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- Herausgeber: Bentham Science Publishers
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
For centuries, legumes have been used as pulses or grains serving as the most critical sources of major protein/oil-producing crops for both human and animal consumption, and for providing raw materials for industrial processing. They are highly valued as soil-building crops, improving soil quality through their beneficial involvement in biological nitrogen fixation, a symbiotic partnership with rhizobia. Advances in Legume Research: Physiological Responses and Genetic Improvement for Stress Resistance serves as a unique source of information on the distinct aspects of basic and applied legume research for general readers, students, academics, and researchers. The book gives several insights on the morphological, physiological, and genetic responses to stresses via 8 concise chapters covering all aspects of legume growth, utilization, and improvement. The included chapters present research findings and succinct reviews concerning the strides continuously made in the improvement of legumes against biotic and abiotic stress factors. This comprehensive new legume reference book disseminates key information pertaining to genetic diversity, conservation, cultivation, manipulation through mutagenic techniques, plant transformation, and other breeding technologies. The book, therefore, continues to build on the need to acquire new knowledge about legume crops and ways to improve their existing agricultural yield for a sustainable and secure food market.
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Seitenzahl: 348
Veröffentlichungsjahr: 2020
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PREFACE
Legumes fall within the group of pod producing grain plants, belonging to the family Fabaceae . Plant species found inside this taxon are of high significance globally because of their relatively higher amounts of quality proteins, carbohydrates, fibre, and essential oils, that are contained within the seeds. Legume seeds contain less fats and no cholesterol, and they are a potential source of innumerable food/pharmaceutical supplements, feed manufacturing ingredients, biofuel, and are positively related to sustainable agriculture due to their association with nitrogen fixing microorganisms. It is, therefore, due to the above mentioned reasons that; they serve as model crops for functional studies in the trait (growth and yield) improvement of crops and the physiological/genomic development of resistant varieties against climate change-induced stress.
Of all the ways that climate change inflict harm on crops, legumes are among the most vulnerable and highly sensitive groups of oilseed crops worldwide. Abiotic stress and biotic stress-based reductions in the growth and yield of these crops, particularly, soybean, mung bean and cowpea, cause greater negative impacts on food security, health, and the import-export rates in many countries. Although these crops were recently rated just around 767th of the most traded products according to the OEC (Observatory of Economic Complexity, 1995‒2017), they all remain a positive driver for sustainable growth and development of many countries’ gross domestic product (GDP), especially in the developing African region.
All authors believe that readers will, therefore, receive and appreciate the insights provided by Advances in Legume Research- Physiological Responses and Genetic Improvement for Stress Resistance from different individuals with high expertise, and who are specialists in the area. The authors are of different scientific backgrounds, which is very important for the diversity of views, bringing new ideas and sharing new important original information covering perspectives from various fields of legume physiology and genetics. Legumes research should undoubtedly be continued, particularly because these crops also have a narrow genetic pool, consequently making them highly sensitive to various stress factors.
Numerous genetic modification tools gained popularity in recent years, but, inefficiencies and losses of beneficial genes still persist in many breeding systems. Some approaches are widely criticised, and others appear as viable alternatives, simply because they receive considerable attention from consumers and researchers. There are widespread untested assumptions of genetic instability of genetically modified organisms, and possible carcinogenic effects believed to emanate from genetically modified plants. Therefore, investigations on the evaluation of chemical compositions, growth, development and reproduction of GMOs and non-GM legume crops is perhaps the most fundamental service to mankind and the increasing populations.
Acknowledgements
The paramount goal of delivering a comprehensive book that clearly elucidates the understanding of mechanisms involved in plant’s genetic and physiological responses to stress was made possible by all authors. As such, we are very much grateful to all the authors and everyone who provided their meaningful contributions. I express my special thanks and appreciation to Dr. Phatlane William Mokwala and Prof. Roumiana Vassileva Nikolova for their continued mentorship and support. We express our special thanks and appreciation to Fariya Zulfiqar, Publication Manager for the support and help in making this goal achievable.
