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

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.

Understanding genetic diversity is essential for achieving genetic improvement and conservation of grain legumes. These crops serve as important pulses grown and consumed all over the world, especially in Africa, south east Asia and America. Legumes serve as a good source of carbohydrates, oil, fibre and proteins. They contain all essential amino acids, nutritionally important unsaturated fatty acids such as linoleic and oleic acids, and mineral elements such as K, Ca, Mg, P and Zn. The seeds contain all important vitamins such as riboflavin, niacin, thiamine, vitamin A precursor ß-carotene and folate. As with other crops like cereals, legumes face reduced genetic diversity, which impact negatively on the production of newly improved varieties showing stress tolerance. Conservation of wild genetic resources and cultivated germplasm will provide genetic materials for future breeding programmes. Genome sequencing libraries and bioinformatics tools could be used to screen and select genotypes with desirable traits, even according to the geographical patterns. The results will be of major importance for conservation genetics and breeding of newly improved cultivars that exhibit high resilience to adverse global climate patterns and plant pathogens.

Soybean (Glycine max L.) is one of the most important leguminous crop plants worldwide. A lot of attention has been focused on soybean cultivation in South Africa. Its production is affected by several biotic and abiotic stress factors which reduce the yield and quality of the crop. The aim of this study was to evaluate drought tolerance in South African soybean cultivars that have the potential for cultivation in areas where water is a limited resource. Six South African (LS677, LS678, Mopani, Sonop, Knap and Pan1564) and two American (R01416 and R01581) cultivars were carefully studied for morphological and physiological markers using a greenhousebased study in a randomised block design. The results showed that several morphological (stem length, leaf area, flowers, and seeds) and physiological (chlorophyll, moisture content, phenolics, flavonoids, ureide content and antioxidant activity) parameters were affected by drought stress. However, cultivars with high phenolic and flavonoids content were associated with high antioxidant activity and slightly increased their yields than other varieties. The anatomical analysis also showed some interesting differences in response to reduced water treatment, with the sizes of vascular tissues and sclerenchyma tissues decreasing under drought stress. In conclusion, this study indicated that reduced stem length and inability to reduce leaf area by soybean plants could lower plant growth and yield response under drought stress. In addition, increased chlorophyll and secondary metabolite content can improve soybean growth under limited water conditions.

The challenge for agriculture is to address the need for adequate food provision and a sustainable future for crop productivity. The solution for increasing food production can probably only be obtained through the expansion of arable land by increasing irrigation practices, or by increasing harvestable yields on available land through the improvement of agricultural technology. With regard to the latter approach, field experiments were conducted to determine the effect of Maxiflo and Trykoside; liquid formulations of Azospirillum and Trichoderma based products, respectively, on vegetative growth and yield of Pisum sativum L., Brassica oleracea var. capitata, Lactuca sativa, Solanum tuberosum, Lycopersicon esculentum and Triticum aestivum. A randomized complete block design with six treatments (Control, Maxiflo, Trykoside, Maxiflo + Trykoside, ComCat® and Kelpak®) was applied in all cases. Maxiflo and Trykoside were applied either separately or together. Two commercially available natural biostimulants, ComCat® and Kelpak®, also served as positive controls. Results showed that peas were most responsive to treatment with the bio-products in terms of the increase in yield obtained compared to other crops. Growth of cabbage and lettuce were not affected, but a significant increase in head mass was observed. Combined treatment of Maxiflo and Trykoside increased fruit yield in tomatoes as well as tuber sizes in potatoes. However, in both crops, the total yield was not significantly affected, especially compared to the yield in peas. In wheat, root growth was stimulated significantly by treatment with Trykoside, but no significant yield increases were observed compared to peas.

