Genetic Improvement of Solanaceous Crops Volume 2: Tomato PDF

Preface

Tomato is one of the most consumed vegetables in the world and is the dietary source of vitamins, minerals and fiber, which are important for human nutrition and health. Fresh fruits are used in salads, various culinary preparations, juices, or processed in the form of purees, concentrates, condiments and sauces. Tomato plants are grown worldwide in the field, or in greenhouses. Genetic improvement of this Solanaceous crop has been an on-going process with the objective of gaining high fruit yield, enhanced fruit nutritive value, controlled fruit maturation and ripening, and developing resistance to phytophagous insects, microbial pathogens, and various abiotic stresses. More importantly, with the increase in the world population, the quantum of tomato consumption has considerably increased and farmers, agronomists and horticulturists have had to walk a tight rope to enhance yield without losing sight of the production quality to meet the demands of the fresh market and the processing industry. Of the nearly 3 million hectares under vegetable cultivation the tomato crop occupied one-third of this global area with total tomato production in 1994 reported as 77.5 Mt, averaging 27 t ha^-l. Most of the production increases hitherto have been achieved using conventional methods of selection and breeding coupled with improved growth practices: use of fertilizer, improved irrigation, and pest management. Other advancements have been possible through the application of molecular markers to ease selection process and technological innovations such as development of genetically enhanced tomatoes engineered for high quality and resistance to disease and extreme environments. This book presents a critical appraisal of the state-of-the-art findings on this crop in the form of overviews, emphasizing various approaches and strategies used for its improvement through research conducted at various research institutes, organizations and universities world over.

Improvement of a particular crop can best be envisaged when comprehensive information is known of its origin and available genetic resources. The controversy over the taxonomic status of the cultivated tomato (Lycopersicon esculentum Mill.) has been resolved. Application of molecular breeding techniques (RAPDs, RFLPs ) and genomics research has now convinced the research community to place tomato under the genus Solanum, namely, Solanum lycopersicum L. ( http://www.sgn.cornell.edu/about/solanum_nomenclature.pl) . This implies that one could explore the gene pool among all Solanum species for improvement of this and other Solanaceous crops. Conservation of all tomato genetic resources is, therefore, all the more necessary. Chapter 1 starts with history, origin and early cultivation of tomato. Interestingly, Peru is considered to be a likely place of domestication of tomato and yet another hypothesis sees its first domestication in Mexico. Tomato gene banks have been established in USA and other countries where currently more than 75000 accessions of tomato are preserved. These gene banks maintain data concerning the reproductive biology of conserved accessions, world production scenario of fresh-market as well as processed tomatoes, and descriptive list of characteristics of various collections. Information dealing with these aspects is given in Chapter 2. Role of cytogenetics in evolution and selection of tomato variants with improved traits is elaborated in Chapter 3. This chapter further provides an overview of tomato genomics through genetic maps constructed by applying conventional and molecular breeding techniques in order to assess variability among various tomato genetic resources such as mutants, wild species, intra- and inter-specific populations as well as introgressed lines derived from recombination experiments. By integrating classical gene linkage maps with the high-resolution molecular maps it is now possible to evaluate the degree of similarity in basic genomic structure of tomato sp. Some plants maintain superior traits vigorously only in hybrid form. This phenomenon called heterosis is recognized as one of the primary factors contributing to manifestation of superiority in respect of some quantitative traits of tomato. Chapter 4 describes the strategies for using heterosis for developing tomatoes with certain quantitative traits, whereas Chapter 5 elaborates on improvement of quality traits using traditional and enhanced breeding methods. Tomato fruits are major dietary sources of antioxidant lycopene, and vitamins A and C, besides other micronutrients/antioxidants, which largely contribute to tomato fruit quality. Tomato breeders have been examining a wealth of genetic variability available in the present day heirloom cultivars, land races, and related wild tomato species in respect of various dietary sources. Approaches to genetically enhance tomato fruit's nutritive value are highlighted in Chapter 6. Molecular markers have proven very useful in selection of elite tomato germplasm, and efforts to best utilize various molecular genetic approaches have led to an understanding of the physiological basis of drought resistance response in tomatoes. An overview of research done on these aspects is provided in Chapters 7 and 8. Recent advances in plant genetic engineering have made it possible to produce transgenic tomato plants with characteristics for a number of improved traits. A general account of genetic engineering technology applied for production of tomatoes transformed for various traits is given in Chapter 9. Hormonal control of fruit maturation, its molecular basis, and future prospects of applying microarray analysis, as well as proteomics, essentially for producing designer-tomato fruits with enhanced shelf-life are discussed in Chapter 10. Biochemical and molecular mechanisms in fruit ripening have projected insights on existence of molecular links between distinct fruit ripening types in tomato. A number of genes involved in ethylene biosynthesis as well as light signaling are implicated, which reportedly provide targets for manipulation of fruit color, nutrient content, and cell-wall breakdown during the process of ripening. Details of these, based on model systems proposed for fruit ripening of both climacteric and nonclimacteric fruits, are summarized in Chapter 11. An inherent problem encountered in crops, including tomato, is the huge annual yield losses incurred due to diseases caused by pathogens and pests. Molecular breeding coupled with application of transgenic technology has to a greater extent the potential to circumvent this problem for tomato by producing cultivars resistant to bacteria, fungi, viruses, and insects as well as mite pests. Recent findings on these aspects of tomato resistance are reviewed in Chapters 12-15. Finally, the considerable progress made toward understanding the physiological bases of plant tolerance to different abiotic stresses and characterization of tolerant tomato genotypes to stresses, such as salinity, cold and heat, are discussed in Chapter 16.

There has been a long-felt need to have documented, comprehensive information on the improvement of tomato in one place. This book attempts to accomplish this goal. It should be useful not only to breeders, or other specialists, but equally benefit teachers as well as students seeking information on aspects of tomato biology, genetics and biotechnology. It will be apparent to the readers that some authors have used the revised classification of tomato as Solanum lycopersicum while others have kept to the older nomenclature, viz., Lycopersicon esculentum. We have let both these usages in the book till the transition over the next few years is completed.

This compendium has become a reality only through the expert contributions of the 33 authors from 6 countries and generous support and encouragement from the publishers. We sincerely thank them all.