R genes are a key component of genetic interactions between plant and biotrophic bacteria and they regulate resistance against bacterial invasion. The most common R proteins contain a nucleotide-binding site and leucine-rich (NBS-LRR) domain. Some NBS-LRR genes in the soybean genome have also been reported to function in disease resistance. Here, we show the correlation between the number of NBS-LRR genes and the number of disease resistance quantitative trait locus (QTL) on each chromosome of soybean. We also surveyed the functional redundancy of disease resistance on recently duplicated regions known to harbor NBS-LRR genes. Moreover, we analyzed NBS-LRR gene expression in the bacterial leaf pustule (BLP)-induced soybean transcriptome.
A total of 319 genes were determined to be putative NBS-LRR genes in the soybean genome and the number of NBS-LRR genes on each chromosome was highly correlated with the number of disease resistance QTL on each chromosome. In addition, the recently duplicated regions contained duplicated NBS-LRR genes and duplicated disease resistance QTL. These recently duplicated regions possessed either an uneven or even number of NBS-LRR genes for each pair of regions. By supporting the disease resistance functions of the NBS-LRR genes in BLP-induced transcriptome, significant differences in the expression of NBS-LRR genes between resistant near isogenic line (NIL) and susceptible NIL supports the disease resistance function of NBS-LRR genes induced by BLP.
The correlation between the number of NBS-LRR genes and disease resistance QTL for each chromosome and for each recently duplicated region indicates that NBS-LRR genes have a disease resistance function in soybean. In addition, NBS-LRR gene expression was significantly different in the BLP-resistant NIL compared to the BLP-susceptible NIL. Moreover, the recently duplicated regions, which have each undergone a different duplication history with respect to the NBS-LRR gene content, showed putative functional diversification of the genes based on different QTL information on the duplicated regions. We concluded that NBS-LRR genes in soybean confer disease resistance in a direct and indirect way and also presented evolutional models of these genes. Therefore, NBS-LRR genes could be used to develop markers for resistance against diverse diseases in order to develop an elite cultivar with marker assisted breeding.