Overexpression of Pto activates defense responses and confers broad resistance

The tomato disease resistance (R) gene Pto specifies race-specific resistan ce to the bacterial pathogen Pseudomonas syringae pv tomato carrying the av rPto gene. Pto encodes a serine/threonine protein kinase that is postulated to be activated by a physical interaction with the AvrPto protein. Here, w e report that overexpression of Pto in tomato activates defense responses i n the absence of the Pto-AvrPto interaction. Leaves of three transgenic tom ato lines carrying the cauliflower mosaic virus 35S::Pto transgene exhibite d microscopic cell death, salicylic acid accumulation, and increased expres sion of pathogenesis-related genes. Cell death in these plants was limited to palisade mesophyll cells and required light for induction. Mesophyll cel ls of 35S::Pto plants showed the accumulation of autofluorescent compounds, callose deposition, and lignification. When inoculated with P. s. tomato w ithout avrPto, all three 35S::Pto lines displayed significant resistance an d supported less bacterial growth than did nontransgenic lines. Similarly, the 35S::Pto lines also were more resistant to Xanthomonas campestris pv ve sicatoria and Cladosporium fulvum. These results demonstrate that defense r esponses and general resistance can be activated by the overexpression of a n R gene.