Natural selection serves the demands of nature and the plant itself resulting, e.g. in fragile ears, long stolons or toxic seeds to secure survival of the species. At this point man has to interact to select the plant that most serves his needs as food, feed or industrial raw material. Consequently this is the central step of the breeding process. Success depends on the ease and speed with which the superior plants within a segregating population can be identified. In classical plant breeding programmes, selection is carried out on huge populations normally in the field. Field selections are strongly influenced, however, by environmental conditions, and are uncertain and lengthy, especially in the case of breeding for quantitative characteristics with a polygenic background. Such traits usually show only slight changes per selection cycle and it may easily take 10–20 years to improve a desired agronomic trait.

In vitro selection

Simultaneously with the progress in plant biotechnology since the 1980s, new methods in plant pathology have been developed. This review summarizes papers that cover basic research on the effects of selective agents on in vitro cultures of host plants, as well as applications of agents in regeneration systems that result in lines with increased variability in resistance or susceptibility. The first part of the study deals with theoretical aspects of the interactions between plants and toxin-producing pathogens, mode of phytotoxic action, and host- and non-host-selective toxins. The second part lists and describes various agents used for selections in vitro. In the last two decades more than 100 publications focused on these selections for the improvement of resistance to plant pathogens. Over 30 plant species were examined to utilise various selection agents extracted from about 40 plant pathogens. The review covers basic research studies and methods that elucidate the relationships between in vitro and in vivo mechanisms of resistance, but also try to develop practical applications to obtain resistant breeding lines. Such methods often utilise some type of explant cultures of the host plants that are treated with various selective agents (culture filtrates, toxins, elicitors), which then elicit typical reactions that parallel those by the pathogens. Their application successfully resulted in resistant lines in banana, carnation, grapevine, strawberry and wheat. Nowadays, these techniques are an important complement to classical breeding methods.

In Vitro Selection for Improved Plant Resistance to Toxin-Producing Pathogens

Somaclonal variation, i.e. the variation induced by cell and tissue culture, offers an opportunity to broaden the genetic variation of crops. As a result of somaclonal variation a wide range of plant characteristics can be altered. However, the selection of agronomically important traits, e.g. disease resistance, has many limitations. The efficiency of selection can be increased by the application of in vitro selection procedures. Selection strategies that may be applied to obtain disease resistant somaclonal variants are described. Their merits and limitations, in relation to the efficiency of the procedures, the frequency of disease resistant variants and the genetics of the resistance obtained, are discussed.

Application of cell and tissue culture and in vitro selection for disease resistance breeding — a review

Publisher Summary The selection of regenerants with good agronomic performance depends on the relative effects of induced and introduced variation during the in vitro process. This variation can arise from any number of processes, but the key to successful selection of desirable regenerants with improved traits is the expression of a wide diversity for the traits, trait stability over generations, adequate population density to have a reasonable chance of visually selecting the desirable variants in the field, and the proper field environment that will foster trait expression. These criteria are identical to those required for all conventional and mutation breeding programs. The application of tissue culture regeneration in selection strategies must be advantageous to the breeder before it will be adopted. Only a synergistic and concerted effort by traditional breeders and plant biotechnologists can bring this technology to the potential level of efficiency and application that will result in enhanced breeding efforts and cultivars with improved value-added traits that can be grown profitably by producers. Finally, as with any breeding program, the variants must be evaluated in multiple environments to ensure that all desirable traits are stable over generations. Useful variants have been released, but compared to conventional breeding methodologies, the results thus far have been limited and disappointing. The reasons for the limited success are discussed in this chapter.

Tissue Culture-Induced Variation and Crop Improvement

Potato is the third most important global food crop and the most widely grown noncereal crop. As a species highly amenable to cell culture, it has a long history of biotechnology applications for crop improvement. This review begins with a historical perspective on potato improvement using biotechnology encompassing pathogen elimination, wide hybridization, ploidy manipulation and applications of cell culture. We describe the past developments and new approaches for gene transfer to potato. Transformation is highly effective for adding single genes to existing elite potato clones with no, or minimal, disturbances to their genetic background and represents the only effective way to produce isogenic lines of specific genotypes/cultivars. This is virtually impossible via traditional breeding as, due to the high heterozygosity in the tetraploid potato genome, the genetic integrity of potato clones is lost upon sexual reproduction as a result of allele segregation. These genetic attributes have also provided challenges for the development of genetic maps and applications of molecular markers and genomics in potato breeding. Various molecular approaches used to characterize loci, (candidate) genes and alleles in potato, and associating phenotype with genotype are also described. The recent determination of the potato genome sequence has presented new opportunities for genomewide assays to provide tools for gene discovery and enabling the development of robustly unique marker haplotypes spanning QTL regions. The latter will be useful in introgression breeding and whole-genome approaches such as genomic selection to improve the efficiency of selecting elite clones and enhancing genetic gain over time.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/pbi.12099

Applications of biotechnology and genomics in potato improvement

Because plant cells cultured in vitro express genetic variability and since they can be regenerated into functional plants, procedures have been designed to use this system for the production of plants with new important agronomic characteristics, particularly for disease resistance. For barley, wheat, and potato somaclones have been found that were less susceptible to a toxin of Helminthosporium, fusaric acid, Fusarium coeruleum, F. sulphureum, or Phytophthora infestans, when screened in the first in-vitro-derived generation. Here the progeny of such somaclones is evaluated after natural and artificial infection, using greenhouse-grown or field material. The progenies of the same somaclones did not express detectable differences, which indicated that no heterozygous mutations occurred. Most lines and clones differed in their level of susceptibility to the pathogen compared to the level of the starting material, but these data were in no instance significant. It is discussed here whether this lack of significance is due to a lack of genetic differences or whether the test procedures are in adequate for detecting and securing the slight, probably quantitative, alterations.

G. Wenzel

Papers

Progeny tests of barley, wheat, and potato regenerated from cell cultures after in vitro selection for disease resistance.