Dalia Mohamedkheir Khojely,Seifeldin Elrayah Ibrahim,Enoch Sapey,Tianfu Han*

aMOA Key Lab of Soybean Biology(Beijing),Institute of Crop Sciences,Chinese Academy of Agricultural Sciences,Beijing 100081,China

bGezira Research Station,Agricultural Research Corporation(ARC),Wad Medani,Sudan

cOil Palm Breeding and Genetics,CSIR-Oil Palm Research Institute,Ghana

1.History of soybean introduction and cultivation in SSA

Sub-Saharan Africa(SSA)is geographically the area of the African continent that is situated south of the Sahara,approximately between 15°N and 35°S.SSA comprises 48 countries and has a total area of 21.2 million square kilometers and 600 Mha of arable land,of which＜10%is currently cultivated.Thus,SSA is the largest underutilized land reserve in the world.

Soybean has a relatively brief history of introduction and commercial cultivation in SSA countries[1].It was introduced to SSA in the 19th century by Chinese traders along the east coast of Africa[2].The first record of cultivation of soybean in South Africa was in 1903[3].Soybean was first cultivated in Tanzania in 1907 and Malawi in 1909[2].In 1908,soybean was introduced to Nigeria[3]and was cultivated as an export crop in a small area in Benue state,where the introduced variety‘Malayan' was adopted.The crop is usually grown in smallholdings in mixed cropping with sorghum or maize or as an intercrop in citrus orchards.Like Nigeria,Zaire has a long history of soybean production by indigenous farmers.Soybeans were introduced and promoted first by missionaries before the nation gained independence,and are considered a medicinal food to prevent and cure the wasting effects of malnutrition in Zaire.Soybean was first introduced to Sudan in 1910[3].Further introductions in the country were made in 1912,and in 1949,soybean was planted in southwest Sudan to prevent severe malnutrition among infants,children,and pregnant and lactating women[4].Further soybean introductions were continued[5]with special efforts to encourage soybean cultivation in SSA.

2.Current status of soybean production in SSA

Agriculture is the primary industry and the development driver in SSA.There is a wide range of agro-ecological regions in SSA,including three major zones:moist savanna,Sudan savanna,and mid-altitude zones,representing respectively 29%,31%,and 40%of the total cropland in SSA.The three zones are considered to be of high potential for growing soybean[6].The moist savanna zone,which has high potential for crop and livestock production and is widely viewed as the emerging breadbasket of SSA,is characterized by a growing period of 150 to 270 days,adequate rainfall(900–1100 mm),high solar radiation,fertile soils,and low disease and insect pressures.Late-maturing soybean varieties are most suitable for this zone.The Sudan savanna has annual rainfall of 600 mm[7].This zone is frequently subject to drought stress during crop growth and development,and extra-early-maturing and drought tolerant soybean varieties are grown and perform well in this zone.The mid-altitude regions also have conditions favorable for high soybean yields,including cool temperatures(permitting good crop growth),adequate rainfall(700?900 mm)in most areas,and some fertile volcanic soils.

The soybean planting area in SSA has increased dramatically,from 20,000 ha in the early 1970s to 1,500,000 ha in 2016.This increase has been the primary factor in raising soybean production in SSA by 177-fold,from 13,000 t in the early 1970s to 2,300,000 t in 2016[8].Average soybean yield in SSA has stagnated at 1.1 t ha?1in the last four decades,in contrast to the world average of 2.4 t ha?1.The low soybean yields in SSA may be accounted for largely by poor-performing varieties and lack of sustained rhizobial inoculant use and fertilizer application[9].

