Keywords

8.1 Sweetpotato and Its Production

Ipomoea, the largest genus in the Convolvulaceae family, encompasses 600–700 species, including I. batatas (L.) Lam. (sweetpotato), which is extensively cultivated worldwide as a food crop (Hirakawa et al. 2015; Austin et al. 2015). Sweetpotato ranks seventh globally among the most valuable food crops, following wheat, rice, maize, potato, barley, and cassava (CIP 2020; https://cipotato.org/sweetpotato/sweetpotato). Cultivated in over 115 countries, sweetpotato boasted an annual production of 91.8 million metric tons in 2022 (FAOSTAT 2022). Asia leads in sweetpotato production, accounting for 82 million tons (81.4%), primarily driven by China, followed by Africa with 17 million tons (15.2%) (FAOSTAT 2022). Notably in Africa, Malawi, Nigeria, United Republic of Tanzania, and Uganda rank among the top most sweetpotato producers after China (FAOSTAT 2022), highlighting its significance as a secondary staple food root crop, alongside cassava, and its substantial role in human diets (van Jaarsveld et al. 2005; Low et al. 2009).

Because of its ability to thrive in nutrient-poor soils with minimal input, sweetpotato is cultivated across diverse agroecological and microclimatic zones, spanning from tropical to temperate climates (Niringiye et al. 2014). Sweetpotato plays multiple roles in the global food system, each of which carries significant implications for meeting food needs, alleviating poverty, and enhancing food security (Low et al. 2017; El-Sheikha and Ray 2017). The roots of sweetpotato possess higher levels of carbohydrates, minerals, and protein compared to other tropical root and tuber crops (Ji et al. 2015). While the protein content of sweetpotato, like most tropical root and tuber crops, is relatively low (around 2%), it surpasses that of cassava and plantain (Woolfe 1992).

8.2 Nutritional and Health Benefits of Sweetpotato

Sweetpotato roots are rich in secondary metabolites that offer significant nutritional benefits and exhibit remarkable sensory versatility, encompassing taste, texture, and flesh color. The flesh color of the roots range from white to cream, yellow, orange, and purple (Fig. 8.1). In sub-Saharan Africa, selective breeding efforts have resulted in the development of varieties characterized by storage roots with high dry matter (>25%) and varying flesh colors, including white, cream, and yellow. These varieties also boast higher starch content and a “mealy firm” texture upon cooking. Starch serves as the primary carbohydrate in sweetpotato storage roots, with the composition, size, and shape of starch granules playing pivotal roles in determining eating quality (Kitahara et al. 2017; Reeve 1967; Lv et al. 2019).

Fig. 8.1
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Sweetpotato flesh color range from white to yellow, orange, and purple. Photo Credit: Mercy Kitavi, 2019

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Sweetpotato cultivars featuring orange flesh color (referred to as orange fleshed; OFSP) are renowned for their rich content of non-digestible dietary fiber, minerals, vitamins, and antioxidants (Neela and Fanta 2019; Dako et al. 2016). Notably, they exhibit high levels of β-carotene but relatively low dry matter content (18–25%). These varieties are typically characterized by a sweet flavor and moist texture post-cooking, making them a popular choice commercially, particularly in the USA (Islam et al. 2016; Grace et al. 2014; Liao et al. 2008).