Tepary bean is an important food legume cultivated in semi-arid areas in many parts of the world, including sub-Saharan Africa. This crop is highly tolerant to drought and pests, but it is also generally low yielding. Similar to many other legumes, the genetic improvement of the tepary bean is also limited by its narrow genetic base. Techniques, such as mutation breeding, particularly chemical mutagenesis with ethyl methanesulfonate (EMS) have been successfully tested to induce genetic variability in this crop. As part of the few reports of chemical mutagenesis in tepary bean, this paper evaluated the seedling performance of the M1 EMS mutagenised plants and adult plant performance of the subsequent generations of tepary bean under different conditions. Based on the results, EMS induced some dominant mutations that were detectable in the M1 generation. These effects were further extended to the early mutagenised generations (M2 to M4) of the tepary bean under field conditions, revealing novel information regarding the response of tepary bean to chemical mutagenesis at both the seedling and adult plant stages.

Soybean is a valuable crop cultivated as an excellent source of proteins, dietary fibre, and a variety of micronutrients. Rhizobia reside as symbiosomes in the infected cells of soybean nodules to fix atmospheric nitrogen. Such association ensures optimal yield and may be beneficial in quenching the use of fertilizers. However, premature nodule senescence remains one of the major problems affecting soybean growth and yield. A clear understanding of the molecular level on the resultant effects of factors that promote early nodule senescence is important. Cysteine proteases are the proteases directly involved in the commencement of early tissue senescence. Nevertheless, the studies performed on the involvement of these proteases in nodule senescence have not reached a consensus. Besides, the specific family and isoforms of cysteine proteases expressed under normal well-watered, waterlogging and water deficit conditions during nodule senescence are not clearly understood. Consequently, there is a need for conducting intensive molecular studies on the involvement of cysteine proteases in nodule senescence. Changes in the whole proteome of a healthy and a senescing nodule need to be understood to make conclusive findings on the molecular events that occur during senescence. The use of proteomic approaches to investigate the level and characteristics of the proteins in general, by preparing a protein map and its application to functional analysis in soybean nodule senescence is important in delaying premature senescence.

Cowpea, Vigna unguiculata L. is a very important grain legume crop that is grown in the tropic and sub-tropical regions. It provides a strong support to the livelihood of the rural poor people and small scale farmers through contributions to their nutritional security, income generation and the improvement of soil fertility. However, its production yield can be adversely affected by abiotic and biotic constraints. The stress affecting cowpea creates the need to develop and implement breeding strategies that can alleviate the devastations caused by biotic and abiotic constraints. Breeders employ pedigree, backcross, marker-assisted breeding, genome editing (CRISPR-Cas9) and other modern biotechnological techniques for genetic manipulation of cowpeas, including legumes such as soybean, chickpea and common bean. These useful strategies have brought about major opportunities for breeders to develop cowpea cultivars with improved tolerance to a wide range of growth and yield inhibiting stress factors.

Recombinant DNA technology remains one of the best tools that still presents a great potential to enhance genetic improvement in many recalcitrant crops since its discovery more than two decades ago. This chapter intends to provide a comprehensive review of the applications of genetic transformation techniques in legumes, useful for both growth and yield improvements, especially under abiotic and biotic stress conditions. This technology is very promising in mitigating the current and future challenges in agriculture, with proven records in the development of a number of cereal and legume crops, such as maize, sorghum, soybean, lentils, peas, chickpeas, common beans, and alfalfa. In the midst of all reported advantages, this technology is also faced with several concerns, criticism, and possible shortcomings emanating from its adoption and production of novel genetically modified cultivars, especially at farm and market levels. Issues such as the genetic integrity of the transgenic cultivars, undesirable mutations, biosafety, and moral beliefs regarding the production and consumption of GM crops are among the controversial topics faced by this biotechnological tool. In addition, a large number of genotypes still persist in being recalcitrant to genetic manipulations, pending a cost-effective, precise, highlycompetent, and robust approach for the generation of fertile transgenic plants. However, this technology remains widely utilised in many countries despite the numerous speculative concerns raised by some scientists, health professionals, and environmentalists.