In SSA,South Africa is the largest soybean producer in 2016,followed by Nigeria,Zambia,and Uganda[8].Other SSA countries,including Zimbabwe,Malawi,Ghana,Sudan,and Ethiopia,have also experienced sizeable commercial soybean expansion.Planting area and production in the leading soybean-producing countries in SSA during 1987–2016 are shown in Figs.1 and 2,respectively.Average yields in South Africa,Nigeria,Zambia,and Uganda during 2012 to 2016 are presented in Table 1.In 2016,South Africa(2290 kg ha?1)and Zambia(1940 kg ha?1)had the highest average yields,followed by Nigeria(960 kg ha?1)and Uganda(600 kg ha?1)in SSA.During the last two decades,Nigeria,South Africa,and Uganda have maintained their positions as leading soybean-producing countries in the region.The reasons behind their successful soybean production include(1)governmental policies to produce food locally rather than depending on imports;(2)progress in research to improve varieties and practices adapted to a wide range of agro-climaticzones;(3)emphasis on developing recipes that substitute or incorporate soybeans in traditional foods;(4)willingness of local manufacturers to use soybeans in baby foods and for vegetable oils and animal feeds;and(5)promotion of soybean production and utilization by organizations,agricultural development projects,hospitals,schools,and local governments.South Africa and Nigeria offer the best examples in SSA of the potential for soybean production and use.Soybean production has increased dramatically,from 84,000 t in 1987 to 1,320,000 t in 2016 in South Africa and from 40,000 t in 1987 to 680,000 t in 2016 in Nigeria(Fig.2).

Fig.1–Soybean planting area(1000 ha)in the leading soybean-producing countries in SSA,South Africa,Nigeria,Zambia,and Uganda from 1987 to 2016.

To meet the rapidly rising market demand,SSA countries imported 6.8 Mt.of soybeans annually at the cost of 4.4 billion USD during 2013 to 2016.They have imported annually 1.7,3.3,and 1.8 Mt.of soybean grain,meal and oil,respectively,during 2012 to 2016.South Africa is the leading soy meal importer in SSA,with annual imports of over one half million tons[10].By increasing soybean yield and production,SSA countries will become soybean demand-driven rather than supply-driven,given that soybean demand is still growing rapidly in SSA.

3.Soybean utilization in SSA

Fig.2–Soybean production(1000 t)in the leading soybean-producing countries in SSA,South Africa,Nigeria,Zambia,and Uganda from 1987 to 2016.

Table 1 –Average yield(kg ha?1)in the leading soybeanproducing countries in SSA during 2012–2016.Data source:USDA/FAS,2017 and Index Mundi,2017.

Soybean is a productive oil crop and constitutes an important component of smallholder cropping systems,with considerable potential for enhancing household food and nutrition security in SSA.Soybean meal has recently been used extensively in the poultry industry in SSA[11],because of its high protein content(＞40%)and its excellent profile of highly digestible amino acids.South Africa and Nigeria are the leading soybean-crushing countries in SSA.Soybean crushing has increased dramatically,from 25,000 t in 1986 to 1,000,000 t in 2016 in South Africa and from 5000 t in 1986 to 350,000 t in 2016 in Nigeria(Fig.3).In recent years,South Africa has made large investments in domestic soybean crushing capacity.The annual soybean crushing in South Africa was increased to 2,200,000 t in 2016.

Since the value of soybean as a high-protein food source has been recognized,the utilization and consumption of soybean-based foods are becoming popular in SSA[12].Food products such as nshima,also called bidia,dawadawa,mahewu,soy-ogi(fortifying maize with soybean),soy kebab(spicy tofu),biscuits,soy flour,soy yogurt,and soymilk have been accepted by local people in many SSA countries[12–14].Nshima,a staple porridge made from maize,soybean and cassava flour,is consumed widely in Zambia and Malawi.The earliest known use of soy-based food in the Nigerian diet in the rural region was in the form of dawadawa,a fermented soup condiment traditionally made from locust bean seeds[15].Presently,most of the dawadawa produced in Nigeria uses soybean as its raw material.Mahewu is a fermented beverage that contains mostly maize and soy flour.When reconstituted with water,it forms an opaque,cereal-tasting beverage.The drink is consumed daily by thousands of SSA residents.In other SSA countries such as Ghana,roasted soybean seeds are used in the home preparation of tuubani.They are ground,mixed with water to form a paste,and then steamed inside a folded leaf.In SSA,soymilk and soup are the daily meals of malnourished children and HIV/AIDS-infected patients in public schools and hospitals.Nigeria offers the best example in SSA for promoting utilization of soy-based food products in rural and urban areas.The high utilization and consumption of soy-based food products in Nigeria is attributed to development of home-level and small-scale processing technologies for soy-based foods,training of technical and extension staff,training of local farmers and village leaders,dissemination of food processing technologies,and support from the public and private sectors.