FOREWORD
Legumes form a major component of daily meals all over the world , especially in the developing world where in some cases, they are the stable food. They are also used in animal feeds. As the world population increases, more demand is placed on basic foods like legumes. Global warming has a detrimental effect on agricultural food production. In addition, people have become more concerned about the quality of the food they consume. These necessitate more research in agricultural food production, with legumes being no exception.
This book, “Advances in legume research – physiological responses and genetic improvement for stress tolerance” offers a reference to those who want to improve their legume production, those who seek better legumes to produce and those who seek foods with improved nutritional quality. Research topics covered include usage, cultivation, basic research, as well as genetic transformation in order to induce biotic and abiotic stress tolerance.
Genetic manipulation of food crops is still not fully acceptable by some people and countries. To those who have this view, the book shows that some basic physiological research methods can still be used to improve legume crops.
List of Contributors
Breeding of Legumes for Stress Resistance
Phetole Mangena*Abstract
As grain legumes continue to be used for various food and health forms, after suitable processing and manufacturing of legume-based products, aspects such as growth, yields, physiological stress and genetic manipulation remain significant topics for the enhancement of their utilisation, to explore new potential and diversify their genetic resources. Future research focusing on the physiological response and genetic improvements of legumes need to be prioritised to improve the utilisation and nutritional quality. The purpose of this chapter is to serve as an introduction to advances made in grain legumes, that are presented in various chapters of this book. The discussion is generalised and intended to provide a comprehensive view on the effect of stress on legume growth and yields. Included in this chapter are (a) a brief discussion on legume origin and classification, (b) brief survey on legume growth, yield and the impact of stress (biotic or abiotic stress) and (c) overview on breeding strategies available for genetic improvement of grain legume species, both conventional and non-conventional technologies.
THE LEGUME (FABACEAE)
The Fabaceae family contains over 650 genera and 20,000 species. This plant family is of greatest importance to world agriculture after the Poaceae family. In this volume, I will focus on this Fabaceae family, but only pay a special emphasis on plant species in this family that are used as edible bean seeds (Fig. 1). These selected bean species are used as food crops, directly or indirectly in the form of ripe-mature or unripe-immature pods, as well as mature and immature dry seeds. Most cultivated grain legumes belong to the two natural tribes; the Vicieae and Phaseoleae, both consisting of species and exhibiting phylogenetic characters as indicated in Table 1.
Both the Vicieae and Phaseoleae species have a combination of hypogeal/epigeal germination system and the herbaceous plant habit [1].
Biological characters such as those highlighted above, clearly indicate a simple inherent genetic control, additionally signifying the fact that many species within the tribes are restricted to one system or may interchange between epigeal and hypogeal germination systems among represented species. These and other key diagnostic characters (Table 1) are a representation of residual traits evolved from ancestral associations [2, 3]. The fruits, which are of pod type vary from dehiscent to indehiscent with a morphological diversity, which is translated into notable variations in seed dispersal mechanisms, such as ornithochory, hydrochory, autochory, anemochory etc [4]. Much of the diversity is exploited in agriculture, especially for the nine (9) annual grain species widely cultivated for commercial or domestic purposes, that include the dry bean, common bean, pea, lentil, mung bean, faba bean, cowpea, pigeon pea, and soybean. All these crop species have grain quality that is suitable for industrial processing.
Domestication of Grain Legumes
The crop species represented above form part of what is now known the civilisation and initiated human dominion over natural plant genetic resources on earth. The gathering and domestication of desirable wild plant species began over 10,000 years ago, leading to well-coordinated breeding practices where species were selected and propagated for greater and more convenient food, as well as medicinal supply. According to phylogenetic evidence-based descriptions provided by Schrire et al. [5], Lopez et al. [4] and Moteetee and Van Wyk [6] in legumes, the selection of crop species for domestication was based on ancestral associations, their usefulness in the primitive economy and the ease of domestication, especially on the simplicity in which selected species could be propagated. The majority of the earliest domesticated species could be identified by their native and endemic traditional uses in certain areas. The distribution and utilisation of cultivated crop species differ according to the age of domestication, period enabling novel introduction to other areas for similar purpose and areas of wider distribution and occurrence of the wild ancestral populations.
