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 gen es Avr4 and Avr9. Three nematode responsive promoters (4xB5+A, Delta0.3TobR B7 and 35S) in different combinations were used to control the expression o f these transgenes. According to previous descriptions of the activity of t hese 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 transgeni c plants which express a hypersensitive response, induced by nematode feedi ng, specifically at the feeding site could be engineered. However, transgen ic plants that harbored both resistance and avirulence gene constructs unde rwent spontaneous necrosis, revealing new features of promoter activity. We show that the pattern of hypersensitive response was dependent upon the co mbination of promoters used to control the expression of the Cf and Avr gen es, transgene position effects and growth conditions. Although activity of the of genes has only previously been reported from aerial parts of the pla nt, here we present evidence that both Cf-9 and Cf-4 are active in tobacco roots.