Purple-fleshed sweetpotatoes, known for their abundance of anthocyanins, are a specialty variety particularly popular in Asia. These sweetpotatoes display an appealing purple-red hue and are characterized by high levels of anthocyanins, total phenols, and antioxidant activity (Steed and Truong 2008; Yoshinaga et al. 1999). Across different sweetpotato varieties, at least 27 anthocyanin pigments have been identified (He et al. 2016; Wang et al. 2018; Lee et al. 2013). Compared to orange-fleshed sweetpotatoes, purple-fleshed varieties exhibit significantly higher anthocyanin content (Kurnianingsih et al. 2020), akin to other anthocyanin-rich crops such as blueberries, blackberries, cranberries, and grapes (Bridgers et al. 2010). Furthermore, purple sweetpotatoes serve as an economical source of natural anthocyanin pigments (Jansen and Flamme 2006). The anthocyanins present in purple-fleshed sweetpotatoes encompass various chemical structures, primarily cyanidins and peonidins; these acylated forms offer heat and light stability properties alongside antioxidant activity (Mu et al. 2021), making them desirable natural pigments for food additives (Odake et al. 1994; Xu et al. 2015). Additionally, purple-fleshed sweetpotato varieties exhibit higher dry matter content (up to 38.96%) compared to OFSP, with a negative correlation observed between anthocyanin levels and water content (Steed and Truong 2008).

8.3 β-Carotene is an Important Vitamin a Source for Humans

Carotenoids play diverse roles in human health, ranging from acting as antioxidants to supporting vision and immune function (Eggersdorfer and Wyss 2018). Dietary compounds with vitamin A activity encompass both preformed all-trans-retinol (referred to here as retinol for simplicity) and retinyl esters, along with provitamin A carotenoids like β-carotene or β-cryptoxanthin (Fraser and Bramley 2004; Krinsky and Johnson 2005; von Lintig 2012; Scott and Ewell 1992). Apart from certain aphids that naturally produce the carotenoid torulene (Moran and Jarvik 2010), animals lack the ability to synthesize these essential nutritional molecules de novo and therefore depend on dietary sources of this vital vitamin (Goodwin 1984).

The primary precursors of vitamin A (VA) in the human body include β-carotene, α-carotene, and β-cryptoxanthin (Arscott and Tanumihardjo 2010). The β-carotene (BC) found in OFSP plays a significant role as a long-term food-based strategy for combating vitamin A deficiency, as evidenced by recent studies in Africa (Low et al. 2017; World Food Prize Foundation 2016). Dietary carotenoids such as α-carotene and β-carotene have beneficial effects on human health, including antioxidant activity, supporting immune function, and reducing the risk of chronic diseases (Eggersdorfer and Wyss 2018; Fiedor and Burda 2014). Furthermore, lutein and zeaxanthin, both carotenoids, serve as macular pigments that aid in protecting the eyes and reducing the risk of age-related macular degeneration and cataracts (Sauer et al. 2019). Many OFSP varieties contain up to 276.98 μg of β-carotene per gram of fresh weight (Low et al. 2007; Kang et al. 2017; Grune et al. 2010). Utilizing OFSP can help improve vitamin A status and enhance the bioavailability of various micronutrients such as iron, zinc, calcium, and magnesium, thereby reducing the risk of vitamin A deficiency (Islam et al. 2016; Vimala et al. 2011; Gurmu et al. 2014). Moreover, β-carotene, as the provitamin A carotenoid with antioxidant properties and the highest vitamin A activity, has been associated with boosting the immune system and reducing the risk of cancer (Fiedor and Burda 2014).

In many developing nations, sweetpotato serves as a secondary staple food, bridging nutritional gaps and bolstering the intake of essential vitamins and minerals, particularly in combating vitamin A deficiency among children, pregnant women, and lactating mothers (Han et al. 2022; Low and Thiele 2020). Orange-fleshed sweetpotato varieties, rich in β-carotene, have proven successful in providing provitamin A biofortification in sub-Saharan Africa (SSA) (Low et al. 2009, 2017; Neela and Fanta 2019). However, sweetpotato cultivation in SSA has traditionally centered on varieties preferred for their high starch content, such as white and yellow-fleshed types, which, despite their high dry matter content, offer lower nutritional value (Low et al. 2017). These varieties have been specifically chosen for their elevated starch levels. Moreover, local consumers favor starchy sweetpotato varieties for their distinct textural attributes after cooking (Jenkins et al. 2018), influenced by factors like texture and sweetness, which are contingent on the composition and quantity of carbohydrates, including cellulose, hemicellulose, pectin, starch, and sugars (Reeve 1967).