Hartmann et al. [7], reported that peas and lentils were the earliest domesticated legume crops, together with wheat and barley cereals in the eastern part of the world. In the far east, millet appears to be the first domesticated crop followed by rice, meanwhile squash and avocado were the first domesticated crops in the central and southern parts of America. These plants are followed by corn, bean, pepper, tomato and potato in the same region, which now serve as some of the major commodities and widely cultivated crops worldwide. Domestication and spread of legume crops across the world did not only enable sustainable food and medicine supply, but also permitted effective capture and recycling of energy from the sunlight [8].
Raven [8] indicated that these may include the capture of CO2 (for incorporation into carbon skeletons used for carbohydrate synthesis and synthesis of other carbon-containing primary and secondary metabolites with a carbon backbone) and improvement in soil fertility through nutrient recycling, when legumes die and replenish the mineral nutrients back in the soil. The spatial distribution of these legume crops still influences the structure and functioning of other plant populations and communities, particularly due to a mutualistic symbiotic relationship with nitrogen fixing bacteria.
GROWTH, YIELD AND STRESS
The growth and development of legume crops from a small grain seed to a mature plant requires a precise and highly organised succession of cellular, genetic, physiological and morphological events. Starting as a single fertilised gamete, plant cells divide, grow and differentiate into an astonishingly complex miniature plant called an embryo, packaged within a seed. In the end, the seed will germinate and give rise to the complex organisation of seedling tissues and organs that divide and grow into a mature plant that later flowers, bears fruits, disperses the seeds, senesces and eventually dies. According to Raven [8], all these events, including the biochemically and environmentally modulated processes constitute plant development and growth. Understanding these processes is one of the major goals of crop physiology. The pattern of changes experienced by cells, tissues and organs is more genetically controlled. Like other plants, legumes also experience a well-coordinated growth of tissues, which is subjected to control at various distinct levels. Such control levels include intrinsic control operating at both intracellular and intercellular level (e.g. gene or protein expression and hormonal regulations), and extrinsic extracellular controls outside the organism, functioning to convey information about the environment [9].
Legumes likewise often encounter unusual or extreme environmental conditions like any other plants. Crops in the northern latitudes for example, experience extreme low temperature, while those in the tropical Savannas may experience scorching temperatures, and high levels of harmful UV radiations. These effects are much felt by farmers of agricultural crops, whose plants may experience a period of extended drought (thus leading to disease outbreak and uncontrollable spreads) or their roots subjected to high salt concentrations in the soil. Unfortunately, plants are rooted in the soil and consequently they cannot escape adverse environmental conditions in their vicinity. Rather, they use various stress response strategies to adapt, survive and grow under these hostile conditions [10]. A major problem is that both natural and anthropogenic activities continue to add and exacerbate the number of stress factors that plants must cope with in their environment (Fig. 2). Dakhovskis et al. [11] reported the physiological adaptation of cultivated plants following exposure to naturally and anthropogenically induced environmental stress. The stress factors affected individual biological characteristics of the plants, emphasising a strong impact on plant homeostatic mechanisms and weakening of plant response to the induced stress.
Fuchs et al. [12] also analysed genome damage, infertility and meiotic abnormalities caused by agricultural expansion and increased utilisation of agrochemicals, releasing heavy metals into the environment, pathogen spread and contamination by pharmaceutical or industrial residues. Thus, all cropping systems need to elaborate on the system’s productivity and sustainability in addition to profitability. All stakeholders should be concerned about conserving the quality of the environment and maintaining soil fertility as much as they pay attention on the quality and quantity of yields. The yield potential of many grains is seldom achieved due to unsuitable cultivated species and inadequate crop management to cope with stresses [13]. The development of fruit pods and seeds is strictly genetically and physiologically modulated. Therefore, the exposure of flowering plants to stress usually cause ovaries and embryo development to abort, or slowed down by hormones that are responsible for the coordination of normal seed and fruit development. The formation of fruits and seeds or the overall yields in grain legumes is linked with irreversible anatomical changes and an aging process. Physiological effects associated with these major changes may include total dry matter, leaf area, photosynthetic rate, stomatal conductance, respiration rates, internal CO2 metabolism and leaf water potential, all, which have negative impacts on yields [14].
