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Review

Advances in Sorghum Improvement for Climate Resilience in the Global Arid and Semi-Arid Tropics: A Review

by
Andekelile Mwamahonje
1,*,
Zamu Mdindikasi
2,
Devotha Mchau
2,
Emmanuel Mwenda
3,
Daines Sanga
2,
Ana Luísa Garcia-Oliveira
4 and
Chris O. Ojiewo
5
1
Tanzania Agricultural Research Institute (TARI) Head Quarters, Dodoma P.O. Box 1571, Tanzania
2
Tanzania Agricultural Research Institute (TARI) Makutupora Centre, Dodoma P.O. Box 1676, Tanzania
3
Tanzania Agricultural Research Institute (TARI) Ilonga Centre, Kilosa P.O. Box 33, Tanzania
4
Genetic Resources Program, Alliance Bioversity International and International Center for Tropical Agriculture (CIAT), Cali 6713, Colombia
5
International Maize and Wheat Improvement Center (CIMMYT), Nairobi P.O Box 1041-00621, Kenya
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(12), 3025; https://doi.org/10.3390/agronomy14123025
Submission received: 26 September 2024 / Revised: 9 November 2024 / Accepted: 13 November 2024 / Published: 19 December 2024
(This article belongs to the Section Crop Breeding and Genetics)
Browse Figure
Figure 1
<p>A graphical representation of top 10 sorghum producers in the world.</p> ">
Versions Notes

Abstract

:
Sorghum is a climate-resilient crop which has been cultivated as a staple food in the semi-arid areas of Africa and Asia for food and nutrition security. However, the current climate change is increasingly affecting sorghum performance, especially at the flowering stage when water availability is critical for grain filling, thus lowering the sorghum grain yield. The development of climate-resilient, biotic and abiotic stress-tolerant, market-preferred, and nutrient-dense sorghum varieties offers a potentially cost-effective and environmentally sustainable strategy for adapting to climate change. Some of the common technologies for sorghum improvement include mass selection, single seed descent, pure line selection, and marker-assisted selection, facilitated by backcrossing and genotyping using molecular markers. In addition, recent advancements including new machine learning algorithms, gene editing, genomic selection, rapid generation advancement, and recycling of elite material, along with high-throughput phenotyping tools such as drone- and satellite-based images and other speed-breeding techniques, have increased the precision, speed, and accuracy of new crop variety development. In addition to these modern breeding tools and technologies, enhancing genetic diversity to incorporate various climate resilience traits, including against heat and drought stress, into the current sorghum breeding pools is critical. This review covers the potential of sorghum as a staple food crop, explores the genetic diversity of sorghum, discusses the challenges facing sorghum breeding, highlights the recent advancements in technologies for sorghum breeding, and addresses the perceptions of farmers on sorghum production under the current climate change conditions.

1. Introduction

Global food security is under unprecedented pressure due to the expanding global population and climate change, which has caused more frequent catastrophic disasters. It is predicted that the global population will rise to 9.7 billion by 2050, which will necessitate an increase in food production by 70% to feed the growing population [1]. According to the WHO, the COVID-19 pandemic contributed to an increase of 83 to 132 million undernourished people in 2020. This increased the vulnerability of the world’s food supply and stagnated the progress towards achieving zero hunger [2]. Hence, there is an urgent need not only to increase food productivity per unit of production inputs but also to develop crop varieties that will grow under more fragile climatic conditions [3,4]. While we acknowledge that there is no single-bullet solution to achieving food and nutrition security for the growing global population and solving the extreme hunger situations exacerbated by changing climates, identifying and adapting crop varieties for healthy diets and climate resilience can make a critical contribution.
Sorghum was identified by the Vision for Adapted Crops and Soils (VACS) Project as a climate-resilient opportunity crop due to its water-use efficiency and tolerance to drought and high temperatures. Sorghum is the world’s fifth most important cereal crop after maize, wheat, rice, and barley and has historically been produced in arid and semi-arid environments [4].
Sorghum belongs to the Poaceae family and has 24 distinct species, including both cultivated and wild relatives [5]. About 65% of the grain’s endosperm is starch, while 11% is protein, with the germ containing about 4% lipids relative to the grain’s total bulk. It is a rich source of nutraceutical components, including phenolic compounds, minerals, and vitamins, along with dietary fibers [6]. Sorghum protein contains 80% kafirin, which is classified as a prolamin since it is usually soluble in polar solvents like aqueous alcohol, and 20% salt- and water-soluble proteins (albumin and globulin). This crop is slowly gaining popularity as a functional food constituent due to its beneficial properties, which include low protein and starch digestibility, high levels of dietary fiber, polyphenols, and particularly condensed tannins [7].
In addition to its use as food, sorghum is used in a variety of industries, such as brewery, the manufacture of paper, animal feed, sugar production, dyeing chemicals, and in the development of energy sources [8,9]. The most impoverished regions of Africa rely on sorghum as a staple grain due to its versatility. The genetic diversity of sorghum is wide and accounts for its diverse adaptation to various agroecological conditions and changing climates [10].
Although sorghum grain feeds more than 500 million people in more than 30 countries in Africa and Asia, its use as animal feed accounts for a majority of the sorghum grain worldwide. The growing importance of sorghum as a potentially tolerant crop to heat and drought, and the excellent functional qualities of sorghum grain in healthy diets point to a greater emphasis on the development of new sorghum-based foods [11].
Therefore, the aim of this review is to highlight the modern tools available for accelerating the development of new sorghum varieties that can tolerate the biotic and abiotic stresses caused by climatic change. It also covers the potential of sorghum as a staple food crop, explores its genetic diversity, discusses the challenges facing sorghum breeding, examines the recent advancements in breeding technologies, and considers the perception of farmers on sorghum production in the context of current climate change.

