Expression of tomato Cf genes and their corresponding avirulence genes in transgenic tobacco plants using nematode responsive promoters

In this study two plant resistance genes, Cf-4 and Cf-9, were expressed in transgenic plants together with their corresponding pathogen avirulence genes Avr4 and Avr9. Three nematode responsive promoters (4xB5+A, Delta0.3TobRB7 and 35S) in different combinations were used to control the expression of these transgenes. According to previous descriptions of the activity of these promoters, for the promoter combinations used, co-expression would be expected to occur only in the feeding sites of root-knot nematodes, and not in uninfected plants. This being the case, it was predicted that transgenic plants which express a hypersensitive response, induced by nematode feeding, specifically at the feeding site could be engineered. However, transgenic plants that harbored both resistance and avirulence gene constructs underwent spontaneous necrosis, revealing new features of promoter activity. We show that the pattern of hypersensitive response was dependent upon the combination of promoters used to control the expression of the Cf and Avr genes, transgene position effects and growth conditions. Although activity of the of genes has only previously been reported from aerial parts of the plant, here we present evidence that both Cf-9 and Cf-4 are active in tobacco roots.