Fig. (2)) An overview of the relationship between environmental stress (biotic and abiotic stress) threating plant survival, growth or yield.BREEDING OPPORTUNITIES FOR SPECIFIC ADAPTATIONS
Grain legumes continue to occupy a crucial position in legume-based diet of many population’s nutrition, health and welfare, mainly as a source of proteins, minerals, carbohydrates and vitamins. It has been confirmed that the consumption of grains contributes to a balanced diet and can prevent the progression of cancer and other chronic diseases. The intensification of legume agriculture has also led to major characteristic changes in the agroecological systems [1, 14], enhancing pathogen generation as well as the spread of many biotic and abiotic stress factors. Abiotic and biotic stress agents adversely affect crop growth, cause rapid depletion of natural genetic resources, cause reduction in arable land and are the basis for accumulation of pollutants in the environment. Human population are still yet to face challenges on mitigating the damage caused by myriad of anthropogenic activities, including those caused by unsustainable agricultural practices. These factors already cause major negative impacts on the natural environment and to human/animal health [15].
These effects are, furthermore, exacerbated by the consequences of climate change. The only apprehension for breeders and scientist is that, positive identification and selection of superior genetic resources showing resistance to these stress constraints are required. So far various traditional and modern methods have been used to recover plants that have unique and desirable genetic properties, for example, plants modified through genetic transformation. Researchers worldwide have to continue optimising breeding techniques to expand the number of legume species amenable to genetic improvement.
BREEDING OF LEGUMES FOR ABIOTIC STRESS
In 2006, the United Nations conference on climate change predicted extended drought seasons in most parts of Africa, due to climate change. Furthermore, it was highlighted that agricultural production will suffer more as a consequence of this frequent climate fluctuations [16]. However, since their initial domestication, more legume crops have been subjected to intensive selections and breeding of varieties that contain crucial agronomic traits. These included a set of characters that made them adaptive to adverse environmental conditions and enhanced their growth, quality and quantity of yields. Such improved plant characters included changes in apical dominance, production of enlarged sizes and numbers of roots, stems, leaves, fruits and seeds. But, it is common knowledge that abiotic stress is responsible for major growth and yield losses in many legume crops. Amongst these, drought has been recorded as the most damaging kind of abiotic stress and most crop plants are highly susceptible and sensitive to drought than any kind of abiotic constraint condition [8, 9, 17].
Herbaceous plant crops such as soybean and cowpea are among the grains that easily get injured by moderate or brief exposure to drought stress, immediately exhibiting one or more metabolic dysfunctions. Freitas et al. [14], reported reduction on several growth parameters of cowpea following moderate and severe water restrictions. In addition, Wijewardana et al. [18] evaluated whether the effects of water deficit stress on parental soybean plants may be transmitted to the F1 generation. The results showed that, seed germination and seedling development in F1 generation were affected by the lasting effects of soil moisture stress that took place originally on affected parent plants. The findings emphasised a key role played by seed weight and storage reserves during germination and seedling growth. Thus, concluding that, optimal water supply during fruiting and seed filling period is beneficial for enhancing seed quality and vigour/ viability characteristics. As predictions continue to estimate that climate change will be responsible for 20% increase in water scarcity due to the occurrence of poorly distributed torrential rains, droughts, and high temperatures. This will severely affect crop development and yield, as already seen in China, India and the United States which all serve as the largest global producers of grain crops [19]. Thus, the breeding of legume crops should include genetic improvement for salinity, heat, light, metal toxicity and chilling stress tolerance.