4.Soybean research in SSA

Soybean research in SSA first began in Nigeria in the mid-1960s.Research on soybean improvement in SSA has been conducted by the International Institute of Tropical Agriculture(IITA),located in Nigeria,and national research programs,universities,and the private breeders.Since the inception of soybean improvement in 1974,IITA has played a leading role in developing tropical high-yielding soybean varieties and improved agronomic technologies and in promoting soybean processing and utilization suitable for smallholder farmers in SSA[16,17].Private companies,such as Pannar Seed,Agricol,and DuPont Pioneer in South Africa,SeedCo in Zimbabwe,and ZamSeed in Zambia have also contributed to soybean variety development in SSA.

4.1.Scientific staffing of national soybean improvement programs in SSA

Fig.3–Soybean crushing(1000 t)for the leading soybean crushing countries in SSA,South Africa,and Nigeria from 1987 to 2016.

Investments in and impacts of soybean improvement research across SSA are historically low and continue to lag behind those in other soybean-producing regions in the world.Despite isolated cases of strong and concerted effort in soybean varietal improvement,human resource capacity remains limited across SSA countries.Scientific staff time allocated to soybean research in SSA ranged from 0.8 to 14.6 full-time equivalents[16].Nigeria has the highest number of full-time equivalents in SSA.To our knowledge,the number of full-time and well-qualified soybean breeders with the Ph.D.degree in national soybean research programs across SSA countries is very low.This situation largely accounts for the negligible contribution of national soybean research programs in SSA to developing improved soybean varieties;almost all soybean varieties commercially grown across SSA have been developed by either IITA or private breeders.Nigeria has also a relatively high research investment in soybean improvement,consistent with its position as one of the leading soybean-producing countries in SSA.

Over the last four decades,several research projects have been designed and implemented in SSA with funding from external donors to improve soybean productivity at the farm level.These include the International Soybean Variety Experiment(supported by USAID,USA),N2Africa Project(supported by Bill&Melinda Gates Foundation and Howard G.Buffett Foundation),the Tropical Legumes-II Project(supported by CGIAR),and the USAID Soybean Value Chain Research Project(supported by USAID and carried out by USAID Soybean Innovation Lab,University of Illinois,USA,in collaboration with five national soybean research programs in Ghana,Mozambique,Zambia,Malawi,and Ethiopia).

4.2.Breeding tropical soybean for SSA

SSA stretches over 50 degrees of latitude,ranging approximately from 15°N to 35°S,meaning that the majority of the SSA regions are tropical and subtropical.

As a short-day plant[18],soybean is sensitive to photoperiod[19–21].Varieties introduced from high latitudes in temperate regions and cultivated under short-day conditions at low latitudes in tropical and sub-tropical regions often flower too early,resulting in poor yield because of insufficient vegetative growth prior to the reproductive phase of the crop.Thus,soybean varieties that are relatively insensitive to photoperiod are needed in SSA regions.