8.4 Carotenoid Biosynthetic Pathway in Higher Plants

Carotenoids are tetraterpene pigments that derive their name from the carrot (Daucus carota), a plant renowned for accumulating high levels of these pigments in its roots. While carotenoids commonly impart color to flowers, fruits, and seeds in plants (Hirschberg 2001), their accumulation in underground organs like tubers and roots represents an exception. Primarily, carotenoids serve in light-harvesting processes by safeguarding the plant’s photosynthetic machinery against photo-oxidative damage (Zakar et al. 2016). Nature consists of hundreds of carotenoid structures, broadly classified into carotenes (hydrocarbons capable of cyclization at one or both ends of the molecule) and xanthophylls (oxygenated derivatives of carotenes) (Ruiz-Sola and Rodríguez-Concepción 2012). Carotenes, predominantly β-carotene, abound in the photosystem reaction centers, while xanthophylls are most prevalent in the light-harvesting complexes, (Davison et al. 2002; Pogson et al. 1998).

The carotenoid biosynthetic pathway (Fig. 8.2a) has been extensively elucidated in numerous plant species, including Arabidopsis (Arabidopsis thaliana) (Ruiz-Sola and Rodríguez-Concepción 2012), tomato (Solanum lycopersicum) (Bramley 2002), maize (Zea mays) (Vallabhaneni and Wurtzel 2009), and rice (Oryza sativa) (Beyer et al. 2002). Carotenoids in higher plants are synthesized through the condensation of geranylgeranyl pyrophosphate (GGPP) from the methylerythritol 4-phosphate (MEP) pathway into phytoene (Auldridge et al. 2006). This initial committed step, catalyzed by the enzyme phytoene synthase (PSY), is considered the principal bottleneck in the carotenoid pathway (Cazzonelli and Pogson 2010; Sandmann et al. 2006). Subsequently, through desaturation and isomerization processes involving enzymes like phytoene desaturase (PDS), 15-cis-ζ-carotene isomerase (Z-ISO), ζ-carotene desaturase (ZDS), and carotenoid isomerase (CRTISO), the plant carotenoid backbone is synthesized (Britton 1995), ultimately leading to the formation of the linear carotenoid lycopene, which imparts a red color (Burton and Ingold 1984; Ruiz-Sola and Rodríguez-Concepción 2012). The first divergence in the pathway occurs when lycopene undergoes cyclization, catalyzed by lycopene β-cyclase (LCY-β) and/or lycopene ε-cyclase (LCY-ε), resulting in the production of orange α-carotene and β-carotene, representing the α- and β-branches of the pathway, respectively. α-carotenoids possess one ß ring and one ε ring (α-carotene), whereas ß-carotenoids feature two ß rings (Chen et al. 2010). α-Carotene, β-carotene, and β-cryptoxanthin are considered provitamin A carotenoids, as they can be converted by the body into retinol. Further modifications of carotenes and xanthophylls lead to the synthesis of various species-specific carotenoids (Giuliano 2014, 2017). Notably, lutein, zeaxanthin, and lycopene are non-provitamin A carotenoids and cannot be converted to retinol.

Fig. 8.2
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Copyright 2018 by Creative Commons CC BY Reprinted with permission

a Carotenoid biosynthesis and degradation pathways. Multi-step reactions are shown by dashed arrows and red arrows indicate degradation reactions. Genes in red rectangles were those found with significantly enriched SNPs at p < 0.005 (Fisher’s exact test). PSY, phytoene synthase; PDS, phytoene desaturase; Z-ISO, ζ-carotene isomerase; ZDS, ζ-carotene desaturase; CRTISO, carotenoid isomerase; LCYE, lycopene ε-cyclase; LCYB, lycopene β-cyclase; β-OHase, β-ring hydroxylase; CYP, cytochrome P450; VDE, violaxanthin de-epoxidase; ZEP, zeaxanthin epoxidase; CCD, carotenoid cleavage dioxygenase. b Allele frequencies (p = 0.005; dotted lines and p = 0.05; solid lines) for putative carotenoid biosynthesis loci between orange and white-fleshed accessions. SNPs from the same gene in each chromosome are indicated by a different color. c Expression profiles of genes involved in carotenoid biosynthesis in different types of roots of orange-fleshed cultivar, “Beauregard” during development. Reprinted from “Genome sequences of two diploid wild relatives of cultivated sweetpotato reveal targets for genetic improvement” Wu et al. (2018).