Breeding for Salinity Stress Tolerance
Soil salinity remain one of the major factors adversely affecting crop yields worldwide. Predictions estimates that, about 1 billion hectares of irrigated land is affected by salinity and the problem is increasing at a rate of about 10% per annum. Legume crops show high sensitivity during vegetative and reproductive stages primarily from the abundance of sodium chlorite (NaCl) from irrigated soil or natural accumulation [20, 21]. Salinity stress cause metabolic dysfunction by causing nutritional imbalances, osmotic stress effects, ion toxicity, decreased photosynthetic rates and cause severe necrosis and chlorosis [21]. From the physiological and genetic aspects, salinity stress is a complex trait, therefore, an integrated approach that use the existing genetic resources, diversity and novel sources to create new varieties is required. Sehrawat et al. [22] reported that, continued screening should be frequently adopted to select salt-tolerant germplasm to develop better performing genotypes.
Breeding for Temperature Stress Tolerance (Heat and Chilling Stress)
Plants exhibit a wide range of sensitivities to extreme temperatures. Both chilling stress and heat stress have detrimental effects on plant growth and productivity. Heat stress has deleterious effects on the morphology, physiology and reproductive growth of plants. But, the reproductive phase is the most vulnerable stage during the period of stress, which reduces crop yields. According to Wang et al. [23] and Bita and Gerats [24], the impact that heat stress have on plant reproduction include reduced pollen viability/mortality, ovule infertility, flower abortion, impaired fertilisation and reduced seed filling leading to decreased seed sizes and yield losses. These observations confirm reports, that daytime temperature above 35°C caused substantial reductions in anthesis and pod setting leading to complete failure of the reproductive phase [25]. Each legume crop species has its unique set of temperature requirement for growth and development. This includes an optimum low temperature at which the plant grows and performs most efficiently without sustaining chilling stress injuries. Under cold stress, vegetative growth of many legumes get severely affected at temperature ranging between 4 to 15°C. Legumes such as chickpea and pigeon pea show high sensitivity to chilling and frost induced stress. Injuries sustained completely inhibit photosynthesis and cause production of reactive oxygen species (ROS) [26]. Therefore, genotypes surviving and reproducing under this temperature range (˂ 15°C) are highly desirable and may be selected as potential genetic resources for the development of chilling stress tolerant cultivars.
Fig. (3)) Anthracnose symptoms on soybean fruit pods as indicated by Tom Allen, Extension Plant Pathologist. (A) & (B) Soybean plants showing a possible viral infection. (C) Seeds attacked by soybean podworm. (D) Example of soybean plants grown in a field without any observable disease symptoms.BREEDING OF LEGUMES FOR BIOTIC STRESS RESISTANCE
Climate change has increased challenges experienced in agriculture by intensifying the spread of diseases affecting legume and cereal grain crops (Fig. 3). It has been widely reported that biotic stresses occur at different intensities across all cultivated agricultural lands worldwide. For example, Anthracnose infections caused by Colletotrichum spp. can infect stems, leaves and pods of soybean causing minimal effects on yields (Fig. 3A) [27]. The occurrence of diseases caused by bacteria, fungi, viruses and constant crop attacks by insects, weeds and nematodes is also increasing at a very alarming rate. These stress factors apparently cause reductions in the growth and yields of many crops. Therefore, for farmers and consumers to cope with biotic stress, plant breeding programmes have to adopt new strategies to rapidly and efficiently develop new cultivars with resistance.