2. Global Production of Sorghum

Sorghum is a significant cereal crop that is grown all over the world for food, especially in the context of gluten-free grain, feed, and other uses. It is commonly grown in the arid and semi-arid tropics, which are prone to drought. The crop is grown in almost 100 nations, with 66 of them producing more than 1000 metric tons each in over 1000 hectares. This is particularly important in regions where land is limited or in developing countries, where this amount of land can represent a substantial endeavor and because over 90% of the world’s sorghum crop is found in developing nations, primarily in Asia and Africa. The United States of America is the largest (13.38 million t) producer of sorghum followed by India (8.23 million t), Nigeria (7.65 million t), and Mexico (6.09 million t) (Figure 1) [12]. However, in terms of productivity, none of these nations have high yields compared to global yield records [13].

3. Potentials of Sorghum as a Staple Food Crop

Smallholder farmers in SSA are currently confronted with issues which call for the comprehensive adoption of crops with a variety of socioeconomic and agronomic significance [14]. Modified temperature and precipitation patterns function as a mediating factor for climatic oscillation, which is expected to affect the stability of agricultural production and pose a danger to food and nutritional security. Wild relatives of sorghum which tolerate harsh conditions have a huge potential which can be exploited for introgression into cultivated sorghum varieties [15,16]. Sorghum can adapt to a wide range of agronomic and environmental conditions, including salt, low precipitation, and insufficient irrigation water supply. It has been reported that sorghum is less susceptible to weather fluctuations, which enables it to sustain high productivity under changing and variable climates [11]. It is a highly nutritious crop that is devoid of gluten and is favored by health-conscious consumers. Therefore, when other crops fail and the region experiences a food deficit and famine, this crop is utilized to assure productivity, food, and nutrition security [17,18]. The genetic diversity found in the nutritional and antinutritional traits of sorghum presents a potential avenue for improving grain quality through breeding thus mitigating malnutrition [19,20]. Although sorghum naturally adapts to harsh and resource-poor areas, there is potential to increase production under more ideal circumstances [21].

4. Perception of Farmers on Sorghum Production Under the Current Climate Change

Considering that sorghum is cultivated by many small-scale farmers in semi-arid areas as a source of food security, several obstacles have prevented it from reaching its potential productivity of 4 to 5 tons per ha. An essential factor to be considered is the perspectives, desires, and limitations of farmers and stakeholders regarding sorghum yield and production [22].
For example, farmers in Ethiopia believe that bird attacks limit sorghum yields. The two most crucial characteristics that farmers value in sorghum cultivars are resistance to disease and tolerance to bird attacks [23]. However, according to a study conducted in Nigeria, the main traits preferred by farmers in sorghum are high yield, drought tolerance, and Striga [24]. Additionally, in Zimbabwe, farmers and service providers fear that rising temperatures may jeopardize food security by reducing and fluctuating sorghum yields below 0.5 t/ha and increasing the pressure from grain storage insect pests [25]. Hence, enhancing productivity and encouraging the adoption of improved sorghum cultivars require the careful consideration of breeding varieties that incorporate the features which farmers prefer [23]. For instance, farmers in Namibia perceive that sorghum production constraints include recurrent drought, declining soil fertility, insect pest damage, high cost of production inputs, unavailability of quality seeds, lack of alternative improved varieties with preferred traits, insufficient organic manure, limited access to market, and limited and/or inadequate extension services.
The preferred traits by farmers in a new sorghum variety include high grain yield, early maturity, and tolerance to drought, as well as resistance to storage insect pests. It is recommended that genetic improvements and the deployment of new sorghum varieties with the described farmer-preferred traits be prioritized to enhance the sustainable production of sorghum [26].