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8.5 Functional Identification of Carotenoid Biosynthesis Genes Controlling Carotenoid and Starch Content in Sweetpotato

Access to genome sequences of a wild diploid sweetpotato relative, Ipomoea trifida along with other Ipomoea genomes (https://ipomoea-genome.org/) (Wu et al. 2018; Wadl et al. 2018; da Silva Pereira et al. 2020), has facilitated the understanding of the genetic architecture and identification of genes involved in starch and carotenoid biosynthesis (Gemenet et al. 2020). Varietal flesh color is correlated with the amount of β-carotene content with OFSP having the highest amount of β-carotene (Gemenet et al. 2020). Using a diversity panel of orange- and white-fleshed sweetpotato, possible loci (single nucleotide polymorphisms) involved in the accumulation of β-carotene in OFSP were identified in key carotenoid biosynthetic genes [phytoene synthase (PSY; itf03g05110), phytoene desaturase (PDS; itf11g08190) and ζ-carotene isomerase (Z-ISO; itf04g12320) (Fig. 8.2b)]. These loci can be used as targets for marker-assisted selection of crosses for beta carotene. A comparison on root types (Fig. 8.2c) showed an upregulation of the PSY only in the orange-fleshed storage roots suggesting its involvement in conferring the orange color in OFSP storage roots (Gemenet et al. 2020). An expression bias in the PSY “orange” alleles was observed in OFSP during the later stages of storage root development, suggesting a correlation with carotenoid accumulation (Gemenet et al. 2020). Likewise, in the initial month of growth, young carrot roots appear pale but gradually accumulate carotenoids, reaching peak levels around three months later, just prior to the completion of secondary growth (Baranska et al. 2006).

New linkage and quantitative trait loci (QTL) mapping methods for polyploids (Mollinari et al. 2020; da Silva Pereira et al. 2020) have aided in identification of a QTL (Gemenet et al. 2020) unraveling the genetic basis for the negative association between β-carotene and starch. Carotenoid accumulation was observed in both storage and fibrous roots of OFSP, whereas it was not reported in white-fleshed varieties (Gemenet et al. 2020). A major QTL co-localized on LG3 and LG12 of the integrated genetic map explained variation in dry matter (DM), starch content, β-carotene levels, and flesh color (FC) (Fig. 8.3). Both parental lines contributed major alleles with comparable effects on traits located on LG12, while only the OFSP parent, Beauregard, exhibited significant allelic effects on traits at the LG3 QTL. Additionally, dry matter (DM), starch, β-carotene (BC), and flesh color (FC) traits are influenced by additive allele effects. The same contributing haplotypes responsible for reducing DM and starch content were also associated with an increase in BC and FC, elucidating the observed negative correlation between starch and BC in sweetpotato, a phenomenon akin to that observed in cassava (Rabbi et al. 2017).

Fig. 8.3
figure 3

Copyright 2020 by Creative Commons CC BY. Reprinted with permission

Quantitative trait loci (QTL) profiles as LOP =  − log10(p) for β-carotene (BC), dry matter (DM), starch, and flesh color (FC in Peru and Uganda; denoted by P and U, respectively) along the genetic map of a biparental (Beauregard × Tanzania) hexaploid sweetpotato population. QTL location peaks are marked by triangles. Reprinted from “Quantitative trait loci and differential gene expression analyses reveal the genetic basis for negatively associated β-carotene and starch content in hexaploid sweetpotato [Ipomoea batatas (L.) Lam.]” Gemenet et al. (2020).

