Advances in Legume Research: Physiological Responses and Genetic Improvement for Stress Resistance

Among the different environmental challenges that affect crop production, biotic stress factors are more devastating. They reduce crop yield and pose serious threats to food security. Legumes constitute a large number of crop varieties that are seriously affected by different biotic stress factors. To enhance their growth in the face of these different stressful factors and preserve their useful genomic and functional growth properties, leguminous crops are subjected to continuous genetic manipulations for stress resistance. Successful breeding of stress-tolerant varieties for cultivation under different farming systems may result in reduced crop losses and production costs, limited use of agrochemicals, and eventual yield increases. Crops that are resistant to biotic stress also exhibit better growth and yield characteristics. As established several decades ago, the revolution in genomic research led to the development of many sophisticated and advanced crop improvement techniques that can be applied across a whole range of leguminous crop species such as cowpea, faba bean, lentil, mungbean, pea, soybean, etc. However, interest in genetic engineering, chemically-or-physicaly-based mutation breeding, marker-assisted selection, quantitative trait loci and genome editing (CRISPR-Cas) have expanded research beyond biotic stress resistance. These techniques play a key role in applications such as the manufacturing of bioenergy, and crop engineering for the expression of valuable bioactive compounds and recombinant proteins. This chapter briefly reviews the diversity of biotic stress factors (bacteria, fungi, insects, parasitic nematodes and viruses) and possible ways in which these stress factors can be managed and eradicated using various breeding methods. The review shows that the biotechnological tools mentioned above provide beneficial functions in pest management through genetic, physiological and morphological improvements, especially when coupled with other farming practices. 

Legumes are economically important crops for the achievement of food security status in many countries in the tropical and subtropical regions of the world. Among various environmental stresses, biotic constraints to the production of grain legumes such as cowpea and soybean are becoming increasingly significant with the recurring change in climatic patterns and diverse environmental alterations. The economic impact of biotic factors such as disease-causing pathogens (fungi, bacteria, viruses and nematodes), insect pests and parasitic weeds has become overwhelming. These biotic stressors induce a wide range of damage symptoms which include stunting, wilting of stems, defoliation, root rots and premature death of plants. Yield losses due to the activities of biotic stress factors have been very significant. Hence, it is imperative to be informed of the various biotic stressors that affect the growth and yield potential of cowpeas and soybeans in various cropping systems. This review seeks to highlight existing pests and diseases in cowpea and soybean and evaluate their impact on the growth and productivity of these crops. It is hoped that the review will further spur scientific research into how these biotic factors can be managed or even manipulated to ensure agricultural sustainability, high economic returns, and global food security. 

Microorganisms found in plants exist as epiphytes or endophytes. Most epiphytes remain on plant surfaces and the latter may be intracellular pathogens, opportunistic and adapted microbial colonisers that originate from the surrounding environment. The main purpose of agricultural practices is thus, to develop disease-free varieties by propagating plants under controlled environmental conditions. Such conditions should be optimal for plant production and reduce disease development. The former requires strict certification schemes via several routes that include (i) indexing with subsequent removal of infected or contaminated materials from the production chain (ii) meristem and other tissue culture production systems and (iii) the use of thermo or chemotherapy for phytosanitation. Other methods also require balancing and proper adjustments in fertilizer usage and crop rotation. Therefore, this chapter reviews the role of microbial pathogen indexing as a means of controlling bacterial, fungal, and viral diseases that have a significant role to play in agriculture.

Legumes are very important food crops that are widely cultivated for their high-quality proteins, oils, and vitamins throughout the world. In total, 168 viruses are officially assigned by the International Committee on Taxonomy of Viruses. These viruses belong to 39 genera in 16 families and have been reported to affect various leguminous crops in different parts of the world. Among these viruses, TSWV (Tomato Spotted Wilt Virus), CMV (Cucumber Mosaic Virus), TMV (Tobacco Mosaic Virus), BYMV (Bean yellow mosaic virus), and BCMV (Bean Common Mosaic Virus) have global economic importance. This review therefore focuses on the economic importance of the abovementioned viruses influencing legume growth and development by looking at aspects such as viral traits, transmission, viral biology, plant host symptoms and the options used to control and manage some viruses such as the CMV (Cucumber Mosaic Virus), TMV (Tobacco Mosaic Virus), BYMV (Bean yellow mosaic virus), and BCMV (Bean Common Mosaic Virus).

One of the constraints to crop production across the world is vertebrate pests. They have been implicated as the most destructive pests which inflict both pre-harvest and post-harvest damages on agricultural production. Legumes are one of the crops usually attacked by vertebrate pest species, though the degree of depredation varies from one crop type to another. Meanwhile, there has been a misconception among farmers, especially in some of the developing countries, that vertebrate pest species belonging to the order Rodentia are very difficult to control. This is evident in their crop cultivations whereby two rows are planted in addition to every eight rows of crop, for rodent pest species that may come and inflict damage on the cultivated crop. Some of the rodent pest species that cause economic damage to legumes on the field include Arvicanthis niloticus, Xerus erythropus, Cricetomys gambianus, Rattus rattus, R. norvegicus, and Mus sp., while avian pest species include Francolinus bicalcaratus, and Ploceus cucullatus. There is a need to effectively manage these vertebrate pest species. Some of the rodent pest management approaches include the use of sanitation measures, exclusion of the vertebrate pest species, and modification of their habitat, and Trap Barrier System, while some of the avian pest management approaches include cage, nets or synthetic fibres, bird scarers, chemical repellents, sound-making devices, chemical poisoning, and trapping. 

