivars and 2x hybrids); 15 4x progenies from crosses between 4x European cultivars and 4x American cultivars; 11 4x progenies from cultivars x Dts and 15 4x progenies from open pollinated (OP) cultivars. The diploids used as male parents were Solanum phureja-haploid S. tuberosum and S. chacoense-haploid S. tuberosum hybrids, which formed 2n pollen by parallel spindles (first division restitution or FDR).

In order to comply for entry to a National List a variety must be distinct, uniform and stable (DUS), and be of value for cultivation and use (VCU). The DUS requirement is also necessary to establish eligibility for Plant Variety Rights. In the UK there are both statutory and advisory VCU systems. The statutory National List Trials are largely used to eliminate inferior varieties, and the advisory Recommended List Trials aim to select the superior varieties.

NATIONAL LIST

National List testing is conducted on an interdepartmental basis by the agricultural departments for England and Wales, Scotland and Northern Ireland. The Scottish Department coordinates the work, collates the results and prepares the reports. The National Institute of Agricultural Botany acts as agent for the Ministry in England and Wales. A National List DUS/VCU Group makes recommendations on the distinctness and merits of new potato varieties to the UK National List Committee.

Both DUS and VCU tests take place over 2 years. Distinctness tests are based on morphological characters, which closely follow the guidelines of the International Union for the Protection of New Varieties of Plants (UPOV). Uniformity and stability rarely present problems in potatoes.

VCU tests include yield trials and tests for disease resistance and quality. Yield trials consist of two replicates of 100 plants at three centres – Cambridge, Edinburgh and Belfast. Seed is provided by breeders.

INTRODUCTION

Wild species are products of natural evolution in centres of diversity. They are not manipulated on purpose or used by man. Their evolution is brought about by the interaction of abiotic and biotic factors with the genetically variable plant populations. This has resulted in the wealth of variation found among and within species. In the centres of diversity the species may coexist but remain largely separated by external and internal barriers developed in the course of evolution.

Domesticated or cultivated plants have derived from the wild species and are manipulated by man both agronomically and genetically in order to improve their adaptation to human needs. The potato has its main centre of diversity in the mountainous regions of Latin American countries. Here the potato was domesticated and has been grown for several millennia. These primitively cultivated potatoes comprise eight Solanum species: the diploids S. phureja, S. stenotomum, S. goniocalyx and S. ajanhuiri, the triploids S. chaucha and S. juzepczukii, the important tetraploid S. tuberosum ssp. andigena and the pentaploid S. curtilobum.

Not until the latter half of the 16th century was the potato introduced into countries outside Latin America, first into Europe, from there into North America and later on all over the world. The immediate ancestor of our present-day autotetraploid S. tuberosum ssp. tuberosum cultivars is the autotetraploid S. tuberosum ssp. andigena. Although initially only few genotypes of S. tuberosum ssp. andigena were introduced, a large number of different S. tuberosum ssp. tuberosum cultivars adapted to various conditions has been derived from that material.

INTRODUCTION

Glycoalkaloids are a group of toxic compounds found in potatoes. They are present at high concentrations in the leaves and shoots of potato plants and in tubers which have been exposed to light. Fortunately in the normal tubers of current popular varieties the level of glycoalkaloid is quite low (Parnell et al. 1984). It is thought that the function of glycoalkaloids might be related to disease and pest resistance.

Although the level of glycoalkaloids in popular varieties is low, there is a danger that modern breeding techniques may result in elevated levels in newly-bred varieties. The reason for this stems from the practice of introducing disease resistance characters from wild plants or primitive cultivars. For example, a variety of potato bred for resistance to Colorado beetle (Leptinotarsa decemlineata) showed unacceptably high levels of glycoalkaloid in the tubers (Sinden & Webb 1974).

Glycoalkaloid level is expressed as milligrams per 100g fresh tuber. Potatoes with less than 20mg/100g are considered entirely acceptable, potatoes with more than 20mg/100g often have a bitter, metallic taste and potatoes containing 50-100mg have caused severe illness.

Trials have consistently shown significant varietal influence on glycoalkaloid content. In view of this and the toxicity effects it is felt that glycoalkaloid screening is of considerable importance.

METHODOLOGY

At the National Institute of Agricultural Botany all National List candidate varieties of potato are screened for glycoalkaloid content. Samples from each of three sites are analysed in triplicate.

In potato breeding programmes much of the testing for disease and pest resistance consists of laboratory or field trials of clonal material, which are costly of time and effort. The selection pressure that has therefore been imposed for these characters has been weak. The Scottish Crop Research Institute (SCRI) has been investigating ways of increasing the efficiency of selection by identifying parental clones which pass on their resistance with an high probability.

Several economically important resistance traits are governed by single dominant effective factors inherited in a disomic manner, e.g. H1 gene conferring resistance to Globodera rostochiensis (Rol); Rx genes giving immunity to potato virus X (and B) and Ry genes providing extreme resistance to potato virus Y. Selective breeding combined with test crossing of resistant clones with susceptibles and assessing the progenies can produce parental material with increased “copies” of such genes. Clones which have a single copy of the gene (simplex) are intercrossed and by test crosses with susceptibles, duplex clones are identified amongst their progeny (Figure 1). The duplex clones are then intercrossed or selfed and the resulting clones again crossed to susceptibles to identify those with three or four copies (triplex or quadruplex) of the gene (Dale & Mackay 1982; Solomon & Mackay person, comm.).

The number of clones that need to be tested to distinguish between the various ratios is readily calculated (Mather 1938).

