The soybean resistance gene Rag1 does not protect against soybean cyst and root-knot nematodes.

The soybean cyst nematode (SCN), Heterodera glycines, is the main soybean (Glycine max) pest in the United States (3); in addition, root-knot nematodes (RKN), Meloidogyne spp., also cause substantial yield losses. Currently, the most feasible option for nematode management is the use of resistant soybean cultivars; however, only a few different sources of resistance to SCN have been identified and introduced in commercial cultivars (3). Another important pest of soybean is the soybean aphid (SBA), Aphis glycines. SBA causes decreases in yield that can reach up to 40%. Aphid management relies on the use of insecticides, although resistant soybean cultivars are currently being tested. The aphid resistance present in the soybean cultivar Dowling is the best characterized (1) and is controlled by Rag1, a single dominant gene that produces a strong antibiosis-type resistance. This gene may belong to the NBS-LRR type of resistance genes (2), but it has not yet been cloned. The only well-characterized gene controlling aphid resistance is Mi-1.2, a tomato gene that confers resistance not only to aphids but also to RKN (4). Mi1.2 belongs to the NBS-LRR family and functions as a single dominant gene. Based on these similarities between Mi-1.2 and Rag1, we hypothesized that Rag1 could also provide resistance to nematodes. Thus, we evaluated two related soybean lines, LD16060 and SD01-76R (provided by Dr. Brian Diers, University of Illinois), that carry the Rag1 gene or no resistance gene, respectively, for susceptibility or resistance to SCN and RKN. For SCN, the soybean cultivars were planted in soil infested with SCN. The soybean cultivar Jack was used as a nematode-resistant control and the cultivar Kenwood 94 was a susceptible control. The number of SCN females that developed on SD01-76R and LD16060 were not significantly different (Fig. 1A), indicating that Rag1 does not affect susceptibility to SCN. A similar experiment was conducted with RKN. Two tomato cultivars, susceptible or resistant to RKN, were used as controls to verify that RKN infection was occurring normally. The tomatoes and the two soybean lines, with or without Rag1, were grown in soil infested with Meloidogynes incognita. There was no significant difference in the number of galls on roots (Fig. 1B) of the soybean plants with or without Rag1.