Submitted to: Journal of Nematology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 15, 2008
Publication Date: August 15, 2008
Citation: Nahed, R., Macdonald, M.H., Matthews, B.F. 2008. Protease inhibitor expression in soybean roots exhibiting susceptible and resistance reactions to soybean cyst nematode. Journal of Nematology. 40:138-146.
Interpretive Summary: The soybean cyst nematode is the major pathogen of soybean in the U.S. and causes almost $1 billion in damages annually. Protease inhibitors are proteins that interfere with digestion of protein and have been used to inhibit the growth of insects that feed on leaves. We determined that genes encoding six protease inhibitors are expressed in the soybean roots when infected by the nematode. Surprisingly, three of the six protease inhibitors were expressed in much higher levels when the soybean root was susceptible to the nematode as compared to levels found in resistant roots. The other three protease inhibitors were expressed at similarly low levels in resistant and susceptible roots. Thus, none of these protease inhibitors appear to be good candidates for increasing resistance of plants to nematodes. These results are of interest to scientists working to genetically engineer nematode resistance into plants.
Protease inhibitors play a role in regulating proteases during cellular development and in plant defense against insects and nematodes. We identified, cloned and sequenced cDNAs encoding six protease inhibitors expressed in soybean roots infected with soybean cyst nematode. Four of these protease inhibitors have never been reported before. Using RT-PCR, we measured the relative transcript levels of each protease inhibitor in roots of soybean cv. Peking inoculated with either SCN TN8, to which Peking is susceptible, or with SCN NL1-RHg, to which Peking is resistant. Plant roots were harvested at 12h, 1, 2, 4 and 8 dpi and RNA was extracted to measure relative transcript levels using RT-PCR. Within 12 to 24 hours mRNA transcripts encoding of five of the six protease inhibitors were more highly elevated in soybean roots inoculated with SCN TN8 (susceptible reaction) than in roots inoculated with NL1-RHg (resistant reaction). Transcripts of two protease inhibitors possessing Kunitz trypsin inhibitor domains were induced 37- and 27-fold in the susceptible reaction within 1 dpi, but were induced only 5- to 7-fold in roots displaying the resistant reaction. The transcript level for a third soybean root protease inhibitor possessing domain similar to potato inhibitor I was induced over 11-fold at 1 dpi compared to resistant roots, which exhibited no change. A fourth protease inhibitor, similar to soybean serine protease inhibitor p20-1, was only 2-fold induced. The fifth and sixth protease inhibitors possessed Bowman-Birk inhibitor domains. One was induced over 4-fold at 1 dpi, while the other protease inhibitor was not induced. Current literature indicates that SCN initiates feeding at 18 to 48 h after inoculation. Our results indicate that soybean roots recognize different soybean cyst nematode populations presumably before feeding and accordingly express soybean protease inhibitors differentially. Interestingly, these protease inhibitors were generally less abundant in roots exhibiting the resistance reaction.