This paper describes a technique for screening breeding material in the first clonal year for resistance to Globodera rostochiensis, G. pallida and Phytophthora infestans. Seedlings were grown in pots in an aphid-proof glasshouse and after harvest, when tubers with poor colours, extremely long stolons or very poor shape were rejected, tubers were stored at 5°C until dormancy had broken. Tubers were removed from store at weekly intervals and kept at room temperature to induce sprouting. One small tuber was planted on the surface of 100g of compost at 30 % water content in each closed transparent plastic container, as described by Phillips et al.(1980). The inoculum used was a suspension of equal numbers of eggs/larvae of both G. rostochiensis Rol and G. pallida Pa2–3 collected from beds where they had been maintained on the appropriate differential host variety. The containers were kept in a dark store room (15 to 20°C) for 6 to 7 weeks while the cysts were forming. Recording commenced when the majority of the cysts of G. rostochiensis were yellow and G. pallida were white. A few mature cysts were brown and hence not assignable to species, while some G. rostochiensis cysts were still white and wrongly attributed to G. pallida, but as only clones with dual resistance were selected this was not of practical importance.

INTRODUCTION

In the cultivated potato, meiotic polyploidization by means of 2n-gametes has received considerable attention, both in relation to the development of more efficient alternatives to current breeding programmes (Mendiburu et al. 1974; Peloquin 1982; Hermsen 1984b) and the production of well-performing and relatively uniform hybrid progenies for the new technology of growing potatoes from true seeds (Peloquin 1983). As to the latter, the possibility of using 2n-egg formation in attempts to induce gametophytic apomixis has recently been contemplated (Hermsen 1980; Iwanaga 1982; Hermsen et. al. 1985).

In gametophytic apomixis an unreduced embryo sac is formed that can be of either diplosporic (sexual) or aposporic (somatic) origin (Rutis hauser 1967). As there are strong suggestions from earlier literature (Powers 1945; Petrov 1970; Asker 1980; Hermsen 1980; Matzk 1982) that both aposporic and diplosporic apomixis comprise distinct and genetically regulated elements, the experimental induction of gametophytic apomixis might well be achieved by a combination of them.

The elements of diplosporic apomixis that can be distinguished are a strongly reduced crossing over in megasporogenesis, the formation of unreduced megaspores and embryo sacs, and parthenogenetic development of the unreduced egg cell. In aposporic apomixis, parthenogenetic development should be combined with the development of a somatic cell of the ovule into an unreduced embryo sac. In potato, the genetically controlled elements of displosporic apomixis are available (Hermsen 1980; Hermsen et al. 1985). With apospory, however, the aforementioned development of a somatic cell, as claimed to occur in diploid potato hybrids (Iwanaga 1980, 1982), has not yet been reported (Jongedijk 1985).

INTRODUCTION

Dr. N.W. Simmonds began his programme of selection for adaptation of Solanum tuberosum ssp. andigena (Andigena or adg) to the environment of the United Kingdom in 1959 (Simmonds 1966). His motivation was based strongly on the theory that Andigena is a rich source of genetic variability for temperate breeding programmes. He proposed and initiated a project of selection for adaptation within a germplasm collection that has few precer dents in other crops. Most efforts to use unadapted germplasm have initiated the process by crossing to adapted genotypes followed by further backcrossing or recurrent selection. Simmonds' scheme was also unique in that it was directed toward the utilization of the total Andigena genotype, not just one or a few genes.

The arguments presented by Dr. Simmonds were one of two reasons for initiating a similar programme in New York in 1963. The second reason is the consequence to US potato breeding of the efforts of Chauncey E. Goodrich with Andigena between 1846 and 1864. In the belief that potato varieties were “running out” and needed an infusion of new parents, he obtained potato varieties from the American consulate in Panama. It is likely that these were Andigena. One of these he called “Rough Purple Chili”. From it he produced a seedling “Garnet Chili”. Subsequently, this clone gave rise to the variety “Early Rose” in 1861 (Clark & Lombard 1946).

It in turn produced the berry from which the “Burbank Seedling” was selected by Luther Burbank in 1876 (Clark & Lombard 1946).

INTRODUCTION

“Breeding strategy” is defined as the determination of breeding objectives and the means necessary to reach these objectives. In Poland the potato is a field crop of major economic importance (Swiezynski 1982). To improve it an efficient breeding strategy is required. After a short description of present-day potato breeding in Poland, some possible ways of increasing its efficiency will be considered.

THE OBJECTIVES AND STRUCTURE OF POTATO BREEDING IN POLAND

Potatoes are grown in Poland under various conditions and the crop is utilized in various ways. It follows that there must be a number of breeding objectives. Recently Polish breeders agreed that it is desirable to improve the following characters: table quality, dry matter content, resistance to viruses, resistance to blight (Phytophthora infestans), resistance to cyst nematode (Globodera spp.), resistance to storage diseases and adaptation to light, water-deficient soils. The breeding work and associated activities may be grouped as follows:

Potato collection Cultivars and breeding lines as well as wild and primitive cultivated species are being maintained and evaluated. It is important to make available to breeders new cultivars developed in countries with extensive potato breeding programmes.

Parental line breeding Parental lines with multiple resistance to pathogens and outstanding in some agronomic characters are being supplied to breeders (Swiezynski 1983, 1984a). Every year each breeder receives a list of potential lines and every 5 years the results of parental-line breeding are summarized (Roguski 1971, Kapsa 1977, Swiezynski 1984a).