5. Sorghum as a Climate-Resilient Crop

A chronically unstable and deteriorating agricultural output in semi-arid areas has resulted from a series of droughts in some parts of the world, particularly in Africa, along with other socioeconomic constraints [27]. The production of staple food crops such as sorghum is negatively impacted by changes in extreme weather and climatic events, namely high-temperature episodes, including extreme rainfall events, droughts, and flooding, which put the ecosystem resilience at risk [28].
However, due to its drought resistance and adaptability to a variety of soil conditions, sorghum has the potential to alleviate extreme food insecurity [29,30]. Its significance is highlighted by its capacity to function admirably in both favorable and unpredictably adverse weather conditions, which are common in the SSA [31]. Sorghum is capable of withstanding spells of flooding [32]. Therefore, the crop could contribute to improved food and income security given the rising trends in climate change and global warming [33,34]. The two most pressing issues facing sub-Saharan Africa (SSA) are water stress and food insecurity, both of which are expected to worsen [35].
The percentage of arid and semi-arid land in the SSA is 43%, and climate change is expected to increase this percentage. In the arid and semi-arid regions of the SSA, small-scale rainfed agriculture is the primary source of income. Over 95% of agricultural land is used for rainfed agriculture, making water scarcity a key production constraint [36]. To meet food demand, crop production, especially sorghum as one of the staple cereal crops, will need to adapt to water scarcity and increase water productivity [37]. In Sub-Saharan Africa’s dry and semi-arid agroecological zones, where a lack of water is a key constraint on cereal production, sorghum, as a drought-tolerant cereal crop, should be included and promoted [38].
Because of its high and constant water-use efficiency, great genetic diversity, comparable nutritional value, and established food value chain in Sub-Saharan Africa (SSA), sorghum production is particularly well suited to these areas [39]. However, in some regions of SSA, sorghum is underutilized both geography and socioeconomically due to either climate and agroecological variability, as well as limitations related to technology, infrastructure, market demand, and dietary preferences. The inclusion and promotion of sorghum in the SSA’s semi-arid and arid regions, particularly among subsistence farmers, can boost food security and water productivity [40]. For developing nations, creating climate-smart cultivars may be one effective strategy for adapting to climate change [29].
The effects of climate change could have a major negative influence on sorghum production in the future if climate-smart cultivars are not adopted. However, this impact can be mitigated, and sorghum yield can be increased by 1 t/ha by substituting the climate-smart ideotypes for the reference cultivar. Under anticipated climatic change conditions, climate-smart cultivars may enhance sorghum grain yield by lengthening the vegetative and reproductive cycles, improving grain filling, increasing the relative leaf size, and optimizing resource allocation to the head [41]. In addition, the development of new sorghum varieties that use water efficiently, the use of drip irrigation systems and the use of sensors to monitor moisture content are some of the climate-smart techniques that can enhance sorghum production [42].
The use of tied ridges, pits, and contour farming has been reported to retain moisture, which supports sorghum growth and final grain yield, thereby ensuring food security, especially in semi-arid areas where sorghum is a major staple food crop [43]. Therefore, there is a need to develop and/or identify sorghum varieties that are demand-driven and tailored to specific region or country, as consumer preferences differ from one location to another.