4.2.1.Introduction of soybean germplasm to SSA

Diverse soybean accessions with specific adaptation traits,such as insensitivity to short daylength and tolerance to drought and extreme temperatures,were introduced from low and high latitudes in temperate and tropical regions to the IITA soybean improvement program based in Nigeria and to many national agricultural research systems in SSA during the last five decades[22].The diverse germplasm was introduced from the USDA Soybean Germplasm Collection,the Asian Vegetable Research and Development Center,the Rural Development Administration of the Republic of Korea,India,Brazil,and Argentina.From 1973 to 1985,hundreds of soybean accessions,especially southern USA and Brazilian varieties,were introduced to＞25 SSA countries through the International Soybean Variety Experiments(ISVEX)initiated by the INTSOY soybean varietal testing program at the University of Illinois,IL,USA.Field studies indicated that the introduced tropical soybeans were highly adapted to SSA's agro-environmental conditions,whereas the temperate germplasm flowered too early under short-day conditions in SSA,resulting in poor vegetative growth,short plant height,and low yield[4].

4.2.2.Variety development and adoption in SSA

High and stable grain yield was the main objective from the outset of soybean improvement in SSA and is still a top priority in soybean breeding and crop management in SSA.Other constraints of importance to soybean breeding include seed viability,pod shattering resistance,and susceptibility to nodulation with Rhizobium strains present in SSA soils.

Between 1970 and 2011,195 soybean varieties have been released in SSA[16].Zimbabwe(31),Zambia(30),Nigeria(20),Cote d'Ivoire(18),Malawi(14),Benin(12),Uganda(11),Cameron(10)and Kenya(10)account for the highest numbers of released soybean varieties,representing 80%of the total releases in SSA.Among the 195 releases in SSA,119,71,and 5 varieties were released by private breeders,IITA,and national research programs,respectively[16].The high level of released varieties is attributed to the involvement in soybean genetic improvement of private companies,such as SeedCo in Zimbabwe and ZamSeed in Zambia[23].Largely because of private-sector participation,only Zimbabwe,Zambia,and Malawi have demonstrated the ability to develop soybean varieties from their own crosses in SSA.The total dependence on IITA elite and advanced soybean materials is expected in very small national research programs in SSA,but is surprising in Nigeria with 14.6 full-time-equivalent scientists.National soybean improvement programs in SSA countries have relied heavily on introduction of elite lines and varieties from Asia,Latin America,and North America and from international providers such as IITA,Nigeria,and INTSOY,USA,with very low emphasis on soybean breeding.Adoption of released soybean varieties is high in SSA[16,17,24],largely because soybean itself is a new crop and its cultivation is made possible through improved varieties.Uganda,Nigeria,and Ghana have released variety adoption levels of 97%,96%,and 94%,respectively,of the total area allocated to soybean production in 2010[16].Participatory varietal selection has also improved adoption of soybean varieties in Ethiopia[25].Of the 1.3 Mha used for soybean planting in SSA,about 0.7 Mha(52%)were occupied by IITA-related varieties in 2010[16],reflecting the important role that IITA plays in soybean genetic improvement in SSA[6,17,26].

South Africa is the only SSA country that has approved genetically modified(GM)soybean varieties tolerant to glyphosate herbicides for commercial production.In 2000,the first Roundup-Ready(RR)soybean variety(A5409RG)was registered in South Africa.By 2015,a total of 89 RR and 35 conventional soybean varieties had been registered[27],and＞90%of all soybean acreage is used to grow RR varieties in South Africa in 2017.PannarSeed,Agricol,DuPont Pioneer,and Monsanto are the leading seed companies developing and releasing soybean GM varieties in South Africa.The national soybean variety evaluation program was initiated by the Agricultural Research Council-Grain Crops Institute(ARC-GCI)in 1978 under government directives in order to stimulate and support the local soybean industry in South Africa[28].The first root-knot nematode tolerant variety,‘Egret',was developed by ARC-GCI and released in 2002.

In 2011,H7 and H17,two soybean varieties introduced from South China Agricultural University were released in Mozambique with support from the McKnight Foundation.High grain yields of 5.2 and 3.0 t ha?1were obtained by H7 and H17,respectively.Dias and Amane[29]studied the yield response of soybean genotypes to different planting dates in Mozambique and found that H7 had a maximum grain yield(3683 kg ha?1)when sown in early November to late December.These results showed the high yield potential of Chinese soybean varieties[49]in SSA.

