Soybean’s wild relative,
G. soja, has been studied mainly to understand soybean domestication, but its high genetic diversity is known to contain desirable traits for crop improvement, including SCN resistance
[46]. GWAS was conducted on 1032
Glycine soja accessions in order to have a better understanding of wild soybean resistance against SCN
[47]. Ten SNPs significantly associated with resistance to SCN were found on chromosomes 2, 4, 9, 16 and 18, three of which were previously identified, but none of which were among the
rhg1 or
Rhg4 QTLs. These regions contained 83 gene models, and some were compatible with plant resistance against disease including: calcium-dependent phospholipid-binding protein, NB-ARC domains containing protein, LRR protein, cytochrome P450, and ethylene-responsive element binding factor. One specific gene, Glyma.18G102600, an NB-ARC domain containing protein, located in a strong linkage disequilibrium block on Chr. 18 seemed highly promising. A transcriptomics database of the response of resistant and susceptible
G. soja accessions to SCN was also created
[48]. Another GWAS on
G. soja lines identified SNPs on chromosomes 18 and 19 as being significantly associated with resistance to SCN (HG 2.5.7), as well as identified 58 gene candidates
[49]. From these, 16 were related to disease resistance, encoding LRR proteins, ring/U-box, receptor-like protein, and MYB transcription factor. Other authors compared transcript expression of the resistant
G. soja line NRS100 to the well-known
G. max Williams 82 (susceptible) and Peking (resistant). The resistant
G. soja (NRS100) did not show any significant differential expression at SHMT, SNAP paralog or SNAP18 which are found in
rhg1 and
Rhg4. The proposed defense mechanism in NRS100 included reduced JA signalling which allowed SA signals to induce a defense response, along with increased polyamine metabolism triggering H
2O
2 regulation and induction of PR proteins which defend the integrity of the cell walls and hinder pathogen invasion
[50]. Finally, a cross between
G. max and
G. soja along with chromosome segment substitution lines (CSSLs) for QTL mapping of SCN resistance was performed
[51]. Thirty-three QTLs were detected on 18 different chromosomes with high significance in relation to SCN resistance. The CSSLs combining positive alleles were highly resistant to SCN in absence of
rhg1 and
Rhg4. These studies shed light on the importance of
G. soja germplasm and new strategies for resistance breeding.