6. Challenges of Sorghum Breeding

In the context of climate change, one of the biggest challenges remains developing cereal crops that are resistant to drought [44]. While sorghum is a C4 crop with inherent drought tolerance, when in the presence of extreme or severe drought conditions, this crop can be significantly impacted, particularly if initially grown under rainfed conditions. Climate-resilient crop breeding uses high-throughput phenotyping tools in conjunction with effective screening approaches [45]. Plants can adapt to drought stress by undergoing through various morphological, anatomical, and physiological changes. Drought resistance is known to be facilitated by several processes, including osmotic adjustments, maintaining green leaf cover, leaf rolling, stem waxiness, root shape and architecture, transpiration efficiency, and the secretion of soluble solutes. Numerous breeding techniques have been suggested, and one technique might not be effective for all crop species.
Producing plants that are resistant to drought requires recombination, mutation, and molecular breeding techniques [46]. Sorghum production and productivity worldwide are affected by various stresses, notably biotic factors such as diseases, weeds, and insect pests. Anthracnose is caused by the fungal pathogen Colletotrichum sublineolum, Henn. (formally known as C. graminicola [Ces.] G.W. Wilson), grain mold, leaf blight (Setosphaeria turcica), rust (Puccinia purpurea), and covered kernel smut (Sporisorium sorghi) and is one of the most important diseases of sorghum, while stem borers, shoot flies, termites, and birds are important pests of this crop. High temperatures and humidity can lead to epidemic levels of sorghum anthracnose, causing yield losses of up to 67% in vulnerable varieties.
Sorghum exhibits considerable genetic variation, with a large number of landraces having been exploited in breeding for disease resistance and better agronomic performance. However, the technologies for exploiting traits of interest are a limiting factor of breeding demand-driven sorghum varieties, especially in sub-Saharan Africa [47]. Therefore, there is a need to equip breeders with the necessary skills for genotyping new crosses to verify quality and ensure quality assurance before advancing to further stages of evaluation.

7. Recent Advancements in Technologies to Surpass Challenges in Sorghum Breeding

Inter-disciplinary scientists have been using a variety of approaches to understand and dissect the mechanisms of plant tolerance to drought stress and related traits, with a focus on the genotype to speed up the advancement of sorghum breeding programs; however, success has been limited [48].
Sorghum is a self-pollinated crop, with an average cross-pollination range of 5–15% [49]. Breeding techniques for cross-pollination are applied to improve sorghum. Before starting their work, plant breeders set out to define the demand-driven breeding objectives for farmers, millers, bakers, and target habitats. These objectives include considerations about biotic and abiotic stressors, consumer preferences, the processing sector, and the farmers’ production methods. Sorghum breeding in Africa focuses on improving grain protein quality, reducing anti-nutritional factors, enhancing yield and yield stability, and developing resistance to diseases and pests. Additionally, breeders aim for tolerance and resistance to drought, salinity, and high temperatures, earliness, photo- and thermo-insensitivity, wider adaptability, high biomass, and high sugar content, aside from several agronomic trait improvements [50]. For instance, the developments in precision phenotyping, breeding techniques, and modern genomic and genetic tools facilitate a more efficient deciphering of the genes and metabolic pathways conferring drought tolerance in crops [51]. In sorghum, the types of varieties that are intended for development determine the choice of breeding methodology. Breeding techniques relevant to self-pollinated crops are often used in the production of lines and varieties in typically cross-pollinated crops such as sorghum. However, the availability of male sterility sources has enabled breeders to take advantage of heterosis through hybridization, allowing for some degree of cross-pollination [52].
In Africa, and during the 1970s, a population improvement program utilized a first selfing (S1) and subsequent generations obtained by selfing the S1 plant (S2) with the relevant selection techniques, including ms3 and ms7 male-sterile genes, to enhance several broad-based populations, such as US/R, US/B, and Fast Lane [53]. The lines obtained from these populations through head-to-row selection underwent extensive testing in several sites in Asia and Africa, with the objective to select for broad adaptability across a range of characteristics, including red and white grain. Subsequently, a greater emphasis was placed on pedigree and backcross breeding techniques to introduce relatively small gene sets into superior, white-grained backgrounds.
The primary criterion for selection in population, pedigree, and backcross breeding techniques was grain yield [52]. However, starting in the 1980s, breeding for tolerance to different biotic and abiotic challenges was given a lot of weight in each of the regions. Pedigree and backcross techniques were widely used for particular adaptations within each region in the later part of the 1980s. Beginning in 1990, a trait-based pedigree breeding strategy was employed, in which families served as the selection units for resistance response, while individuals within the resistant families served as the selection units for grain yield [54]. Additionally, since 1990, simple mass selection has been employed in Africa and Asia to enhance populations and create trait-based gene pools, such as the ICSP-high tillering population.
To enhance male-sterile lines for particular resistance traits and high grain production, trait-based breeding programs simultaneously tested and backcrossed the selected maintainer plants while selecting for resistance traits and grain yield [55]. In recent studies, the parents and recombinant inbred lines (RILs) obtained from the resistant × susceptible crosses were examined to identify simple sequence repeat (SSR) markers linked to resistance to shoot flies, Striga, and stay-green. Through the advancement of head-to-row generations, RILs were developed. Additionally, Bt-genes and T1 transgenics, which are typically evaluated in greenhouses, have been deployed through genetic transformation to confer stem borer resistance [55].
Different tools have been developed to support sorghum breeding, leading to the development of numerous improved varieties. These tools include machine learning algorithms, gene editing, genomic selection, the rapid generation advancement and recycling of elite sorghum materials, and high-throughput phenotyping tools for sorghum improvement. The aim is to shorten the breeding cycle with high accuracy of information gathered from the new crosses’ materials.