Recently,substantial advances in soybean breeding have been made in the national soybean improvement programs of Ghana,Ethiopia,and Sudan.The USAID Soybean Innovation Lab(SIL)(http://soybeaninnovationlab.illinois.edu/),in collaboration with research scientists from the University of Illinois and the national soybean improvement program in Ghana have been investigating how maturity,long juvenile period,and stem architecture traits influence the adaptation of soybean to low latitudes,specifically in northern Ghana.A total of 360 experimental lines derived from six populations were evaluated in Ghana over two growing seasons in 2016 and 2017,and promising lines with traits favoring adaptation to tropical low-latitude environments will be selected for further evaluation and future variety releases to farmers.In response to low phosphorus availability in SSA soils,which has strongly constrained tropical soybean yield potential across SSA,SIL research scientists at the University of Illinois and the national soybean improvement program of Ethiopian Institute of Agricultural Research(EIAR)in Ethiopia have been working on developing low-phosphorus-tolerant soybean varieties that will thrive in the nutrient-poor soils common in SSA.In Sudan,the national soybean breeding program of the Agricultural Research Corporation(ARC),Wad Medani,has been conducting for over a decade research on adaptation of soybean to irrigated and rainfed cropping systems in Sudan[4,30,31].Four non-GM soybean varieties introduced from IITA were released for commercial production in Sudan under the names‘Sudan 1',‘Sudan 2',‘Soya 3',and ‘Soya 4'.Sudan 1 and Sudan 2,released in 2012,are late-maturing varieties(120 days),and Soya 3 and Soya 4,released in 2017,are early-maturing varieties(100 to 105 days).However,the narrow genetic base of the ARC soybean breeding pool is a major constraint to maintaining consistent rates of genetic gain for both economic and agronomic traits,particularly to meet the future challenges posed by climate change[31].A total of 1094 F5lines derived from different six soybean crosses:Sudan 1×PI 416937(N=247),Sudan 2×PI 416937(N=51),Sudan 1×N7001(N=130),Sudan 2×N7001(N=139),Sudan 2×N01-11424(N=427),and Sudan 1×N05-7266(N=100)were developed at ARC[31].N7001,N01-11424,N05-7266,and PI416937 are elite and exotic USDA-ARS breeding lines with yield and drought-tolerance traits[32–34].Recombinant inbred lines and the six parents will be evaluated genotypically and phenotypically,with the aim of identifying single-nucleotide polymorphisms that can be associated with specific traits and further used in marker-assisted-breeding to improve drought adaptation,yield potential,and grain quality in soybean.

Knowledge of genotype×environment interaction is invaluable to soybean breeders in selecting desirable variety.Multiple-year and multiple-location testing is a key approach for identifying and selecting high-yielding and stable soybean varieties adapted to diverse or specific agro-environment conditions in SSA[26,35].SIL partners,in collaboration with the Syngenta Foundation for Sustainable Agriculture,IITA,and the African Agricultural Technology Foundation have been engaging in the pan-African soybean variety trials to implement the first third-party testing of soybean in SSA.The trials “fast-track”the introduction and testing of commercial soybean varieties in order to provide local seed companies,farmers,seed buyers,and others with access to a broader selection of seed than is currently available.The trialing platform provides testing and registration of modern varieties and establishes distribution systems for quality seed in SSA.To date the pan-African soybean variety trials have been conducted with multiplelocation testing in Malawi,Mali,and Kenya,and more testing locations in SSA countries will include in 2018.

