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© 2009 Plant Management Network.
Accepted for publication 4 March 2009. Published 1 April 2009.


The Soybean Resistance Gene Rag1 Does Not Protect Against Soybean Cyst and Root-knot Nematodes


Matthew Studham and Gustavo C. MacIntosh, Biochemistry, Biophysics and Molecular Biology, Felicitas Avendaño, David Soh, and Gregory L. Tylka, Plant Pathology, Iowa State University, Ames, IA 50011


Corresponding author: Gustavo MacIntosh.  gustavo@iastate.edu


Studham, M., MacIntosh, G. C., Avendaño, F., Soh, D., and Tylka G. L. 2009. The soybean resistance gene Rag1 does not protect against soybean cyst and root-knot nematodes. Online. Plant Health Progress doi:10.1094/PHP-2009-0401-01-BR.


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). Mi-1.2 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.


   

 

Fig. 1. Effect of Rag1 on nematode infections. (A) Two related soybean lines, LD16060 and SD01-76R, that carry the Rag1 gene or no resistance gene respectively, were sowed in soil infested with the soybean cyst nematode (SCN). Plants were collected after 45 days, and SCN females were removed from roots and counted. The soybean varieties Kenwood 94 and Jack were used as susceptible and resistant controls, respectively. (B) A soybean line carrying the Rag1 gene (LD16060) and a related line without Rag1 (SD01-76R) were sowed in soil infested with the root-knot nematode (RKN, Meloidogyne incognita). Plants were collected after 45 days of growth, roots were stained, and galls were quantified. RKN susceptible (Early Girl hybrid) and resistant (Beefmaster hybrid) tomato plants were used as controls. In both experiments ten plants of each line were used, and the experiment was repeated three times with similar results. Results from a representative experiment are shown. Means with the same letter were not significantly different (Fisher’s least-significant-difference test, a = 0.05). * Data were normalized by root volume.

 

Finally, to confirm that Rag1 provided efficient protection against SBA in the soybean line tested, we performed a non-choice growth chamber experiment. The line carrying Rag1 was very poorly colonized by SBA, whereas there were high numbers of SBA on the susceptible line, which lacked Rag1 (Fig. 2). This result confirmed that Rag1 provides strong antibiosis-type resistance against SBA.


   
 

Fig. 2. Effect of Rag1 on soybean aphid infestation. A soybean line carrying the Rag1 gene (LD16060) and a related line withoutRag1 (SD01-76R) were grown in growth chamber. When plants reached the V2 stage (plants with two sets of unfolded trifoliate leaves), 30 aphids were placed on the central leaflet of the V2 leaf, and plants were covered with nets. The number of aphids on each plant was quantified over time. Ten plants of each line were used, and the experiment was repeated three times. A representative experiment is shown. * = statistical difference between lines (a = 0.005).

 

Our results suggest that Rag1 will not be useful as protection against SCN or RKN, although it provides good protection against SBA. However, we cannot discard other effects of Rag1 when both nematodes and SBA are present on the plant simultaneously. Our results also suggest that Rag1 confers resistance to SBA by a mechanism that differs from that of Mi-1.2. Thus, it will be important to dissect such mechanism to understand soybean resistance to SBA.


Literature Cited

1. Hill, C. B., Li, Y., Hartman, G. L. 2006. A single dominant gene for resistance to the soybean aphid in the soybean cultivar Dowling. Crop Sci. 46:1601-1605.

2. Li, Y., Zou, J., Li, M., Bilgin, D. D., Vodkin, L. O., Hartman, G. L., Clough, S. J. 2008. Soybean defense responses to the soybean aphid. New Phytol. 179:185-195.

3. Niblack, T. L., Lambert, K. N., and Tylka, G. L. 2006. A Model Plant Pathogen from the Kingdom Animalia: Heterodera glycines, the Soybean Cyst Nematode. Annu. Rev. Phytopathol. 44:283-303.

4. Rossi, M., Goggin, F. L., Milligan, S. B., Kaloshian, I., Ullman, D. E., Williamson, V. M. 1998. The nematode resistance gene Mi of tomato confers resistance against the potato aphid. P. Natl. Acad. Sci. USA 95:9750-9754