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The contrasting traits exhibited by the two parents suggest that the interaction of alleles between the two QTLs dictates the presence or absence of β-carotene accumulation in sweetpotato storage roots. Additionally, the rate-limiting PSY gene (itf03g05110) in carotenoid biosynthesis was found to be positioned between the two co-localized QTL peaks on LG3, specifically at 2,994,719 bp (associated with β-carotene and flesh color) and 3,185,578 bp (linked with dry matter and starch) in I. trifida. The starch gene sucrose synthase (SuSY; itf03g05100) exhibited early expression during root initiation (10–20 days after transplanting) in both white and orange-fleshed cultivars. However, it continued to be expressed in white-fleshed sweetpotato beyond 50 days after transplanting. Moreover, the PSY gene and SuSY genes are situated within a 12.2 kb region with no intervening genes.

The orange (OR) protein plays a pivotal role in regulating carotenoid accumulation through post-transcriptional regulation of PSY, facilitating the formation of carotenoid-sequestering structures, and preventing carotenoid degradation (Chayut et al. 2017; Zhou et al. 2015). Intriguingly, the Or gene (itf12g24270), situated 5.7 kb from the LG12 QTL peak, has been linked with β-carotene accumulation in sweetpotato (Gemenet et al. 2020). Evidence suggests that Or modulates PSY, enabling the transformation of amyloplasts into chromoplasts in various crops such as cauliflower (Lu et al. 2006), tomato (Yazdani et al. 2019), Arabidopsis (Bai et al. 2016), corn (Berman et al. 2017), melon (Tzuri et al. 2015), and sweetpotato (Kim et al. 2013; Park et al. 2015), including purple-fleshed sweetpotato cultivars. Post-translational mechanisms governing OR and PSY protein stability contribute to increased carotenoid levels in yellow cassava (Jaramillo et al. 2022).

8.6 Breeding for β-Carotene Content and Future Directions

Although research has prioritized the fortification of sweetpotato with provitamin A carotenoids (Low and Thiele 2020; Low et al. 2017), the mechanisms underlying carotenoid accumulation in underground storage roots remain poorly understood across various crops, including sweetpotato (Carvalho et al. 2016). Carotenoids are synthesized within various plastids, including proplastids, amyloplasts, etioplasts, chloroplasts, and chromoplasts (Jarvis and López-Juez 2013). Among these, all plastids except proplastids play crucial roles in regulating carotenogenic activity, carotenoid stability, and pigment diversity (Li et al. 2016). Amyloplasts are predominantly found in starchy organs such as wheat, rice, barley, and maize seeds, as well as potato tubers and cassava roots (Jarvis and López-Juez 2013). They primarily synthesize and accumulate carotenoids, particularly xanthophylls like lutein, zeaxanthin, and violaxanthin (Sun et al. 2018; Wurtzel et al. 2012; Wurtzel 2019). Several factors, including biosynthetic capacity, plastid ultrastructure, and metabolic channeling, may limit carotenoid biosynthesis and accumulation in amyloplasts. In contrast, chromoplasts exhibit superior capabilities for carotenoid sequestration and storage by forming carotenoid-lipoprotein sequestering substructures (Li and Yuan 2013). These substructures are proposed to act as a sink for sequestering excess carotenoids, ensuring stable storage and preventing an overload of the carotenoid biosynthetic pathway products.

Among root and tuber crops, sweetpotato distinguishes itself by its ability to induce the transformation of amyloplasts into crystalline-type carotenoid sequestration substructures known as amylochromoplasts (Drapal et al. 2022). This process alters both the capacity for carotenoid storage and biosynthesis, leading to increased accumulation of β-carotene (Zhang et al. 2014; Drapal et al. 2022). The observation of a mutually exclusive relationship between carotenoid accumulation and starch granule development in tobacco floral nectaries and carrot roots implies that increased carotenogenesis may act as a developmental cue, guiding the transition from amyloplasts to chromoplasts (Kim et al. 2010; Horner et al. 2007).