4.2.3.Breeding “promiscuous”varieties for nodulation with Rhizobium

In 1974,IITA initiated soybean improvement in Nigeria,with the main focus on developing high-yielding tropical soybean varieties that can nodulate with indigenous Rhizobium strains in SSA soils:so-called “promiscuous”varieties[6,36,37].The improvement program was initiated using breeding materials from southern USA and Brazil and a limited number of southeast Asian landraces.The IITA soybean improvement effort has played a leading role in international efforts aiming at developing and disseminating well-adapted varieties and elite germplasm to farmers and national soybean research programs in SSA.In the early 1980s,IITA scientists observed that indigenous rhizobia in SSA soils are effectively symbiotic with legumes such as cowpea but could not be utilized by improved soybean varieties from the southern USA,the principal germplasm source of tropically adapted soybean varieties[38–40].Based on a promiscuous nodulation genetic source,a traditional soybean cultivar from southeast Asia,characterized by poor yielding,lodging,a vining habit,and high nodulation,the promiscuity character has been incorporated into improved southern USA soybean varieties[40–42].Incorporation of promiscuous nodulation has enabled the improved southern USA soybean varieties to nodulate effectively with indigenous Rhizobia in SSA soils[40,41].Development of IITA promiscuous soybean varieties,Tropical Glycine Cross(TGx)varieties,in the late 1980s was a key breakthrough in overcoming the most significant constraint impeding the expansion of soybean production in SSA,given that the soil bacterium Bradyrhizobium japonicum is not indigenous in most SSA soils and seed inoculation is needed before sowing.In addition,most farmers do not have access to B.japonicum inoculant that can be used to inoculate seed[6,17],because most SSA countries do not have industries able to produce viable inoculants at prices that smallholder farmers can afford.

4.2.4.Use of the long juvenile(LJ)trait in tropical soybean breeding Identification and introduction of the LJ trait into soybean varieties in the 1970s[43]enabled the crop to expand dramatically in the northern and southern American tropical regions[44–47].Brazil has expanded its soybean production from 1 Mha in 1970 to over 33 Mha in 2016[47].This trait delays flowering time and thereby ensures sufficient vegetative growth prior to reproductive development.The LJ trait has been widely deployed across continents to overcome the poor adaptation and low yield that had hitherto prevented economically viable soybean production in low latitudes in tropical regions[48].In SSA,the vast majority of tropical soybean varieties were originally developed from the southern USA genetic background,which is the principal germplasm source of tropical soybean varieties[38–40].Incorporation of the LJ trait has greatly facilitated the cultivation of tropical soybean in SSA.The LJ trait,in parallel with the promiscuity trait,has opened up a new era of expansion of tropical soybean in SSA.The recent identification and cloning of the J gene conferring the LJ trait in soybean[47,49]provides opportunities for soybean breeding in SSA to develop LJ varieties adapted to a wide range of agro-ecological zones in SSA by using biotechnology tools such as marker-assisted-selection,genomic selection,and genome editing.

4.3.Soybean crop management and soil fertility in SSA

Despite the progress that has been made in genetic improvement of tropical soybean yield in SSA,poor crop management practices and low soil fertility have constrained successful and sustainable tropical soybean production in smallholder farming systems of SSA.As a result of ISVEX trials across SSA,low soybean yield potential in many SSA countries was affected by poor management practices rather than by variety adaptation to environment.During the last four decades,research-based recommendations have been made for agronomic practices in soil fertility management[50–54],promiscuous nodulation and nitrogen fixation[1,55–61],sowing date[30,62,63],row and plant spacing and plant population[4,64,65],and crop rotation and intercropping[66,67].