Breeding for Insect Pests
Grain legumes remain the most important source of food and medicine for the increasing populations, currently estimated to have reached 7.8 billion people on earth. The main objective for plant breeders and researchers should be to develop insect-resistant legume crops, that are well-adapted to a diverse range of climates, soil types, and widely cultivated throughout the temperate, tropical and sub-tropical climates. However, the demand for food has been increasing very rapidly, especially with the increasing spread of crop diseases that lead to more than 20% yield losses almost every year. Biotic stress affect the most vulnerable and highly susceptible widely cultivated legumes, such as cowpea. Cowpea is grown in many parts of the world, including west, south and east Africa, Latin America, United States and south east Asia [28]. Although, crop plants naturally express phytoalexins (collection of isoflavonoids and other secondary metabolites) to ward off insects and disease outbreaks, this defense system already proved inefficient for the current evolved sets of attackers (Fig. 3C).
Cowpea probably suffer most from insect pests, because all parts of the plant at all stages, from seedling to mature plant ready for harvest get infested by insects. A large number of insects pests, which mainly belong to the phytophagous taxa have the ever evolving and increasing gene pool. Its speciation is somehow believed to be intimately linked to their hosts. The insect taxon include Coleoptera, Diptera, Homoptera, Heteroptera, Hymenoptera, Lepidoptera, Orthoptera and Thysanoptera which comprise about 76 predominate phytophagous families containing at least 20 species of insects each [29]. Modern agriculture should address the lack of genetically improved cultivars and good crop management systems, in order to increase crop yield under such gigantic gene pool of insect pests. Improved cultivars are necessary to achieve abundant food production, high crop quality and make food prices affordable for the poor masses of starving people, particularly living in developing African countries.
Breeding for Pathogen Resistance
One of the most notorious crop pathogens to have caused over 80% yield losses in Europe (1845) is Phytophthora infestans. This etiological agent could cause close to 100% yield losses in agricultural production fields, accompanied by huge direct monetary costs of pest control and lost productions estimated at more than $ 3 billion per year globally [30]. Similar Phytophthora species that attack pasture legumes were reported by Irwin et al. [31]. According to the report, Australia experienced an annual production loss exceeding $ 200 million due to individual species belonging to this taxa. Another devastating pathogen that also cause more than 80% yield losses each year is Aphanomycetes euteiches, of the family Saprolegniaceae, order Saprolegniales. According to Gaulin et al. [32], this pathogen infects the cortical tissues in primary and lateral roots of crop legume seedlings. It forms oospores within the root’s cortex tissues that causes yellowing of cells followed by browning on root cells, and subsequently leading to the blackening of seedling’s hypocotyl. Crop rotation with faba bean, pea and any species of lupin usually leads to a build-up of soil-borne pathogens with long field persistence [30, 31]. Recently, some studies illustrated that, should resistant cultivars not be developed, a large amount of fungicides with agroeconomic and environmental negative effects will not be reduced. Furthermore, billions of dollars of income losses for legume crop growers will also not be avoided, with no possibilities of business recoveries. As long as the spectrum of biotic stresses that may cause crop yield losses is large and continue to diversify as already demonstrated in Phytophthora taxa. This threatens the efforts made in providing food, feed, fibre and bioenergy for the increasing world population [32].
AVAILABLE IMPROVEMENT STRATEGIES AND TECHNIQUES
Legumes constitutes a large number of varieties in which many of them are bred and developed for both subsistent and commercial farming purposes worldwide. These crops are improved for pathogen resistance, drought or salinity stress resistance, competitiveness against weeds, high heritability, additive genetic control and shared better performance of individual lines or populations. According to Fritsche-Neto et al. [33] selection or hybridisation is often based on hybrid performance for allogamous species when a trait of low hereditability or when its inheritance is based on non-genetic effects. However, many traits will be quantitatively determined by the interactions of a large number of genes expressed uniquely under different environments. Increased variations provide further opportunity for selection and possible hybridisation to produce new genotypes adapted to specific environmental niches.
A recently established revolution in genetic research has led to the development of the most far-reaching applications across the whole range of applied plant biology, especially in the selection and breeding of crop species to sustain plant growth during biotic and abiotic stress. Biotechnology continues to provide and produce improved varieties mainly through transgenesis and other techniques, which allow the introduction of one or more genes for stress tolerance [33]. Some of these methods have become more efficient and highly rapid for the development of new stress resistant cultivars, especially for major crop commodities such as cotton, soybean, rice, maize, wheat and sorghum. These crops are targets of significance for food, beverage, and health industries, as well the potential production of a clean efficient bioenergy, worldwide. Furthermore, according to Haile et al. [34] wheat, corn, rice and soybean serve as very important staple commodities and remain crucial for the fight against global food insecurity.
