Expression of 35S :: Pto globally activates defense-related genes in tomato plants

The tomato (Lycopersicon esculentum) resistance gene Pto confers resistance to the bacterial pathogen Pseudomonas syringae pv tomato carrying the avir ulent gene avrPto. Overexpressing Pto under the control of the cauliflower mosaic virus 35S promoter constitutively activates defense responses in the absence of pathogen infection and nonspecifically enhances disease resista nce. To elucidate the mechanisms underlying this resistance, we isolated cD NAs corresponding to transcripts that accumulated in 35S::Pto plants. By us ing suppression subtractive hybridization, we isolated 82 unique cDNA clone s, most of which corresponded to differentially expressed transcripts. Mos. of the genes examined were also induced by pathogen inoculation. Sequence analysis showed that a large number of genes encode defense-related protein s, and most had not been previously isolated from tomato. The isolated cDNA s also include those with a putative role in the oxidative burst, proteolys is, the hypersensitive response, signal transduction, and a number of genes with unknown functions. The isolation of these cDNAs of diverse functions will assist in the characterization of defense pathways activated during di sease resistance.