**13. Breeding for reduced cadmium content in soybean**

Based on the importance of soybean as a staple food crop, the development of low Cd soybean cultivars should be a priority. The genetic variability for Cd accumulation within a species provides an opportunity to select soybean genotypes with low Cd

**67**

*Food Grade Soybean Breeding, Current Status and Future Directions*

concentration. In soybean grain, Cd concentration was found to be controlled by a single gene, with low Cd dominant in the crosses studied [134]. Lines with the low Cd trait had restricted root-to-shoot translocation, which limited the Cd accumulation in the grain. Genetic variability in soybean [19, 135] has been reported. An understanding of genetics and heritability of the Cd accumulation is essential in designing the breeding strategy to incorporate gene(s) controlling low Cd accumulation in modern cultivars. However, identifying low Cd phenotypes by analysis of the grain is challenging due to the high cost of analysis [136]. Developing inexpensive methods would assist in transferring the low Cd accumulation traits with other desirable traits.

**14. Developing markers for marker-assisted selection of low Cd** 

concentration of 166 RILs ranged from 0.067 to 0.898 mg kg<sup>−</sup><sup>1</sup>

tive adagio-like protein, and plasma membrane H+

vicinity. The presence of protein kinase and plasma membrane H+

utilized in MAS for developing soybean cultivars with low Cd content.

Marker-assisted selection (MAS) could be an alternative to phenotypic selection. In soybean, DNA markers linked to low Cd accumulation were identified using RIL population (*F*6:8) derived from the cross AC Hime (high Cd accumulation in seeds) and Westag-97 (low Cd accumulation in seeds). The distribution of Cd

(SSR) markers, SatK138, SatK139, SatK140 (0.5 cM), SatK147, SacK149, SaatK150, and SattK152 (0.3 cM), were reported to be linked to *Cda1* in soybean seed. It was also reported that all the linked markers were mapped to the same linkage group (LG) K. SSR markers closely linked to *Cda*1 in soybean seeds have the potential to be used for MAS to develop low Cd-accumulating cultivars in a breeding program [134]. In a similar mapping approach, Benitez et al. [137] identified a major QTL cd1 on chromosome 9 (LG-K) across years and generations which accounted for 82, 57, and 75% of the genetic variation. Near-isogenic lines (NILs) were used to confirm the effect of the QTL and the peak of the QTL that was located in the vicinity of two SSR markers, Gm09:4770663 and Gm09:4790483. Both the studies revealed a major QTL for seed *Cd* content, *Cda*1 at a similar genomic location, suggesting that *cd*1 and *Cda1* may be identical. Candidate genes related to heavy metal transport or homeostasis were located in the vicinity of the identified QTL (Cda1). Protein kinase, puta-

the tightly linked SSR markers suggests that the regulation of this enzyme may play a vital role in Cd stress [134]. This was later supported by a major QTL-controlling Cd concentration (*cd*1) identified in soybean [137]. The gene was designated as *GmHMA1.* In *GmHMA1a*, one base substitution from G to A at nucleotide position 2095 resulted in a loss of function of the ATPase and was found to be associated with Cd uptake [137]. The SSR markers linked to the *Cda*1 and*Cd*1gene(s)/or QTLs and the SNP marker in the P1B-ATPase metal ion transporter gene in soybean can be

Breeding for soybean seed composition traits is a complicated process; fortunately, ample genomic resources and tools are now available to soybean breeders/ researchers for dissection of seed composition traits. The combination of conventional breeding strategy and genomic approaches will help to identify genomic loci, haplotypes, and FMs in breeding for improvement of seed composition traits. For improvement of protein, the major protein QTL, which was repeatedly mapped

[134]. Using the RIL population, seven simple sequence repeat

, with a mean of



*DOI: http://dx.doi.org/10.5772/intechopen.92069*

**accumulation**

0.268 ± 0.013 mg kg<sup>−</sup><sup>1</sup>

**15. Future directions**

*Food Grade Soybean Breeding, Current Status and Future Directions DOI: http://dx.doi.org/10.5772/intechopen.92069*

