Approximately 70% of the population in the semi-and arid tropics reside in rural areas and depend on agriculture for their livelihood. Crop production is primarily focused on a few starchy staple crops. While this can ensure adequate calories, it inadvertently neglects the need for dietary diversity. Consequently, food and nutritional insecurity remains prevalent in the semi-and arid tropics. We reviewed the legume value chain with the aim to identify opportunities and challenges to unlocking

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... eir value and promoting them in the tropics. Several grain legumes are rich in proteins and micronutrients. They also possess adaptability to marginal environmental conditions such as drought and low input systems which typify rural landscapes. Adaptability to abiotic stresses such as drought makes them key to agriculture in areas that will receive less rainfall in the future. However, this potential was currently not being realized due to a range of challenges. Aspects related to their seed systems, production, post-harvest handling and marketing remain relatively under-researched. This was especially true for minor legumes. There is a need for trans-disciplinary research which will address the entire value chain, as has been done for major starchy crops. This could also unlock significant economic opportunities for marginalized groups such as women. This will unlock their value and allow them to contribute meaningfully to food and nutrition security as well as sustainable and resilient cropping systems. Sustainability 2017, 9, 60 2 of 25 and sweet potato (Ipomea batatas) are the staple crops. These crops have been the subject of significant research and government attention [7]. This has led to breeding of high-yielding and drought-tolerant cultivars of common cereals and root and tuber crops. Cereals and root and tuber crops, which are starch rich, mainly provide calories to address energy requirements but lack dietary diversity to ensure adequate nutrition [8] . Dietary diversity is a strategy that involves including a variety of food groups to the diet such as fruit and vegetables, legumes, starch and animal products [9] . Meat, fruit and vegetables are the major sources of proteins and micronutrients, respectively, but they are not always accessible to the rural poor. Meat remains expensive while fruit and vegetables are generally affordable, only when in season, but unaffordable when out of season. In this regard, the use of grain legumes as alternative sources of protein and other micronutrients [10] could assist in improving dietary diversity of poor rural households. The promotion of grain legumes has been mainly linked to them being rich sources of protein, low in saturated fat, as well as possessing certain important micronutrients (zinc, folate and calcium and tocopherols) [11] [12] [13] . In this regard, legumes could contribute significantly to diets of rural households if consumed as compliments to starch. While history shows that early Khoikhoi and Indian settlers in the semi-and arid tropics utilized indigenous legumes as a major component of their diets [14] , this status has since changed. The "Green Revolution" shifted attention to cereal crops. While this resulted in improvements to crop production and energy supply, it inadvertently resulted in stagnation of production and crop improvement of legumes [15] . The promotion of legumes which are adapted to the semi-and arid tropics will contribute to the diversity of cropping systems and diets of people living in these areas. However, there is need to address critical knowledge gaps that will allow for the promotion and reinstatement of legumes within food systems. To date, there has been separate attempts by crop scientists [16] [17] [18] [19] [20] [21] [22] and nutritionists [11] [12] [13] to address the knowledge gap on legumes. These efforts have been disciplinary and the information is yet to be consolidated so as to make meaningful impact on policy. The emerging interest on minor legumes, indigenous to semi-and arid tropics, should also be considered [23] . As the world celebrated the International Year of Pulses in 2016, there was a need to re-conceptualize the possible role that legumes can play in the post-2015 agenda. The aim of this review was to provide a holistic perspective on the potential of legumes. This was done through focusing on the legume value chain and identifying challenges and opportunities for unlocking the value of legumes. A mixed-method review approach, which included combining quantitative and qualitative research or outcomes with process studies, was used to compile the review. Scientific journal articles, book chapters, technical reports and other forms of literature were used for the review. The review focused primarily on literature describing sub-Saharan Africa and South Asia; the two regions share similar development trajectories, challenges and opportunities, thus making them comparable. The review was then structured as follows; Section 2 provides an overview of water scarcity in SSA and SA and its effect on agricultural production. Furthermore, Section 2 also highlights food and nutritional security status in SSA and SA using selected indicators such as stunting, wasting, anemia and obesity. Section 3 discusses grain legumes, with a focus on their diversity and adaptability to the semi-and arid tropics. Section 4 discusses the progress and gaps in research on grain legumes. A value chain approach was used to categorize research into four components, namely, (i) breeding and crop improvement; (ii) agronomy; (iii) processing and utilization; and (iv) marketing. Lastly, Sections 5 and 6 present the challenges, opportunities and recommendations concerning promoting legumes in semi-and arid tropics. Setting the Scene-South Asia and Sub-Saharan Africa South Asia refers to the southern part of Asia which is dominated by the Indian tectonic plate which rises above sea level as Nepal and extends to the south of the Himalayas and the Hindu Kush. Sub-Saharan Africa refers to the regions that are fully or partially located south of the Sahara desert. The two regions are climatically alike according to the Köppen-Geiger climate classification. Sustainability 2017, 9, 60 3 of 25 They are described as semi-and arid climates due to actual precipitation being less than actual evapotranspiration [24] . These two regions are also considered the poorest regions in the world [25] . Approximately 70% of the population in these regions reside in rural areas and rely on agriculture for their food and livelihood [26] . However, agricultural activities are primarily challenged by water scarcity. Water Scarcity Most countries in South Asia and sub-Saharan Africa experience some form of water scarcity (Figure 1 ). Rainfed agriculture is the primary source of food production in the semi-and arid tropics. The amount of arable land under rainfed production ranges from 60% to 95% [27]; making water is the most limiting factor in crop production. The uncertainties in rainfall distribution and occurrences and the high frequency of dry spells and droughts [28] frequently result in significant yield losses and crop failure for rural farmers. Most of them are incapable of recovering from such disturbances. This alludes to the importance of promoting resilient cropping systems in these areas. Sustainability 2017, 9, 60 3 of 25 managing the research through WRC Projects No. K5/2274//4 "Determining water use of indigenous cereal and legume food crops" (WRC, 2014) and WRC Project No. K5/2493//4 "Water use and nutritional water productivity of food crops for improved nutrition and health in poor rural households."