Soybean cyst nematode (SCN, Heterodera glycines) is a devastating pathogen in soybean‐producing regions globally. SCN resistance in soybean is a multigenic and quantitative trait. In the presence of strong resistance genes, the minor genes are often covered and cannot be detected. The advanced generations of soybean chromosome segment substitution lines (CSSLs) carrying a unique recurrent parent background represent a permanent population and provide ideal resources for crop improvement and quantitative trait loci (QTL) mapping, especially for minor QTL mapping.
Currently, soybean response to SCN is evaluated only with female index (FI) associated with cyst number per plant but not associated with root weight. However, the plants carrying same FI value but with small size of root weight provide less breeding value when compared with big size of root. The great difference in root weight between two parental lines for CSSLs make it suitable for mapping cysts per gram root (CGR).
A research team led by Professor WANG Congli from the Northeast Institute of Geography and Agroecology of the Chinese Academy of Sciences, in collaboration with the research team led by Professor CHEN Qingshan from the Northeast Agricultural University, for the first time investigated SCN transgressive resistance mechanism in soybean using CSSLs.
The researchers demonstrated a wide range of distribution and transgressive lines in CSSLs for FI and CGR through phenotypic screening.The association between FI and CGR was very low, indicating different genes controlling FI and CGR. Whole-genome resequencing of CSSLs displayed 3766 hyplotype blocks containing 580,524 SNP (single nucleotide polymorphism) markers.
Following the single marker analysis nonparametric and multiple-QTL model (MQM) mapping tests revealed 38 QTLs contributing 5.6-36.2% phenotypic variance to FI and CGR on 20 chromosomes. Gene interactions from both susceptible parents inhibit nematode reproduction which produces transgressive resistance, indicating multiple minor effect QTLs.
Therefore, both FI and CGR should be considered for breeding purposes in the absence of strong resistance genes and the identified CSSLs with low FI and CGR might be potential candidates for replacement of susceptible varieties when resistant varieties are not available.
The findings not only directly confirm the presence of multiple minor QTLs in the soybean accessions either through GWAS or biparental population analysis, even in the susceptible genotype, and also indirectly explain the wide range of phenotypic variation among soybean accessions even carrying the same major resistance genes rhg1 and Rhg4.
Further, the study represents the first time for SCN evaluation with CGR, and the first to use CSSLs to evaluate SCN FI and CGR, and provides a new way to explore transgressive resistance sources from the positive allele combination from susceptible parents.
This work has been published as an article entitled “Transgressive resistance to Heterodera glycines in chromosome segment substitution lines derived from susceptible soybean parents” in Plant Genome.
This work was supported by the National Natural Science Foundation of China and funding from Heilongjiang Province.