*Legume Crops - Prospects, Production and Uses*

acid content are preferred [13].

of genetic engineering, 18:1 levels of about 80% total lipid have been achieved [42]. In general, soybean varieties with unique fatty acid composition such as high oleic acid content, high stearic acid content, low linolenic acid content, or low palmitic

Assessment of agronomic traits has been used to evaluate phenotypic diversity in 20,570 Chinese soybean accessions and it was reported that seed coat color had the highest diversity index among the qualitative traits [126]. Plant's height had the most variation among quantitative traits, and followed by seed size, protein content, growth period, and oil content. The seed size of those accessions ranged from smaller than 2 to as large as 46 g/100-seeds. The protein content ranged from 30 to 53%; and oil content ranged from 10 to 25%. The variances of seed size, protein content, and oil content of the U.S. cultivars were lower than the Chinese cultivars [127]. The Southern U.S. soybeans were more variable in oil and protein contents and less variable in seed size than the Northern U.S. soybeans. The food-grade soybean breeding aims to increase the nutritional content and quality of protein and oil [128]. Greater genetic diversity of protein content, seed hardness, calcium content, and seed size uniformity than other quality traits in both small and large-seeded genotypes were evaluated [128]. The U.S. soybean genotypes with small seed were more diverse and exhibited higher swell ratio and oil content but lower stone seed ratio and protein content than the Asian accessions [128]. Among the large-seeded accessions, the U.S. genotypes had higher stone seed ratio and oil content but lower swell ratio and protein content, and were less diverse than the Asian genotypes [128]. The characterization of diverse food grade

soybeans will facilitate parent selection in specialty soybean breeding [1].

Soybean germplasm PI542044, also known as Kunitz soybean, contains the null allele of KTI, i.e., kti that encodes a truncated protein and it was developed in a backcross program involving Williams 82 and PI157440 [129]. Introgression of kti is complicated by a number of factors viz., (i) kti being recessive in inheritance, each conventional backcross generation would be requiring selfing followed by estimation of KTI content in the seeds so as to identify a target plant. However, three recessive null alleles, viz. Kunitz trypsin inhibitor, soybean agglutinin, and P34 allergen null were stacked in the background of "Williams 82" and were termed as "Triple Null" [130]. Three SSR markers, viz. Satt228, Satt409, and Satt429 have been reported to be closely linked (0–10 cM) with the null allele of Kunitz trypsin inhibitor [131]. These SSR markers was also validated in the mapping population generated using Indian soybean genotypes as the recipient parent (*TiTi*) and PI542044 (*titi*) as the donor for the null allele [132]. Further, a gene-specific marker has also been designed from the null allele of KTI from genotype PI157440 [15] and has been deployed in the selection of plants carrying the null allele of KTI derived from PI542044 [121]. The null allele of KTI from PI542044 was introgressed into the cultivar "JS97–52" (recurrent parent) through marker-assisted backcrossing using the SSR marker Satt228, tightly linked with a trypsin inhibitor *Ti* locus. An introgressed line JS97–52 with reduced trypsin

**12. Breeding for reduced trypsin inhibitor**

inhibitor (68.8–83.5%) content was developed [133].

**13. Breeding for reduced cadmium content in soybean**

Based on the importance of soybean as a staple food crop, the development of low Cd soybean cultivars should be a priority. The genetic variability for Cd accumulation within a species provides an opportunity to select soybean genotypes with low Cd

**66**

concentration. In soybean grain, Cd concentration was found to be controlled by a single gene, with low Cd dominant in the crosses studied [134]. Lines with the low Cd trait had restricted root-to-shoot translocation, which limited the Cd accumulation in the grain. Genetic variability in soybean [19, 135] has been reported. An understanding of genetics and heritability of the Cd accumulation is essential in designing the breeding strategy to incorporate gene(s) controlling low Cd accumulation in modern cultivars. However, identifying low Cd phenotypes by analysis of the grain is challenging due to the high cost of analysis [136]. Developing inexpensive methods would assist in transferring the low Cd accumulation traits with other desirable traits.