8. Rapid Generation Advancement and Recycling of Elite Sorghum Materials

Sorghum is becoming a model crop for the functional genomics and genetics of tropical grasses, with a wide range of applications, including fuel, feed, and food. Unfortunately, numerous biotic and abiotic stressors have a detrimental effect on agricultural outputs. However, there are various strategies that can be used for the rapid generation advancement and recycling of elite sorghum materials, including marker-assisted breeding, the selection of agronomically useful transgenic plants, hybrid production and the use of diverse, adapted germplasm in breeding as will be discussed below. With marker-assisted breeding, it is possible to develop sorghum varieties that are tolerant to diseases, high yielding, and resilient to changing climate conditions. This type of selection method has reduced the breeding cycles for releasing new climate resilient varieties [56] and is important for exploitation of traits from the donor parents to the improve target traits in the recurrent parents of sorghum [57].

8.1. Genomic Selection

Genomic selection is a tool which uses genome wide markers to predict the genomic estimate of breeding genetic value for identifying favorable traits in crop improvement. This method complements the efficiency of marker-assisted selection (MAS) in crop improvement, which is trait-specific based on QTL mapping at gene loci. GS covers the entire genome thus capturing information on the correlation between the genotypic and phenotypic data for both major and minor gene effects, thereby enhancing the genetic gain in each breeding cycle of sorghum [58]. This tool allows for the ranking of individual genetic values to identify candidates for advancing the next generation of plants.
GS has high accuracy in predicting traits such as drought tolerance, pest, and disease resistance, which in turn increases the rate of genetic gain and has been reported to accelerate crop productivity by reducing the generation interval. Genomic selection has higher efficiency in the selection of phenotypic traits when compared to marker-assisted selection (MAS), the methodology of which is based on GWAS analyses and may not identify [59,60]. The model of genomic selection considers both additive and non-additive genetic variances which account for several genetic components that contribute to the small effect sizes of expressing traits [61]. GS considers effects of all genetic markers which account for complex traits, while MAS is effective for traits which are controlled by few genes based on the Mendelian principle [62]. Furthermore, GS requires a sufficient sample size for the accurate estimation of the association between genotype and phenotype, which increases the selection intensity of the studied population. However, the cost of achieving GS is expensive as it is associated with the high cost of developing the genotyping array and the costs of genotyping a sufficient sample size of individuals for effective selection within the population.
The steps in genomic selection (GS) for sorghum improvement include the collection of germplasm, high-quality reference genome sequences, and the establishment of sorghum association panels for genome-wide association studies of traits prioritized by the market and consumer preferences, as well as those adapted to the current climate change. However, the global focus of sorghum improvement through GS emphasizes food and bioenergy production, the development of mutant lines to accelerate the discovery of genes expressing the desired phenotypes for sorghum improvement, the creation of gene expression atlases, and the use of online databases that integrate tools which facilitate breeding and genomic studies [63].
GS in sorghum uses models that incorporate naturally available germplasm and improved crosses to predict the variations within the population [64,65]. For a successful GS in sorghum, accurateness of genotyping and phenotyping need to be critically maintained in order to generate new improved varieties with distinct traits, uniformity, and stability of the seeds over time [3]. Therefore, the application of GS is suggested for accelerating breeding cycles and increasing genetic gain in sorghum (Table 1), similar to what has been achieved in wheat, maize, and rice crops. The GS model developed by [66] has reported an increase in genetic gain of 33–39% in sorghum.

