Handbook of New Technologies for Genetic Improvement of Legumes PDF

Preface

Legumes, belonging to the family Fabaceae, follow only the cereals in importance. Apart from being sources of nutritious protein, species belonging to this group have applications of economic importance. For example, the peanut has the distinction of being a source of oil as well as protein; its seed is consumed even without processing and is a favorite snack for humans. No other crop has this distinction, even though the soybean has the distinction of being a rich source of protein and oil as well. The other applications include forage and feed in the species of Medicago and Trifolium, highly preferred vegetable oils in soybean and peanut, paper pulp in Leucaena leucocephala, firewood in many species, hardwood for construction purposes, mulching material, improving soil fertility, and so on.

As experimental material, the importance of legumes need not be exaggerated. A member of this legume family, the pea, offered itself as the study material to the father of genetics, Mendel, who proposed the laws of inheritance, which subsequently became the basis for modern genetics. In terms of molecular biology too, legumes did not lag behind the model system for molecular biology studies, Arabidopsis thaliana, which lacks an important biological activity, the interaction with soil rhizobia that results in atmospheric nitrogen fixation.With suitable characteristics like short generation time, relatively small genome size (though not as small as Arabidopsis), suitable plant morphology, low chromosome number, availability of easy regeneration and genetic transformation system, and the capacity to fix atmospheric nitrogen through the interaction with rhizobium, Medicago truncatula started posing a threat to the supremacy of Arabidopsis. Intense efforts are being made to sequence the genome of this species, and already as many expressed sequence tags from this species have been deposited in the GenBanks as for Arabidopsis. Along with these features, M. truncatula is a favorable material for plant-rhizobial interaction. It is the dream of scientists to transfer the trait of atmospheric nitrogen fixation from the legumes to other economically important crops like cereals, making their cultivation independent of chemical fertilizers. Simultaneously, other legume species like Lotus japonicus are also trying to grab attention from Arabidopsis as models for molecular biology studies. Julia Frugoli has reviewed this aspect and the importance of Medicago truncatula in this volume, followed by a contribution from P. Ratet, and A. Kondorosi, and others on reverse genetic approaches that are applicable to this model legume species. M. Chabaud has dealt with the transformation technology and the various possibilities in this material.

Most of the legumes are cultivated under adverse conditions, and the cultivation is generally rain fed. In such a situation, the crop is dependent on the vagaries of nature. Various stresses in the form of insect predators, disease- causing pathogens, and environmental stresses including drought, extended rainfall, salinity, chilling, and so on cause considerable damage to the crop. Some environmental stresses also make the plant susceptible to diseases, resulting in serious loss of productivity. When adequate genetic variability is available for the crop, different stresses can be tackled through suitable crop improvement programs. Unfortunately, many legumes suffer from a serious lack of adequate genetic variability. This can be tackled by recourse to genetic engineering and introducing suitable alien genes into crop cultivars. However, this is incumbent on the availability of suitable genetic transformation and regeneration technology for legumes.

Traditionally, legumes have been branded as recalcitrant to in vitro regeneration, making plant genetic engineering difficult for this set of species. Very few legumes are amenable to regeneration through the callus phase. However, the successful deployment of cotyledonary node and embryo axes explants in genetic transformation experiments has hastened the process of genetic engineering in legumes. To date, the cotyledonary node-mediated transformation has been the preferred method of genetic manipulation in legumes. Of late, there is an increasing involvement of these two explant systems in genetic manipulation in legumes.

Champa Sengupta-Gopalan in Chapter 1 has summarized the current trends in the genetic manipulation of forage legumes, whereas Qi and Valentin (Chapter 5) discuss the recent trends in nutritional enhancement of soybeans. The species belonging to the legume family treated in this compendium are Medicago and Trifolium species (Chapter 2), soybean (Chapters 6 and 7), pigeon pea (Chapter 8), peanut (Chapters 16 and 17), chickpea (Chapter 9), Lathyrus (Chapter 12), pea (Chapters 13, 14, and 15), Phaseolus and Vigna (Chapter 10), Lotus species (Chapter 20), azuki bean (Chapter 11), broad bean (Chapter 19), Leucaena (Chapter 21), and Robinia (Chapter 22). As there are efforts from various laboratories across the world aimed at simplifying the transformation procedure for different species, Chapter 18 summarizes in planta genetic transformation methodology with reference to legumes. An attempt has been made in various chapters to give suitable protocols and hints.

Along with the tissue culture-plant regeneration technology for some selected legumes, some of the recent developments in the field of molecular biology and genomics as applicable to legumes are also summarized in this volume.

The analysis of transgenic plants ismore technically demanding than their production. It is a fundamental principle in plant biotechnology that transgenic plants carrying single copies of introduced genes with optimum expression should be developed to make the technology successful. Chapter 26 details the strategies for analyzing transgenic plants in detail. Real-time polymerase chain reaction (RT-PCR) is fast becoming a standard technique in quantifying gene expression. Chapter 30 provides detailed information on the use of quantitative real-time-PCR in the analysis of transgenic plants. Chapters 28, 29, and 31 discuss techniques based on complementary cDNA for characterizing differential gene expression and cloning novel genes.

Plant biotechnology is often limited by the lack of promoters for proper tissue-specific expression of foreign genes in transgenic plants and only a few promoters are available for deployment in plants. Hence, the isolation and characterization of novel promoters should be given utmost priority. Chapter 27 explains the strategies in tagging regulatory elements for successful biotechnological applications.

Genetic maps occupy a very important place in modern genetics and dense molecular genetic maps with very closely placed DNA markers have often been used in cloning genes of agricultural importance. The importance of synteny between closely related species is being appreciated. It essentially means that detailed molecular genetic maps available for one species can be utilized in related species for which such information is not readily available. Chapters 32 and 34 give extensive information about the various options available for developing molecular markers and compiling them into molecular genetic maps. Chapter 25 summarizes the fluorescence in situ hybridization techniques for characterizing chromosomes in legumes, the information from which ultimately is utilized in placing genes and gene blocks on chromosomes.

Legumes are a rich source of proteins, oil, and other essential biochemicals. Hence, they are prone to attack by various invaders. To protect themselves from the impending attack, they develop proteins of immense agricultural value. Chapter 37 summarizes the recent information on defensins in legumes. Chapter 36 describes recent trends in legume-rhizobium interaction. Chapter 35 gives elaborate information on manipulating abiotic stress tolerance in legumes.

With these chapters, it is hoped that the handbook becomes a handy reference with protocols for research workers in the field of legume genetic manipulation and young workers, who are getting initiated into this area.

I thank all the contributors for their valuable contributions and for making this project a success by sparing their time. I thank my associate, Ms. M. R. Beena, for helping me in formatting the volume suitably.