Resistance to bacterial speck disease in tomato occurs when the Pto ki
nase in the plant responds to expression of the avirulence gene avrPto
in the Pseudomonas pathogen. Transient expression of an avrPto transg
ene in plant cells containing Pto elicited a defense response. In the
yeast two-hybrid system, the Pto kinase physically interacted with Avr
Pto. Alterations of AvrPto or Pto that disrupted the interaction in ye
ast also abolished disease resistance in plants. The physical interact
ion of AvrPto and Pto provides an explanation of gene-for-gene specifi
city in bacterial speck disease resistance.