8.2. High-Throughput Phenotyping Tools for Sorghum Improvement

Plant phenotyping is a tedious and time-consuming process. However, significant advancements have been made in the development of high-throughput phenotyping technology, where thousands of field-grown plants cultivated in a variety of settings can have the exact measurements of the desired features made of them. This is a crucial step in selecting more productive, disease-resistant, and stress-tolerant lines, which speed up crop development programs [75]. Determining the genetic basis of complex features linked to plant growth and development as well as targeted attributes is made easier with the use of high-throughput phenotyping tools and platforms. A vast amount of data has been collected through high-throughput phenotyping (HTP) [76]. An unmanned aerial system (UAS) is used to gather and analyze phenotypic data for the purpose of optimizing and evaluating two main features in grain sorghum, namely grain yield and plant health [77]. Imaging chambers are utilized for measuring the nodal root angle of sorghum, which is important for the architecture of the root system in this crop [78]. It has a significant impact on the mature plant roots’ spatial distribution in the soil profile, which has an effect on drought tolerance in plants [79]. An imaging box is an appropriate phenotyping tool for quickly, non-destructively, and digitally measuring the nodal root angle at the seedling stage. The nodal root angle can therefore be precisely measured from digital photographs using the free program open GelPhoto.tcl [80]. This platform is applied in sorghum breeding initiatives that seek to enhance drought tolerance by modifying the architecture of the root system [46]. Previously, unmanned aerial vehicles (UAVs) have been used to conduct high-throughput phenotyping of sorghum plant height and its response to nitrogen supply [81]. Therefore, UAV remote sensing holds great promise to improve throughput in sorghum improvement [82].

8.3. Gene Editing

Gene editing is a tool of crop improvement which is gaining momentum due to its applicability. Gene editing is among the technologies which speed up the improvement in target traits which are expressed by single or few genes, different from those related to GM crops [83]. The application of CRISPR/Cas in sorghum enables us to understand gene function in relation to the phenotypic expression of the plants, which allows for an easy selection of the best materials [84]. CRISPR/Cas can develop new lines of sorghum by modifying gene sequence with specific traits for the development of new sorghum products. However, this applies to sorghum lines which have a well-known genome and its loci with related traits [84,85]. A number of studies on gene editing for sorghum improvement have been conducted by employing CRISPR/Cas9-mediated gene editing for establishing allelic variants into the elite plant for improvements in nutritional value, yield, resistance to pests and diseases, and tolerance to various abiotic stresses [86,87,88,89,90]. This is contrary to conventional breeding methods where traits from the donor parents are crossed with the recurrent parents and advanced by backcrossing to retain the donor parents’ traits to the crossed materials [83]. Genome-editing studies in sorghum have been carried out through CRISPR/Cas9, specifically focusing on two genes, cinnamyl alcohol dehydrogenase (CAD) and phytoene desaturase (PDS), with the positive achievement of the TX430 sorghum genotype [91]. These achievements were attained after sequencing PCR products of the sorghum genome by genetic engineering through bombardment. For a positive performance of the CRISPR/Cas9 system in sorghum genome editing, an efficient transformation system, the effective expression of CRISPR components including Cas9 and gRNA, and the design of targeted gene sequence for gRNA factors should be taken into consideration [86,92]. The example of genome-editing information in sorghum is presented in Table 2, therefore, gene editing has potential for the exploitation of gene validation and understanding gene function for sorghum improvement to address the challenges facing the crop.

9. Genetic Diversity of Sorghum

Aside from these modern breeding tools and technologies, enhancing genetic diversity for the various climate resilience traits including heat and drought stress into the current sorghum breeding pools is critical. Sorghum is one of the crops with the largest genetic diversity which originated in Ethiopia [98]. The wide range of habitats over which the genus is distributed likely contributes to the diversity of the sorghum genus. Therefore, sorghum’s wild gene pool may contain a large number of beneficial genes for both biotic and abiotic stress tolerance, which have been used as parents to enhance crop diversity [99]. Nearly a quarter of a million sorghum accessions are gathered and preserved by national and international genebanks worldwide. The International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) and the United State Department of Agriculture (USDA) are the two of the largest repositories of sorghum germplasm in the world [100]. Together, they preserve nearly one-third of the global collection of sorghum germplasm. Most accessions have been characterized for the identification of specific traits within accessions for breeding programs [100]. These data are available for researchers to browse and select materials that suit the demands of their projects. Importantly, new sources of variants have been identified for use in sorghum improvement initiatives employing core and mini-core collections, or genotype-based reference sets, which represent the diversity present in the complete germplasm [101].