An agroecosystem refers to a complex system comprising a couple of different interacting factors, involving species, ecological, and management processes. This system contains lesser species diversity of both plants and animals than a natural ecosystem. The variation in species of plants and insects is critically important to serve as a complex food chain and web whose interactions function to stabilise this ecological unit. However, among the groups of herbivores and predators found in agroecosystems, spiders play a key role in most crop fields by preying on a variety of pests. Besides this, the current pace of research on this subject shows that the role of spiders in regulating pest species and serving as potential biological control agents has been largely ignored. So far, information on agricultural spider communities, diversity and their role as biological pesticides remain scant in various parts of the world with the exception of countries such as the United States of America, Australia, and some parts of the Middle East Asia. Thus, this chapter outlines the most relevant information on the diversity, abundance and effect of arthropodous spiders on agroecosystems, particularly those that are involved in the cultivation of legume crop species. The paper also discusses current relevant threats to spiders, conservation measures, the threat of species extinction, and the role that these arthropods play in agriculture, especially by reducing the growth and productivity of species such as soybean (Glycine max L.) and cowpea (Vigna unguiculata).

Soybean is a crucial crop that is recognised globally for its high-value protein, vitamins, carbohydrates, fibre, and oils. However, the production of soybeans is frequently influenced by biotic stress factors such as bean leaf beetles (Cerotoma trifurcate), grasshoppers (Schistocerca americana), corn earworms (Helicoverpa zea) and stinkbugs (Halyomorpha halys). However, these insect pests were discovered to be both beneficial and harmful to crop growth and productivity, particularly, in soybeans. According to the literature, the rise in temperature causes an increase in insect pest populations, thereby severely influencing the growth, and yield quality of many crops. Less precipitation also contributes to drought stress, and plants undergoing water-deficit stress produce fewer secondary metabolites rendering them vulnerable to attacks by these insects. Similar effects were also revealed due to the rise in atmospheric CO2 levels that led to the adverse weather effects that caused enhanced reproduction and spread of pest insects. This chapter, therefore, explores the role of climate change-induced factors, such as temperature, precipitation patterns and rising atmospheric CO2 on insects’ distribution, and reproductive patterns, as well as their subsequent influence on crop growth and productivity in soybeans. The review also briefly discusses the chemical, biological and biotechnological approaches of insect pest control that have been employed effectively to combat losses of crop production. Side effects, cost effectiveness and the ability of new biotechnological methods to target specific pests are also discussed in this chapter.

Environmental stress generally causes considerable yield loss in leguminous crop production. This stress could be biotic (Insect pests, disease pathogens, weeds, vertebrate pests, etc.) or abiotic (Drought, heat, cold, salinity, flooding, heavy metal contamination, etc.). Either biotic or abiotic stress, both are capable of causing total yield loss. Unfortunately, crops are simultaneously exposed to these stress factors on the field. The response and level of tolerance to both stress factors, however, depend on the crop's genetic and nutritional status. The level of infection or infestation is determined by the cropping system and soil nutrient status. The induction of defense mechanisms by plants in response to pathogenic attack is dependent on environmental conditions like plant nutrient status. It means that there is a complex signaling network with crop nutrition that enables the plants to recognize and protect themselves against pathogens and other environmental stresses. The disease severity could be reduced by adequate crop nutrition due to host nutrient availability, plant composition of secondary metabolites, and the effect on the plant defense mechanisms. Shortages in essential nutrients on their own can predispose plants to attack by pests and pathogens. Therefore, the only sustainable method for growing crops in the face of different environmental stresses is good crop nutrition. A well-fed crop is more resistant to environmental hazards than poorly-fed crop. Though leguminous crops can fix atmospheric nitrogen themselves, the nutritional requirements for healthy crop production are more than just one element. The ability to fix nitrogen, if combined with appropriate crop nutrition will place the plant in a better position to withstand environmental stresses. This chapter discusses some of the different nutrient elements required by leguminous crops and their functions, crop nutrition abiotic stress tolerance, and mechanisms of nutrient-induced resistance in leguminous crops. 

Legumes are considered the second most important source of food after cereals, and their production can be affected by abiotic and biotic stresses. The incidence of biotic and abiotic stress conditions resulting from climate change is expected to increase in the future and may affect legume production drastically. Abiotic stresses could result in escalated biotic stress occurrence. Although responses to abiotic and biotic stress differ in most cases, combined abiotic and biotic stress responses could be expressed in synergistic or opposing forms. In view of the impending escalation in climate change, responses of legumes to stressful environments are expected to vary among crops. However, collective information on combined biotic and abiotic stress in legumes is not readily available. This paper seeks to gather available information on the responses of legumes to biotic, abiotic, and combined stress with a focus on physiological responses. This review will, therefore, help in providing information and encourage further research into combined stress factors in legumes