**10. Breeding for carbohydrate content in soybean seeds**

#### **10.1 Sucrose content**

Breeders aim to increase the sucrose content in soybean seeds which contribute to the sweet taste of soy foods, especially for tofu, soy milk, and edamame. The sucrose content in soybeans ranges from 1.5 to 10.2%, and germplasm with even higher content, 13.6%, has been identified [13]. Varieties that target a specific component of the carbohydrate fraction are varieties high in sucrose content and varieties low in oligosaccharides [13]. Compared to conventional soybeans, highsucrose soybeans contain 40% more sucrose but 90% less stachyose and raffinose. High-sucrose soybeans are used to produce tofu, soymilk, beverages, baked goods, puddings, cheese, and meat analogs [13]. The genotypic correlation between sucrose and 100-seed weight is positive and significant, as well as the genotypic correlation of 1000-seed weight with protein. Moreover, the heritability for 1000-seed weight is high. Hence, the breeding program selection on 100-seed weight would result in a good response on relative protein and sucrose content.

#### **10.2 Oligosaccharides content**

Stachyose and raffinose are not readily digestible and cause flatulence when soy foods are consumed. Therefore, breeders aim to develop soybean seeds with reduced oligosaccharide content. Stachyose and raffinose content among soybean germplasm range from 1.4 to 6.7%, and 0.1 to 2.1%, respectively. Breeding lines with less than 1% stachyose and raffinose have been developed [13]. Soybean germplasm "V99–5089" was developed with high sucrose, low raffinose, and low stachyose content to use as a parent in food-grade soybean breeding programs [118]. The genetic variability of seed sugars has significant allelic difference in the genes controlling the biosynthetic enzymes. QTL mapping of soluble sugars in soybean seed were reported and of which 28 were for seed sucrose (**Table 1**). These 28 QTLs were mapped on LGs A1 and E; 3 QTLs on A2, I, and F, and 3 QTLs on L, M, and B1 [73], two QTLs on L, D1b, 7 QTLs on L [74], and B2, D1B, E, H, J [75]. The genomic regions associated with sucrose, raffinose, and stachyose were identified in segregating F2–10 RILs [74].

Alkond et al. [76] reported 14 significant QTLs associated with sucrose and oligosaccharides that were mapped on 8 different linkage groups (LGs) and chromosomes (Chr). Seven QTL were identified for raffinose content on LGs D1a (Chr1), N (Chr3), C2 (Chr6), K (Chr9), B2 (Chr14), and J (Chr16). Four QTL for stachyose content were identified on LG D1a (Chr1), C2 (Chr6), H (Chr12), and B2 (Chr14) [76]. Three QTL for seed sucrose content were identified on LGs N (Chr3), K (Chr9), and E (Chr15). The region of Chr15 (LG E) that has been reported to be associated with sucrose was detected by others [73, 75, 77, 78], but the position of the QTL was different [76]. The two of the regions underlying seed sucrose QTLs identified on LG N (Chr3) and K (Chr9) are additions to the loci previously reported on LGs D1b (Chr2), A1 (Chr5), M (Chr7), A2 (Chr8), B1 (Chr11), H (Chr12), F (Chr13), G (Chr18), J (Chr16), L (Chr19), and I (Chr20) [73, 75, 77, 78]. The selection for beneficial alleles of these QTLs could facilitate breeding strategies to develop soybean lines with higher concentrations of sucrose and lower levels of raffinose and stachyose.