Use of Genebanks to Support Sorghum Breeding

Sorghum is among the crops which are available in the genebank both in situ and ex-situ and can be accessed by plant breeders for the development of new climate resilience varieties [102]. In addition, sorghum germplasm is conserved in the national agricultural research centers responsible for sorghum breeding programs and farmers in the local areas in most of the developing countries. National Research Institutions (NARIs) in Africa have collaborated with other stakeholders in the world to modernize breeding through sharing knowledge, skills, germplasm, facilities, and capacity building.
Over the past 20 years, significant global policy changes, such as the 2004 International Treaty on Plant Genetic Resources for Food and Agriculture (ITPGRFA) and, more recently, the 2014 Nagoya Protocol on Access and Benefit Sharing (NP)—a follow-up to the 1993 Convention on Biological Diversity (CBD)—have enhanced genetic resource conservation, use, and the sharing of genetic resources across the globe. To make inferences about the possible impacts of the NP, emphasis has been given to understanding how changes in national and international policy related to the CBD and ITPGRFA are correlated, as well as related to the mobility of genetic resources [103]. For seven crops that are crucial to food security in many developing nations, the analysis considered the historical patterns in genebank acquisitions that are crop-specific, how germplasm exchange networks have changed over time, and the correlations between these exchanges. Following the implementation of the CBD in 1993, a substantial downturn in genebank acquisitions, which was followed by a decrease in the size of germplasm exchange networks, was observed [104]. After the ITPGRFA went into effect in 2004, certain crops faced a shift in these trends and patterns. It was also found that in contrast to the ITPGRFA membership, a nation’s membership in the CBD was strongly linked with declines in genetic resource flows. The results indicated that both the CBD and ITPGRFA had a significant effect on these dynamics, despite significant variability among the crops and countries. This suggests that the NP could have an unintentional and harmful impact on global PGRFA flows [105].
As mentioned above, sorghum is well-known for its ability to endure stress and for displaying great genetic diversity that can be used as a source of both biotic and abiotic stress resistance [106,107]. Fortunately, in recent years, a vast array of genetic and genomic resources in sorghum has becoming available in this crop, giving researchers the chance to link sequence variants with desired phenotypic traits and apply these to sorghum improvement initiatives This is an example on how the effective breeding of drought-tolerant sorghum varieties has shown promise through the use of genomic and molecular marker technologies [108,109,110].

10. Increasing Farmer Production Through Sorghum Hybrid Seed Production

Due to the availability of a heritable and stable cytoplasmic male sterility (CMS) mechanism, that permits large-scale, cost-effective hybrid seed production and a sufficiently high magnitude of heterosis over a range of production settings for economic purposes, sorghum has been able to commercially exploit its heterosis [49,111]. Nevertheless, the majority of sorghum farmers in sub–Saharan Africa cultivate local and open pollinated varieties (OPVs) compared to hybrid. The reasons behind this choice include the lack of access to quality seed from agro-dealers at affordable prices, the non-existence of a relatively good established seed system and the lack of possibility to recycle seeds [112]. While hybrid sorghum varieties have a high potential in terms of grain and forage productivity which surpasses that of the OPVs, the global focus of breeding has been on hybrid seed production, where the genetic gains and returns have a higher cost benefit compared to OPV seeds [113,114]. This is also a common trend in maize hybrid seeds, where most farmers in sub–Saharan Africa cultivate hybrid varieties that yield more than twice that of the OPV seeds. The adoption of hybrid sorghum varieties could increase production due to higher heterosis of the parent lines compared to OPVs and local varieties thus contributing to food security [115,116]. Experience of hybrid sorghum production has been reported in developed countries such as United States of America (USA), Australia, and China, where 95% of cultivated area is planted with hybrid varieties and 85% for other countries including India [117]. Farmers are often willing to invest in sorghum hybrid seeds with higher yield potential and consistently more stable market prices compared to OPVs and local seeds. In Tanzania, for instance, there has been a higher uptake of the hybrid sorghum seed PAC 501 compared to OPVs, driven by the high demand from breweries which triggered the adoption. In addition, sorghum farmers in East Africa are willing to pay for improved (both hybrids or OPVs) sorghum varieties that exhibit tolerance to environmental stresses, high yield, early maturing, and fetch higher grain prices, particularly those with white grains [117]. However, small holder farmers still cultivate local sorghum seeds because of the possibility to share local seeds season after season, reliance on subsistence farming, inconsistent market conditions, high prices for improved seeds compared to farmers’ capital, and the low research priority given to sorghum crop by a majority of the African governments [118]. Therefore, efforts to create awareness and promote the increased adoption rate of improved sorghum varieties including hybrids are essential to enhance the production of sorghum in Africa.

11. Conclusions and Future Perspectives

Sorghum is a crop with enormous untapped potential that can be used to reduce the challenge of food and nutrition insecurity in prone areas of sub-Saharan Africa. The application of modern breeding tools, such as marker-assisted selection, genomic selection, genetic diversity, and gene editing, integrated with conventional bereding methods and high-throughput phenotyping is highly emphasized in the current breeding program to accelerate variety development with the traits of stakeholder interest. The tactics used to revive the crop as a traditional high value crop and the expanding sorghum market thus increasing the production. Sorghum may possess qualities that set it apart from other basic grains, suggesting that it could find novel use in processed foods. Breeders should create more dependable hybrid and OPV varieties that not only tolerate rapidly changing climate conditions but also show resistance to pests and diseases across the tropics. To make sorghum palatable to a larger global population than that of today, further research is needed to clarify the health implications of the grain when ingested by demographic groups that have not historically consumed it. Sorghum’s ability to adapt to climate change could be enhanced by incorporating more genetic diversity into the current breeding programs for enhancing climate-resilient seeds.