Low and declining soil fertility has long been recognized as a major impediment to intensifying agriculture in SSA[53,68,69].Biological nitrogen fixation(BNF)in soybean is economically and ecologically beneficial in SSA,where most soils are deficient in nitrogen and nitrogen fertilizers are not affordable to farmers,owing to their economic conditions.Besides producing valuable grain,soybean fixes between 44 and 300 kg N ha?1,making a high nitrogen contribution to intercropped and rotated cereal crops.For example,Peoples and Cras well[70]estimated the improvement of a maize crop following a soybean crop at between 0.5 and 3.5 t ha?1or 30%–350%relative to maize–maize sequences.Intercropping is one of the most valuable practices for improving land use[71–74].Using maize–soybean strip intercropping,grain yields of 12,750 kg ha?1for maize and 1650 kg ha?1for soybean,and a land equivalent ratio of 2.2 were obtained[74].This ratio indicates that the output value of intercropping maize with soybean is high compared to maize monocropping[74,75].

SSA is characterized by 15 diverse farming systems[76].Highland Perennial,Maize Mixed,Cereal–Root and Tuber Crops,Agro-Pastoral,and Highland Mixed farming systems are the major farming systems in SSA[77–79].In the five farming systems,maize is the most widely grown crop,occupying＞33 Mha each year[80],and it is a staple food for an estimated 50%of the population in SSA[81].The high risk of extensive cereal monocropping practices in SSA necessitates the diversification and intensification of the cereal-based systems in SSA through inclusion of grain legumes such as soybean[82,83].In this context,a maize–promiscuous soybean rotation and/or intercropping provide alternative approach for enhancing the productivity and sustainability of farming systems in SSA.Moreover,soybean-maize intercropping can help in suppressing infestation by striga(Striga hermonthica),a parasitic weed that infests over 60%of farmland under cultivation in SSA[84],whereas the continued cultivation of maize in the field provides a host for striga[85,86].Economic analysis of these systems shows an increase of 50%to 70%in gross income compared to those of farmers still following the current practices,mainly continuous maize cultivation[87].

In SSA,where tropical soybean is primarily a rainfed crop,planting date is of paramount importance because early plantings stimulate excessive vegetative growth,which will later lead to lodging problems without yield advantages.On the other hand,late plantings result in insufficient vegetative growth,a low bottom pod height,and lower yield.For instance,soybean is recommended to be planted in early to mid-November in South Africa and in early June to early July in Nigeria for best yield.Soybean production is characterized by low plant density per unit area across almost all SSA countries.This practice may partially account for the low average yield of SSA countries.Row spacings of 60 to 80 cm and plant spacings of 5 to 10 cm are widely used on smallholder soybean farms in SSA.

5.Potential of the soybean industry in SSA

SSAis one of the regions where the human population increases faster than food production[88].Food insecurity is a major concern in SSA.Protein deficiency exacts a greater toll from infants,children,and pregnant and lactating women in SSA than anywhere else in the world,partially because starchy foods are widely consumed and animal protein often is too expensive and out of reach for low-income families.Increases in protein production are needed in SSA countries to address the challenges of food security by increasing income and improving human nutrition at the household level.To fill the gap between the demand and supply of food,especially protein,growing soybean as a tropical crop in SSA is an ideal solution[12,89–91].In addition,soybean has the capacity to fix nitrogen even with native Bradyrhizobium strains and fits well into current crop rotations[92,93].

Global soybean demand continues to surge.New soybean frontiers are thus likely to develop.SSA is one candidate region for such expansion[90,94].Although the areas potentially suitable for tropical soybean in SSA vary from 140 to 270 Mha,＜3%of this area is currently being utilized[3,94].With advances in biotechnology tools,the LJ gene can be incorporated into locally adapted varieties in SSA,and new releases of LJ varieties can be expected.These new releases of LJ varieties will lead to the expansion of soybean production across SSA countries.The scaling up of soybean production and utilization in SSA countries can be further promoted through strengthening the entire soybean value chain and networking research scientists,policy makers,extension services,non-governmental organizations,and public and private breeders.

Acknowledgments

This work was supported by the Agricultural Science and Technology Innovation Program of CAAS and China Agriculture Research System(CARS-04)awarded to T.Han of CAAS and S.E.Ibrahim of Agricultural Research Corporation(ARC),Soybean Research Program,Wad Medani,Sudan.

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