Transgenics
Long-term significant genetic improvement efforts through genetic engineering have been taking place in many private, parastatal and government research laboratories worldwide. Some of these national and international laboratories have thus far registered and distributed a range of improved legume crop lines around the world, accomplishing a number of stress resistance factors and other crop qualities. Such include disease and pest resistance, improved seed quality, biotic and abiotic stress resistance. According to Chandra and Pental [35], among the many different techniques tested for gene delivery to plant cells, only Agrobacterium-mediated genetic transformation and particle bombardment have been extensively employed. These techniques can be utilised for or coupled with transient gene expression studies, functional genomics and even CRISPR-Cas9 genome editing technology. CRISPR-Cas9 in short, refers to clustered regularly interspaced short palindromic repeats and CRISPR-associated protein 9.
Gene editing predominantly induces non-homologous end joining (NHEJ), which generates altered genome by random insertions or deletions, and precise recombination products by homology-directed repairs [36].
Particle bombardment-mediated transformation remains one of the most expensive procedures, but rather highly rapid, efficient and suitable for genetic manipulation of many recalcitrant plant species. This method utilises a gene gun employed to allow penetration of the cell wall by foreign particle coated genetic materials containing the gene of interest to be transferred into the host cells [37]. Different plant tissues or organs targeted include pollen grains, embryos, callus cells, seedling epicotyls and hypocotyls, fruits, flowers, and roots [38]. Ivo et al. [39] generated stable transgenic cowpea (Vigna unguiculata) plants showing a Mendelian transgene inheritance. The transgenic plants were mutated with ahas gene coding for acetohydroxyacid synthase conferring high level resistance to herbicide imazapyr. Cicer arietinum (L.) was also genetically transformed with an insecticidal crystal protein gene (crylAc) taken from Bacillus thuringiensis [40]. Particle gun method was, furthermore, used for the genetic manipulation of pigeon pea and soybean using different explant types. Reported studies evaluated the use of different promoters and stacking of genes for various biotic and abiotic stresses [41].
It has been more than two (2) decades since the introduction of genetically modified plants predominantly established through in vitro Agrobacterium-mediated transformation. In soybean, the first successful transformation was reported by Hinchee et al. [42], using cotyledonary explants with Agrobacterium pTiT37-SE plasmid harbouring pMON9749 gene for herbicide glyphosate tolerance. The success of this method depended upon several factors, which included tissue culture conditions, Agrobacterium strain and selected host plant genotypes aimed at receiving the transgenes. To date, this technique has succeeded in the production of high yielding transgenic cultivars, particularly for corn, chickpea, rice, cowpea, as well as a few soybean genotypes [43-45]. According to Patel et al. [46] and Mehrotra et al. [45], this technique is considered the most economic and highly effective method of genetic modification that has been reported so far. The method holds the potential and promise to efficiently regenerate transgenic plants, especially in recalcitrant legume crops.
Mutagenesis
Mutations are primary sources of spontaneous or induced genetic variability resulting from DNA changes that alter the plant’s genome. This approach has been used to identify gene function or cause genetic manipulation in plants by involving the use of chemicals, ionizing radiation or specific DNA sequence insertions [33]. Spontaneous mutations are rare and non-targeted as the chance of occurrence is very minimal, meanwhile, targeted mutations are induced via treatment of plant tissues with various mutagenic agents. A variety of legume mutant lines, such as those conferring weed control by herbicide tolerance, and those used as germplasm collection for further breeding in the domestication of species for agriculture are already in use and available. Mutation breeding of agronomically important legumes, usually used as models to study legume crop genetics and genomics has also been reported.
