The primary objective of this study was to investigate the roles of plant proteinase inhibitors in plant-nematode (Meloidogyne spp.) interactions. Transgenic tomato and tobacco plants were employed to examine the effects of proteinase inhibitor I or II transgene on nematode disease development. In the first part of this study, tomato and tobacco root cultures and seedlings aseptically grown in agar medium were developed to test the roles of proteinase inhibitor transgenes in enhancing plant resistance against nematodes. Root galling in cultured root and seedlings expressing inhibitor I or II gene was reduced as compared with controls. Nematode development was also retarded in proteinase inhibitor-expressing root cultures. In the second part of this study, the effects of high expression of proteinase inhibitor I or II transgene on nematode disease development were examined in whole plants grown under greenhouse conditions. It was found that both root galling, nematode egg and egg mass production were inhibited in transgenic tomato plants during the early infection stage. However, this inhibition ceased during the late infection stage. The suitability of cauliflower mosaic virus (CaMV) 35S promoter used for transgene constructs was evaluated in this study. It was found that the expression of proteinase inhibitors, driven by the CaMV 35S promoter, decreased in root tissues of transgenic plants during late nematode infection stage. The developmental expression pattern of proteinase inhibitors in root tissues was clearly correlated with nematode disease development. In addition, the GUS gene, driven by CaMV 35S promoter, was not expressed in gall tissues containing feeding nematodes during the late infection Stage. The results of this study suggested that CaMV 35S promoter might not be suitable for engineering nematode resistant crop plants. Additional experiments were performed to identify the proteolytic activity present in root-knot nematodes at different developmental stages. Both trypsin and chymotrypsin activities were detected in second-stage juvenile extracts. Only trypsin activity was found in female extracts. Both tomato proteinase inhibitor I and II were induced in root tissues in response to nematode infection. The preliminary results of this study further confirmed the involvement of proteinases and proteinase inhibitors in plant-nematode interactions. / Ph. D.