Moreover, alterations in chromoplast morphology observed in plants engineered for enhanced carotenoid production suggest an adaptation of cellular structures to facilitate the sequestration of newly synthesized carotenoids (Horner et al. 2007; Kim et al. 2010). However, the storage of carotenoids in modified amyloplasts leads to competition for carbon resources between starch and carotenoid biosynthesis, resulting in a negative correlation, as observed in sweetpotato (Gemenet et al. 2020; Yada et al. 2017). Similar phenomena have been documented in other crops, including citrus (Cao et al. 2015), potato (Mortimer et al. 2016; Fernandez-Orozco et al. 2013), and cassava (Olayide et al. 2020).

Biofortification of sweetpotato landraces has been a continuous process leading to improved carotenoid content attributable to the adoption of advanced breeding techniques including the screening of large numbers of genotypes for nutritional quality, agronomic traits, yield traits, and the selection of progenies with the optimal traits for further breeding (Yada et al. 2017; Gemenet et al. 2020). Efforts to biofortify sweetpotato have focused on increasing β-carotene content and improving organoleptic qualities of commonly consumed varieties. Replacement of white-fleshed sweetpotato with orange-fleshed varieties has benefited ∼50 million children <6 years of age at risk of VA deficiency (Low et al. 2017; van Jaarsveld et al. 2005). Furthermore, OFSP clones are being selected for other health traits such as increased Fe and Zn. Nonetheless, the negative starch/β-carotene correlation and the yet undefined textural characteristics has limited the actual adoption of improved orange-fleshed varieties. Therefore, comprehensive analysis of the genetic architecture of the negative association between starch and β-carotene could aid in advances of breaking this linkage as an important objective of breeding programs targeting sweetpotato for food and nutritional security.

Marker technology, including the utilization of SNPs and the determination of allele dosage, can be inferred through polyploid genotype calling methods (Pereira et al. 2018; Zych et al. 2019). Understanding the distribution of dosage-dependent key genes and alleles associated with high β-carotene content in cultivated sweetpotato can facilitate more efficient improvement of the crop. Currently, breeders can employ marker-assisted selection (MAS). For instance, findings from a dosage study in maize revealed a consistent increase in the concentrations of lutein, zeaxanthin, β-cryptoxanthin, and total carotenoids with the addition of each dominant Y1 allele to the endosperm, with the highest concentration observed at three doses (Egesel et al. 2003). MAS during the seedling stage is not only cost-effective (Slater et al. 2013) but also presents an attractive option for addressing recessive alleles, such as combining homozygous zeaxanthin epoxidase (zep) with dominant β-carotene hydroxylase to produce orange-fleshed tubers with significant zeaxanthin content in tetraploid potato (Wolters et al. 2010).

With a hexaploid genome, the breeding of β-carotene in sweetpotato can be evaluated using more advanced technologies. Strategies for metabolic engineering of the carotenoid pathway to increase β-carotene or enhance total carotenoid accumulation have been successfully implemented in plants (Bhatia and Ye 2012; Giuliano 2014). Three distinct and complementary strategies have been employed to enhance β-carotene accumulation in plants: overexpression of biosynthetic gene(s) (“push”), blocking the α-carotene branch pathway that competes with β-carotene biosynthesis and/or inhibiting the conversion of β-carotene to downstream products (“block”), and creating a sink for β-carotene accumulation by modulating the formation of chromoplasts or other carotenoid-sequestering structures (“sink”). Additionally, CRISPR/Cas-based gene editing can be utilized to fix desirable allelic variants, generate novel alleles, disrupt deleterious genetic linkages such as the beta carotene and starch, support pre-breeding efforts, and facilitate the introgression of favorable loci into elite lines.