Author Contributions

Conceptualization, A.M.; Methodology, A.M.; Validation, D.M., E.M., D.S., A.L.G.-O. and C.O.O.; Writing—Original Draft Preparation, A.M. and Z.M.; Writing—Review and Editing, D.M., E.M., D.S., A.L.G.-O. and C.O.O. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by Bill and Melinda Gates Foundation through Accelerated Variety Improvement and Seed Delivery of Legumes and Cereals, funding number-INV-049752-AVISA Transion.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agronomy 14 03025 g001
Figure 1. A graphical representation of top 10 sorghum producers in the world.
Figure 1. A graphical representation of top 10 sorghum producers in the world.
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Table 1. Number of stay-green QTLs identified in the developed sorghum inbred lines.
Table 1. Number of stay-green QTLs identified in the developed sorghum inbred lines.
S/NParent PopulationLine TypePopulation SizeNumber of QTLs IdentifiedReference
1E36-1*N13RILs22621[67]
2SC283*BR007RILs1004[68]
3E36-1*IS9830RILs22619[67]
4TX7000*SC56RILs12514[69]
5TX7000*B35RILs984[70]
6B35*M35-1RILs24543[71]
7IS18551*296BRILs1689[72]
8QL39/QL41RILs1525[73]
9TX430*B35RILs967[74]
Table 2. List of genome-editing studies in sorghum.
Table 2. List of genome-editing studies in sorghum.
Promoter sgRNA/CasNo of gRNADelivery MethodTarget GeneSMEdit Efficiency (%)PhenotypeReference
OsU6/OsAct11AgrobacteriumMDsRED2nptIINRDsRED 2 expression[93]
ZmU6/ZmUbi11AgrobacteriumSb-CENH3nptII37–40NR. Biallelic frameshift mutations potentially lethal[94]
TaU3/ZmUbi1AgrobacteriumK1C gene familynptII92.4Partial opacity in T1 seeds, reduced α-kafirin, improved grain protein digestibility and lysine content[95]
OsU6/ZmUbi12AgrobacteriumSbFTSbGA2ox5bar33.3, 83.3Delayed flowering, No phenotype, Biallelic mutations potentially lethal[96]
OsU6/CaMv35S2AgrobacteriumSbLG1nptII33.3Altered leaf inclination angle, ligule, and auricle size. Distinct phenotypes for WT, Mono allelic and biallelic mutations[97]
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Mwamahonje, A.; Mdindikasi, Z.; Mchau, D.; Mwenda, E.; Sanga, D.; Garcia-Oliveira, A.L.; Ojiewo, C.O. Advances in Sorghum Improvement for Climate Resilience in the Global Arid and Semi-Arid Tropics: A Review. Agronomy 2024, 14, 3025. https://doi.org/10.3390/agronomy14123025

AMA Style

Mwamahonje A, Mdindikasi Z, Mchau D, Mwenda E, Sanga D, Garcia-Oliveira AL, Ojiewo CO. Advances in Sorghum Improvement for Climate Resilience in the Global Arid and Semi-Arid Tropics: A Review. Agronomy. 2024; 14(12):3025. https://doi.org/10.3390/agronomy14123025

Chicago/Turabian Style

Mwamahonje, Andekelile, Zamu Mdindikasi, Devotha Mchau, Emmanuel Mwenda, Daines Sanga, Ana Luísa Garcia-Oliveira, and Chris O. Ojiewo. 2024. "Advances in Sorghum Improvement for Climate Resilience in the Global Arid and Semi-Arid Tropics: A Review" Agronomy 14, no. 12: 3025. https://doi.org/10.3390/agronomy14123025

APA Style

Mwamahonje, A., Mdindikasi, Z., Mchau, D., Mwenda, E., Sanga, D., Garcia-Oliveira, A. L., & Ojiewo, C. O. (2024). Advances in Sorghum Improvement for Climate Resilience in the Global Arid and Semi-Arid Tropics: A Review. Agronomy, 14(12), 3025. https://doi.org/10.3390/agronomy14123025

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